National Academies Press: OpenBook

MASH Railing Load Requirements for Bridge Deck Overhang (2023)

Chapter: Chapter 6 - Overhangs Supporting Deck-Mounted Steel Posts

« Previous: Chapter 5 - Overhangs Supporting Concrete Posts
Page 154
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 154
Page 155
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 155
Page 156
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 156
Page 157
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 157
Page 158
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 158
Page 159
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 159
Page 160
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 160
Page 161
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 161
Page 162
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 162
Page 163
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 163
Page 164
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 164
Page 165
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 165
Page 166
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 166
Page 167
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 167
Page 168
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 168
Page 169
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 169
Page 170
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 170
Page 171
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 171
Page 172
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 172
Page 173
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 173
Page 174
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 174
Page 175
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 175
Page 176
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 176
Page 177
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 177
Page 178
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 178
Page 179
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 179
Page 180
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 180
Page 181
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 181
Page 182
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 182
Page 183
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 183
Page 184
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 184
Page 185
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 185
Page 186
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 186
Page 187
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 187
Page 188
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 188
Page 189
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 189
Page 190
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 190
Page 191
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 191
Page 192
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 192
Page 193
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 193
Page 194
Suggested Citation:"Chapter 6 - Overhangs Supporting Deck-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.
×
Page 194

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

154 Overhangs Supporting Deck-Mounted Steel Posts In applications where improved visibility, reduced railing weight, or rapid construction is desired, some state agencies favor steel post-and-beam railings over barriers. Due to the small footprint of steel-post base plates, this railing type exerts highly concentrated flexural and tensile demands on the overhang, often resulting in significant deck damage. In this chapter, all aspects of NCHRP Project 12-119 regarding overhangs supporting 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. One impact test was performed. The results of this test 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 load distribution patterns through the barrier and 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 deck-mounted steel railings in order to inform the analytical and testing programs. Key results of this preliminary data collection 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 Significant slab damage is common during testing of steel post-and-beam railings mounted directly to the deck surface. Common damage mechanisms observed in testing include punching shear damage and wider, arching cracks that radiate longitudinally and toward the field edge C H A P T E R 6

Overhangs Supporting Deck-Mounted Steel Posts 155   of the slab from the trac-side anchor bolt line or the eld edge of the supporting element. Common damage mechanisms are shown in Figure 222. Deck-mounted steel post-and-beam railings that use weak posts, such as W6×9 or S3×5.7 sections have been shown to perform adequately within the target test level without damaging the slab. Top-mounted weak-post systems have been successfully crash tested to MASH TL-2 and TL-3, including two variants of the TxDOT T631 bridge railing. At a post spacing of 75 in., the system passed MASH TL-2 criteria; at a post spacing of 37.5 in., the system passed MASH TL-3 criteria (34, 35). e MASH TL-3 TxDOT T631 bridge railing specimen aer MASH test designation no. 3-10 is shown in Figure 223. A bridge railing variant of the Midwest Guardrail System (MGS) was successfully crash tested to MASH TL-3 criteria, although this system was side-mounted. Side-mounted railings are not considered within the scope of NCHRP Project 12-119. Existing BDS Design Methodology e existing AASHTO LRFD BDS, 9th Edition (2) contains a methodology for designing/ analyzing overhangs supporting deck-mounted steel posts. Two limit states are described in TxDOT T101 (32) New York Two-Rail (33) Figure 222. Damage to strong-post crash-test specimens. Figure 223. Damage to TxDOT T631 weak-post crash-test specimen (35).

156 MASH Railing Load Requirements for Bridge Deck Overhang the methodology for lateral loading: a flexural limit state in which the slab bending strength at the traffic face of the base plate must be greater than or equal to the plastic moment capacity distributed over an assumed length; and a punching shear limit state in which the slab must be able to support the yield force of the post compression flange spread over an assumed base plate compression zone. These limit states are demonstrated in Figure 224. Alternative Design Methodologies The existing design methodology of the AASHTO LRFD BDS, 9th edition considers only the contribution of transverse slab steel in the ability of the overhang to support the attached post. Although the contributions of longitudinal slab steel have not been directly quantified in previous research, they are qualitatively anticipated by some state agencies, such as TxDOT, whose standard deck design uses larger longitudinal slab bars in the bottom mat than in the top mat. Further, in research performed by Mander et al. (30), a slab yield-line mechanism considering the effects of longitudinal steel was proposed for slabs resisting wheel loads. Objectives and Scope of Analytical and Testing Programs The primary objective of the analytical and testing programs for overhangs supporting deck-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 such, emphasis was placed on accurately predicting the ultimate failure mechanism and capacity of the overhang when subjected to the lateral and flexural loads associated with the plastic moment strength of the post. Secondary objectives of the analytical and testing programs for overhangs supporting deck- mounted steel posts included quantifying effective tensile and flexural demands at Design Region B-B. Further, the effects of varying certain design parameters on the overhang capacity and load distributions were briefly investigated. This investigation was not performed to the extent that it was for barriers, as barriers are significantly more common, and the sensitivities of system behavior to certain parameters were deemed negligible in that investigation and therefore did not require reevaluation. Flexure Punching shear Figure 224. Existing AASHTO LRFD BDS, 9th Edition, failure mechanisms for deck-mounted steel posts (2). E 5 distance from edge of slab to centroid of compressive stress resultant in post (in.). B 5 distance between centroids of tensile and compressive stress resultants in post (in.). h 5 depth of slab.

Overhangs Supporting Deck-Mounted Steel Posts 157   This chapter contains the results of testing and modeling of steel posts mounted directly to the slab. Results for steel posts mounted on curbs are presented in Chapter 7. Side-mounted railings were not considered in this project. Impact Test of Deck-Mounted Steel-Post Specimen An impact test was performed on a deck-mounted steel-post specimen to measure the longitudinal distribution of impact loads through the deck overhang. In addition to providing a physical data point, test results were used to evaluate the accuracy of the accompanying LS-DYNA model. Further, as it is recommended that slabs supporting steel posts are designed to resist the demands associated with the full plastic moment capacity of the post, the test results were also used to evaluate the ultimate capacity of the slab. Test Specimen Details Test specimen details are shown in Figure 225. As the barrier and steel-post specimens were installed on the same deck overhang, the reinforcement configuration was unchanged between the two, with the exception of the transverse bar spacing being reduced from 6 in. to 4 in. The W6×20 steel post was welded to a 1-in.-thick steel plate with a 0.25-in.-all-around fillet weld. Before testing, a second pass was added to the weld, increasing its effective throat to roughly 0.375 in. Additional weld material was added based on past experience; due to prying action associated with plate bending or the combined stress state at the post base, unexpected weld failures have been observed in similar designs. Post anchorage was achieved by through-bolting four 0.875-in.-diameter A325 bolts through the slab and into a bottom-face washer plate. Although some states employ an alternative anchorage mechanism in which the washer plate is embedded in the deck slab under the top mat of steel, a through-bolt configuration was selected for this test to ensure vertical anchor breakout did not control the specimen capacity. Assuming nominal material properties, the intended target mechanism was slab flexure, as a higher-than-anticipated capacity would have provided physical justification for increasing longitudinal distributions of flexural loads in the slab. However, as the as-tested yield stresses of the post and transverse slab steel were 56 ksi and 84 ksi, respectively, the punching shear limit was expected to control in the test. Figure 225. Deck-mounted steel-post test specimen.

158 MASH Railing Load Requirements for Bridge Deck Overhang It should be noted that the specimen tested herein does not demonstrate good practice and was instead configured to simplify the load transfer from the post into the slab. The traffic-side bolt line was aligned with the traffic-side post flange such that the tensile load in the bolt line would be roughly equal to the flange tension, which reduced uncertainty in some aspects of the analysis. If the anchorage design previously noted was implemented into an actual, in-service system, the system’s performance would be improved by moving the traffic-side bolt line farther from the field edge, increasing the effective lever arm of the base plate. The design compressive strength of the slab 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 15. 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 bolt line and at Design Region B-B, over the field edge of the grade beam. Primary strain gages operational during the deck-mounted steel-post test are shown in Figure 226. Due to the more local behavior and lack of edge-stiffening effect provided by a concrete barrier, a significantly less extensive distribution of strain gages was used in the steel- post testing. 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 12.3 mph. As the stiffness and failure energy of the Nominal As-Tested Mpost = 63 k-ft 70 k-ft Plastic moment capacity of post, FyZx He = 29 in. 29 in. Load application height above deck surface Ppost = 26 kips 29 kips Post base shear associated with Mpost Mst = 24 k-ft/ft 32 k-ft/ft Transverse bending capacity of slab Figure 226. Longitudinal distribution of transverse slab-bar strain gages. Red dots indicate strain gages near Design Region A-A. Blue dots indicate strain gages at Design Region B-B. Table 15. Nominal and as-tested parameters for deck-mounted steel-post specimen.

Overhangs Supporting Deck-Mounted Steel Posts 159   specimen was significantly less than that of the concrete barrier specimen, bogie crush tube heights were reduced from 10 in. to 5 in. The test setup for the deck-mount steel-post test is shown in Figure 227. General Specimen Response Three sequential photos of the deck-mounted steel-post impact test are shown in Figure 228. In the event, the post successfully contained the bogie vehicle, and the slab sustained moderate damage. The crush tubes exhausted their entire stroke length during the impact event. The force exerted on the bogie by the post as measured via onboard accelerometers, is shown in Figure 229. The curves have been passed through a CFC-60 filter. The peak CFC-60 average force measured in the test was approximately 40 kips. The peak 50-ms average force measured in the test was 36 kips. Crush tube yielding began in the test around 10 ms after the point of first contact. Around 30 ms after contact, strain hardening in the crush tubes resulted in an increasing lateral load exerted between 30 ms and 80 ms. Beyond this point, significant deck softening occurred due to punching shear and other cracking, resulting in a roughly constant load exerted between 80 ms and 125 ms. The bogie began to rebound from the specimen after 125 ms. Specimen Damage The damage state of the deck slab after the test is shown in Figures 230 and 231. Critical deck damage appeared consistent with the expected punching shear mechanism. In addition to punching shear cracking, a wider, roughly trapezoidal cracking mechanism also developed on the top and field surfaces of the slab, although this cracking was minor. Damage to the bottom of the slab is shown in Figure 232. The most apparent damage mecha- nism on the bottom face of the slab was punching shear. However, inverted, diagonal cracking consistent with a yield-line flexural mechanism was also observed. Figure 227. Deck-mounted steel-post pretest condition.

160 MASH Railing Load Requirements for Bridge Deck Overhang Figure 228. Sequential images of deck-mounted steel-post test. Figure 229. Lateral load exerted on the bogie in the deck-mounted steel-post test. Figure 230. Deck-mounted steel-post test slab damage.

Overhangs Supporting Deck-Mounted Steel Posts 161   Figure 231. Deck-mounted steel-post test slab damage. Figure 232. Damage to the bottom of the slab.

162 MASH Railing Load Requirements for Bridge Deck Overhang Evidence of post yielding is shown in Figure 233. As shown, the compression flange of the post was significantly deformed during the test, indicating that the base moment was greater than or equal to the yield moment of the post. In addition to the visible bending of the com- pression flange, galvanization flaking was also observed on the field face of the post, further evidencing plastic deformation. Base plate deformation sustained in the test is shown in Figure 234. The base plate bent about both the transverse and longitudinal axes of the bridge. The primary bending mechanism was about the longitudinal bridge axis—the traffic-side bolts yielded, and the base plate bent upward about the field-side flange of the post. The maximum residual lift measured at the front of the base plate was 0.75 in., and the lift at the corners of the base plate was 0.25 in. As shown in Figure 235, minor weld cracking occurred during the test. The additional weld pass, which was added after the initial post-to-plate weld, separated from the post, indicating a poor bond to the galvanized post face. The weld failure did not significantly affect the behavior of the post, as the additional weld pass was added only for conservatism to ensure weld failure did not control the post capacity. The original 0.25-in.-fillet weld underneath the weld shown in Figure 235 was not visibly damaged. Damage to the slab underneath the base plate and lower washer plate is shown in Figures 236 and 237. While punching shear was the dominant mechanism in the slab, cracking consistent with a trapezoidal yield-line mechanism also developed in the test. Additionally, minor shear/tensile breakout cones developed on the field side of the traffic-side bolts. Figure 233. Plastic deformation of the post. Figure 234. Base plate deformation.

Overhangs Supporting Deck-Mounted Steel Posts 163   Figure 235. Weld damage at the base of the post. Figure 236. Slab top-surface damage under the base plate. Figure 237. Slab bottom-surface damage under the washer plate.

164 MASH Railing Load Requirements for Bridge Deck Overhang Strain Gage Data Linear strain gages were fastened to specimen reinforcement at two locations: top-mat transverse slab steel at Design Region A-A, and top-mat transverse slab steel at Design Region B-B. Strain gage measurements on slab bars at Design Region A-A at first yield and the peak measured strain are shown in Figures 238 and 239. Peak strain gage measurements at Design Region B-B are shown in Figure 240. Discussion of Test Results Results of the deck-mounted steel-post test were consistent with expectations. The slab was able to develop the full capacity of the post but sustained significant punching shear damage in the event. Prior to punching shear failure, slab strains under the post were also consistent with expectations. After punching shear failure occurred, extreme bar strains (3–4%) were measured under the post. At Design Region B-B, bar strains were distributed over a greater length and were of lesser magnitude. Peak strains measured at Design Regions A-A and B-B were 3.4% and 0.12%, respectively. Figure 238. Design Region A-A bar strains at point of first yield in the slab. Figure 239. Peak Design Region A-A bar strains measured in the deck-mounted steel-post test.

Overhangs Supporting Deck-Mounted Steel Posts 165   Calibrated Deck-Mounted Steel-Post Model Using the data produced in the physical test of the deck-mounted steel post, the accuracy of the LS-DYNA model 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. Calibration Process The first iteration of the LS-DYNA model created prior to the deck-mounted steel-post test produced an acceptably accurate result. Therefore, no calibration was required. Overall Response Accuracy The force-time history of the calibrated LS-DYNA model is compared to the physical test result in Figure 241. As shown, the model produced an accurate representation of the post’s Figure 240. Peak Design Region B-B bar strains measured in the deck-mounted steel-post test. Figure 241. Comparison of LS-DYNA model force response to physical test measurement.

166 MASH Railing Load Requirements for Bridge Deck Overhang resistance, predicting the peak load within 5% of the measured value. e slight deviation between the model and the physical test result between 60 and 80 ms is believed to be due to localized eects at one bolt in the physical test caused by a minor construction error not accounted for in the model. Predicted Damage Damage progression calculated in the calibrated LS-DYNA model of the event is shown in Figure 242. As shown, punching shear was the controlling damage mechanism in the model, which was consistent with the physical test results. Figure 242. Progression of damage in the calibrated LS-DYNA model.

Overhangs Supporting Deck-Mounted Steel Posts 167   Post and base plate deformations in the LS-DYNA model are shown in Figures 243 and 244. As in the physical test, the post compression ange yielded in the model, as evidenced by the nonzero plastic strain contours on the eld face of the post. Additionally, the model accurately predicted the extent of the base plate and trac-side bolt yielding. Concrete damage on the bottom face of the slab is shown in Figure 245. Bottom-face damage was consistent with the damage observed in the physical test. Signicant damage concentrations occurred at the projection of the punching shear surfaces, and inverted, diagonal cracking consistent with the early stages of a yield-line mechanism also formed. Comparison to Strain Gage Measurements LS-DYNA strains are compared to strain gage measurements at the point of rst yield in Figure 246. As shown, the LS-DYNA model produced accurate estimates of transverse bar strain at Design Region A-A. Figure 243. Slab failure mechanism and post deformation. Figure 244. Base plate and bolt deformation.

168 MASH Railing Load Requirements for Bridge Deck Overhang Discussion of Calibrated LS-DYNA Model As the deck-mounted steel-post test model exhibited an acceptably accurate prediction of the overall force-deflection response of the specimen, the post-test damage profile, and strain gage measurements, the model was deemed adequately calibrated. As such, the model was 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 are required. For concrete barrier and concrete-post tests, a quasi-static pushover variant of the calibrated model was created to more clearly characterize load distributions in the overhang. Converting to static loading was advantageous for those systems, as it mitigated the inertial effect, which obscures the relationship between the load exerted by the bogie vehicle and the load mechanically resisted by the overhang at the base of the railing. For the deck-mounted steel post, only the post underwent significant deflections. As such, the inertial effects for this test are minor, and the impact model was deemed acceptable for relating slab demands to the applied lateral load. Regions A-A and B-B flexural and tensile demands acting on the slab at the peak lateral load are shown in Figures 247 and 248, respectively. Peak slab moments are compared to estimates made by the proposed slab design methodology in Figure 249. In the figure, the peak moment demand experienced in the slab at any point in the model is shown. As shown, slab punching shear damage interrupted the longitudinal distribution of flexural loads, resulting in a higher- magnitude demand at Design Region A-A. Figure 245. Damage to the bottom face of the slab. Figure 246. Comparison of LS-DYNA strain distribution to physical test result at point of first yield.

Figure 247. Transverse slab moments at peak load in the deck-mounted steel-post model. Figure 248. Transverse slab tensions at peak load in the deck-mounted steel-post model. Figure 249. Comparison of Region A-A historical distribution methods to model demands at post plastic moment and after slab damage.

170 MASH Railing Load Requirements for Bridge Deck Overhang Assuming flexural demands distribute from the field edge of the base plate at 45 degrees resulted in a conservative estimate of the peak slab moment, even after slab damage occurred. However, as the proposed methodology will use a yield-line mechanism at Design Region A-A rather than a single flexural failure surface, this historically adopted distribution method is only mentioned anecdotally here. Results in Figure 249 are shown for informational purposes only. Extrapolative Modeling—Load Distributions As overhangs supporting deck-mounted steel posts are subjected to significantly more concentrated loads than those supporting barriers, characterizing load distribution patterns is less important for developing an effective design methodology. Overhang steel configurations will almost always be governed by local demands at post locations, rendering design consid- erations at Design Region B-B inconsequential if the at-post steel configuration is extended to the exterior girder. Further, at Design Region A-A, the objective of the analytical and testing programs was to accurately describe the slab capacity required to develop the ultimate strength of the post without sustaining significant damage. However, a modeling effort was performed to investigate the effects of varying selected design parameters on load distribution patterns and overall overhang performance. Basic Load Distribution Basic flexural and tensile demand distributions in the slab at the ultimate strength of the calibrated post model are shown in Figures 247 and 248 of the previous section, respectively. Both the Design Regions A-A and B-B moment demands were conservatively predicted by using a 45-degree longitudinal distribution angle in the slab. Effect of Overhang Thickness For decks supporting steel posts, overhang thickness has two effects: increased overhang thick- ness provides greater punching shear strength, and increased overhang thickness increases deck stiffness, potentially affecting distributions of demands throughout the overhang. The greater punching shear strength provided by increasing overhang thickness is dem- onstrated in Figure 250 in which the thickness of the deck ranged from 8 in. to 12 in., and a W6×15 post was loaded until plastification. At an equal load, the 8-in. deck sustained signifi- cant breakout/shear damage and moderate flexural distress, the 10-in. deck sustained virtually no breakout/shear damage and minor flexural distress, and the 12-in. deck sustained only minimal flexural distress. The effect of deck thickness on load distributions for Regions A-A and B-B is shown in Figures 251 and 252. As shown, at the point of post plastification, increased deck thicknesses resulted in reduced deck cracking and significantly decreased longitudinal distributions and, consequently, slightly increased demands (normalized for self-weight) on Region A-A. Increased demands at Region A-A were trivial when considered together with corresponding increased capacities at increasing thicknesses. At Region B-B, both the distribution and magnitude of the flexural demands were less sensitive to deck thickness. Effect of Overhang Cantilever Distance The cantilever distance has a pronounced effect on the stiffness of the deck overhang. However, for deck-mounted steel posts, this effect is reduced, as local damage mechanisms govern behavior

Overhangs Supporting Deck-Mounted Steel Posts 171   (a) 8-in. deck (b) 10-in. deck (c) 12-in. deck Figure 250. Comparison of deck damage at plastification of W6315 post for varying slab thicknesses. Figure 251. Design Region A-A moment demands at plastification of W6315 post for varying slab thicknesses.

172 MASH Railing Load Requirements for Bridge Deck Overhang and limit demands, rather than one-way cantilever or two-way plate bending. To observe the effect of cantilever distance on the behavior of deck-mounted steel posts, the baseline 3-ft cantilever model was compared to a reduced 1-ft cantilever model at the failure of a W6×25 post. This variation, shown in Figure 253, increased Region A-A demands by 5%. The moment demands acting on Region A-A in each model are shown in Figure 254. In both models, deck failure in torsion/punching shear was the dominant mechanism, and the post did not plastify. Figure 252. Design Region B-B moment demands at plastification of W6315 post for varying slab thicknesses. Figure 253. Varying cantilever distance models: 1-ft cantilever (top) and 3-ft cantilever (bottom).

Overhangs Supporting Deck-Mounted Steel Posts 173   Effect of Longitudinal Deck Steel To investigate the effects of varying longitudinal deck steel, moment demands were com- pared between W6×25-post models using #4 longitudinal deck bars and #6 longitudinal deck bars. This comparison is shown in Figures 255 and 256. When the longitudinal bar size was increased from #4 to #6, a trend similar to that of increasing the deck thickness was observed— peak Region A-A moment was increased by 7%, and peak Region B-B moment was increased by 3%. Additionally, while the deck with #6 longitudinal bars remained insufficient to completely plastify the W6×25 post, the lateral load inducing failure in the deck increased by 12% relative to the baseline deck using #4 longitudinal bars. Distance from post center (ft) D es ig n R eg io n A- A m om en t ( k- ft/ ft) Figure 254. Design Region A-A moment demands at failure of W6325 post for varying cantilever distances. Distance from post center (ft) R eg io n A- A m om en t ( k- ft/ ft) Figure 255. Design Region A-A moment demands at equal load for W6325 post with varying longitudinal deck bars.

174 MASH Railing Load Requirements for Bridge Deck Overhang Effect of Transverse Deck Steel Transverse deck steel is another parameter affecting the stiffness of the deck overhang whose effect is magnified when no stiffening element is included. To investigate this effect, the transverse bars in the W6×15-post model were reduced from #5 bars to #3 bars. When this change was made, although the flexural strength of the deck was reduced, the deck remained able to fully plastify the post. Moment demands at Regions A-A and B-B at plastification of the W6×15 post in the baseline (#5) and reduced (#3) models are compared in Figures 257 and 258. Before any significant deck damage or transverse bar yielding, the amount of transverse deck steel had a substantial effect on peak demands at Region A-A. When the transverse steel was reduced from #5 bars at 4 in. to #3 bars at 4 in., the peak moment demand on Region A-A was reduced by 18%. Distance from post center (ft) R eg io n B- B m om en t ( k- ft/ ft) Figure 256. Design Region B-B moment demands at equal load for W6325 post with varying longitudinal deck bars. Distance from post center (ft) R eg io n A- A m om en t ( k- ft/ ft) Figure 257. Design Region A-A moment demands at plastification of W6315 post for varying transverse deck bars.

Overhangs Supporting Deck-Mounted Steel Posts 175   End Regions When a steel post is placed at an end region, both the longitudinal distribution of flexural demands and the critical punching shear perimeter are restricted to one direction. Currently, AASHTO LRFD BDS does not explicitly discuss steel-post behavior at end regions. To demonstrate the end-region behavior of a deck-mounted steel post, a W6×25-post was placed with its center- line 1 ft away from the free edge of the deck and loaded to failure. When placed at the free edge, the post’s capacity was 76% of the interior region’s capacity. The damage state of the system at the point of first yield in the transverse bars is shown in Figure 259. Based on the moment demands at this state, which are shown in Figure 260, it is inferred that this yielding was the result of shear or torsional mechanisms, rather than longitudinal-axis flexure, as the peak moments acting on the section were roughly half of the Distance from post center (ft) R eg io n B- B m om en t ( k- ft/ ft) Figure 258. Region B-B moment demands at plastification of W6315 post for varying transverse deck bars. Figure 259. Damage to the deck overhang at point of first yield in transverse bars, end region.

176 MASH Railing Load Requirements for Bridge Deck Overhang calculated deck capacity. At the free edge, the punching shear critical perimeter takes the shape of an L, rather than a U, losing one of its three resisting concrete planes. As such, the punching shear capacity of the deck at the end region is reduced significantly. When loaded beyond the point of first yield, the deck overhang eventually failed in punching shear, in a mechanism characterized by a complete through-depth shear of the overhang adjacent to the post base plate on the interior side. The damage state and distribution of moment demands at this point are shown in Figures 261 and 262, respectively. Extrapolative Modeling—Overhang Capacity Although the purpose of NCHRP Project 12-119 is not explicitly to evaluate the capacities of deck overhang systems, but rather to characterize the demands acting on them under MASH loading, the distinction between capacity and loading is less in the case of steel posts (both deck-mounted and curb-mounted) than for barriers. For steel-post systems, the overhang is not designed to resist some portion of the lateral impact load. Instead, the overhang is designed to Figure 260. Design Regions A-A and B-B moments at the point of first yield in transverse bars, end region. Figure 261. Damage to the deck overhang at loss of load-bearing capacity, end region.

Overhangs Supporting Deck-Mounted Steel Posts 177   support the post’s full plastic capacity, such that the assumptions of the inelastic method used to determine the post-and-beam system’s overall capacity remain valid. As such, it was deemed valuable to investigate the ability of the deck overhang to develop various post sections’ plastic capacities, track the flexural and shear damage developed in each case, and compare the perfor- mance of the deck to the predictions set by following the current AASHTO LRFD BDS (2). Effect of Post Section on Deck Damage Through a survey of each state’s bridge design manual and standard details, it was determined that the most common steel-post section used in steel post-and-beam bridge rails in the United States is the W6×25. The W6×25 was followed in popularity by the W6×20, the W6×9, and the W6×15. As such, for this portion of the analytical program, models were made in which the deck and base plate configuration were held constant, and different post sections ranging in strength from W6×9 to W6×25 were “welded” to the plate using nodal constraints. In total, models were made with W6×9, W6×12, W6×15, W6×20, and W6×25 shapes. Each model was loaded to either post plastification or punching shear failure in the deck. Other sections, such as HSS sections and custom shapes were not considered for this study. The deck damage and transverse deck-bar stresses at the failure (post plastification, transverse deck-bar yielding, or punching shear) of each case, as the section was increased from a W6×9 to a W6×25, are shown in Figures 263 to 267. The baseline deck overhang design was able to support the plastification of all modeled post sections except for the W6×25—the deck over- hang failed in punching shear prior to the full development of the W6×25’s full plastic moment strength. The W6×9 through W6×20 posts were able to plastify without any yielding of the transverse deck bars. Moment demands calculated at Region A-A and Region B-B at post failure for the W6×9 through W6×25 models are shown in Figures 268 and 269. As anticipated, distribution lengths at both Design Regions were substantially smaller for deck-mounted steel posts than for concrete barrier or concrete-post systems. Figure 262. Design Regions A-A and B-B moments at loss of load-bearing capacity, end region.

178 MASH Railing Load Requirements for Bridge Deck Overhang Peak bar stress = 13 ksi Peak bar stress = 29 ksi Figure 263. Deck damage and bar stresses at plastication of the W639 post. Figure 264. Deck damage and bar stresses at plastication of the W6312 post.

Overhangs Supporting Deck-Mounted Steel Posts 179   Peak bar stress = 42 ksi Peak bar stress = 54 ksi Figure 265. Deck damage and bar stresses at plastication of the W6315 post. Figure 266. Deck damage and bar stresses at plastication of the W6320 post.

180 MASH Railing Load Requirements for Bridge Deck Overhang Peak bar stress >60 ksi Figure 267. Deck damage and bar stresses deck failure of the W6325 post. Figure 268. Comparison of Region A-A demands at failure of various post shapes.

Overhangs Supporting Deck-Mounted Steel Posts 181   Parametric Variations of Calibrated Post Model Variations of the calibrated LS-DYNA model of the bogie impact test were created to further characterize the damage mechanisms and capacity of the overhang. Base plate edge distance and overhang slab thickness were varied in this investigation, as they are the primary geometric parameters affecting overhang capacity. Edge Distance Increasing the post-edge distance results in a wider effective edge-beam resisting the com- pressive force underneath the base plate. With larger edge distances, more longitudinal slab bars are engaged by the post-anchorage system, increasing the capacity significantly. Although not accounted for in the LS-DYNA models due to the manner in which reinforcement is con- strained in the concrete, increasing edge distance also improves transverse bar development under the post. Models were created in which the edge distance was varied from 2 in. to 16 in. Damage contours at the peak load for the 2-in.- and 16-in.-edge distance models are shown in Figure 270, and the ultimate capacity of each model relative to the baseline calibrated model, which had an Figure 269. Comparison of Region B-B demands at failure of various post shapes. 2-in.-edge distance 16-in.-edge distance Figure 270. Effect of varying edge distances on overhang damage at peak load.

182 MASH Railing Load Requirements for Bridge Deck Overhang edge distance of 4 in., is shown in Figure 271. It should be noted that, in these models, the post was modeled as elastic, such that failure occurred in the slab, rather than at the post base. Overhang Thickness Increasing deck thickness results in increased slab bending strengths, greater shear resistance, and steeper compression strut under the post, all of which reduce the demand on the critical deck concrete directly under the post. Further, the demand on the horizontal tension tie is reduced. Models were created in which the deck thickness was varied from 6 in. to 12 in. Damage contours for the 6-in.- and 12-in.-thick slab models are shown in Figure 272, and the ultimate capacity of each model relative to the baseline (8 in.) is shown in Figure 273. The overhang capacity increased linearly with increasing slab thickness up to 12 in. Effect of Post Attachment Type Through a survey of state details, it was found that through-bolt, embedded-plate, and epoxy attachment mechanisms are used to attach steel posts to deck overhangs. Models were created Baseline 6-in.-thick slab 12-in.-thick slab Figure 271. Effect of edge distance on overhang capacity. Figure 272. Effect of varying slab thickness on overhang damage at peak load.

Overhangs Supporting Deck-Mounted Steel Posts 183   with through-bolt and embedded-plate anchors to compare their performance. Epoxy anchors were not included in this study due to a relative lack of use compared to through-bolt and embedded-plate details and the anticipated complications associated with properly modeling epoxy anchorages in LS-DYNA. To evaluate dierences in the behavior of a through-bolt and an embedded-plate attachment mechanism, the lower washer plate in the W6×15 post model was lied and constrained into the concrete just below the top mat of transverse steel. en, the model was loaded to post failure, and the damage states between the two models at failure were compared. As shown in Figure 274, whereas the through-bolt W6×15 post was able to plastify, the embedded-plate W6×15 post failed prior to post plastication due to extensive deck damage. e deck damage, shown in prole in Figure 275, was a combination of a breakout of the washer plate through the top surface of the deck and a downward punching shear of the deck edge. Baseline Figure 273. Effect of slab thickness on overhang capacity. Deck failure at 20.5 kips Post failure at 30.1 kips (a) Through-bolt (b) Embedded-plate Figure 274. Comparison of deck damage at post failure for varying attachment mechanisms.

184 MASH Railing Load Requirements for Bridge Deck Overhang Effect of Transverse Bar Termination Type To investigate the capacity benefit of hooking transverse slab bars, a variant of the baseline model was created in which the transverse slab bars were unhooked and tapered to represent incomplete development. In the baseline model, the overhang capacity was sufficient to develop the full strength of the W6×15 and sustained minor cracking consistent with early stages of a yield-line flexure mechanism. In the straight-bar model, the overhang was able to develop just 70% of the post plastic moment and sustained extreme punching shear damage. Model steel details and damage contours near the peak load are shown in Figure 276. Conclusions of Deck-Mount Steel-Post Testing and Analytical Program Key findings of the deck-mounted steel-post testing and analytical program are summarized in this section. Findings are based on results of an impact test of an instrumented deck-mounted steel post and overhang specimen and calibrated analytical models. Overhang Damage Mechanisms Punching shear failures of the slab were observed in both the physical test and analytical models of deck-mounted steel-post impacts. The observed punching shear mechanism dimensions were largely consistent with the critical perimeter specified in the AASHTO LRFD BDS, 9th edition (2). However, in many cases, wider cracking patterns more consistent with a yield-line mechanism were observed. The mechanisms that controlled the ultimate capacity of the over- hang were system-dependent. An example of damage consistent with a yield-line mechanism is shown in Figure 277. Overhang Capacity Based on the results of parametric variations of the calibrated test model, it was found that base plate edge distance and slab thickness were the geometric parameters with the greatest effect on overhang capacity. Comparisons of observed effects to the effects predicted in the existing AASHTO LRFD BDS are discussed below. Figure 275. Deck bulging and bar stresses at post failure for embedded-plate anchor model.

Overhangs Supporting Deck-Mounted Steel Posts 185   (b) Straight, tapered transverse bars(a) Hooked transverse bars ld per AASHTO LRFD Figure 276. Effect of transverse bar termination type on deck damage at failure. ld 5 straight-bar development length. The dots in various colors indicated longitudinal rebar. Top-surface cracking Bottom-surface cracking Figure 277. Yield-line flexural cracking of overhang at ultimate load in pushover model.

186 MASH Railing Load Requirements for Bridge Deck Overhang In the existing AASHTO LRFD BDS guidance, increasing edge distance results in only marginal capacity increases in the punching shear mechanism. The flexural limit state is typically unaffected unless straight transverse bars are used, in which case increased edge distance pro- vides improved bar development at the critical region. Modeling results indicated that increasing the base plate edge distance results in overhang capacity increases that are independent of trans- verse bar development and significantly greater than those predicted by the existing AASHTO LRFD BDS. For the physically tested slab design, increasing the base plate edge distance from 2 in. to 4 in. resulted in a capacity increase of 42%. These results suggest that evaluating the slab in one-way flexure is highly conservative, as it neglects the longitudinal bending strength of the slab, which is significantly increased for greater edge distances. Edge distance has long been assumed to provide greater capacities for overhangs supporting post-and-beam railings (13), and this effect is noted in the existing AASHTO LRFD BDS, although it is not quantified. The proposed methodology developed in this project, which is presented in the following section, includes an overhang yield-line mechanism, which allows for consideration of the slab longitu- dinal bending strength. Overhang thickness was also found to be a significant factor in the capacity of overhangs supporting deck-mounted steel posts, although its effect was less significant than edge distance. For example, for the tested deck-mounted steel-post design, increasing the slab thickness from 8 in. to 10 in. resulted in a capacity increase of 20%. Effects of overhang thickness were captured to a reasonable degree of accuracy in the existing AASHTO LRFD BDS. Last, the transverse bar termination type was found to have a pronounced effect on overhang capacity. Relative to straight transverse bars, hooked transverse bars provide improved concrete confinement near the field edge of the slab as well as improved bar development under the base plate. With ample edge distance and hooked bars, analytical results suggested that the post compression flange yield force can be transferred through the slab via a diagonal compression strut rather than in pure shear, which significantly decreases slab damage and increases capacity. Slab Joint Damage In analytical models of the impact test, as well as variations of that model, diagonal tension damage underneath the base plate was observed. This observation was consistent with the find- ings of the analytical programs for overhangs supporting barriers and open concrete railings. Depending on the slab configuration, this damage is due to either transverse splitting of a diagonal compression strut due to the Poisson effect or punching shear. If slab joint damage occurs, the bottom cover of the slab is delaminated, resulting in reduced slab bending strengths due to a loss of flexural depth in the slab. Overhang Tensile Demands Physical test and modeling results indicated that tensile loads in the overhang directly under the base plate were greater than initial expectations. It was found that tensile loads do not effec- tively distribute longitudinally with inward transmission through the slab in the same manner as flexural loads. Tensile demands for overhangs supporting deck-mounted steel posts were most accurately predicted by dividing the applied lateral load over the width of the base plate at both Design Regions A-A and B-B. Distributed Loads at Design Region B-B Unlike tensile demands, which were concentrated at post locations, flexural demands in the overhang were found to extensively distribute along the bridge span. It was found that distributed flexural loads at Design Region B-B can be conservatively approximated by distributing the

Overhangs Supporting Deck-Mounted Steel Posts 187   loads at a 45-degree angle from the traffic-side bolt line of the base plate to Design Region B-B. This distribution is narrow enough that, for most railing and overhang configurations, loaded regions of adjacent posts will not interact significantly. Proposed Methodology The results of the analytical and testing programs for overhangs with deck-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 D. It should be noted that the design methodology for overhangs supporting deck-mounted steel posts is virtually identical to that of overhangs supporting concrete posts. Notable changes between the two methods include an increased emphasis on the punching shear evaluation, replacement of the concrete-post compression force with the post compression flange yield force, and reconfiguration of load application geometries to represent the base plate load patch. Nomenclature Variables used in the design methodology for overhangs supporting deck-mounted steel posts are summarized in Table 16. Interior Posts Step 1. Identify Critical Overhang 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 280), and Design Region B-B, which is over the critical section of the exterior girder. These Design Regions are shown in Figure 278 in which a flanged steel girder is shown for demonstration purposes. Step 2. Establish Ultimate Post Capacity 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, Y– iii. Lateral load at Y– creating Mpost at deck 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 P b post = W (72) Step 3. Evaluate Slab Joint for Diagonal Tension Damage Prior to estimating bending strengths of the slab, the portion of the slab under the base plate must be evaluated for diagonal tension damage. This check must be performed because diagonal tension damage typically causes delamination of the bottom slab cover, which affects directional bending strengths in the trapezoidal yield-line mechanism of the slab.

188 MASH Railing Load Requirements for Bridge Deck Overhang ap = Concrete post compression block depth (in.) bo = Critical perimeter of punching shear mechanism (in.) cc,bot = Slab bottom cover (in.) cc,top = Slab top cover (in.) Cp = Yield force of compression flange (kips) db = Distance from field edge of base plate to traffic-side anchor bolt line (in.) dbt = Slab transverse bar diameter (in.) ds = Distance from field edge of base plate to center of traffic-side tension anchorage eb = Base plate edge distance (in.) f’c = Design concrete compressive strength (ksi) Fv = Vertical vehicle load on top of rail (kips) fy = Design steel reinforcing yield stress (ksi) 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) Lv = Distribution length of vertical force Fv (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) N = Distributed tensile demand along longitudinal yield-line at post base (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 = Base plate edge distance longitudinally to end of deck (in.) XA = Distance from field edge of slab to traffic-side anchor bolt line (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 16. Nomenclature for design methodology for overhangs supporting deck-mounted steel railings. Figure 278. Critical regions for overhangs supporting deck-mounted steel posts.

Overhangs Supporting Deck-Mounted Steel Posts 189   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 17. 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 American Institute of Steel Construction (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 deck surface is =ap c b pC 0.85f Wl (73) If the evaluation performed in Table 17 fails (i.e., the appropriate inequality is violated), diagonal tension damage is expected, and the flexural strength of the slab must be penalized Using a Strut-and-Tie Model Using a Punching Shear Model Failure mechanism: strut splitting i. Angle of compression strut (74) ii. Bearing length of CCT node (75) iii. Vertical component of max strut load (76) iv. Check compression strut capacity (77) Failure mechanism: punching shear i. Effective concrete shear strength (78) ii. Critical perimeter (79) Where: (80) iii. Shear capacity (81) iv. Check shear capacity (82) Table 17. Evaluation of deck joint for diagonal tension damage under post.

190 MASH Railing Load Requirements for Bridge Deck Overhang 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. Positive and negative longitudinal bending strengths of the slab act as shown in Figure 279. The longitudinal bending strength of the slab associated with bottom-mat longitudinal bars is not affected. 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 penal- ized 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 17 succeeds or that only the top mat participates if the evaluation performed in Table 17 fails. Longitudinal bending strength of the slab outside of the traffic-side bolt line, Msl, is also calcu- lated in this step. Step 4. Calculate Yield-Line Capacity of Slab The yield-line mechanism in the slab is shown in Figure 280. The horizontal yield-line passes through the traffic-side bolt line. Diagonal yield-lines and transverse yield-lines engage the negative and positive longitudinal bending strengths of the slab, respectively. Loading is applied across the entire width of the base plate and is conservatively placed at its field edge. Figure 279. Activation of positive (Msl,p) and negative (Msl,n) longitudinal slab bending strengths with downward post deflection. Figure 280. Yield-line mechanism for overhang supporting a deck-mounted steel post.

Overhangs Supporting Deck-Mounted Steel Posts 191   The critical length of the yield-line mechanism is L 12 8 M M 12 X cs b st,A sl A= + W J L KK N P OO (83) The maximum post moment able to be supported by the slab in the yield-line mechanism is M C M X e X M X M X 12L M L 12 8 Mpost,eff p post A b A str,A A b st,A A cs b sl cs b post#= - + - + - W W W J L KK J L K K K N P OO N P O O O (84) If straight transverse bars are used, Mst,A should be calculated using the average bar embed- ment depth over the diagonal yield-lines. Step 5. 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 is shown in Figure 281 and calculated as L 12 2 X e 1B b B b = + -W ` j (85) 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 (86) The effective distribution length at Design Region B-B for Design Case 2 moment is L 12 2 X e 2B b B b = + -W ` j (87) Figure 281. Effective load distribution pattern for lateral and vertical loads through overhang.

192 MASH Railing Load Requirements for Bridge Deck Overhang 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 M2B v v 2B b b sw,B= - + (88) Step 6. 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$ (89) For Design Case 2, Design Region B-B: M M2st,B B$ (90) 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 adequately anchored, incomplete bar development should be considered when calculating slab bending strengths. End Posts Evaluation of the overhang for deck-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. The modified end-region yield-line mechanism is shown in Figure 282. Figure 282. Overhang yield-line mechanism for end-region, deck-mounted steel post.

Overhangs Supporting Deck-Mounted Steel Posts 193   The critical length of the end-region yield-line mechanism is eL 12 M M 12 X cs b st,A sl A= + W W+ J L KK N P OO (91) The maximum post moment able to be supported by the slab in the end-region yield-line mechanism is e e e M C M X e X M X M X 12L L 12 M Mpost,eff p post A b A str,A A b st,A A cs b cs b sl post#= - + - + - W W W + + + W W W J L KK J L K K KK `N P OO N P O O OO j (92) 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. Overhang Punching Shear Capacity At end regions, one resisting plane in the punching shear capacity of the overhang is removed. Therefore, the critical perimeter calculation is adjusted to b t e a 2 t o b e s b p s= + + + + +W W J L KK` N P OOj (93) 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 L 12 X e 1B b e AB b= + + -W W (94) L 12 X e 2B b e AB b= + + -W W (95) Evaluation of Methodology The methodology described in this section was evaluated using the parametric variation models described in previous sections. The proposed methodology predicted the failure load of

194 MASH Railing Load Requirements for Bridge Deck Overhang each model within 15% and predicted peak demands at Design Region B-B within 10%. Errors in the methodology were underpredictions for local capacities and overpredictions for distributed demands. Comparison of Methodology to Existing AASHTO LRFD BDS For deck-mounted steel post-and-beam railings, the proposed methodology is more stringent than the existing methodology, largely due to the modification of the punching shear load patch aspect ratio. Additionally, a trapezoidal yield-line mechanism was included in the methodology, though this limit state will rarely govern designs, as the punching shear limit state is often more critical. With these changes, the methodology provides results that are more consistent with in-service railing and crash-test specimen observations—overhangs with deck-mounted steel posts are commonly damaged in impact events. Further, inclusion of a trapezoidal yield-line mechanism allows for direct quantification of the effects of longitudinal overhang steel on the ultimate capacity of the attached post. Design Example A full design example demonstrating the methodology described is presented in Appendix D. The design example includes the full analysis of the TxDOT T631 railing and accompanying TxDOT overhang design. The methodology predicted that the overhang would be able to develop the full plastic moment strength of the post, which was 8.9 k-ft, with minimal damage. The predicted yield-line capacity of the slab was 12.9 k-ft. An LS-DYNA pushover model of the system, which included an elastic post to isolate the failure strength of the overhang, indicated an overhang yield-line capacity of 15.5 k-ft. In the model, the stress of partially developed, straight transverse slab bars was used to account for incomplete bar development in the yield-line region.

Next: Chapter 7 - Overhangs Supporting Curb-Mounted Steel Posts »
MASH Railing Load Requirements for Bridge Deck Overhang Get This Book
×
 MASH Railing Load Requirements for Bridge Deck Overhang
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

READ FREE ONLINE

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!