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Pages 114-153

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From page 114...
... 114 Overhangs Supporting Concrete Posts In applications where improved hydraulic performance or visibility is desired, open concrete railings are favored by some state agencies over barriers. Open concrete railings transfer impact loads into the overhang as concentrated flexural and tensile demands at post bases, often resulting in more significant overhang damage in impact events.
From page 115...
... Overhangs Supporting Concrete Posts 115   It should be noted that the TxDOT T224 railing is installed on a curb, therefore strengthening the overhang significantly. Existing BDS Design Methodology Deck overhangs supporting concrete post-and-beam rails are not discussed explicitly in AASHTO LRFD BDS (2)
From page 116...
... 116 MASH Railing Load Requirements for Bridge Deck Overhang as the railing is effectively a plate. Concrete post-and-beam railings are effectively space frames, and characterizing the distributions of railing loads into individual posts in a manner conducive to specification language was not possible within the bounds of NCHRP Project 12-119.
From page 117...
... Overhangs Supporting Concrete Posts 117   post would sustain the least severe damage in the full-scale crash testing series. Then, linear strain gages were applied to transverse deck bars near the selected post, and the system was constructed.
From page 118...
... 118 MASH Railing Load Requirements for Bridge Deck Overhang height of 29 in. to provide load application dimensions consistent with the MASH TL-4 recommendations of NCHRP Project 22-20(2)
From page 119...
... Overhangs Supporting Concrete Posts 119   e force exerted on the bogie by the post, as measured via onboard accelerometers, is shown in Figure 175. e curves shown were passed through a CFC-60 lter.
From page 120...
... Figure 174. Progression of field-edge slab damage.
From page 121...
... Overhangs Supporting Concrete Posts 121   Specimen Damage During the impact test, the post was undamaged, and the slab sustained severe damage somewhat similar to a yield-line mechanism, as shown in Figure 176. Elliptical failure surfaces extended outward from the traffic side of the post, and a flexural crack was opened through the section coincident with the traffic-side vertical steel.
From page 122...
... 122 MASH Railing Load Requirements for Bridge Deck Overhang Figure 178. Deflection of longitudinal slab bars.
From page 123...
... Overhangs Supporting Concrete Posts 123   This point in time was selected because it represents the maximum strain state recorded before probable strain gage damage. As most of the strain gages on the right side of the post did not survive the construction process, data from the left side of the post were mirrored for readability.
From page 124...
... 124 MASH Railing Load Requirements for Bridge Deck Overhang damage incurred during the previously performed full-scale crash tests, and the friction coefficient between the post and slab was reduced from 0.4 to 0.2. Overall Response Accuracy The force-time history of the calibrated LS-DYNA model is compared to the physical test result in Figure 182.
From page 125...
... Overhangs Supporting Concrete Posts 125   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 physical test strain gage measurements. Top-mat slab-bar strains calculated at Design Region A-A are compared to corresponding strain gage measurements at the point of first yield in Figure 185.
From page 126...
... 126 MASH Railing Load Requirements for Bridge Deck Overhang Discussion of Calibrated LS-DYNA Model As the concrete-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 127...
... Overhangs Supporting Concrete Posts 127   Extrapolative Modeling -- Load Distributions As overhangs supporting concrete 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 considerations at Design Region B-B inconsequential if the at-post steel configuration is extended to the exterior girder.
From page 128...
... 128 MASH Railing Load Requirements for Bridge Deck Overhang 4.4 ft, indicating an effective distribution angle of 49 degrees between the field face of the post and Design Region A-A. The peak Design Region B-B moment corresponded to an effective distribution length of 18.9 ft, indicating an effective distribution angle of 61.5 degrees.
From page 129...
... Overhangs Supporting Concrete Posts 129   Midspan and At-Post Loading The behaviors discussed in the previous section corresponded to a load applied directly at a post centerline. Models were also created in which the load was applied at the midspan between posts to evaluate potential differences in overhang demands or damage between the two cases.
From page 130...
... 130 MASH Railing Load Requirements for Bridge Deck Overhang Effect of Beam-and-Post Steel Configurations As the concrete beam is the primary load-transferring element in the concrete post-andbeam system, it was inferred that modifying the longitudinal beam steel would affect the load transfer to adjacent posts and, to a lesser extent, further stiffen the deck edge. Models including baseline #6 longitudinal bars and #4 longitudinal bars were subjected to loading at post and midspan locations.
From page 131...
... Overhangs Supporting Concrete Posts 131   longitudinal distribution of the loads would increase, and peak moments in the deck would be reduced. To investigate this effect, models using #5 post verticals were compared to models using #3 post verticals under both at-post and midspan loading conditions.
From page 132...
... 132 MASH Railing Load Requirements for Bridge Deck Overhang Moment demands at Design Region A-A were increased by 59% when the load was moved to the outermost post and by 14% when the load was moved to the outermost midspan. Moment demands at Design Region B-B were increased by 196% when the load was moved to the outermost post and by 160% when the load was moved to the outermost midspan.
From page 133...
... Overhangs Supporting Concrete Posts 133   36-in.-tall MASH TL-4 Optimized Concrete Bridge Rail (4CBR)
From page 134...
... 134 MASH Railing Load Requirements for Bridge Deck Overhang Region B-B Position along span (ft) Load application length R eg io n BB m om en t ( kft/ ft)
From page 135...
... Overhangs Supporting Concrete Posts 135   the calibrated model included edge distances varying from 0 in. to 24 in., overhang thicknesses ranging from 6 in.
From page 136...
... 136 MASH Railing Load Requirements for Bridge Deck Overhang 14 modeled systems were able to develop the full plastic moment capacity of the post due to diagonal tension failure of the joint. This damage mechanism is accounted for in the proposed methodology.
From page 137...
... Overhangs Supporting Concrete Posts 137   Models were created in which the edge distance was varied from 0 in.
From page 138...
... 138 MASH Railing Load Requirements for Bridge Deck Overhang Baseline 8-in.-thick slab 18-in.-thick slab Baseline Figure 206. Effect of edge distance on overhang capacity.
From page 139...
... Overhangs Supporting Concrete Posts 139   capacity increased linearly with increasing slab thickness up to 12 in. Beyond 12 in., capacity increases became negligible, as the capacity of the system began to be limited by shear breakout of the post anchor bars.
From page 140...
... 140 MASH Railing Load Requirements for Bridge Deck Overhang the concrete failures to the sides of the post result from a complex combined stress state of shear breakout and torsion, in addition to vertical punching shear. These results suggest that adding steel similar to Option 2 is strongly preferable and may result in significantly less deck damage and improved post performance.
From page 141...
... Overhangs Supporting Concrete Posts 141   When subjected to an isolated couple at the post base, the overhang was able to support a 194-kip downward load, which was 213% higher than the maximum load developed in isolated punching shear. In this model, a linearly increasing downward load was applied to the load patch, and an equal tension force was applied to the vertical post bars extending from the slab.
From page 142...
... 142 MASH Railing Load Requirements for Bridge Deck Overhang Conclusions of Concrete-Post Testing and Analytical Program Key findings of the concrete-post testing and analytical program are summarized in this section. Findings are based on the results of an impact test of an instrumented concrete post and overhang specimen and calibrated analytical models.
From page 143...
... Overhangs Supporting Concrete Posts 143   Factors Affecting Overhang Capacity By parametrically varying the design of the LS-DYNA model calibrated to the physical test, sensitivities of the overhang capacity to selected design choices were characterized. It was found that edge distance and overhang thickness had the most pronounced effect on overhang capacity and damage under post loading.
From page 144...
... 144 MASH Railing Load Requirements for Bridge Deck Overhang the same test level. For this system, the beam was 27 in.
From page 145...
... Overhangs Supporting Concrete Posts 145   load permitted in BDS Article 3.6.1.3.4 for railings with "structurally continuous concrete railing[s]
From page 146...
... 146 MASH Railing Load Requirements for Bridge Deck Overhang 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.
From page 147...
... Overhangs Supporting Concrete Posts 147   in the deck. Positive and negative longitudinal bending strengths of the slab act as shown in Figure 216.
From page 148...
... 148 MASH Railing Load Requirements for Bridge Deck Overhang if the evaluation performed in Table 14 fails. Longitudinal bending strength of the slab outside of the traffic-side vertical post steel, Msl, is also calculated in this step.
From page 149...
... Overhangs Supporting Concrete Posts 149   Figure 218. Effective distribution of lateral and vertical loads through the overhang.
From page 150...
... 150 MASH Railing Load Requirements for Bridge Deck Overhang Overhang Yield-Line Capacity The overhang yield-line capacity must be modified to account for the free end of the overhang adjacent to the concrete post. The modified end-region yield-line mechanism is shown in Figure 219.
From page 151...
... Overhangs Supporting Concrete Posts 151   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.
From page 152...
... 152 MASH Railing Load Requirements for Bridge Deck Overhang the concrete post and overhang specimen tested in this project, the calibrated LS-DYNA model indicated an ultimate static strength of 28 kips. As the nominal capacity of the post was 40 kips, this result suggested that overhang failure limited the ultimate load able to be exerted on the post.
From page 153...
... Overhangs Supporting Concrete Posts 153   7% lower than the observed failure load in the model. At the post yield-load of 16 kips, the peak Design Region B-B moment in the LS-DYNA model was 3.7 k-ft/ft; the methodology predicted a Region B-B moment of 6.3 k-ft/ft, indicating a significant degree of conservatism.

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