April 6, 2005

TO:                  Bridge & Structures Office Design Staff

FROM:            J. Kapur - 47340

PHONE:          705-7209

SUBJECT:       Steel Reinforced Elastomeric Bearing Design

Many designers have reported difficulty in satisfying the rotational design requirements specified by AASHTO for designing steel reinforced bearings. This has been particularly true when applying the 0.005 radian tolerance for uncertainties to larger width bearings used for prestressed concrete girder sections having new wider bottom flanges. Accommodating additional rotation generally requires more elastomeric layers, resulting in a taller bearing. The resulting taller bearings often do not then meet stability design criteria. This issue is not unique to our office, having been identified by other transportation agencies as well.

The current AASHTO empirical provisions for rotational design capacity are believed to be particularly conservative, having been developed without the benefit of any substantial experimental research. Recognizing this deficiency, NRHRP Research Project 12-68 Improved Rotational Limits of Elastomeric Bearings is currently underway to develop more realistic criteria for both rotational capacity and rotational design of these bearings. As guidelines, even of a preliminary nature, resulting from this research are more than a year away, some guidance for designers is needed in the interim.

With respect to the 0.005 radian tolerance, the commentary in the third edition of the AASHTO LRFD code states "An owner may reduce the fabrication and setting tolerance allowances if justified by a suitable quality control plan; therefore, these tolerance limits are stated as recommendations rather than absolute limit." Rotational uncertainties arise from construction tolerances, girder camber deviations, and live loading. The latter two affect qx, not qy. Elastomeric bearing failure modes associated with fatigue are directly related to qx only. Recognizing these facts, it is acceptable to use 0.002 radians for qy. The uncertainty for qx should be maintained as 0.005 radians. Consult with the Expansion Joint and Bearings Specialist in instances where these less stringent criteria do not result in a solution.

If you have any questions regarding this issue, please contact Ralph Dornsife at 705-7199.



cc:   J. Weigel, Bridge and Structures - 47340

       F. Higgins, Bridge and Structures 47340