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the structural response and retrofit methods are being studied. The article will present a summary of
the study of a selected number of the critical issues.
DESCRIPTION OF THE VIADUCT
The viaduct consists of two main 22-span continuous structures for the westbound and eastbound
traffic on Interstate 95 in North Las Vegas (Fig. 1). In addition a 7-span on ramp and two off ramp
badges, one with two spans and the other with three spans are connected to the viaduct. The
superstructure is cast-in-place muiticell box girder in all the structures and has seven in-span hinges in
the main structure. The main bridges are supported on multi-column piers with two to four diamond
shape columns and the ramp bridges are supported on irregular octagonal single-column piers some
with a pedestal. Figures 2 and 3 show the cross section of the columns. Single columns are detailed
vuth a one-way hinge at connection to the footing and columns of the multi-column bents are detailed
with two-way hinges at the base.
The viaduct was constructed in 1969 and was widened in 1984 by adding one row of columns and a
new box cell on the north side with little seismic load and detailing consideration. The columns are
supported on spread footings. Based on the soil blow counts at the site of the bridge, approximately
the west third of the bridge is on a relatively soft soil while the rest of the structure is on medium firm
soil.
Seismic codes place Las Vegas in areas of moderate seismicity category. To determine how critical a
bridge is with respect to the need to upgrade its seismic performance, however, the average daily
traffic (ADT) is also factored in. As a result the Nevada Department of Transportation has identified
the viaduct as the most critical bridge in Nevada with respect to priority for seismic retrofit because of
its high ADT in addition to having seismic deficiencies. Seismological studies of the Las Vegas area
have pointed out the possibility of strong ground motion due to soil amplification despite the fact that
the area is categorized as having only a moderate probability of experiencing high seismic forces (Su
etal. 1998).
CRITICAL ISSUES
The original structure and the subsequent expansion were not designed to resist significant seismic
forces. As a result there are many deficiencies in the bridge with respect to seismic detailing. These
include inadequate lateral steel in the columns (Fig. 2 and 3), short anchorage length for longitudinal
bars, insufficient shear steel in the beams, very low amount of bottom steel in the beams at
connections to columns, a lack of top mat of steel in footings, short seat widths at hinges, and a lack of
shear steel at beam column connections. Considering the length of the viaduct and variation in soil
properties, there is also uncertainty about the performance of the bridge under incoherent ground
motions at different supports.
The study to address these deficiencies include (1) a three dimensional detailed nonlinear modeling of
the viaduct subjected to earthquakes to determine the performance level under different loading
scenarios, (2) a reduced linear modeling of the bridge to study the effect of incoherent ground motions,
(3) shake table studies of the as-built and retrofitted models of single-column bents, (4) shake table