Seismic retrofit screening of highway bridges in Nevada, USA has identified a major viaduct in downtown Las Vegas as a structure with highest priority for retrofit. The bridge has 24 spans and 110 columns of various shapes. The seismic performance, vulnerability, and retrofit details of the bridge and its adjacent ramps are being evaluated. Five major aspects of study are in progress. The focus of this article is on the effects of incoherent ground motion on the viaduct, detailed nonlinear dynamic analysis of the bridge, and shake table studies of single column piers. It is shown in the paper that combination of geometric incoherency, wave passage, and site characteristics effects can increase the base shears considerably. The columns in the main bridge are found particularly vulnerable to the longitudinal motion of the bridge. The shake table tests of the as-built octagonal columns have shown that the pedestals are unable to work with the columns and separate from the column under large drifts.
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