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Bridge Deck Overhangs with MASH-Compliant Railings

Description:

Agencies across the country are upgrading standards, policies, and processes to satisfy the 2016 AASHTO/FHWA Joint Implementation Agreement for Manual for Assessing Safety Hardware (MASH). Many existing bridge deck overhangs were designed to accommodate a barrier with a 10-kip collision design load while the current TL-3 and TL-4 test railing call for a 54k-load. When a barrier upgrade is required the bridge deck overhang must often be assumed to have adequate over-strength, or retrofitted in order to accommodate the new requirements. The latter is costly and the traveling public is inconvenienced by the construction. In many cases, limitation of work space and strength of existing reinforced concrete make retrofit of overhang impractical to satisfy the new higher collision load requirements.

FHWA mandated use of the AASHTO LRFD Bridge Design Specifications (LRFD) in 2007. These Specifications include higher design loads than the previous AASHTO Standard Specifications for different test levels (TL) of bridge railings. The deck overhang design is specified in LRFD Section A13; three design load cases are given including the over-turning moment caused by the collision load and portion of the vehicle on the overhang. California, New Jersey, and other states have gone even further in requiring 20% additional strength in the overhang.

In 2009, AASHTO Manual for Assessing Safety Hardware (MASH), 1st Edition, was published followed by the 2nd Edition in 2016. Crash-testing of various safety shapes for the new requirements has taken place, but the test surface to which the shape is anchored is often over-designed in order to verify performance the barrier. Anecdotal review of crash data supports the premise that bridge deck overhangs perform well during collision events.

Limited recent research has shown that the force effect in the overhang/barrier connection due to a lateral static load may be less than the resistance required by the current AASHTO LRFD Bridge Specifications (LRFD), Section A13. In a study by Purdue University the collision load distribution length for two solid parapet-type barriers was shown to be much greater than that specified in LRFD. Furthermore, for various amounts of bridge deck overhang reinforcement the failure mode in the overhang was shown to be shear under the barrier.

New technologies exist to analyze and enhance the strength of bridge deck overhangs. Refined analysis may be able to quantify energy dissipation via crushing of the concrete and elongation of the bar reinforcement in the barrier, deformation of the barrier or post connection, and potentially the overhang itself. Alternatively, some bridge owners have used composite strips and other new technologies to enhance the strength of bridge deck overhangs. The latter can have less impact to the traveling public—and be cost competitive.

Objective:

The primary objective of this research is to recommend changes to current LRFD bridge deck overhang design specifications for both solid and “see-through” post-beam barriers subjected to TL4 and TL5 collision loads. The changes may be based on refined or nonlinear analysis methods but lead to a practical and quickly performed flexural design. Current methods may be acceptable with development and justification of an increased the load distribution, alternative concrete strengths for existing bridges, or revised barrier-deck connections. Potential shear failure and detailing recommendations for the bridge deck overhang must also be provided. The deck overhang must always be able to carry the vertical load of the vehicle causing the damage.

The research is to survey past as well as present practice of existing deck overhang designs and details, and barrier connection details. Recommendations from this research must be applicable to the broadest cross-section of practice possible for both new and existing structures in terms of configuration, strength, and barrier anchorage. Experimental testing of prototype overhang-barrier systems may be determined based on literature survey and analytical study.

The final objective is to develop design examples and guidance on when to retrofit an overhang, methods for doing so, and design examples. The goal is to keep both bridge and vehicle safe in a cost-effective way.

Tasks:

The following tasks are envisioned:

  1. Survey of accident records to discover circumstances of any deck overhang damage and failures due to a collision load.

  2. Survey of literature as well as owners to learn what applicable crash-testing has been done or is planned.

  3. Survey of existing bridge deck overhangs throughout the US to learn of design criteria and common details used in the last five decades.

  4. Development of a research plan including but not limited to: (a) Breadth of barrier types; deck overhang lengths, thickness at face of exterior girder, face of barrier, amount of transverse reinforcement; connection details (b) Studies to be done; method(s) of analysis and experimental study (if necessary) (c) If needed, supplementing deck overhang strength with composite strips (d) Number and description of design examples to be developed (e) Approval of the plan by the research oversight panel is required before proceeding.

  5. Carry out the studies, modelling, and experimental studies (if necessary).

  6. Recommend specification changes.

  7. Draft guidance for implementation of the recommended changes to the specifications; recommend detailing for new overhangs and retrofit of existing overhangs; demonstrate all recommendations through design examples

  8. Write and submit a report

Relevance:

The research is of urgent need to ensure public safety and satisfy the AASHTO MASH and LRFD requirements in cost-effective way. State DOTs across the country are striving to upgrade standards, policies, and processes to meet the 2016 AASHTO/FHWA Joint Implementation Agreement for MASH (MASH), where it is stated that for contracts on National Highway System with a letting date after December 31. 2019, only safety hardware evaluated using the 2016 edition of MASH criteria will be allowed for the new permanent installations and full replacements.

During the implementation of the AASHTO/FHWA agreement, MASH compliant bridge rails and concrete deck overhang should be considered as a system for design to keep both motorists safety and sufficient bridge capacity. Reliable determination of existing bridge overhang/barrier connection performance and strength under vehicle impact loads will ensure sufficient safety and avoid overhang replacements i.e. save time, labor cost, and work zone safety for construction.

The successful research will meet AASHTO Strategic Plan for Bridges and Structures in following aspects: - Maintain and Enhance AASHTO Specifications for Improved Structural Performance; - Enhance and Optimize Structural System; - Enhance National Policy;

The research outcomes will potentially be valuable not only for MASH barrier upgrades on existing bridges, but also for new bridge design and construction. The new policy and process based on successful research will be aligned with AASHTO and FHWA requirements for roadway safety and bridge structural safety. The implementation may start with a pilot study or studies, identification of a “champion” affiliated with the AASHTO Committee on Bridges and Structures, and dissemination of the research recommendations at conferences attended by practicing bridge design engineers.

Sponsoring Committee:AKB10, Innovative Highway Structures and Appurtenances
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Richard Dunne, PE
Date Posted:04/13/2018
Date Modified:04/20/2018
Index Terms:Bridge decks, Overhangs, Bridge design, Strength of materials,
Cosponsoring Committees: 
Subjects    
Design
Bridges and other structures

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