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From page 19...
... 19   Overhangs Supporting Concrete Barriers Concrete barriers transfer impact loads into the overhang as distributed flexural and tensile demands. Additionally, this railing type provides a significant edge-stiffening effect, resulting in expansive longitudinal distribution of demands and reserve overhang capacity beyond the instance of the first slab-bar yielding.
From page 20...
... 20 MASH Railing Load Requirements for Bridge Deck Overhang To inform future revisions of the AASHTO LRFD BDS Section 13, state DOT representatives provided commentary on observed shortcomings of the current bridge deck overhang evaluation methodology. In this free-response question, the most common concerns regarded overhangs supporting barriers.
From page 21...
... Overhangs Supporting Concrete Barriers 21   railing were not included in the summary. Specimen damage in these tests was generally minor, consisting mostly of diagonal barrier cracking.
From page 22...
... 22 MASH Railing Load Requirements for Bridge Deck Overhang concrete barrier about its longitudinal axis at its base, Mc,base. The unit-length tensile force acting on the overhang section kips per foot (k/ft)
From page 23...
... Overhangs Supporting Concrete Barriers 23   the vertical load may be applied at the lateral location of the barrier centroid, as suggested in the NHI course, or conservatively at the rear face of the barrier. The design case and design region combinations presented in NHI Course No.
From page 24...
... 24 MASH Railing Load Requirements for Bridge Deck Overhang in which the design load, βFt, acts at some distance H2 above the deck surface to create a moment that distributes with downward transmission through the barrier at a 45-degree angle. β is an optional overstrength factor adopted by some agencies to amplify the transverse vehicle impact force, Ft, specified by the AASHTO LRFD BDS and thereby reduce the potential for deck damage from vehicle impacts by overdesigning the deck relative to the supported barrier.
From page 25...
... Overhangs Supporting Concrete Barriers 25   2 T L H F 1 A i c tb= + (4)
From page 26...
... 26 MASH Railing Load Requirements for Bridge Deck Overhang Dakota, Ohio, and Washington State DOTs. Similarly, flexural demands should be limited to Mc, as the cantilever bending strength of the barrier is the maximum flexural demand that can be transferred to the deck overhang in an impact event.
From page 27...
... Overhangs Supporting Concrete Barriers 27   strength was lower than the cantilever bending capacity of the barrier. Tests performed on deck overhang capacities with flexural capacities less than Mc are summarized in Table 5.
From page 28...
... 28 MASH Railing Load Requirements for Bridge Deck Overhang were performed to quantify effective distribution angles through the barrier (θA) and overhang (θB)
From page 29...
... Overhangs Supporting Concrete Barriers 29   Instrumentation The barrier and overhang specimen were instrumented with linear strain gages installed on the traffic-side vertical barrier bars and the top-mat transverse slab bars. Barrier bar strain gages were installed 2 in.
From page 30...
... 30 MASH Railing Load Requirements for Bridge Deck Overhang 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 barrier at 20 mph and at an impact angle of 90 degrees.
From page 31...
... Overhangs Supporting Concrete Barriers 31   to select an interior impact location that would produce a full, interior yield-line mechanism without interfering with the end-region test. The impact location selected to produce an interior behavior without developing a cracking pattern that would extend into the expected cracking pattern of the end-region test was 12.5 ft from the north end of the barrier.
From page 32...
... 32 MASH Railing Load Requirements for Bridge Deck Overhang Figure 38. Sequential images of interior barrier test.
From page 33...
... Overhangs Supporting Concrete Barriers 33   (40 ms and 55 ms) were roughly triangular and are shown in Figure 41.
From page 34...
... 34 MASH Railing Load Requirements for Bridge Deck Overhang Following initial documentation, barrier and slab cracks were marked for visibility. The marked cracking pattern of the barrier and slab is shown in Figure 44.
From page 35...
... Overhangs Supporting Concrete Barriers 35   Traffic-side face Field-side face Figure 45. Summary of cracking for interior barrier test.
From page 36...
... 36 MASH Railing Load Requirements for Bridge Deck Overhang Figure 47. Maximum cracking severities on the traffic face and top (top photo)
From page 37...
... Overhangs Supporting Concrete Barriers 37   Strain Gage Data Linear strain gages were fastened to specimen reinforcement at three locations: the traffic-side vertical barrier steel 2 in. above the slab surface; top-mat transverse slab steel at Design Region A-A; and top-mat transverse slab steel at Design Region B-B.
From page 38...
... 38 MASH Railing Load Requirements for Bridge Deck Overhang 40 ms 55 ms Figure 51. Strain gage measurements, traffic-side vertical barrier bars.
From page 39...
... Overhangs Supporting Concrete Barriers 39   the proximity of the impact point to the free end of the barrier. It should be noted that these strain gage measurements do not include self-weight strains, as gages were zeroed before testing.
From page 40...
... 40 MASH Railing Load Requirements for Bridge Deck Overhang more accurate result than the K&C model; therefore, the CSC model is considered the calibrated model for this event. Using the calibrated LS-DYNA model, additional data describing the event were extracted, including the static behavior of the system, direct separation of slab demands into flexural and tensile components, and damage inside the system not visible in video or photo records of the event.
From page 41...
... Overhangs Supporting Concrete Barriers 41   the CSC model was more accurate than the K&C model. A more accurate K&C model result was not able to be achieved by making justifiable modifications; therefore, the CSC model was used as the calibrated LS-DYNA model for further investigation.
From page 42...
... 42 MASH Railing Load Requirements for Bridge Deck Overhang Comparison to Strain Gage Measurements After it was determined that the LS-DYNA model had predicted the overall force and damage response of the specimen to a reasonable degree of accuracy, slab-bar strains calculated in the LS-DYNA model were compared to the physical test strain gage measurements. Top-mat slab-bar strains calculated at Design Region A-A are compared to corresponding strain gage measurements during the first and second force peak in Figures 60 and 61, respectively.
From page 43...
... Overhangs Supporting Concrete Barriers 43   Traffic-side face Field-side face Figure 57. Comparison of damage contours to test cracking patterns.
From page 44...
... 44 MASH Railing Load Requirements for Bridge Deck Overhang Diagonal cracking of slab at field edge Longitudinal cracking on bottom face of slab Figure 58. Comparison of deck damage between the physical test and model.
From page 45...
... Overhangs Supporting Concrete Barriers 45   Figure 59. Diagonal tension failure of slab below barrier.
From page 46...
... 46 MASH Railing Load Requirements for Bridge Deck Overhang Figure 62. Comparison of peak Design Region B-B strain gage data to LS-DYNA model strains.
From page 47...
... Overhangs Supporting Concrete Barriers 47   LS-DYNA model as well as the validity of end-region barrier models used in the analytical program. Details of the test specimen, impact conditions, and test results are presented in this section.
From page 48...
... 48 MASH Railing Load Requirements for Bridge Deck Overhang and barrier and the critical length and redirective capacity of the barrier yield-line mechanism are summarized in Table 7. The expected end-region yield-line mechanism, which is consistent with the recommendations of NCHRP Project 22-41, is shown in Figure 66.
From page 49...
... Overhangs Supporting Concrete Barriers 49   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 barrier at 20 mph and at an impact angle of 90 degrees.
From page 50...
... 50 MASH Railing Load Requirements for Bridge Deck Overhang Figure 70. Sequential photos of end-region barrier test, side view.
From page 51...
... Overhangs Supporting Concrete Barriers 51   loads. The peak 50-ms average force measured in the test was 67 kips.
From page 52...
... 52 MASH Railing Load Requirements for Bridge Deck Overhang Traffic-side face Field-side face Traffic-side face Field-side face Traffic-side face Field-side face Figure 73. Barrier and deck damage after end-region barrier test.
From page 53...
... Overhangs Supporting Concrete Barriers 53   The estimated critical yield-line pattern that formed in the barrier is shown in Figure 76. The cracking pattern that developed in this test was somewhat consistent with the pattern proposed in NCHRP Project 22-41, although the apparent critical length was significantly shorter than the expected value.
From page 54...
... 54 MASH Railing Load Requirements for Bridge Deck Overhang In addition to the severe damage at the eld edge of the slab, distributed longitudinal and diagonal cracking was also observed on the top and bottom surfaces of the slab. Top-surface slab cracking at Design Region B-B (over the eld edge of the grade beam)
From page 55...
... Overhangs Supporting Concrete Barriers 55   Figure 80. Top-surface slab cracking at Design Region B-B.
From page 56...
... 56 MASH Railing Load Requirements for Bridge Deck Overhang Strains measured at Design Region B-B, which is over the field edge of the grade beam, are shown in Figure 84. It should be noted that these strain gage measurements do not include selfweight strains, as gages were zeroed before testing.
From page 57...
... Overhangs Supporting Concrete Barriers 57   Calibrated End-Region Impact Model The accuracy of the LS-DYNA model for the end-region barrier impact event was evaluated using the physical test data. Models created using both the K&C and CSC concrete models produced reasonably accurate representations of the physical test.
From page 58...
... 58 MASH Railing Load Requirements for Bridge Deck Overhang 35 ms 55 ms 80 ms Traffic-side face Field-side face Figure 86. LS-DYNA force-deflection comparison to physical test result.
From page 59...
... Overhangs Supporting Concrete Barriers 59   pattern and diagonal fracture occurred; on the field-side face of the barrier, inverted diagonal cracking developed. The total length of barrier cracking in the LS-DYNA model was 17.3 ft, while the total length of cracking in the physical test was 13.4 ft.
From page 60...
... 60 MASH Railing Load Requirements for Bridge Deck Overhang 38 ms after impact 44 ms after impact 60 ms after impact Figure 89. Comparison of barrier-slab joint damage progression.
From page 61...
... Overhangs Supporting Concrete Barriers 61   e LS-DYNA model also predicted vertical barrier bar rupture, which was observed in the physical test. As shown in Figure 90, the second, third, and fourth vertical bars from the barrier edge were fractured in the model.
From page 62...
... 62 MASH Railing Load Requirements for Bridge Deck Overhang as a baseline for other investigative models, such as static loading and design variation models. Further, as only minor, test-specific adjustments were made to the model in the calibration process, no adjustments to the models created in the preceding analytical program were required.
From page 63...
... Overhangs Supporting Concrete Barriers 63   in resistance between the static and dynamic models suggests a substantial inertial contribution, which dissipates impact energy and reduces the effective load that must be resisted by the barrier and slab. As shown in Figure 94, specimen damage in the quasi-static pushover model was similar to that sustained in the dynamic event shown in Figures 87 and 88.
From page 64...
... 64 MASH Railing Load Requirements for Bridge Deck Overhang load of 74 kips. This load state was chosen for identification of design demands as it is consistent with the alternative design procedures used by several agencies in which the slab demands are specified by dividing the total applied moment over an effective distribution length.
From page 65...
... Overhangs Supporting Concrete Barriers 65   The effective distribution length at Design Region A-A, LA, is calculated by dividing the total applied moment by the maximum moment demand, MA,max, calculated in the LS-DYNA model with the moment due to self-weight, Msw,A, removed. For this model: • • .
From page 66...
... 66 MASH Railing Load Requirements for Bridge Deck Overhang As the peak Design Regions A-A, TA, and B-B, TB, tension demands are nearly equal, no distribution between the two regions was considered. Therefore: .14 8T T ft kips B A= = (17)
From page 67...
... Overhangs Supporting Concrete Barriers 67   Figure 99. Effective distribution angles for calculation of the slab design moment.
From page 68...
... 68 MASH Railing Load Requirements for Bridge Deck Overhang difference between the Regions A-A and B-B moments was less significant than for interior loading. After removing self-weight moments, the demands shown correspond to barrier and overhang distribution angles of 45.5 degrees and 61.2 degrees, respectively.
From page 69...
... Overhangs Supporting Concrete Barriers 69   lower longitudinal deck bars in each model, and top transverse deck bars transitioning from round hooks to square hooks to account for the increased deck depth. Moment demands calculated at Design Regions A-A and B-B for interior loading models of varying deck thicknesses are shown in Figures 104 and 105.
From page 70...
... 70 MASH Railing Load Requirements for Bridge Deck Overhang Moment demands observed at Regions A-A and B-B for end-region loading models of varying deck thicknesses, which showed similar trends as those of the interior region, are shown in Figures 106 and 107. As for interior regions, increasing deck thickness increased peak moment demands on both Regions.
From page 71...
... Overhangs Supporting Concrete Barriers 71   (a) 1-ft cantilever (b)
From page 72...
... 72 MASH Railing Load Requirements for Bridge Deck Overhang However, for the 1- cantilever system, Regions A-A and B-B became coincident, as the face of the barrier and the face of the supporting element were separated by less than an inch. As such, in the results shown in this section, moment demands at both Regions are identical for the 1- cantilever model.
From page 73...
... Overhangs Supporting Concrete Barriers 73   Sensitivity to Barrier Height With increasing barrier height, the magnitude of the deck-edge stiffening effect increases, consequently increasing the longitudinal extent of demand distributions and reducing moments exerted on the deck slab. Additionally, increasing barrier height results in greater lengths over which longitudinal distribution can occur as loads travel downward from the application point to the deck surface.
From page 74...
... 74 MASH Railing Load Requirements for Bridge Deck Overhang parameters. erefore, for TL-3 barriers, increasing barrier height simply increases the stiening eect at the deck edge without changing the impact load magnitude, application length, or application height.
From page 75...
... Overhangs Supporting Concrete Barriers 75   Figure 114. Force-deflection responses for variable-height TL-3 models.
From page 76...
... 76 MASH Railing Load Requirements for Bridge Deck Overhang Height Variation Within Test Level 4 For TL-4 barriers, design parameters are dependent upon barrier height, as the engagement of the 10000S test vehicle's cargo box is aected by the barrier height. erefore, as barrier height increases, the stiening eect at the deck edge increases, but design parameters also change, resulting in deck demand trends that are not as consistent as those of TL-3 barriers.
From page 77...
... Overhangs Supporting Concrete Barriers 77   post-overlay behavior. As for the TL-3 models, reinforcement was configured such that each barrier height could withstand the applied lateral load without modification.
From page 78...
... 78 MASH Railing Load Requirements for Bridge Deck Overhang demands at Regions A-A and B-B increased by only 10% and 34%, respectively. is reduction in deck demands relative to the applied moment is primarily due to greater vertical distances over which loads must travel before reaching the deck surface and greater edge-stiening eects but is partially owed to the 1- increase in the load application length that occurs when the barrier height is increased beyond 36 in.
From page 79...
... Overhangs Supporting Concrete Barriers 79   Figure 121. Design Region A-A moments for TL-5 barriers of varying height.
From page 80...
... 80 MASH Railing Load Requirements for Bridge Deck Overhang distribution of deck demands expanded, and demand magnitudes increased, due to the increasing load magnitude and application height. As for TL-4 barriers, increases in deck demands were significantly outpaced by increases in the total applied moment due to the additional edge-stiffening effect of taller barriers.
From page 81...
... Overhangs Supporting Concrete Barriers 81   Moment demands calculated at Region A-A and Region B-B for end-region loading models of varying barrier steel are shown in Figures 125 and 126. The trends observed in the interior loading of these models were maintained.
From page 82...
... 82 MASH Railing Load Requirements for Bridge Deck Overhang In order to investigate the effect of this parameter, the transverse deck-bar size was varied from #3 to #6. It should be noted that as transverse deck steel is the primary load-bearing component in the deck resisting lateral barrier loads, bar yielding and deck damage occurred at different loads and to different extents in these models.
From page 83...
... Overhangs Supporting Concrete Barriers 83   models showed virtually no sensitivity to the influence of elastic stiffness variation with alternative portions of transverse deck steel. Moment demands calculated at Region A-A and Region B-B for end-region loading models of varying transverse deck steel are shown in Figures 129 and 130.
From page 84...
... 84 MASH Railing Load Requirements for Bridge Deck Overhang varied from #3 to #6. The quantity and position of longitudinal deck bars were left unchanged, except for the vertical position of the bars, which required slight modification due to changes in bar diameter.
From page 85...
... Overhangs Supporting Concrete Barriers 85   Figure 132. Design Region B-B moment distribution variation with longitudinal deck steel.
From page 86...
... 86 MASH Railing Load Requirements for Bridge Deck Overhang Sensitivity to Deck-Barrier Interface Type In the early modeling of overhangs supporting concrete barriers, it was observed that the manner in which the barrier was secured to the deck surface affected the location of Design Region A-A. In models where the barrier simply rested on the deck surface and was only secured to the deck via the vertical bars, the critical section was located at the position of the vertical barrier steel; in models where the barrier was fixed to the deck surface by sharing nodes, the critical section was located at the barrier face.
From page 87...
... Overhangs Supporting Concrete Barriers 87   inferred that the tensile strength of concrete is too low for this parameter to behave consequentially relative to the effect of vertical barrier steel. Sensitivity to Span Length A key factor in the longitudinal distribution of deck demands is the distance over which they can spread.
From page 88...
... 88 MASH Railing Load Requirements for Bridge Deck Overhang Figure 138. Effect of span length on Design Region A-A moment demand.
From page 89...
... Overhangs Supporting Concrete Barriers 89   demand curve is changed such that the peak moments occur at the deck edge and begin to rise drastically with further decreases in span length, as the deck overhang begins to act as a pure cantilever rather than a cantilevered plate. Moment demands calculated at Region A-A and Region B-B for end-region loading of these models are shown in Figures 142 and 143, respectively.
From page 90...
... 90 MASH Railing Load Requirements for Bridge Deck Overhang overhang distribution begins to change drastically at Region B-B at roughly 40–50 ft of span. For a curbed steel-post system, which is discussed in Chapter 7 of this report, the breakpoint span is 15–20 ft, as the stiffening effect of the 8-in.-tall curb is substantially less intense than that of the 39-in.-tall barrier.
From page 91...
... Overhangs Supporting Concrete Barriers 91   elastic region. However, if weak-deck and strong-deck models are compared at the design load of 80 kips, and deck yielding is permitted, the load distributions observed in these models are clearly different due to the progressive yielding and widening load distribution that occurs when the deck is too weak to resist the concentrated moment that occurs at the horizontal yield-line in the barrier.
From page 92...
... 92 MASH Railing Load Requirements for Bridge Deck Overhang than Mc, significant yielding of the transverse deck bars occurred at Region A-A. Rather than failing catastrophically, however, as the load increased, the yielding progressed longitudinally to the extent required to equilibrate the load.
From page 93...
... Overhangs Supporting Concrete Barriers 93   The commentary of the current AASHTO LRFD BDS, Section 13 suggests that if the deck capacity is less than the cantilever bending capacity of the barrier, the yield-line mechanism will not be able to form properly in the barrier. Modeling results were partially consistent with this suggestion.
From page 94...
... 94 MASH Railing Load Requirements for Bridge Deck Overhang Summary of Sensitivity Study Demands acting on Design Regions A-A and B-B were affected substantially by some of the parametric variations imposed on the baseline model. Parameters with significant effects on deck demands were: • Span length.
From page 95...
... Overhangs Supporting Concrete Barriers 95   • Barrier height. As the barrier height was increased, but the total moment acting on the barrier was held constant, demands in the deck at Region B-B were reduced.
From page 96...
... 96 MASH Railing Load Requirements for Bridge Deck Overhang to 88 kips. This reduction in redirective capacity was predicted within 10% by reducing the value of Mc,base in the yield-line equations accordingly.
From page 97...
... Overhangs Supporting Concrete Barriers 97   negligible, and demands at Region B-B peaked at the load center and were distributed in a triangular shape across the full 70 ft span. The magnitude and distributions of vertical loads were also evaluated at the end region of the baseline model.
From page 98...
... 98 MASH Railing Load Requirements for Bridge Deck Overhang Extrapolative Modeling -- Slab Joint Strength An unexpected damage mechanism was identied as a result of the physical tests performed on the barrier specimen. A longitudinal crack formed along the bottom surface of the deck roughly 1  from the eld edge, and a severe crack with a maximum opening width of roughly ½ in.
From page 99...
... Overhangs Supporting Concrete Barriers 99   Figure 156. Diagonal tension damage in the joint visible in the end-region bogie test.
From page 100...
... 100 MASH Railing Load Requirements for Bridge Deck Overhang Due to the frequency with which this damage mechanism is observed in laboratory testing and the cracking patterns observed in physical testing performed in this project, it is believed that this damage mechanism may be common for overhangs supporting barrier railings. In significant in-service impacts, it is possible that this damage mechanism is occurring to a lesser extent than shown above but is obscured by the surrounding concrete.
From page 101...
... Overhangs Supporting Concrete Barriers 101   Basic Load Distribution and Overhang Demands Moment demands at Design Regions A-A and B-B can be conservatively calculated by dividing the total applied moment, FtHe, over effective distribution lengths, which are calculated using effective distribution angles through the barrier and overhang. Through the barrier, flexural demands can be assumed to distribute at 45 degrees with downward transmission; through the slab, flexural demands can be assumed to distribute at 60 degrees with lateral transmission through the overhang.
From page 102...
... 102 MASH Railing Load Requirements for Bridge Deck Overhang moment demands at Design Region B-B than would be estimated using the patterns shown. Tensile demands are independent of span length.
From page 103...
... Overhangs Supporting Concrete Barriers 103   supporting barrier railings. This damage was visible in the end-region barrier impact test and is believed to have also occurred in the interior impact test.
From page 104...
... 104 MASH Railing Load Requirements for Bridge Deck Overhang as listed in Table 8. This requirement affects interior loading only.
From page 105...
... Overhangs Supporting Concrete Barriers 105   Interior Loading For demonstration purposes, it is assumed that the design of the barrier and overhang are known. Thus, the process shown is an analysis methodology, rather than a design methodology.
From page 106...
... 106 MASH Railing Load Requirements for Bridge Deck Overhang performed, because diagonal tension damage typically causes delamination of the bottom slab cover, resulting in a reduced effective bending depth at Design Region A-A. The load transfer mechanism from the compressive zone of the barrier to Design Region A-A is believed to occur either through a strut-and-tie behavior or a vertical shear mechanism.
From page 107...
... Overhangs Supporting Concrete Barriers 107   two layers of reinforcing if the evaluation performed in Table 10 succeeds or applying the steel area reduction only to the top layer if the evaluation performed in Table 10 fails.
From page 108...
... 108 MASH Railing Load Requirements for Bridge Deck Overhang The effective distribution length at Design Region B-B is L L 12 tan 60 1B 1A AB= + 2X c (32) Therefore, the design moment at Design Region B-B is M 12L F H 0.5t M1B 1B t e s sw,B= + + ` j (33)
From page 109...
... Overhangs Supporting Concrete Barriers 109   Step 6. Compare Slab Strength to Distributed Demands At this point, effective slab strengths and distributed moment demands are known.
From page 110...
... 110 MASH Railing Load Requirements for Bridge Deck Overhang Step 5. Calculate Distributed Design Case 1 and 2 Moment Demands As load distribution is restricted to one direction longitudinally at the end region, effective distribution lengths are reduced, and moment demands are magnified.
From page 111...
... Overhangs Supporting Concrete Barriers 111   Comparison of Methodology to Existing AASHTO LRFD BDS For overhangs with barriers, using the proposed load distribution pattern will often result in dramatically reduced moment demands relative to the existing method of the AASHTO LRFD BDS (2)
From page 112...
... 112 MASH Railing Load Requirements for Bridge Deck Overhang that tensile demands are increased in the updated methodology; however, the updated methodology still requires significantly less overhang steel than the existing method for the majority of designs. Design Example A full design example demonstrating the methodology described above is presented in Appendix B.
From page 113...
... Overhangs Supporting Concrete Barriers 113   Figure 167. Force-deflection response of example LS-DYNA model for interior loading.

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