Temporary bridges are often desired to maintain traffic during bridge reconstruction projects and are also utilized on an emergency basis to accommodate structures that are taken out of service by unforeseen circumstances such as nature or vehicular collisions. From a national perspective, the design and construction criteria for temporary bridges are an inconsistent practice among agencies. Some agencies have special provisions covering this work, while other agencies have no such provisions. Design of temporary bridges in accordance with the current AASHTO LRFD Bridge Design Specification or alternative specifications may result in potential over/under design for the following areas:
- Satisfying the Fatigue Limit State, and specifying details with infinite fatigue life, or a specified number of cycles;
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Satisfying the Extreme Event Limit State for seismic, wind, stream, and vessel impacts, where the bridge is not anticipated to be in service for a particular event recurrence interval;
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Satisfying exterior beam, deck overhang, and parapet loading criteria;
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Satisfying bridge width, striping, and end treatment criteria;
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Long term creep or prestressing losses in concrete elements;
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Design of foundations for contraction scour, local scour and long term scour, and required hydraulic opening for a given design flood event;
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Live Load requirements, including special and routinely permitted vehicle classes;
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Inspection frequency and coding requirements subject to NBI compliance;
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Load rating requirements; and
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Limited traffic for temporary bridges where certain vehicles are restricted from use.
It is also desirable to provide definitions and requirements to address the anticipated service durations for “temporary” and “interim” bridges - given that these durations may vary considerably, and may require the need for different structural and safety performance. The need to define requirements regarding single-use temporary bridges/components versus multiple deployment temporary bridges/components also exists.
Obtaining a national perspective of the current design and construction requirements used by agencies for the procurement of temporary bridges may lead to the development of a more consistent and unified set of performance based criteria. To achieve this goal, focused analytical studies of existing temporary bridge specifications along with comparisons to AASHTO LRFD Bridge Design Specifications, 17th Edition Standard Specifications, and the AASHTO Guide Design Specifications for Bridge Temporary Works will need to be performed.
The objective of this research was to develop proposed AASHTO guidelines for design and construction of temporary bridges. The proposed guidelines should provide more consistency in the reliability and performance of temporary bridges, and offer agencies with economically feasible options that will eliminate the concerns of under/over designed temporary structures.
PRODUCTS AVAILABILITY
The recommended AASHTO guidelines is under review by the AASHTO Committee on Bridges and Structures, Technical Committees: T-4 Construction. (NCHRP Staff: W. Dekelbab)
