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Suggested Citation:"Summary." 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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Suggested Citation:"Summary." 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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1   MASH Railing Load Requirements for Bridge Deck Overhang Bridge rail and support guidance provided in the 9th edition of AASHTO LRFD Bridge Design Specifications (LRFD BDS) reflects the requirements of the superseded crash testing standard, NCHRP Report 350: Recommended Procedures for the Safety Performance Evalu- ation of Highway Features, rather than the current standard found in the 2nd edition of the Manual for Assessing Safety Hardware (MASH). A separate project, NCHRP Project 22-41, “Proposed Modification to AASHTO LRFD Bridge Design Specifications, Section 13— Railing,” is charged with revising Section 13 to modernize criteria and guidance to reflect the MASH. The scope of NCHRP Project 22-41 was limited to reviewing and synthesizing research available in the literature, but insufficient information was available to address deck overhang demands. The objective of NCHRP Project 12-119, “MASH Railing Load Requirements for Bridge Deck Overhang,” is to address knowledge gaps identified under NCHRP Project 22-41 and facilitate implementation by (1) characterizing the pattern by which impact loads distribute longitudinally with downward transmission through the railing and with inward transmis- sion through the overhang, (2) identifying slab failure mechanisms and refining capacity evaluation methods for overhangs supporting posts, (3) developing design methodologies for overhangs supporting various railing types, and (4) developing examples demonstrating the application of the proposed modifications. The scope of the research effort encompassed cast-in-place concrete overhangs support- ing concrete barriers as well as open concrete top-mounted steel and curb-mounted steel railings. Emphasis was placed on overhangs supporting concrete barriers, as these represent the majority of inventory in the United States. Only steel reinforcing was considered for both concrete railings and decks. Glass-fiber-reinforced polymer (GFRP) reinforced barriers and decks as well as timber railings and decks were excluded from the scope of work. Skewed and curved bridges and side-mounted steel-post systems were not considered. These limi- tations in scope may be considered in other projects in future research. The research activities included a survey of state agencies, analytical modeling performed using LS-DYNA, and execution of six component tests on instrumented test articles. Test data were obtained from (1) accelerometers mounted to surrogate vehicles to determine impact forces, (2) video analysis and cable extension transducers to determine displace- ments, and (3) strain gages attached to reinforcing cast into concrete. Two critical regions are proposed for deck overhang designs to resist vehicle collision loads transferred through railings: Design Region A-A and Design Region B-B. Design Region A-A is under or near the railing. Design Region B-B is at the exterior element supporting the deck. For decks supporting concrete barriers, moment demands at Design Regions A-A and B-B can be conservatively calculated by dividing the total applied moment, FtHe, over effective S U M M A R Y

2 MASH Railing Load Requirements for Bridge Deck Overhang distribution lengths, which are calculated using effective distribution angles through the barrier and overhang. Through the barrier, flexural demands can be assumed to distribute to Design Region A-A at 45 degrees measured from the extents of the yield-line mechanism, Lc. Flexural demands can be assumed to further distribute from Region A-A to Region B-B at 60 degrees through the overhang. The research also found that local damage due to vertical shear or compression strut bursting under the barrier may influence the ability of the deck to transfer moment between the barrier and deck. For decks supporting open concrete, steel post-on-deck, and steel post-on-curb railings, results of the analytical and testing programs indicated that the overhang ultimate failure mechanisms at Region A-A are typically due to yield-line flexure. Adopting a yield-line approach more accurately captures influences of parameters such as edge distance and longitudinal reinforcing that are either underestimated or neglected in the existing pro- visions. Recommended methodologies for these systems also include evaluations of deck vertical punching shear and potential consideration of compression strut bursting in the deck directly under posts. Furthermore, failure of these concentrated vertical force limits is expected to result in cracking through the overhang, which propagates along the bottom mat of reinforcing for doubly reinforced decks, reducing the effective depth available for transverse deck flexure and also the effectiveness of longitudinal flexural components in the deck yield-line at Region A-A. The presence of a curb was demonstrated to significantly improve deck performance by extending the lengths of both Design Regions A-A and B-B as well as localizing damage to the curb and therefore avoiding damage in the deck. Recommended Research Research needs that were identified while executing NCHRP Project 12-119 include the following: • Characterization of inertial resistance as a component of barrier redirective capacity. • Further investigation and clarification of barrier mechanical behavior and capacity. • Behavior and capacity of open concrete railings. • Applicability and criteria for alternative strength limit state traffic loading on deck overhangs. • Damage mechanisms and capacities for barrier-to-deck joints. • Reinforcing development lengths and efficacy of hooks around perpendicular reinforcing to provide anchorage sufficient for full development. • Modeling methods for partial reinforcing development. • Effects of slab continuity and barrier shear transfer elements at barrier expansion joints or segment ends. • Effects of drainage slots or drop-chutes in decks. NCHRP Project 12-119 was limited in scope to conventional concrete decks reinforced with steel, without significant skew, and supporting top-mounted barriers. Additional research is underway in other in-progress or prospective studies to address alternative materials such as ultrahigh performance concrete (UHPC) and GFRP reinforcing. Further research should investigate other alternatives beyond the scope of NCHRP Project 12-119, such as decks with significant skew, side-mounted steel post-and-beam railings, and timber decks and railings.

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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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