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The ratios of base shear at different column base nodes for the non-uniform and uniform motions are
shown in Fig. 5. In case 3 only the effect of site characteristics was included. Cases 4 and 5 included
the wave passage effect in addition to the local site effect, one for waves traveling from east to west
and the other traveling in the opposite direction. In case 6 the combination of local site effect and
geometric mcoherency was included. Finally cases 7 and 8 accounted for all three parameters, for
waves traveling from east to west and west to east, respectively. A ratio exceeding one on the vertical
axis indicates an increase in force due to incoherent ground motion. It can be seen in Fig. 5 that cases
3,4 and 5 were not generally critical. However, for cases 6 to 8 the base shear ratios exceeded 1 in
man}* piers. The maximum ratio occurred in Pier 7 with ratios being 1.59, 1.72, and 1.84 for case 6, 7,
and 8, respectively. It was concluded that the inclusion of all three mcoherency effects can be the
most critical combination and even eliminating the wave passage effect (case 6) led to force ratios that
exceeded one in one-half of the piers.
STUDY OF SHAPE MEMORY ALLOYS
Superelastic shape memory alloys (SMAs) have found their applications in medical equipment,
aerospace, and dentistry. Their ability to deform well into the yielded state and recover their entire
deformation upon unloading makes them potentially attractive for earthquake resistant design. The
application of SMA rods for bridge seismic restrainers has been explored by DesRoches (2001). As
part of the study of the retrofit alternative for the Las Vegas Downtown Viaducts, the performance of
SMA rods made from Nickel and Titanium (NiTi) used in combination with concrete is being
investigated. Basic characteristics of SMAs in providing concrete confinement and in acting as
longitudinal reinforcement are determined in concrete cubes and beams subjected to cyclic loads.
Figure 6 shows a flag-shape stress strain relationship that is characteristic response of superelastic
SMA materials. A series of 150x150x150 mm concrete cubes have been cast with central pipes in two
directions for the passage of SMA rods. Figure 7 shows the compression load setup for one of the
cubes. The SMA rods are anchored by nuts and stiff steel bearing plates to confine the concrete. The
rods in the setup shown are of 13 mm diameter with a nominal upper yield plateau of 420 MPa and a
lower yield plateau of 140 MPa.
Only a limited number of cubes have been tested thus far. Figure 8 shows a sample result for cyclic
loading of a cube confined with two 13-mm SMA rods. The unconfmed cube strength was 35.9 MPa.
It can be seen that confinement increased the compressive strength to 52.2 MPa, an increase of 45
percent. Using Mander's confined concrete model and the measured confinement stress, the estimated
compresshe strength would be 55.6 MPa. The ultimate strain capacity of the confined concrete was
near 0.029, which is approximately 10 times the nominal failure strain of unconfmed concrete. More
tests are being conducted to compare the permanent damage experienced by SMA-rod confined cubes
and cubes confined by steel rods and to develop confinement models as necessary.
CONCLUSIONS
Several aspects of the behavior and retrofit of a major viaduct are being studied. The results of the
study of the incoherent ground motion effects suggest that internal forces can be significantly higher