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MASH Railing Load Requirements for Bridge Deck Overhang (2023)

Chapter: Chapter 8 - Summary and Conclusions

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Page 241
Suggested Citation:"Chapter 8 - Summary and Conclusions." 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.
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Page 242
Suggested Citation:"Chapter 8 - Summary and Conclusions." 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.
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Page 242
Page 243
Suggested Citation:"Chapter 8 - Summary and Conclusions." 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.
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Page 243

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241   Summary and Conclusions In this project, physical testing and analytical modeling were performed in order to develop modified design methodologies for concrete deck overhangs supporting concrete barriers as well as open concrete deck-mounted steel and curb-mounted steel railings. For each system type, physical testing was performed, LS-DYNA models were calibrated using physical test data, and numerous models were created to investigate the effects of design variations on load distri- butions and overhang capacity. Proposed Modifications to AASHTO LRFD BDS Based on the results of the physical testing and analytical modeling programs performed in NCHRP Project 12-119, proposed modifications to the deck overhang design methodology of AASHTO LRFD BDS, Section 13 were developed. Key updates to the deck overhang design methodology include: • Direct definition of overhang critical regions. • Inclusion of load distribution patterns and design moment and tension equations for overhangs supporting barriers. • Estimation of distributed demands at exterior girder for concrete and steel post-and-beam railings. • Use of a yield-line mechanism to determine the capacity of overhangs supporting posts, which better represents the effects of edge distance and longitudinal steel. • Definition of a new damage mechanism (diagonal tension failure) in the slab below the railing, the consideration of which may significantly reduce overhang damage and increase railing strength. • Quantification of overhang capacity increase granted by a curb. Detailed discussions of key findings for each particular system were previously presented in their respective individual chapters. Refer to the preceding chapters for additional information. Recommended Research Several significant knowledge gaps were identified regarding railing and overhang behavior. Research needs identified during the course of the work performed in NCHRP Project 12-119 are listed below. Inertial resistance. In this project, it was found that concrete barriers and open concrete railings have a significant inertial resistance associated with the force required to accelerate their significant mass. Concrete railings with static capacities less than their corresponding test C H A P T E R 8

242 MASH Railing Load Requirements for Bridge Deck Overhang level’s design lateral impact loads were shown to be able to resist dynamic loads associated with higher test levels due to their inertial resistance. To quantify this effect and eventually directly account for it in official design methodologies, research is recommended to subject concrete barrier and open concrete railings to static and dynamic load tests in which the load rate is the only variable. Quantifying the inertial resistance of concrete railings could result in a drastically more efficient design methodology for both the railings and the over- hangs to which they are attached. Barrier yield-line capacity. Physical testing and analytical modeling results obtained in this project indicated discrepancies when compared to the existing trapezoidal and w-shape yield-line mechanisms which are anticipated to be adopted under NCHRP Project 22-41 to estimate the ultimate strength of barriers. Critical lengths predicted in LS-DYNA were not consistently estimated by either mechanism, and longitudinal steel in the barrier rarely yielded in quasi-static pushover or bogie impact models. It is believed that the torsional capacity of the barrier may have a significant effect on its overall redirective capacity. Capacities calculated using the trapezoidal yield-line mechanism were often significantly lower than capacities determined from LS-DYNA, while those calculated using the w-shape mechanism were often significantly higher. At end regions, the trapezoidal mechanism recommended in NCHRP Project 22-41 overpredicted capacity relative to LS-DYNA predictions on rare occasions but only when both traffic-side and field-side vertical steel rebar were considered in the barrier cantilever moment strength. When only the traffic-side steel was considered, the NCHRP Project 22-41 end-region capacity was conservatively predicted in comparison to LS-DYNA predictions. Furthermore, barriers with high pro- portions of Mw-to-Mc exhibited less conservative strengths when compared to LS-DYNA, although redirective capacity was still conservatively predicted by neglecting the benefits of field-side vertical steel regardless of Mw-to-Mc ratio. Last, it should be noted that even barriers resulting in unconservative end-region capacities in comparison to LS-DYNA were evaluated on a quasi-static pushover basis. Inertia is likely to appreciably increase the redirective capacity of barriers, but research is needed to determine appropriate method- ologies to account for inertial effects, as noted previously. To better understand the behav- ior and more efficiently design this very common railing type, research is recommended to directly investigate the static capacity of barriers. Interior and end-region quasi-static pushover tests of barriers are rare in the literature, despite the widespread use of this system type. Testing of this nature, preferably with internal instrumentation, may result in a more optimized design procedure for barriers. Ultimate capacity of open concrete railings. Analytical modeling performed using models based on the calibrated concrete-post test indicated that the existing inelastic method used to determine the strength of open concrete railings is highly conservative. This high degree of conservatism has resulted in open concrete railings historically exhibiting their expected redirective capacity, despite posts not reaching their full strength due to diagonal tension failure of the slab. Thus, in-service railings have typically provided ample redirective capacity, but the overhang may sustain unexpected damage. If the inelastic method were updated to include the torsional resistance of the longitudinal concrete beam, the method would produce increased—and therefore more accurate—strength estimates. As a result, sufficient redirective capacities could be provided using lower post strengths and, consequently, less deck overhang steel. Quasi-static pushover testing of open concrete railings is recom- mended to directly quantify the ultimate strength of this common railing type and refine the inelastic method used to analyze them. Strength limit state for overhangs. AASHTO LRFD BDS allows for the replacement of the 16-kip design wheel load and 25-kip design tandem load with a 1 k/ft line load if the rail- ing is structurally continuous. In this project, it was found that the 1 k/ft load assumption was valid for overhangs supporting both concrete barriers and open concrete railings. It is

Summary and Conclusions 243   also believed that the edge-stiffening effect of curbs may reduce wheel load demands in the overhang. Research is recommended to refine the analysis methods for overhangs under the strength limit state, as overhangs that are not designed using the 1 k/ft load assumption often require large steel quantities. Refining the strength limit state analysis procedure, particularly relating design loads to the type of installed railing, could result in significantly decreased transverse overhang steel requirements. Further, refining this analysis procedure would allow for the use of significantly wider overhangs. Diagonal tension damage of overhang joint. For each system type, an unexpected diagonal tension damage mechanism was observed in the slab in both physical tests and analytical models. Component testing is recommended to further investigate this damage mechanism. In particular, it is recommended that the cause of the damage, whether compression strut bursting or punching shear, as well as the effects of transverse and longitudinal slab steel on capacity, are investigated. Development of bars hooked around longitudinal slab steel. In this project, it was found from the testing program that #4 and #5 bars were able to exceed their yield stress despite their embedment depth being significantly below the AASHTO LRFD BDS required devel- opment length when hooked around a longitudinal slab-bar. This finding was consistent with testing performed by Williams et al. (11) and Ansley (39). Due to the significant spatial constraints in deck overhangs, particularly near the field edge under the railing, the ability to expect full yielding of bars if hooked around longitudinal slab steel would result in signi ficantly more efficient railing and overhang designs. Further, as the bending strength of barriers and concrete posts constitutes an overhang demand rather than a capacity, assuming that vertical anchor bars cannot reach their yield stress is paradoxically uncon- servative. As it has been physically proven in multiple testing series, a research effort in which this behavior could be further investigated and considered in the specifications is recommended. Modeling rebar development. Currently, there exists no user-friendly method of modeling incomplete rebar development in LS-DYNA. In this project, two methods were used to roughly model this effect—tapering the bar area and tapering the bar yield stress between the free end of the bar and the end of the AASHTO LRFD BDS development length. Due to the lack of direct testing, the accuracy of these methods is largely unknown. Research involving component-level testing and analytical modeling is recommended to evaluate the accuracy of these simplified methods or to develop new, user-friendly methods of modeling bar development. Partial end regions and shear transfer joints in barriers. Barrier expansion joints are often located away from deck slab expansion joints. Cursory modeling performed in this project suggested that deck overhang demands, in this case, are less than at pure end regions, where both the barrier and slab are discontinuous at the same location but greater than in interior regions. Further, it is common for barrier expansion joints to have a shear transfer mecha- nism. Additional research is recommended to characterize slab demands at these locations to further optimize the methodology proposed in this project. Effects of drainage slots and drop-chutes. Overhangs with barriers commonly have horizontal drainage slots at the base of the barrier or vertical drop-chutes through the overhang to allow for drainage off the deck surface. This design type is either a special case of a barrier or a special case of an open concrete railing, depending on the size and spacing of drainage slots. If vertical slots extend through the deck overhang, stress concentrations near the slots are expected. Further research is recommended to quantify the effects of drainage slots and drop-chutes on overhang design demands.

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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.

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