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Pages 195-240

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From page 195...
... 195   Overhangs Supporting Curb-Mounted Steel Posts Due to the small footprint of steel-post base plates, steel railings exert highly concentrated flexural and tensile demands on the overhang, often resulting in significant slab damage. Mounting steel railings on a curb, rather than directly to the deck surface, has been shown to be an effective method of reducing overhang damage while largely maintaining the aesthetic and hydraulic benefits of steel railings.
From page 196...
... 196 MASH Railing Load Requirements for Bridge Deck Overhang cracking of the curb, which may or may not extend into the overhang, is occasionally observed. Examples of overhang damage observed in curb-mounted steel post-and-beam crash-test articles are shown in Figure 283.
From page 197...
... Overhangs Supporting Curb-Mounted Steel Posts 197   Impact Tests of Curb-Mounted Steel-Post Specimen Two impact tests were performed on curb-mounted steel-post specimens in order to measure the longitudinal distribution of impact loads through the curb and deck overhang. In addition to providing physical data points, test results were used to evaluate the accuracy of the accompanying LS-DYNA models.
From page 198...
... 198 MASH Railing Load Requirements for Bridge Deck Overhang Locations of interior and end-region curb-mounted post tests are shown in Figure 285. The interior test was performed before the end-region test.
From page 199...
... Overhangs Supporting Curb-Mounted Steel Posts 199   Interior Test Specimen Response Sequential photos of the interior curb-mounted steel-post impact test are shown in Figures 289 and 290. In the event, the post successfully contained the bogie vehicle, and the curb sustained extreme damage.
From page 200...
... 200 MASH Railing Load Requirements for Bridge Deck Overhang Peak strain gage measurements on slab bars at Design Regions A-A and B-B are shown in Figures 299 and 300, respectively. End-Region Test Impact Conditions Loading was applied to the specimen via a surrogate bogie vehicle impact.
From page 201...
... Overhangs Supporting Curb-Mounted Steel Posts 201   Figure 290. Sequential images of interior curb-mounted steel-post test, front view.
From page 202...
... 202 MASH Railing Load Requirements for Bridge Deck Overhang Figure 292. Interior curb-mounted steel-post test damage, front view.
From page 203...
... Overhangs Supporting Curb-Mounted Steel Posts 203   Figure 296. Curb failure in the interior test (with loose concrete removed)
From page 204...
... 204 MASH Railing Load Requirements for Bridge Deck Overhang Figure 299. Peak strain gage measurements at Region A-A in interior curb-mounted steel-post test.
From page 205...
... Overhangs Supporting Curb-Mounted Steel Posts 205   Figure 302. Sequential images of end-region curb-mounted steel-post test, rear view.
From page 206...
... 206 MASH Railing Load Requirements for Bridge Deck Overhang Crush tube yielding began in the test around 10 ms after the point of first contact. Around 35 ms after contact, strain hardening in the crush tubes resulted in an increasing lateral load exerted between 35 ms and 65 ms.
From page 207...
... Overhangs Supporting Curb-Mounted Steel Posts 207   Figure 305. End-region curb-mounted steel-post test specimen damage with marked cracking.
From page 208...
... 208 MASH Railing Load Requirements for Bridge Deck Overhang Reserve Capacity Test Although a clear strain distribution was observed for Design Region A-A in the end-region curb-mounted steel-post test, recorded strains were small (roughly 20% of yield)
From page 209...
... Overhangs Supporting Curb-Mounted Steel Posts 209   As shown in Figure 309, the post and base plate assembly were completely broken out of the curb. A trapezoidal breakout cone was formed in the curb.
From page 210...
... 210 MASH Railing Load Requirements for Bridge Deck Overhang Figure 311. Curb damage extending into the slab.
From page 211...
... Overhangs Supporting Curb-Mounted Steel Posts 211   The curb was the limiting element in all tests, and deck damage was only sustained in the catastrophic failure produced in the second test of the damaged end-region specimen. Although the curb was designed to be sacrificial, the post was able to exert lateral loads on the bogie vehicle that were 30% higher than the deck-mounted post due to the decreased lever arm provided by the curb.
From page 212...
... 212 MASH Railing Load Requirements for Bridge Deck Overhang surrounding concrete had eroded and interrupted the load transfer mechanism. After applying this change and slightly modifying the concrete erosion strain, an acceptable level of accuracy was achieved.
From page 213...
... Overhangs Supporting Curb-Mounted Steel Posts 213   Comparison to Strain Gage Measurements Peak LS-DYNA strains at Region A-A are compared to peak strain gage measurements recorded in the interior test in Figure 319. As shown, the LS-DYNA model produced accurate estimates of transverse bar strains at Design Region A-A.
From page 214...
... 214 MASH Railing Load Requirements for Bridge Deck Overhang Discussion of Calibrated LS-DYNA Model As the curb-mounted post-test models exhibited acceptably accurate predictions of the overall force-deflection response of the specimens, the post-test damage profiles, and strain gage measurements, the models were deemed adequately calibrated. As such, the models were able to be used as a baseline for other investigative models, such as static loading and design variation models.
From page 215...
... Overhangs Supporting Curb-Mounted Steel Posts 215   Basic Load Distribution in Calibrated Model To characterize the effective moment and tension demands in the slab at the peak load reached by the post, the calibrated impact model was converted to a quasi-static pushover model. Moment and tension demands calculated in the slab at the peak lateral load reached by the post, which was 34.2 kips, are shown in Figure 321.
From page 216...
... 216 MASH Railing Load Requirements for Bridge Deck Overhang (neglecting self-weight)
From page 217...
... Overhangs Supporting Curb-Mounted Steel Posts 217   this project (discussed in a later section, in the proposed Section 13 modifications, and in the accompanying design example) predicted curb yielding at 20 kips.
From page 218...
... 218 MASH Railing Load Requirements for Bridge Deck Overhang linearly with increasing curb height. The proposed methodology also predicted a linear relationship between curb height and post capacity, although predicted capacity increases were conservatively less sensitive to curb height.
From page 219...
... Overhangs Supporting Curb-Mounted Steel Posts 219   therefore, additional increases to slab capacity may have occurred beyond this limit but were not demonstrated in the model. Deck damage was significantly reduced with increased edge distance, as models with low edge distance were susceptible to field-edge spalling (Figure 327)
From page 220...
... 220 MASH Railing Load Requirements for Bridge Deck Overhang Effect of Transverse Slab Steel To investigate the effect of transverse slab steel on system performance, models were created in which transverse steel areas were decreased and increased relative to the baseline system. The baseline system, which had an ultimate capacity of 50 kips, used #5 bars spaced at 4 in.
From page 221...
... Overhangs Supporting Curb-Mounted Steel Posts 221   ultimate capacity is more sensitive to transverse than longitudinal deck reinforcing. Although the proposed methodology does not capture the analytical insensitivity to longitudinal reinforcing, this observation should be taken in context with the condition that the curb reinforcing was significantly greater than typical construction with the intent to examine deck behavior sensitivities.
From page 222...
... 222 MASH Railing Load Requirements for Bridge Deck Overhang rectangular chutes visible at bolt locations represent the transition between concrete material models necessary for fine, distorted meshes at bolt holes. Bolts were discretely modeled as circular solid shafts that pass through the rectangular blocks visible in the figure.
From page 223...
... Overhangs Supporting Curb-Mounted Steel Posts 223   Unlike the baseline deck-mounted W6×25 model in which the deck failed in punching shear, the W6×25 was able to fully develop its plastic moment capacity when mounted to a curb. Moments calculated at Regions A-A and B-B at post plastification are shown in Figure 333.
From page 224...
... 224 MASH Railing Load Requirements for Bridge Deck Overhang Prior to post plastification, the point of first yield occurred in the longitudinal curb bar behind the anchor bolts. The bar stress state at this point is shown in Figure 335.
From page 225...
... Overhangs Supporting Curb-Mounted Steel Posts 225   Attachment Details and Deck Damage As for deck-mounted posts, the attachment mechanism of the post to the deck also affected curb-mounted post behavior. As shown in Figure 338, when the washer plate was moved from the bottom face of the deck to inside the deck just below the upper transverse steel, damage mechanisms in the curb and deck were changed substantially.
From page 226...
... 226 MASH Railing Load Requirements for Bridge Deck Overhang Effect of Span Length on Load Distribution In the barrier analytical program, it was found that demands in the deck overhang are significantly affected by the continuous span length. For barrier systems, as the span length was decreased, demands in the deck increased, and at a certain breakpoint span, the behavior of the system changed dramatically, developing peak Region B-B moments comparable to or potentially exceeding Region A-A moments.
From page 227...
... Overhangs Supporting Curb-Mounted Steel Posts 227   Moment demands calculated at Regions A-A and B-B for the 50-ft-, 30-ft-, and 15-ft-span curb models are shown in Figures 340 and 341. No differences were observed between the 50-ft- and 30-ft-span models.
From page 228...
... 228 MASH Railing Load Requirements for Bridge Deck Overhang Figure 341. Comparison of Region B-B moment demands with varying span lengths and an 8-in.
From page 229...
... Overhangs Supporting Curb-Mounted Steel Posts 229   (a)
From page 230...
... 230 MASH Railing Load Requirements for Bridge Deck Overhang affecting punching shear strengths and load distributions throughout the overhang. While curb and deck damage at post plastification were slightly reduced by increasing the longitudinal curb bar size from #4 to #6, moment demands acting at Regions A-A and B-B were virtually unchanged, as shown in Figures 347 and 348.
From page 231...
... Overhangs Supporting Curb-Mounted Steel Posts 231   • Span length. As the deck-mounted steel-post model did not have a stiffening element on its field edge, it was inferred that the deck-mounted steel-post behavior would not be affected by span length for typical bridge spans of at least 20 ft.
From page 232...
... 232 MASH Railing Load Requirements for Bridge Deck Overhang Effect of a Curb on Overhang Capacity Results of the physical testing and analytical programs for overhangs supporting curbmounted steel posts indicated that the addition of a curb resulted in a significantly increased overhang capacity and more extensive longitudinal distribution of post demands. Curbs increase the capacity of the overhang in punching shear due to the additional planes of resistance and increased size of the load application patch at the top surface of the slab.
From page 233...
... Overhangs Supporting Curb-Mounted Steel Posts 233   Nomenclature Variables used in the design methodology for overhangs supporting curb-mounted steel posts are summarized in Table 19. Interior Posts Step 1.
From page 234...
... 234 MASH Railing Load Requirements for Bridge Deck Overhang Step 2. Establish Ultimate Capacity of the Post and Associated Overhang Demands To establish the design demands acting on the overhang, the ultimate capacity of the post is first calculated.
From page 235...
... Overhangs Supporting Curb-Mounted Steel Posts 235   post in flexure. It should be noted that the transverse bending capacity of the curb is also the maximum moment that can be transferred to the deck slab.
From page 236...
... 236 MASH Railing Load Requirements for Bridge Deck Overhang bending strength, Mstr. Tension should be considered by following the provisions of Section 5 of the AASHTO LRFD BDS, assuming that both top and bottom transverse mats participate for decks with two layers of reinforcing if the evaluation performed in Table 20 succeeds or that only the top mat participates if the evaluation performed in Table 20 fails.
From page 237...
... Overhangs Supporting Curb-Mounted Steel Posts 237   The maximum post moment able to be supported by the slab in the yield-line mechanism is M C Y M Y h X e e X M X 12L M X 12 L L M L L 8 M post,eff p post curb A b c A str,A A curb st,A A cs curb sl cs curb post# = - + + r r J L K KK J L KK a ` N P OO N P O OO k j (110) If straight transverse bars are used, Mst,A should be calculated using the average bar embedment depth over the diagonal yield-lines.
From page 238...
... 238 MASH Railing Load Requirements for Bridge Deck Overhang Therefore, the design moment at Design Region B-B associated with the plastic moment capacity of the post is •M M M 12L P Y 0.5t M1B post post,eff 1B post s sw,B= + + ra k (112) The effective distribution length at Design Region B-B for Design Case 2 moment is L 12 2h 2 X e b 2B b curb B c curb = + + - -W ` j (113)
From page 239...
... Overhangs Supporting Curb-Mounted Steel Posts 239   M C Y M Y h X e e X M X h M X 12L h L 12 h M M post,eff p post curb A b c A str,A A b e curb st,A A cs b e curb cs b e curb sl post# = - + + + - + + + + + W W W W W W r r J L KK J L K K KK a ` ` ` N P OO N P O O OO k j j j (118) It should be noted that for certain combinations of post position and transverse slab steel configurations, the end-region yield-line equation may produce a greater capacity than the interior mechanism.
From page 240...
... 240 MASH Railing Load Requirements for Bridge Deck Overhang 2h t e e a+ + + + +W+W=bos b e curb s b c p (120) For nonzero span-end offsets, We, the end-region critical perimeter equation may result in a greater punching shear strength than the interior equation.

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