# Bruneau and Marson 2004

The adequacy of the existing design provisions for concrete-filled steel pipes subjected to axial forces and flexure is reviewed by comparing the strengths predicted by the CAN/CSA-S16.1-M94, AISC LRFD 1994, and the Eurocode 4 1994 codes and standards against experimental data from a number of investigators. New proposed design equations are then developed in a format compatible with North American practice.

## Analytical Study

A summary of previous comparisons between existing design codes and experimental data demonstrates that current codes underestimate the strength of circular concrete filled tube columns.

A computer program was written to generate a force-deflection curve from the structural characteristics of a concrete-filled steel tube. The program calculates each layer’s individual area, center of gravity, stress, and force corresponding to a given curvature and neutral axis location. Forces from all layers are summed together and the neutral axis is iteratively moved until the sum becomes equal to the applied axial force. The corresponding moment at each curvature is then calculated. Finally, the force is taken as the moment divided by the height of the column and the deflection is calculated by integration of the curvature.

Specimens from another study were used to determine the material models that could best predict the experimentally observed behavior using a simple plasticity framework. The models assumed that the maximum moment occurred at the concrete foundation, and that the column moment linearly decreased from the top of the concrete foundation to the top steel plate; these assumptions were supported by data from the previous study. Steel was modeled by a bilinear stress-strain relationship. Seven concrete axial compression stress-strain models were considered. The best results came from an unconfined concrete model with high ductility.

## Design Formulation

New equations are proposed to calculate the strength of circular CFT columns. Free-body diagrams were used to develop axial and flexural strength equations of circular CFTs. The new proposed design axial-flexure interaction equation appears to predict reasonably well the behavior of concrete-filled steel pipes.

## References

- Bruneau, M. and Marson, J. (2004). ”Seismic Design of Concrete-Filled Circular Steel Bridge Piers.” Journal of Bridge Engineering, 9(1), 24–34. doi: 10.1061/(ASCE)1084-0702(2004)9:1(24)