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Pages 154-194

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From page 154...
... 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.
From page 155...
... 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.
From page 156...
... 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.
From page 157...
... 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.
From page 158...
... 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.
From page 159...
... 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.
From page 160...
... 160 MASH Railing Load Requirements for Bridge Deck Overhang Figure 228. Sequential images of deck-mounted steel-post test.
From page 161...
... Overhangs Supporting Deck-Mounted Steel Posts 161   Figure 231. Deck-mounted steel-post test slab damage.
From page 162...
... 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.
From page 163...
... Overhangs Supporting Deck-Mounted Steel Posts 163   Figure 235. Weld damage at the base of the post.
From page 164...
... 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.
From page 165...
... 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.
From page 166...
... 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.
From page 167...
... 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.
From page 168...
... 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.
From page 169...
... Figure 247. Transverse slab moments at peak load in the deck-mounted steel-post model.
From page 170...
... 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.
From page 171...
... Overhangs Supporting Deck-Mounted Steel Posts 171   (a)
From page 172...
... 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.
From page 173...
... Overhangs Supporting Deck-Mounted Steel Posts 173   Effect of Longitudinal Deck Steel To investigate the effects of varying longitudinal deck steel, moment demands were compared between W6×25-post models using #4 longitudinal deck bars and #6 longitudinal deck bars. This comparison is shown in Figures 255 and 256.
From page 174...
... 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.
From page 175...
... 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.
From page 176...
... 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.
From page 177...
... 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 performance of the deck to the predictions set by following the current AASHTO LRFD BDS (2)
From page 178...
... 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.
From page 179...
... 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.
From page 180...
... 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.
From page 181...
... 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.
From page 182...
... 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.
From page 183...
... 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.
From page 184...
... 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.
From page 185...
... Overhangs Supporting Deck-Mounted Steel Posts 185   (b) Straight, tapered transverse bars(a)
From page 186...
... 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 provides improved bar development at the critical region.
From page 187...
... 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.
From page 188...
... 188 MASH Railing Load Requirements for Bridge Deck Overhang ap = Concrete post compression block depth (in.) bo = Critical perimeter of punching shear mechanism (in.)
From page 189...
... 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.
From page 190...
... 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.
From page 191...
... 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)
From page 192...
... 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)
From page 193...
... 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)
From page 194...
... 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.

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