Report No.: CCEER-04-6

Title: Seismic Performance of RC Bridge Columns Reinforced with Two Interlocking Spirals

Authors: Juan Correal, Saiid Saiidi and David Sanders

Date: August 2004

Sponsoring Agency: California Department of Transportation (CalTrans)

Performing Organization:
Department of Civil Engineering/258
University of Nevada, Reno
Reno, NV 89557


Interlocking spirals are used in bridge columns not only because they provide more effective confinement than rectangular hoops but also because interlocking spirals simplify the column fabrication. The behavior of columns with interlocking spirals has been studied only to a limited extent. A study was conducted at University of Nevada, Reno, on the seismic behavior of double interlocking spirals columns to determine the effect of some of the more critical parameters.

Experimental and analytical studies were conducted on six large scale concrete columns with double interlocking spirals. The primary test variables were the levels shear stress and the limits of the horizontal distance between the centers of the spirals. The specimens were tested under increasing amplitudes of the Sylmar record from the 1994 Northridge Earthquake, in the strong direction of the columns until failure. The tests revealed that the Caltrans upper spirals spacing limit of 1.5 times the radius is satisfactory even under high shear. However, supplementary cross ties are needed to prevent premature vertical shear cracking.

The analytical studies included push-over analysis, development of a plastic shear stiffness model, and development of design recommendation for cross ties. The proposed shear stiffness model improved the calculated shear deformation.