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Interlocking Spiral Columns

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The primary objective of this research was to study the seismic performance of
bridge columns with interlocking spirals subjected to earthquake excitation on a shake
table and to assess the most critical design parameters that were of interest to Caltrans
designers. The level of average shear stress, the horizontal distance between the centers
of the spirals, di as a function of the radius of the spirals, R, and supplementary horizontal
cross ties were the design parameters included in this study.

Six large-scale columns reinforced with interlocking spirals were built based on
the current Caltrans design provisions. Two 1/4-scale specimens with di of 1.0R and
1.5R subjected to low level of average shear stress (shear index of 3) and two 1/5-scale
specimens with di of 1.0R and 1.5R subjected to high level of average shear stress (shear
index of 7) were tested in order to study the effect of the first two design parameters
mentioned above. Two additional variables, one an intermediate level of di and the other
with supplementary cross ties and di of 1.5R were studied after observed vertical cracks
in one of the high shear columns tested with the maximum horizontal spacing between
center to center of the spirals (di of 1.5R). All the specimens were designed to fail in a
ductile mode and they were subjected to increasing amplitude of the Sylmar record from
the 1994 Northridge earthquake. The specimens were tested at James E. Rogers and
Louis Wiener Large-Scale Structures Laboratory at the University of Nevada, Reno.

Only in-plane response of the columns was studied with axial load index of 10%.
Based on the data and analyses in this and other studies, a new model to estimate
the post yield shear stiffness was developed and recommendations were made for
possible adoption by Caltrans.