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MASH Railing Load Requirements for Bridge Deck Overhang (2023)

Chapter: Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts

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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Page 206
Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Page 230
Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
×
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Suggested Citation:"Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts." National Academies of Sciences, Engineering, and Medicine. 2023. MASH Railing Load Requirements for Bridge Deck Overhang. Washington, DC: The National Academies Press. doi: 10.17226/27422.
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195   Overhangs Supporting Curb-Mounted Steel Posts Due to the small footprint of steel-post base plates, steel railings exert highly concentrated flexural and tensile demands on the overhang, often resulting in significant slab damage. Mounting steel railings on a curb, rather than directly to the deck surface, has been shown to be an effective method of reducing overhang damage while largely maintaining the aesthetic and hydraulic benefits of steel railings. In this chapter, all aspects of NCHRP Project 12-119 regarding overhangs supporting curb- mounted steel posts are presented. First, a literature review was performed to identify relevant tested systems and existing design methodologies to inform the objectives of the analytical and testing program. Next, an instrumented test specimen was configured based on preliminary modeling results and subjected to bogie impact testing. Two impact tests were performed. The results of these tests were used to evaluate the accuracy of the corresponding LS-DYNA model and calibrate it as necessary. The calibrated LS-DYNA model was used to characterize the ultimate failure mechanism and load distribution patterns in the overhang. Additionally, the model was extrapolated to evaluate the behavior of other system designs not physically tested. Last, the data pool created in the analytical program was used to develop a proposed design methodology and accompanying specification language. Background and Synthesis of Literature Review A state agency survey and literature review were conducted to collect information regarding overhangs supporting curb-mounted steel railings in order to inform the analytical and testing programs. Key results of these preliminary data collection exercises are briefly summarized in this section. Agency Survey Results Based on survey results from 16 state agencies, top-mounted steel railings were the third- most common railing type used on state inventories. Top-mounted steel railings represented a plurality of railings in Massachusetts and New York. Inventory percentages composed of top- mounted steel railings ranged from 0% to 34%. Top-mounted railings were not separated into deck-mounted and curb-mounted systems in the survey. However, Massachusetts and New York indicated that 100% and 49%, respectively, of their state inventory bridges feature a curb of some kind. Full agency survey results are presented in Appendix A. Observations from Tested Systems Curb-mounted steel post-and-beam railing systems typically cause significantly less overhang damage in impact events than systems mounted directly to the deck surface. However, diagonal C H A P T E R 7

196 MASH Railing Load Requirements for Bridge Deck Overhang cracking of the curb, which may or may not extend into the overhang, is occasionally observed. Examples of overhang damage observed in curb-mounted steel post-and-beam crash-test articles are shown in Figure 283. Existing BDS Design Methodology Design of curb-mounted steel posts is not directly addressed in the existing guidance of the AASHTO LRFD BDS, 9th edition (2). However, the edge-stiffening effect of curbs has been implied in the past. According to Commentary CA13.4.3.1, previous editions of the AASHTO Standard Specifications for Highway Bridges allotted an additional 1.25 ft of longitudinal dis- tribution for posts mounted on curbs. Alternative Design Methodologies No alternative design methodologies for curb-mounted steel posts were identified in the literature. However, design calculations for the curb-mounted MASSDOT S3TL-4 curb-mounted bridge railing were obtained, which demonstrated capacity calculations specific to curb- mounted systems. These calculations included a modified punching shear capacity calculation considering the additional strength granted by the curb and an edge-beam torsional capacity calculation. Objectives and Scope of Analytical and Testing Programs The primary objective of the analytical and testing programs for overhangs supporting curb- mounted steel posts was to better characterize the overhang strength required to develop the full capacity of the attached post without sustaining significant damage. As testing and modeling of deck-mounted steel posts were performed prior to that of curb-mounted posts, testing and modeling of these systems were largely focused on quantifying the capacity benefit granted by the curb. Secondary objectives of the analytical and testing programs for overhangs supporting curb- mounted posts included quantifying effective tensile and flexural demands at Design Region B-B. In particular, a goal of this program was to quantify the extent to which the stiffening effect of the curb resulted in greater longitudinal distribution of impact loads through the overhang. Further, the effects of varying certain design parameters on the overhang capacity and load distributions were briefly investigated. TxDOT T131RC (37) Alaska 2-Tube Railing (38) Figure 283. Damage to curb-mounted steel post-and-beam railing specimens.

Overhangs Supporting Curb-Mounted Steel Posts 197   Impact Tests of Curb-Mounted Steel-Post Specimen Two impact tests were performed on curb-mounted steel-post specimens in order to measure the longitudinal distribution of impact loads through the curb and deck overhang. In addition to providing physical data points, test results were used to evaluate the accuracy of the accom- panying LS-DYNA models. Further, physical test data informed the proposed methodology for analyzing the capacity of curbed slabs supporting steel posts. Test Specimen Details The cross section of the curb-mounted steel-post specimen is shown in Figure 284. Slab steel, post, base plate, and anchor bolt configurations were unchanged from the deck-mounted specimen. The 8 in. × 16 in. curb was anchored to the slab with #4 bars spaced at 12 in. Longi- tudinal curb reinforcement consisted of two #4 bars. Base plate details are previously shown in Figure 225. The test specimen curb was configured to be a sacrificial element that would provide a stiffening effect to the slab edge while limiting the force that could be transferred to the slab. This design philosophy was selected to reduce the uncertainty of the loads transferred to the slab and to physically evaluate the effectiveness of a weak-curb system in reducing slab damage without diminishing the capacity of the post relative to a deck-mounted system. The design compressive strength of the slab and curb concrete was 5,000 psi, and the design yield stress of all reinforcing steel was 60 ksi. Nominal and as-tested bending strengths of the slab and post, as well as the load application height and post base shear, are shown in Table 18. Figure 284. Curb-mounted steel-post specimen. Nominal As-Tested Mpost = 63 k-ft 70 k-ft Plastic moment capacity of post, FyZx He = 21 in. 21 in. Load application height above top of curb Ppost = 36 kips 40 kips Post base shear associated with Mpost Mst = 24 k-ft/ft 32 k-ft/ft Transverse bending capacity of slab Mcurb = 14 k-ft/ft 19 k-ft/ft Cantilever bending capacity of curb Table 18. Nominal and as-tested parameters for curb-mounted steel-post specimen.

198 MASH Railing Load Requirements for Bridge Deck Overhang Locations of interior and end-region curb-mounted post tests are shown in Figure 285. The interior test was performed before the end-region test. LS-DYNA models were used to configure the locations of each test such that interior behavior could be developed for the interior test without influencing the subsequent end-region test. Instrumentation To directly measure specimen deformations during the test, strain gages were installed on transverse reinforcement at two critical sections. A row of strain gages was placed near Design Region A-A, 2 in. inside of the traffic-side vertical curb bars and at Design Region B-B, over the field edge of the grade beam. Primary strain gages, indicated by the red dots, that were operational during interior and end-region curb-mounted steel-post tests are shown in Figures 286 and 287, respectively. Interior Test Impact Conditions Loading was applied to the specimen via a surrogate bogie vehicle impact. In the test, the 5,378-lb bogie vehicle was to impact the post at a target speed of 13 mph and an impact angle of 90 degrees. The actual impact speed was 13.2 mph. To account for the increased stiffness of the curb-mounted system relative to the deck-mounted system, the height of the center crush tube was increased from 5 in. to 10 in. Heights of adjacent crush tubes remained at 5 in. The test setup for the curb-mounted steel-post test is shown in Figure 288. Figure 285. Curb-mounted steel-post test locations. Figure 286. Longitudinal distribution of transverse slab-bar strain gages for the interior test. Figure 287. Longitudinal distribution of transverse slab-bar strain gages for the end-region test.

Overhangs Supporting Curb-Mounted Steel Posts 199   Interior Test Specimen Response Sequential photos of the interior curb-mounted steel-post impact test are shown in Figures 289 and 290. In the event, the post successfully contained the bogie vehicle, and the curb sustained extreme damage. The crush tubes exhausted their entire stroke length during the impact event. The force exerted on the bogie by the curb-mounted post, as measured via onboard acceler- ometers, is shown in Figure 291. The curve is the average of the two accelerometer records passed through a CFC-60 filter. The peak force measured in the test was 53 kips. The peak 50-ms average force measured in the test was 33 kips. Crush tube yielding began in the test around 10 ms after the point of first contact. Around 35 ms after contact, strain hardening in the crush tubes resulted in an increasing lateral load exerted between 35 ms and 65 ms. Between 60 and 70 ms after first contact, a significant amount of curb concrete damage rapidly evolved in the specimen, and the bogie began to rebound from the post. Interior Test Damage Damage to the curb-mounted steel-post specimen after the interior test is shown in Figures 292 through 298. While extreme damage was sustained by the curb, the deck slab sustained minimal damage. The ultimate failure mechanism of the interior specimen was a flexural and/or torsional failure of the curb. Diagonal cracks on the front face of the curb—the first to develop during the test— spiraled around the curb with longitudinal transmission. Traffic-side vertical and longitudinal curb bars were deformed significantly as the embedded washer plate lifted from the deck surface. Based on the undisturbed surface under the washer plate (Figure 297), it is assumed that little cohesive stress was developed between the slab and the washer plate, although the washer plate was cast into the top surface of the slab. The post and base plate remained undeformed. Interior Test Strain Gage Data Linear strain gages were fastened to specimen reinforcement at two locations: top-mat trans- verse slab steel at Design Region A-A and top-mat transverse slab steel at Design Region B-B. Figure 288. Interior curb-mounted post specimen and bogie impact orientation.

200 MASH Railing Load Requirements for Bridge Deck Overhang Peak strain gage measurements on slab bars at Design Regions A-A and B-B are shown in Figures 299 and 300, respectively. End-Region Test Impact Conditions Loading was applied to the specimen via a surrogate bogie vehicle impact. In the test, the 5,378-lb bogie vehicle was to impact the post at a target speed of 8 mph and an impact angle of 90 degrees. e actual impact speed was 7.7 mph. e test setup for the end-region curb-mounted steel-post test is shown in Figure 301. End-Region Test Specimen Response Sequential photos of the end-region curb-mounted steel-post impact test are shown in Figure 302. In the event, the post successfully contained the bogie vehicle, and the curb sustained moderate damage. No deck damage was observed. Bogie crush tubes exhausted roughly half of their stroke length. e force exerted on the bogie by the curb-mounted post, as measured via onboard acceler- ometers, is shown in Figure 303. e curve shown is the average of the two accelerometer records passed through a CFC-60 lter. e peak force measured in the test was 22 kips. e peak 50-ms average force measured in the test was 19 kips. Figure 289. Sequential images of interior curb-mounted steel-post test, rear view.

Overhangs Supporting Curb-Mounted Steel Posts 201   Figure 290. Sequential images of interior curb-mounted steel-post test, front view. Figure 291. Lateral load exerted on bogie in the interior curb-mounted steel-post test.

202 MASH Railing Load Requirements for Bridge Deck Overhang Figure 292. Interior curb-mounted steel-post test damage, front view. Figure 293. Interior curb-mounted steel-post test damage, rear view. Figure 294. Curb failure in interior test. Figure 295. Curb failure in the interior test (with loose concrete removed).

Overhangs Supporting Curb-Mounted Steel Posts 203   Figure 296. Curb failure in the interior test (with loose concrete removed). Figure 297. Undisturbed deck surface under the washer plate. Figure 298. Undamaged base plate and post.

204 MASH Railing Load Requirements for Bridge Deck Overhang Figure 299. Peak strain gage measurements at Region A-A in interior curb-mounted steel-post test. Figure 300. Peak strain gage measurements at Region B-B in interior curb-mounted steel-post test. Figure 301. End-region curb-mounted steel-post test setup.

Overhangs Supporting Curb-Mounted Steel Posts 205   Figure 302. Sequential images of end-region curb-mounted steel-post test, rear view.

206 MASH Railing Load Requirements for Bridge Deck Overhang Crush tube yielding began in the test around 10 ms after the point of first contact. Around 35 ms after contact, strain hardening in the crush tubes resulted in an increasing lateral load exerted between 35 ms and 65 ms. At 100 ms, the bogie began to rebound from the post. End-Region Test Damage Damage to the curb-mounted steel-post specimen after the end-region test is shown in Figures 304 and 305. Moderate damage was sustained by the curb, and no deck damage was observed. Curb damage consisted of minor diagonal cracking consistent with a punching shear or torsional mechanism, minor interface cracking, and a moderate vertical crack on the back face. Further, a minor anchor bolt breakout cone formed adjacent to the traffic-side bolt nearest the free end of the curb. End-Region Test Strain Gage Data Linear strain gages were fastened to specimen reinforcement at two locations: the top-mat transverse slab steel at Design Region A-A and the top-mat transverse slab steel at Design Region B-B. Peak strain gage measurements on slab bars at Design Region A-A are shown in Figure 306. Design Region B-B strains were negligible. Figure 303. Lateral load exerted on bogie in end-region curb-mounted steel-post test. Figure 304. End-region curb-mounted steel-post test specimen damage.

Overhangs Supporting Curb-Mounted Steel Posts 207   Figure 305. End-region curb-mounted steel-post test specimen damage with marked cracking. Figure 306. Peak strain gage measurements at Region A-A in end-region, curb-mounted steel-post test.

208 MASH Railing Load Requirements for Bridge Deck Overhang Reserve Capacity Test Although a clear strain distribution was observed for Design Region A-A in the end-region curb-mounted steel-post test, recorded strains were small (roughly 20% of yield). Further, strains recorded at Design Region B-B were minimal. As such, a second impact test was per- formed on the damaged end-region specimen to further define the slab strain distribution, characterize the ultimate capacity of the post, and evaluate the reserve capacity of the post in a second impact. In the second impact test on the end-region, curb-mounted steel post, the target bogie speed was 11 mph, and the actual impact speed was 11.3 mph. In the test, the post failed to completely arrest the bogie velocity, and bogie brakes were applied after system failure. The final position of the bogie vehicle is shown in Figure 307. The force exerted on the bogie by the curb-mounted post, as measured via onboard acceler- ometers, is shown in Figure 308. The curve shown is the average of the two accelerometer records passed through a CFC-60 filter. The peak force measured in the test was 35 kips. The peak 50-ms average force measured in the test was 17 kips. Although the system produced a higher peak load in the second test, the overall energy dissipation was reduced, and the system experienced a brittle failure of the curb due to bar pullout. Peak 50-ms average forces were similar between the two tests. Figure 307. Final resting place of bogie vehicle after the test. Figure 308. Lateral load exerted on bogie in a test of damaged end-region, curb-mounted steel-post.

Overhangs Supporting Curb-Mounted Steel Posts 209   As shown in Figure 309, the post and base plate assembly were completely broken out of the curb. A trapezoidal breakout cone was formed in the curb. Post assembly breakout occurred due to the pullout of the traffic-side longitudinal and vertical curb bars. The longitudinal curb bar, which had a straight termination on the free end of the curb, pulled out from the end of the curb from underneath the outermost vertical curb bar, interrupting load transfer to that bar. Consequently, a nearly vertical shear failure occurred on the outer edge of the post assembly. Due to the loss of load transfer on one side of the post, the vertical curb bar passing through the anchorage was subjected to significant loading, and the bar unhooked and pulled out from the slab. Evidence of this mechanism is shown in Figure 310. Damage to the deck slab is shown in Figures 311 and 312. As vertical curb bars were deflected backward, top and field-edge cover were spalled from the slab. Field-edge cover spalling occurred on the top half of the deck slab. Cover on the bottom half of the field edge and the bottom face of the slab remained intact. No other slab damage was observed. Strain gage data collected in the second impact test of the end-region post at Design Regions A-A and B-B are shown in Figures 313 and 314, respectively. The peak Region A-A strain recorded in the second test was 50% greater than that of the first test but remained minor. Design Region B-B strains were minimal but reached a magnitude sufficient for confident char- acterization of a longitudinal distribution. Discussion of Test Results Results of the interior and end-region tests of curb-mounted steel posts were consistent with expectations. The intended behavior of the system was curb failure with limited deck damage. Figure 309. Front view of system damage. Figure 310. Pullout of longitudinal and vertical curb bars.

210 MASH Railing Load Requirements for Bridge Deck Overhang Figure 311. Curb damage extending into the slab. Figure 312. Partial-depth field-edge cover spalling of the slab. Figure 313. Peak strains at Design Region A-A in the repeat test of end-region post.

Overhangs Supporting Curb-Mounted Steel Posts 211   The curb was the limiting element in all tests, and deck damage was only sustained in the cata- strophic failure produced in the second test of the damaged end-region specimen. Although the curb was designed to be sacrificial, the post was able to exert lateral loads on the bogie vehicle that were 30% higher than the deck-mounted post due to the decreased lever arm provided by the curb. Test results also suggested a longitudinal distribution angle of post demands through the curb of at least 45 degrees. For the interior test, three vertical curb bars participated in resisting the post demands, although the base plate width was only 16 in., and bars were spaced at 12 in. For the end-region test, premature failure occurred due to longitudinal bar pullout, which restricted the longitudinal distribution of loads through the curb. Calibrated Curb-Mounted Steel-Post Models Using the data produced in the physical tests of the curb-mounted steel posts, the accuracy of the LS-DYNA models created prior to the test was evaluated, and the model was calibrated as necessary. The LS-DYNA model was first evaluated in its ability to predict the overall response of the system, including force-time history and damage, and then in its ability to predict internal rebar strains. After calibration, the LS-DYNA model was used to further investigate the behavior of the test specimen. The second impact of the end-region specimen was not modeled in LS-DYNA, as model accuracy would rely on an accurate representation of physical damage from the previous test, which could not be reliably performed using only available surface observations of damage. Further, the failure mechanism in the second test was bar pullout, which the LS-DYNA models are not configured to capture. Calibration Process and Response Accuracy The LS-DYNA model of the interior test created prior to the event, which used the K&C concrete model, underpredicted the ultimate capacity of the post by roughly 15%. To achieve better agreement with the test data, a contact algorithm was added to allow the embedded washer plate and curb reinforcement to directly contact adjacent reinforcement following erosion of the surrounding concrete. This change was made to the model based on physical test observations. In the test, the washer plate bore on the longitudinal curb bar, which in turn was confined by the vertical curb bars. In the original LS-DYNA model, this mechanism was not captured as the Figure 314. Peak strains at Design Region B-B in the repeat test of end-region post.

212 MASH Railing Load Requirements for Bridge Deck Overhang surrounding concrete had eroded and interrupted the load transfer mechanism. After applying this change and slightly modifying the concrete erosion strain, an acceptable level of accuracy was achieved. The force history calculated by the LS-DYNA model is compared to the physical test measurement in Figure 315. The LS-DYNA model of the end-region test produced an acceptably accurate representation of the event without any further calibration. The force history calculated in the end-region model is compared to the physical test measurement in Figure 316. Predicted Damage Damage predicted by the LS-DYNA models was consistent with the damage observed in the physical tests. In the interior test model, the curb failed in a combination of flexure, torsion, and shear, and the post anchorage was able to rotate significantly, as shown in Figure 317. In the end-region test model, only moderate damage was predicted, as was observed in the physical test. Damage calculated in the end-region LS-DYNA model is shown in Figure 318. Minimal slab damage was predicted by the model, and no slab damage was observed in the test. Figure 315. Model force-history comparison for the interior curb-mounted steel-post test. Figure 316. Model force-history comparison for end-region curb-mounted steel-post test.

Overhangs Supporting Curb-Mounted Steel Posts 213   Comparison to Strain Gage Measurements Peak LS-DYNA strains at Region A-A are compared to peak strain gage measurements recorded in the interior test in Figure 319. As shown, the LS-DYNA model produced accurate estimates of transverse bar strains at Design Region A-A. Further, at Design Region B-B, the LS-DYNA model predicted minimal strains after self-weight strains were removed. The LS-DYNA model predicted a peak Region B-B strain of roughly 100 microstrains (µε), and the maximum physical test value was 81 µε. Peak LS-DYNA strains at Region A-A are compared to peak strain gage measurements recorded in the end-region test in Figure 320. As shown, the LS-DYNA model produced accurate estimates of transverse bar strains at Design Region A-A. Strains calculated at Design Region B-B were negligible after accounting for self-weight, which was consistent with physical test measurements. Figure 317. LS-DYNA model damage prediction for the interior test. Figure 318. LS-DYNA model damage prediction for the end-region test.

214 MASH Railing Load Requirements for Bridge Deck Overhang Discussion of Calibrated LS-DYNA Model As the curb-mounted post-test models exhibited acceptably accurate predictions of the overall force-deflection response of the specimens, the post-test damage profiles, and strain gage measurements, the models were deemed adequately calibrated. As such, the models were able to be used as a baseline for other investigative models, such as static loading and design variation models. Further, as no adjustments were required to produce acceptably accurate results, no adjustments to the models created in the preceding analytical program were required. Inertial effects in the curb-mounted steel-post tests did not become significant until extreme damage had occurred, and large debris fields were accelerated from the curb. As such, the dynamic impact models were deemed sufficient to characterize flexural and tensile demands in the slab. Extrapolative Modeling The calibrated LS-DYNA model of the impact test was modified to investigate load distribu- tions, overhang capacity, and the effects of design variations on system behavior. First, the basic load distribution pattern at Design Regions A-A and B-B was established. Then, the effects of varying design parameters on this distribution were characterized. Figure 319. Comparison of peak LS-DYNA strains and strain gage measurements for the interior test. Figure 320. Comparison of peak LS-DYNA strains and strain gage measurements for end-region test.

Overhangs Supporting Curb-Mounted Steel Posts 215   Basic Load Distribution in Calibrated Model To characterize the effective moment and tension demands in the slab at the peak load reached by the post, the calibrated impact model was converted to a quasi-static pushover model. Moment and tension demands calculated in the slab at the peak lateral load reached by the post, which was 34.2 kips, are shown in Figure 321. At the peak load of 34.2 kips, the total moment acting on the slab was 79.8 k-ft. Therefore, the peak Region A-A moment of 9.4 k-ft/ft corresponded to an effective distribution length of 8.5 ft. As the base plate was 16 in. wide, and the curb was 8 in. tall, this result suggests an extensive longitudinal distribution of post demands through the curb. In this case, the effective distribution angle of loads with downward transmission through the curb from the edges of the plate was 79.5 degrees. The effective distribution angle of loads with inward transmission through the overhang can be calculated using the model results similar to the approach adopted for barriers. At the peak load, which corresponded to a total moment of 79.8 k-ft, the peak Region B-B moment Figure 321. Slab moment (top) and tension demands (bottom) at peak post-load.

216 MASH Railing Load Requirements for Bridge Deck Overhang (neglecting self-weight) was 5.9 k-/. erefore, the eective distribution length at Region B-B was 13.5 . If the distribution angle in the curb is assumed to be 45 degrees, this length cor- responds to an eective distribution angle of 72.9 degrees between the trac-side vertical curb steel and the eld edge of the grade beam. Parametric Variations of the Calibrated Model Eects of design variations on system behavior were evaluated by varying the calibrated model. Twenty models were subjected to quasi-static pushover loading to failure, and their force-deection response and point of rst slab-bar yield were recorded. Design values varied in the models and included curb vertical steel, curb longitudinal steel, curb height, curb edge distance, curb-to-deck interface type (monolithic versus unbonded), washer plate location, slab transverse steel, and slab longitudinal steel. ese models were created in order to develop and rene the design methodology for overhangs supporting curb-mounted steel posts. With the exception of the two models in which the washer plate location was varied, all models shown in this section used post subassemblies, which were anchored with a washer plate embedded just below the bottom face of the curb, as this detail was used in the physically tested system. Further, modeling results had shown that this detail was weaker than attachment details in which the washer plate was embedded within the slab core or under the slab, making it a critical detail that provides a worst-case behavior for curb-mounted systems. A review of state details indicated that this detail is also the most common curb-mounting detail in use on state bridge inventories. e baseline system design, which was used for the physical test, is shown in Figure 322. Additionally, the specimen damage sustained in the interior bogie test is shown. e ultimate failure mechanism in the physical test was a exural failure of the curb. No deck damage was sustained. When modeling the system, nominal material properties were used. Loading was applied 29 in. above the deck surface. Effect of Vertical Curb Steel As the ultimate failure mechanism in the physical test was curb exure and/or torsion, the rst model variation decreased the vertical curb bar spacing from 12 in. to 4 in. is change was implemented to move the failure mechanism from the curb into the slab. To develop more mean- ingful modeling results and investigate the eects of various design modications on overhang behavior, this change was maintained in all the following models discussed in this section. In the baseline model, curb vertical bars began to yield at a lateral load of 20 kips, and the ultimate lateral load exerted by the post was 34 kips. e design methodology developed in Figure 322. Baseline system design and damage in the physical test.

Overhangs Supporting Curb-Mounted Steel Posts 217   this project (discussed in a later section, in the proposed Section 13 modifications, and in the accompanying design example) predicted curb yielding at 20 kips. After reducing the vertical bar spacing from 12 in. to 4 in., the peak lateral load was increased from 34 kips to 50 kips. First slab yielding appeared at 50 kips in the field-edge longitudinal bar. The proposed methodology predicted a slab yield-line failure at 43 kips. Damage contours at the peak load are shown for each LS-DYNA model in Figure 323. As shown, in the baseline model (12-in. curb bar spacing), curb failure governed the ultimate strength of the system, whereas in the modified model, significant deck damage was sustained in the event. It should be noted that improved capacity could be achieved by increasing curb steel in the post region only, rather than over the entire span. Effect of Longitudinal Curb Steel The effect of curb longitudinal steel on system behavior was evaluated by decreasing and increasing the size and/or quantity of bars. The baseline system, which used the 4-in. curb bar spacing discussed in the previous section, included two #4 longitudinal curb bars. The peak lateral capacity in this model was 50 kips. When the longitudinal curb bar size was reduced from #4 to #3, the capacity was reduced to 49 kips. When the longitudinal curb bar size was increased from #4 to #5, and the number of bars was increased from two to five, the capacity was increased to 51 kips. These results suggest that, for the modeled system, longitudinal curb steel had a negligible effect on system behavior. For curbs with less vertical steel, this effect may be more pronounced, but benefits may only be realized after sustaining severe deck damage. Effect of Curb Height As the curb stiffens the deck edge and provides an area over which post loads can distribute before reaching the slab, curb height has a significant effect on system performance. To evaluate this effect, models were created in which the curb height was varied from 4 in. to 16 in. Vertical curb bar spacing was 4 in. for each model, and two #4 longitudinal curb bars were added per 4 in. of curb height to maintain a roughly constant vertical-axis bending strength per unit height of curb. Responses of each model, including the lateral loads at which the first bar yield occurred and the ultimate lateral capacity, are summarized and compared to proposed methodology predictions in Figure 324. As shown, both the yield load and ultimate load increased roughly 12-in. curb bar spacing (first yield = 20 kips) 4-in. curb bar spacing (first yield = 50 kips) Figure 323. Effect of vertical curb steel spacing on system damage at peak load.

218 MASH Railing Load Requirements for Bridge Deck Overhang linearly with increasing curb height. The proposed methodology also predicted a linear relation- ship between curb height and post capacity, although predicted capacity increases were conser- vatively less sensitive to curb height. The underprediction of capacity increases associated with increasing curb height is likely the result of the methodology neglecting the longitudinal bending capacity of the curb, which is increased significantly with increased height. In the methodology, this effect was neglected due to uncertainty in the extent to which the curb and slab act as a composite member in flexure. Damage at the ultimate load for the 4-in. curb and 16-in. curb models is shown in Figure 325. As shown, significantly more damage was sustained in the 4-in. curb model, which had a capacity of roughly 45% of the 16-in. curb model. Further, with increasing curb height, the system behavior began to transition into a damage mechanism resembling a barrier yield-line failure. Effect of Curb Edge Distance Curb edge distance, which is the transverse distance between the field side of the curb and the field side of the slab, was varied from 0 in. to 8 in. In the baseline model, the edge distance was 2 in. As shown in Figure 326, modeling results suggested a roughly linear relationship between curb edge distance and ultimate capacity, which was accurately predicted by the proposed meth- odology. Beyond 4 in. of edge distance, the model system capacity was limited by curb failure; Figure 324. Effect of curb height on yield load and ultimate lateral load. 4-in. curb height 16-in. curb height Figure 325. Effect of curb height on damage mechanism.

Overhangs Supporting Curb-Mounted Steel Posts 219   therefore, additional increases to slab capacity may have occurred beyond this limit but were not demonstrated in the model. Deck damage was significantly reduced with increased edge distance, as models with low edge distance were susceptible to field-edge spalling (Figure 327). Effect of Curb-to-Deck Interface Type As curbs are typically poured after the deck slab rather than monolithically, the effect of curb-deck interface continuity on overall behavior was evaluated using the calibrated model. In the baseline model, the curb-deck interface was unbonded, and any composite behavior between the curb and slab was due to contact friction and steel dowel action. The ultimate capacity of the baseline model with 4-in.-vertical curb bar spacing was 50 kips. When the interface was modi- fied to be continuous (similar to a monolithic pour), the capacity increased to 57 kips. In cases where pouring the slab and curb monolithically is feasible and economical, performance may be improved due to increased composite action and torsional strength. However, as curbed over- hangs are not typically poured monolithically, and accounting for monolithic behavior would require appreciable modifications, this effect was not accounted for in the proposed methodology. 0-in.-edge distance 8-in.-edge distance Figure 326. Effect of curb edge distance on yield load and ultimate lateral load. Figure 327. Effect of curb edge distances on damage mechanism.

220 MASH Railing Load Requirements for Bridge Deck Overhang Effect of Transverse Slab Steel To investigate the effect of transverse slab steel on system performance, models were created in which transverse steel areas were decreased and increased relative to the baseline system. The baseline system, which had an ultimate capacity of 50 kips, used #5 bars spaced at 4 in. Varia- tions on this model included #4 bars at 6 in., #4 bars at 4 in., and #6 bars at 4 in. Yield load and ultimate loads recorded in each model are compared to proposed methodology predictions in Figure 328. As shown, increasing the transverse slab steel did not have a significant effect on the ultimate lateral capacity of the post but was effective in increasing the load at which transverse bar yielding occurred in the slab. Capacities predicted by the proposed methodology were success- ful in capturing this behavior. It is assumed that the ultimate lateral capacity was not sensitive to the transverse slab steel area due to the stiffening effect of the curb. After slab yielding occurs, the curb may be able to distribute loads longitudinally to engage more deck bars, resulting in an effectively unchanged capacity at the expense of deck damage. Effect of Longitudinal Slab Steel Longitudinal slab steel had an appreciable effect on the ultimate capacity of deck-mounted steel posts. This effect was also evaluated for curb-mounted steel posts by varying longitudinal steel configurations in the slab near the field edge. In the baseline model, which had a capacity of 50 kips, longitudinal reinforcement under the post consisted of one #4 bar per mat. Models were created in which the top-mat longitudinal bar was removed, longitudinal bars were increased to #5, and two additional #5 longitudinal bars were added to the bottom mat. As shown in Figure 329, neither the yield load nor the ultimate load was sensitive to the longitudinal slab steel. It is assumed that this is due to the stiffening effect of the curb; with increasing curb height and curb steel, the system behavior approaches that of a barrier. For overhangs with barriers, behavior is not significantly affected by longitudinal slab steel. For low levels of longitudinal reinforcement, the proposed methodology underpredicted the capacity of the post. With increas- ing longitudinal reinforcement, the proposed methodology predictions narrow errors to closely match the model results. Differences between analytical and proposed methodology predic- tions are likely due to underpredicted participation of the curb and distribution of moment and torsional demands along the length with more closely spaced curb hoop reinforcing than was provided in the crash-test specimen. The proposed methodology does not significantly change the footprint of the deck yield-line mechanism as curb hoop reinforcing varies. With closer curb hoop reinforcing, the yield-line is likely to extend to engage a longer base width, so that Figure 328. Effect of transverse slab steel on yield load and ultimate lateral load.

Overhangs Supporting Curb-Mounted Steel Posts 221   ultimate capacity is more sensitive to transverse than longitudinal deck reinforcing. Although the proposed methodology does not capture the analytical insensitivity to longitudinal reinforcing, this observation should be taken in context with the condition that the curb reinforcing was significantly greater than typical construction with the intent to examine deck behavior sensi- tivities. The proposed methodology intends to balance implementable feasibility with reasonably realistic approximate representations of mechanical behavior, not perfectly represent mechanical behavior. Effect of Post Attachment Details As anchorage to the bridge deck with a washer plate resting on the top surface of the slab (just below the curb) is the most common railing attachment detail used on state inventories, all previous models and the physically tested system used this detail. However, railings can also anchor to the overhang with a washer plate embedded within the slab core, under the top steel mat, or with a through-bolt mechanism in which the washer plate is below the slab. Differences in performance between these attachment types were investigated using variations of the base- line model. These models are shown in Figure 330. It should be noted that, in Figure 330, the Figure 329. Effect of longitudinal slab steel on yield load and ultimate lateral load. Slab top pocket (baseline) Plate embedded in core Through-bolt Figure 330. Post attachment details subjected to pushover modeling.

222 MASH Railing Load Requirements for Bridge Deck Overhang rectangular chutes visible at bolt locations represent the transition between concrete material models necessary for fine, distorted meshes at bolt holes. Bolts were discretely modeled as circular solid shafts that pass through the rectangular blocks visible in the figure. Force-deflection responses of each post are shown in Figure 331. As shown, the behavior of the post and overhang was largely unchanged when the anchoring washer plate was moved from the top surface of the slab to within the slab core. However, when the washer plate was moved to the bottom surface of the slab, and the base plate and washer plate effectively sand- wiched the slab and curb, the capacity of the system was significantly increased. It should be noted that in the models used to investigate differences between these details, posts and connection hardware were modeled as elastic to ensure failure occurred in the slab or curb. In physical practice, care should be taken with through-bolted systems to ensure bolt yielding does not occur. Supplementary modeling results in which bolts were allowed to yield sug- gested that through-bolt attachment mechanisms are highly susceptible to bolt bending and lateral anchorage breakout. Parametric Variations of Through-Bolted System Models of a modified version of the tested system in which the lower washer plate was underneath the deck slab, were also created. In these models, bolts extended from the top surface of the curb to the bottom surface of the slab. System details were modified, and the effects of modifications on load distributions were recorded. A through-bolt attachment mechanism was used for this portion of the investigation to ensure curb failure did not affect results. Direct Comparison to Deck-Mounted System To evaluate the extent to which the behavior of a steel post on the deck overhang is affected by the inclusion of a concrete curb, a baseline deck-mounted W6×25 steel-post model was modified to include an 8-in.-tall, 14-in.-wide curb. The baseline curb-mounted steel-post model, which used a through-bolt attachment mechanism, is shown in Figure 332. The deck configura- tion used in this model was identical to the baseline design used in the deck-mounted steel-post models to allow for isolation of the curb’s effect on behavior. The curb was anchored to the deck using #4 bars spaced at 12 in. Curb anchors were moved as needed to allow for placement of the post and base plate assembly. Figure 331. Effect of anchor plate location on force-deflection response of post.

Overhangs Supporting Curb-Mounted Steel Posts 223   Unlike the baseline deck-mounted W6×25 model in which the deck failed in punching shear, the W6×25 was able to fully develop its plastic moment capacity when mounted to a curb. Moments calculated at Regions A-A and B-B at post plastification are shown in Figure 333. Load distributions observed in the curb-mounted post model were similar to those observed in barrier models. Damage to the curb and deck system at plastification of the W6×25 post is shown in Figure 334. At post plastification, significant shear and torsional damage had occurred in the curb, but only minimal damage was incurred by the deck. Figure 332. Baseline curb-mounted steel-post model. Figure 333. Moments at Regions A-A and B-B at plastification of curb-mounted W6325 post.

224 MASH Railing Load Requirements for Bridge Deck Overhang Prior to post plastification, the point of first yield occurred in the longitudinal curb bar behind the anchor bolts. The bar stress state at this point is shown in Figure 335. The baseline behavior of the curb-mounted W6×25 post indicates an extreme reduction of deck demands for curb-mounted posts relative to posts mounted directly to the deck surface. In Figures 336 and 337, moment demands at Regions A-A and B-B are compared between the curb-mounted and deck-mounted W6×25 posts at failure. The addition of an 8-in.-tall, 14-in.-wide curb reduced deck demands at post failure by 66% at Region A-A and 26% at Region B-B. Figure 334. Damage to curb and deck at plastification of W6325 post. Figure 335. Bar stresses at the point of first yield in loading of curb-mounted W6325 post.

Overhangs Supporting Curb-Mounted Steel Posts 225   Attachment Details and Deck Damage As for deck-mounted posts, the attachment mechanism of the post to the deck also affected curb-mounted post behavior. As shown in Figure 338, when the washer plate was moved from the bottom face of the deck to inside the deck just below the upper transverse steel, damage mechanisms in the curb and deck were changed substantially. For the embedded-plate attach- ment, damage to the curb was less severe; however, the deck suffered more damage. It should be noted that, while the embedded-plate detail caused extreme damage to the deck overhang in the deck-mounted application, the detail did not result in severe deck damage in the curb-mounted application. This finding justifies a trend observed in the survey of state DOT standard details— embedded-plate details are very common for curb-mounted systems, while deck-mounted systems are dominated by through-bolt details. Figure 336. Comparison of Region A-A moments at punching shear failure of the deck-mounted W6325 and plastification of the curb-mounted W6325. Figure 337. Comparison of Region B-B moments at punching shear failure of the deck-mounted W6325 and plastification of the curb-mounted W6325.

226 MASH Railing Load Requirements for Bridge Deck Overhang Effect of Span Length on Load Distribution In the barrier analytical program, it was found that demands in the deck overhang are sig- nificantly affected by the continuous span length. For barrier systems, as the span length was decreased, demands in the deck increased, and at a certain breakpoint span, the behavior of the system changed dramatically, developing peak Region B-B moments comparable to or poten- tially exceeding Region A-A moments. To investigate the extent to which that behavior occurs for curbed systems, the span length of the curb-mounted steel-post model was reduced incre- mentally from 50 ft until the span breakpoint was observed. This variation showed that, for this particular combination of overhang cantilever distance (3 ft), curb dimensions (8 in. by 14 in.), and deck thickness (8 in.), the span breakpoint was between 15 and 20 ft. For span lengths less than this breakpoint, demands increased noticeably at Region B-B, as shown in Figure 339. (a) Through-bolt attachment (b) Embedded-plate attachment Figure 338. Comparison of deck damage between through-bolt attachment detail and embedded-plate attachment detail at post plastification.

Overhangs Supporting Curb-Mounted Steel Posts 227   Moment demands calculated at Regions A-A and B-B for the 50-ft-, 30-ft-, and 15-ft-span curb models are shown in Figures 340 and 341. No differences were observed between the 50-ft- and 30-ft-span models. However, when the span length was reduced to 15 ft, both the Region A-A and B-B peak moments increased, particularly at Region B-B. Effect of Overhang Cantilever Distance on Load Distribution In the barrier analytical program, a clear trend was observed for the effect of cantilever dis- tance on deck demands—as the cantilever distance was increased, the deck overhang became more flexible, load distributions expanded, and demands were reduced. This trend was main- tained for curbed systems, although the effects were less significant. Moment demands calculated at Regions A-A and B-B for the curb-mounted W6×25 post on a 3-ft and 5-ft cantilever are shown in Figures 342 and 343. Effect of Curb Height on Load Distribution The curb height (and to a lesser extent, the curb width) has two effects on system behavior: increasing the curb height results in larger planes resisting punching shear failure, and increas- ing the curb height further stiffens the deck edge. As such, it was expected that increased curb heights would result in greater punching shear strengths and reduced deck demands. Damage to the curb and deck at plastification of the W6×25 post is shown for 6-in., 8-in., and 12-in. curbs in Figure 344. As the curb height was increased, curb and deck damage were Figure 339. Comparison of deck damage between 50-ft-span and 15-ft-span curb models. Figure 340. Comparison of Region A-A moment demands with varying span lengths and an 8-in. curb.

228 MASH Railing Load Requirements for Bridge Deck Overhang Figure 341. Comparison of Region B-B moment demands with varying span lengths and an 8-in. curb. Figure 342. Comparison of Region A-A moment demands with varying cantilever distances and an 8-in. curb. Figure 343. Comparison of Region B-B moment demands with varying cantilever distances and an 8-in. curb.

Overhangs Supporting Curb-Mounted Steel Posts 229   (a) 6-in. curb (b) 8-in. curb (c) 12-in. curb Figure 344. Comparison of deck and curb damage for various curb heights at post plastification. reduced dramatically. When the W6×25 was attached to a 6-in. curb, it was nearly unable to plastify, with extreme damage occurring in the deck prior to plastification. Alternatively, when attached to the 12-in. curb, the post was able to plastify with only minimal punching shear damage in the curb and virtually no damage in the deck. Moment demands calculated at Regions A-A and B-B for varying curb heights are shown in Figures 345 and 346. While demands at both regions generally reduced with increasing curb height, the effect was less significant than expected—when the curb height was increased from 6-in. to 12-in., the peak Region A-A moment was virtually unchanged, and the peak Region B-B moment was reduced by 10%. Effect of Longitudinal Curb Steel on Load Distribution Modifying the longitudinal curb steel was considered as a variation complementary to curb height. Greater longitudinal curb bar sizes theoretically strengthen and stiffen the deck edge, Figure 345. Comparison of Region A-A moment demands with varying curb heights.

230 MASH Railing Load Requirements for Bridge Deck Overhang affecting punching shear strengths and load distributions throughout the overhang. While curb and deck damage at post plastification were slightly reduced by increasing the longitudinal curb bar size from #4 to #6, moment demands acting at Regions A-A and B-B were virtually unchanged, as shown in Figures 347 and 348. Summary of Through-Bolted System Modeling A parametric study was performed to evaluate the behavior of the overhang supporting a curb-mounted steel-post system. Parameters with noticeable effects on deck demands were: • Curb height. While the variation of curb height had a minimal effect on deck demands for this system in the elastic range, it had a substantial effect on system damage at post failure. When the baseline curb height of 8 in. was reduced to 6 in., damage from the curb penetrated through the full length of the deck overhang. At 8-in. and 12-in. curb heights, damage was isolated in the curb, and the deck sustained no damage. Figure 346. Comparison of Region B-B moment demands with varying curb heights. Figure 347. Comparison of Region A-A moment demands with varying longitudinal curb steel.

Overhangs Supporting Curb-Mounted Steel Posts 231   • Span length. As the deck-mounted steel-post model did not have a stiffening element on its field edge, it was inferred that the deck-mounted steel-post behavior would not be affected by span length for typical bridge spans of at least 20 ft. However, this parameter was varied for the curb-mounted steel-post model, as the curb acts as a stiffener at the deck edge, so longitu- dinal distribution becomes a factor in the deck demand distribution. As the span length was reduced from 50 ft to 30 ft, no sensitivity was observed. When the span length was reduced from 30 ft to 15 ft, a transition point was observed at which deck demands were increased substantially at both Design Regions. However, this transition point corresponds to a very limited range of span lengths. • Cantilever distance. As the cantilever distance was increased from 3 ft to 5 ft, demands in the deck at Regions A-A and B-B were reduced by 6% and 13%, respectively. Parameters without significant effects on deck demands were: • Post attachment detail. Unlike for deck-mounted systems, changing the post attachment detail from a through-bolt to an embedded-plate detail did not result in a significant change in the magnitude of damage sustained by the deck at post plastification. However, the damage pattern changed significantly. Through-bolt attachment mechanisms are not recommended for curb-mounted railings, as the required length of the bolts makes them susceptible to bending. • Curb reinforcement. Although it was anticipated that the curb reinforcement could affect the stiffness of the deck edge and, therefore, reduce deck demands, no sensitivity to this parameter was observed at the load and damage levels considered. Additional curb reinforcement may enhance system performance in the presence of severe damage, such as might occur with a stronger post. Conclusions of Curb-Mounted Steel-Post Testing and Analytical Program Key findings of the curb-mounted steel-post testing and analytical program are summarized in this section. Findings are based on the results of impact tests of an instrumented, curb-mounted steel post and overhang specimen and calibrated analytical models. Figure 348. Comparison of Region B-B moment demands with varying longitudinal curb steel.

232 MASH Railing Load Requirements for Bridge Deck Overhang Effect of a Curb on Overhang Capacity Results of the physical testing and analytical programs for overhangs supporting curb- mounted steel posts indicated that the addition of a curb resulted in a significantly increased overhang capacity and more extensive longitudinal distribution of post demands. Curbs increase the capacity of the overhang in punching shear due to the additional planes of resistance and increased size of the load application patch at the top surface of the slab. Curbs increase the capacity of the overhang in yield-line flexure due to an increased effective loaded area as well as some extent of longitudinal bending strength directly provided by the curb. Results suggested that post demands can conservatively be assumed to distribute longitudi- nally at a 45-degree angle with downward transmission through the curb. Therefore, whereas the effective load region for deck-mounted steel posts is approximately the width of the base plate, the effective load region for curb-mounted steel posts is approximately the width of the base plate plus twice the curb height. Distribution through the curb results in greater numbers of transverse slab bars being engaged in the flexural mechanism, resulting in significantly less strict requirements on transverse slab-bar spacing to develop the full strength of the post. Effect of Curb on Distributed Loads at Design Region B-B For deck-mounted steel posts, demands distributed through the overhang at an effective angle of 45 degrees. Due to the edge-stiffening effect of the curb, post loads distributed through curbed overhangs at an effective angle of 60 degrees, resulting in reduced flexural demands at Design Region B-B. In systems with tight post spacing and/or long overhang distances, this extensive distribution may result in interaction between the load regions of adjacent posts. If load regions overlap, Design Region B-B demands should be magnified proportionally to the extent of overlap. Effect of Curb on Overhang Damage Results produced in this project highlighted the usefulness of the curb in reducing slab damage without reducing the effective lateral resistance of the post. In the physical test of the curb-mounted steel post, a greater lateral load was exerted on the bogie vehicle than in the deck- mounted steel-post test. However, damage to the slab in the curb-mounted test was minimal, while damage in the deck-mounted test was severe. Curbs provide additional stiffness at the slab edge and improve load distribution, resulting in less concentrated loads acting on the slab. Further, curbs can be designed to be sacrificial, wherein their bending strength is significantly less than that of the overhang, in order to mitigate slab damage. Proposed Methodology The results of the analytical and testing programs for overhangs with curb-mounted steel posts were used to develop a design/analysis methodology intended to ensure expected post behavior and limit overhang damage. This proposed methodology is briefly summarized in this section, and a design example demonstrating its use is provided in Appendix E. The design methodology described herein for curb-mounted steel posts is an expansion of the deck-mounted steel-post methodology, which considers the capacity and load distribution benefits granted by the curb. The overall procedure is largely unchanged between deck-mounted and curb-mounted steel posts. However, the following changes are made: distribution of loads through the curb can be accounted for in the slab yield-line capacity calculation; curb planes and distribution through the curb can be accounted for in the punching shear capacity calculation; and the ability of the curb to transfer loads to the deck surface must be evaluated.

Overhangs Supporting Curb-Mounted Steel Posts 233   Nomenclature Variables used in the design methodology for overhangs supporting curb-mounted steel posts are summarized in Table 19. Interior Posts Step 1. Identify Overhang Critical Regions The overhang must be evaluated at two critical regions: Design Region A-A, which is a trapezoidal yield-line mechanism under the post (visible later in Figure 351), and Design Region B-B, which is over the critical section of the exterior girder. These Design Regions are shown in Figure 349. ac = Depth of compression block at bottom of curb (in.) ap = Depth of compression block at bottom surface of base plate (in.) bcurb = Curb depth (in.) bo,c = Critical perimeter of punching shear mechanism in curb (in.) bo,s = Critical perimeter of punching shear mechanism in slab (in.) cc,bot = Slab bottom cover (in.) cc,top = Slab top cover (in.) Cp = Yield force of compression flange (kips) dbt = Slab transverse bar diameter (in.) ds = Distance from field edge of base plate to traffic-side anchor bolt line (in.) eb = Base plate edge distance from field edge of curb (in.) ec = Curb edge distance from field edge of slab (in.) f'c = Design concrete compressive strength (ksi) Fv = Vertical vehicle load on top of rail (kips) fy = Design steel reinforcing yield stress (ksi) hcurb = Curb height (in.) L = Post spacing (ft) L1B = Effective distribution length for lateral load moment at Design Region B-B (ft) L2B = Design flexural distribution length for vertical loads at Design Region B-B (ft) lb = Bearing length of assumed CCT node in slab (in.) Lcs = Critical length of slab yield-line mechanism (ft) Lcurb = Effective distribution length at base of curb for post loading (in.) L v v = Distribution length of vertical force F (ft) M1B = Lateral load design moment at Design Region B-B (k-ft/ft) M2B = Vertical load design moment at Design Region B-B (k-ft/ft) Mpost = Plastic moment capacity of steel post (k-ft) Mpost,eff = Maximum post moment able to be supported by slab yield-line mechanism (k-ft) Msl = Longitudinal bending strength of slab outside of traffic-side anchor bolt line (k-ft) Mst,A = Basic transverse bending strength of slab at Design Region A-A (k-ft/ft) Mst,B = Basic transverse bending strength of slab at Design Region B-B (k-ft/ft) Mstr,A = Transverse bending strength of slab at Region A-A, penalized with N (k-ft/ft) Mstr,B = Transverse bending strength of slab at Region B-B, penalized with N (k-ft/ft) Msw,B = Self-weight moment at Design Region B-B (k-ft/ft) Mu,curb = Flexural demand at the top surface of the deck to develop full plastic moment of a curb-mounted steel post (kip-ft/ft) N = Distributed tensile demand (k/ft) Pns = = Compression limit in slab compression strut (kips) (Pns)y y (vertical) component of the strut axial force capacity Ppost = Lateral load acting at which creates Mpost at post base (in.) Ppost,eff = Lateral load at corresponding to Mpost,eff (kips) ts = Slab thickness (in.) vc = Effective shear strength of concrete in punching shear mechanism (ksi) Vn = Punching shear capacity of slab (kips) Wb = Base plate width along span-axis of bridge (in.) We = Post base plate edge distance longitudinally to end of deck or curb (in.) XA = Distance from field edge of slab to traffic-side vertical curb steel (in.) XAB = Distance between traffic-side anchor bolt line and Design Region B-B (in.) XB = Distance from field edge of slab to Design Region B-B (in.) = Centroid height of longitudinal railing elements (in.) θs = Angle of compression strut in slab under post, measured from horizontal (deg.) Table 19. Nomenclature for design methodology for overhangs supporting curb-mounted steel railings.

234 MASH Railing Load Requirements for Bridge Deck Overhang Step 2. Establish Ultimate Capacity of the Post and Associated Overhang Demands To establish the design demands acting on the overhang, the ultimate capacity of the post is first calculated. Additionally, loads associated with the ultimate capacity are also calculated. Values calculated in this step include: i. Plastic bending strength of post, Mpost ii. Centroid height of longitudinal railing elements above deck surface, Y– iii. Lateral load at Y– creating Mpost at curb top surface, Ppost iv. Yield force of post compression flange, Cp Distributed tensile demands at each critical region are also calculated in this step. At both Design Regions A-A and B-B, the tensile demand can be calculated as N 2h 12P b curb post = +W (96) Step 3. Evaluate the Ability of the Curb to Transfer Loads from Base Plate into Slab In this step, the capacity of the curb is evaluated to check if it limits the bending strength of the post. Cantilever flexure of the curb must be evaluated—a general limit applicable regardless of detailing. Anchorage limits must also be evaluated in accordance with AASHTO LRFD BDS Article 5.13 (2), but the specific checks required will depend on detailing and differ from system to system. For the cantilever flexure limit state, the load distribution shown in Figure 350 can be used to determine the distributed moment acting at the bottom surface of the curb due to Mpost. L 12 2h curb b curb= +W (97) M L 12P Y u,curb curb post = r (98) If the transverse bending capacity of the curb (about the longitudinal axis of the bridge) at the deck surface is greater than Mu,curb , then the curb is able to develop the full strength of the Figure 349. Critical regions for overhangs supporting curb-mounted steel posts.

Overhangs Supporting Curb-Mounted Steel Posts 235   post in flexure. It should be noted that the transverse bending capacity of the curb is also the maximum moment that can be transferred to the deck slab. Step 4. Check Deck Joint for Diagonal Tension Damage Prior to estimating bending strengths of the slab, the curbed edge must be evaluated for diagonal tension damage. This check must be performed because diagonal tension damage typi- cally causes delamination of the bottom slab cover, which affects directional bending strengths in the trapezoidal yield-line mechanism of the slab. It should be noted that curbed systems will typically not sustain significant diagonal tension damage due to increased punching shear area or longitudinal expansion of the diagonal compression strut. The load transfer mechanism from the compressive zone of the base plate to Design Region A-A is believed to occur either through a strut-and-tie behavior or a vertical shear mechanism. The strut-and-tie mechanism is only available if adequate anchorage of the top-mat transverse bars is provided. As such, this step is divided into two categories, as shown in Table 20. The vertical shear mechanism may be used in all cases but may produce more conservative assessments of deck performance than the strut-and-tie approach, if available. The location of the compressive resultant acting on the base plate is determined using a method adapted from the AISC Design Guide 1, 2nd edition (36). If it is assumed that the traffic- side bolts reach their yield stress simultaneously with post hinging, the depth of the compression block acting on the curb top surface is a pp c b = C 0.85f Wl (99) If the evaluation performed in Table 20 fails (i.e., the appropriate inequality is violated), diagonal tension damage is expected, and the flexural strength of the slab must be penalized in the following calculations. If diagonal tension damage is expected, the transverse bending strength of the slab should be calculated using a reduced slab depth equal to the nominal slab depth minus the bottom cover. Additionally, any contribution of bottom-mat transverse steel to the bending strength should be neglected. Further, the longitudinal bending strength of the slab in negative bending (top-surface tension) is reduced to zero. The longitudinal bending strength of the slab associated with bottom-mat longitudinal bars is not affected. This process is demonstrated in the accompanying design example. After evaluating the slab for diagonal tension damage, the transverse bending strength of the slab, Mst, is calculated. The distributed tensile force, N, is then used to calculate a penalized Figure 350. Effective distribution pattern of loads through the curb.

236 MASH Railing Load Requirements for Bridge Deck Overhang bending strength, Mstr. Tension should be considered by following the provisions of Section 5 of the AASHTO LRFD BDS, assuming that both top and bottom transverse mats participate for decks with two layers of reinforcing if the evaluation performed in Table 20 succeeds or that only the top mat participates if the evaluation performed in Table 20 fails. Longitudinal bending strength of the slab outside of the traffic-side bolt line, Msl, is also calculated in this step. If the slab and curb are poured monolithically or if composite action is proven through calculation of sufficient shear transfer, the curb can be included in the calculation of longitudinal bending strengths. Step 5. Calculate Yield-Line Capacity of Slab The yield-line mechanism in the slab under the curb is shown in Figure 351. The mechanism is identical to that of steel posts mounted directly to the deck surface; however, the load application region and horizontal yield-line lengths are increased by twice the height of the curb. This accounts for an effective 45-degree distribution of loads with downward transmission through the curb. The critical length of the yield-line mechanism is L L 8 M M 12 X cs curb st,A sl A= + J L KK N P OO (109) Using a Strut-and-Tie Model Using a Punching Shear Model Failure mechanism: strut splitting i. Angle of compression strut (100) ii. Bearing length of CCT node (101) iii. Vertical component of max strut load (includes 45-degree distribution of compression zone within curb) (102) iv. Check compression strut capacity (103) Failure mechanism: punching shear i. Effective concrete shear strength (104) ii. Critical perimeter in curb (105) iii. Critical perimeter in slab (106) iv. Shear capacity (107) v. Check shear capacity (108) Table 20. Evaluation of curbed edge for diagonal tension damage.

Overhangs Supporting Curb-Mounted Steel Posts 237   The maximum post moment able to be supported by the slab in the yield-line mechanism is M C Y M Y h X e e X M X 12L M X 12 L L M L L 8 M post,eff p post curb A b c A str,A A curb st,A A cs curb sl cs curb post# = - - - + - + - r r J L K KK J L KK a ` N P OO N P O OO k j (110) If straight transverse bars are used, Mst,A should be calculated using the average bar embed- ment depth over the diagonal yield-lines. Step 6. Estimate Distributed Demands at Design Region B-B In this step, distributed flexural demands at Design Region B-B are calculated. The effective distribution length at Design Region B-B for Design Case 1 moment, illustrated in Figure 352, is L 12 2h 2 X e b tan 60 1B b curb B c curb = + + - -W c` j (111) Figure 351. Overhang yield-line mechanism for curb-mounted steel post. Figure 352. Effective distribution pattern of lateral moment demand through curbed overhang.

238 MASH Railing Load Requirements for Bridge Deck Overhang Therefore, the design moment at Design Region B-B associated with the plastic moment capacity of the post is •M M M 12L P Y 0.5t M1B post post,eff 1B post s sw,B= + + ra k (112) The effective distribution length at Design Region B-B for Design Case 2 moment is L 12 2h 2 X e b 2B b curb B c curb = + + - -W ` j (113) Therefore, the design moment at Design Region B-B associated with vertical impact loading at the back face of the railing is •M L F L L X e e M2B v v 2B B c b sw,B= - - + (114) Step 7. Perform Limit State Checks Limit states are evaluated in this section. For Design Case 1, Design Region A-A, if the yield- line capacity, Mpost,eff , is less than the nominal bending strength of the post, Mpost, the railing performance may be affected, and local slab damage is expected. For Design Case 1, Design Region B-B M Mstr,B 1B$ (115) For Design Case 2, Design Region B-B: M M2st,B B$ (116) It should be noted that the tension penalty is not applied to the slab strength in Design Case 2, as vertical and lateral loading do not act simultaneously. If transverse slab bars are not hooked around a field-edge longitudinal bar, incomplete bar development should be considered when calculating slab bending strengths. End Posts Evaluation of the overhang for curb-mounted steel posts installed near the free ends of the slab must be modified to account for reduced local strength and restricted load distribution patterns. Modifications applied for end-region posts—which are herein defined as posts with 2 ft or less of distance between the deck edge and the side of the base plate—are as follows. Overhang Yield-Line Capacity The overhang yield-line capacity must be modified to account for the free end of the overhang adjacent to the post. For curb-mounted posts, a 45-degree distribution of loads within the curb is considered. Therefore, yield-line mechanisms are slightly different for edge distances greater than or less than the curb height. These mechanisms are shown in Figures 353 and 354, respectively. Although the mechanisms are slightly different in appearance, critical lengths and capacities are calculated in the same manner. For both, the critical length and capacity of the mechanism are L 12 h M M 12 X cs b e curb st,A sl A= + + + W W J L KK N P OO (117)

Overhangs Supporting Curb-Mounted Steel Posts 239   M C Y M Y h X e e X M X h M X 12L h L 12 h M M post,eff p post curb A b c A str,A A b e curb st,A A cs b e curb cs b e curb sl post# = - - - + + + - + + + - + + W W W W W W r r J L KK J L K K KK a ` ` ` N P OO N P O O OO k j j j (118) It should be noted that for certain combinations of post position and transverse slab steel configurations, the end-region yield-line equation may produce a greater capacity than the interior mechanism. For posts that have a span-end offset, We, greater than 2 ft, both the interior and end-region calculations should be performed, and the effective capacity of the post should be taken as the minimum value. If traffic-side bolts are not anchored to the bottom face of the slab and are instead embedded in the curb or slab, longitudinal bars must be hooked or otherwise anchored at the end of the slab. If longitudinal bars are not hooked or anchored, upward loads acting on the embedded plate may cause pullout of the longitudinal bars. Overhang Punching Shear Capacity At end regions, one resisting plane in the punching shear capacity of the overhang is removed. Therefore, the critical perimeter calculations are adjusted to h e a+ + +W+W=boc b e curb b p (119) Figure 353. Overhang yield-line mechanism for end-region, curb-mounted steel post (We > hcurb). Figure 354. Overhang yield-line mechanism for end-region, curb-mounted steel post (We < hcurb).

240 MASH Railing Load Requirements for Bridge Deck Overhang 2h t e e a+ + + + +W+W=bos b e curb s b c p (120) For nonzero span-end offsets, We, the end-region critical perimeter equation may result in a greater punching shear strength than the interior equation. Judgment should be used when applying these equations, and posts that are not directly situated at the span end should be evaluated using both the interior and end-region equations, and the punching shear capacity should be taken as the minimum value. Load Distributions to Design Region B-B At the end region of the slab, longitudinal distributions of demands through the overhang are restricted to one direction. As such, effective length calculations must be modified. The end-region Design Case 1 and 2 distribution lengths at Design Region B-B are h (X − e − b ) tan 60+ + L 121B b curb B c curb= W c (121) h X e b+ + - - L 122B b curb B c curb= W (122) Evaluation of Methodology The methodology described above was developed using data produced in the physical and analytical programs for overhangs supporting curb-mounted steel posts. The methodology predicted the ultimate capacity of selected post, curb, and overhang designs within 10%. Addi- tionally, distributed demands at Design Region B-B were predicted within 15%. Errors in the methodology were underpredictions for local capacities and overpredictions for distributed demands. Comparison of Methodology to Existing AASHTO LRFD BDS The existing AASHTO LRFD BDS (2) does not provide equations for the capacity of over- hangs supporting curb-mounted posts. The updated methodology closes this gap in the speci- fications, providing design equations that quantify the benefits of curbed edges on overhang capacity. The punching shear capacity is increased by the additional curb thickness, and the yield-line capacity is increased by a widened load application region. Design Example A full design example demonstrating the methodology previously described is presented in Appendix E. The design example includes a full analysis of an overhang supporting a modified version of the MASSDOT S3TL4 curb-mounted steel post-and-beam railing. The methodology predicted that the overhang would be able to develop the full plastic moment strength of the post, which was 69 k-ft, with minimal damage. The predicted yield-line capacity of the slab was 79 k-ft. An LS-DYNA pushover model of the example system, which used an elastic post to ensure failure in the slab, indicated an ultimate overhang strength of 81.3 k-ft.

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State highway agencies across the country are upgrading standards, policies, and processes to satisfy the 2016 AASHTO/FHWA Joint Implementation Agreement for MASH.

NCHRP Research Report 1078: MASH Railing Load Requirements for Bridge Deck Overhang, from TRB's National Cooperative Highway Research Program, presents an evaluation of the structural demand and load distribution in concrete bridge deck overhangs supporting barriers subjected to vehicle impact loads.

Supplemental to the report are Appendices B through E, which provide design examples for concrete barriers, open concrete railing post on deck, deck-mounted steel-post, and curb-mounted steel-post.

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