Property modification factors have been introduced into the AASHTO Guide Specification for Seismic Isolation Design to account for likely variations in isolation bearing properties over the life of a seismically isolated bridge. The design of an isolated bridge in the United States must now account for variations in temperature, ageing, velocity (strain rate), travel, contamination, and scragging. In the absence of elastomer compound test data, the Guide Specification provides de facto values for likely variations in temperature, ageing, and scragging for use in design. Experimental data from tests of high-damping and lead-rubber bearings were analyzed to assess the de facto values listed in the Specification. The AASHTO values for the scragging factor should be increased from 1.2 to between 1.5 and 2.0 for HDR-A elastomeric bearings and left unchanged for lead-rubber bearings. Full recovery of virgin (unscragged) properties following scragging should be assumed. No change in the AASHTO ageing factors for high-damping and lead-rubber bearings are recommended at this time. The data support the listing of a velocity (strain rate) factor in the Guide Specification with a value equal to 1.2 for high-damping elastomer compounds.
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