NCHRP Report 596 provides elastomeric bearing design procedures suitable for adoption in the AASHTO LRFD Bridge Design Specifications. The report details the experimental and analytical program used to develop the design procedures. The material in this report will be of immediate interest to bridge designers.
Bridges experience translational movements and rotations caused by creep and shrinkage, thermal effects, traffic loading, initial construction tolerances, and other sources. Bridge bearings are designed and built to accommodate these movements and rotations while supporting required gravity loads and providing the necessary restraint to the structure. Elastomeric bearings accommodate movement and rotation by deformation of the elastomer. The bearing must be designed to control the strains and deformations in the elastomer to assure a long service life and good bearing performance.
AASHTO's current elastomeric bearing design limits were developed in NCHRP Project 10-20, but bearing rotation was not considered a high-priority issue for that project. As a result, virtually no experimental research on elastomeric bearing rotation was performed, and design provisions were developed based on conservative interpretation of past theoretical results. For example, the experiments used to validate the tension limits were done nearly 60 years ago and used very small laboratory samples, which are not at all similar to bridge bearings. The limits on lift off, uplift, and shear strain in the elastomer are known to be controlling criteria for elastomeric bearing rotation, but the equations used in the current AASHTO provisions have not been verified experimentally and are believed to be overly conservative.
The present conservatism in the rotational design limits results in more expensive bearings and may limit their use. The increased bearing cost is due in part to additional elastomeric material and quality-assurance testing. In some cases, expensive high-load multi-rotational bearings must be used to satisfy the design requirements. For these reasons, it was important to re-evaluate rotational design limits and quality-assurance requirements for elastomeric bearings.
The objective of NCHRP project 12-68 was to develop recommended provisions and commentary for rotational design capacity and quality assurance of elastomeric bearings suitable for inclusion in the AASHTO LRFD specifications. This objective has been accomplished. This research was performed by the University of Washington, Seattle, Washington. The report fully documents the experimental and analytical program used to develop the design procedures.
The contractor's uncorrected draft final report (with appendixes) is available as PDF files here: