Caltrans Projects Home

Interlocking Spiral Columns

Research Personnel



Executive Summary

Photos and Video

Publications & Presentations

Research Notes



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