Strain Limits for Concrete Filled Steel Tubes in AASHTO Seismic Provisions

AUTC project number: 510001


Mervyn Kowalsky (NCSU)


Alaska Department Of Transportation (ADOT)

  • Start Date: Jun 1, 2011
  • End Date: May 31, 2013

Project Summary

The use of concrete filled steel tubes in bridge construction is common in the state of Alaska, as well as in other states and regions of the country. One reason for their use is that construction is simplified because the steel tubes serve the dual function of the foundation, i.e., piles below the ground surface, and the above-ground columns for the cap beam. The majority of past research has dealt with concrete filled steel tubes at very small scale and without internal reinforcement. In Alaska, the preferred system utilizes longitudinal reinforcement in addition to the steel tube itself. While available research has shown that the performance of concrete filled steel tubes is satisfactory, a number of important questions remain unanswered for the particular application commonly used in Alaska. The problems that will be addressed in this research project include the impact of reinforcing steel on the behavior of the pile-column, the accuracy of analysis methods for prediction of force-deformation response of the pile-column system, the impact of the ratio of tube diameter to tube thickness (D/t ratio) on the performance of the pile-column at multiple limit states, and the plastic hinge length for the below-ground hinge developed in the pile-column. These problem areas will be examined through a series of 10 large-scale tests on concrete filled pile-columns, and from the development of a finite element model capable of capturing all of the anticipated modes of failure, including local buckling of the steel tube. Two of these tests will be conducted in the environmental chamber at NC State, in an attempt to capture the effects of low temperatures (-40°C) on structural behavior and performance. The research will result in design expressions relating D/t to strains at various limit states, design expressions for plastic hinge lengths for concrete filled pipes with internal reinforcing steel, and modifications to moment-curvature analysis tools, if required, to predict the force-deformation response to cyclic loading.  The research findings will be summarized in a concise design manual appropriate for AKDOT use.