# Lu and Kennedy 1994

Pure bending tests were performed on rectangular CFTs to determine the effect of different width to depth ratios and different shear span to depth ratios. The authors were interested in the effect of different ratios of concrete to steel in the compression zones, and what affect (if any) this would have on the stiffness and ultimate strength of the members. Through investigation of results, it was found that the concrete in the compression zone was able to reach its full cylinder strength rather than 85% of f'_{c} (the common assumption made in ultimate strength design of RC members) The transfer of shearing forces from the steel to the concrete through bond was investigated by loading configurations which induced a variety of shear span to depth ratios. The beams were instrumented to measure bond slip between the two materials during loading at various points in the beam depth. It was found that bond degradation did not occur until the onset of concrete crushing in the compression zones, and did not effect the ultimate load carrying capacity of the CFT beams.

## Experimental Study, Discussion, and Results

*Column Tests.* Five HT stub columns were tested to determine the average steel σ- ε curve. The σ- ε curve for concrete was obtained from cylinder tests. Five CFT stub columns were tested and the σ- ε curve obtained was plotted against the linear superposition of the HT and concrete curves. The results varied, depending upon the D/t of the cross section. CFTs with low D/t ratios carried a higher load than the superposition of steel and concrete, indicating some increase in concrete strength due to confinement. CFTs with high D/t ratios actually carried less load than the superposition of steel and concrete, perhaps due to the biaxial state of stress in the steel coupled with local buckling effects.

*Beam Tests.* 5 HT tests and 12 CFT tests were performed under simple loading conditions. The D/t ratio, L/D ratio, and shear span-to-depth ratio (z) were the main parameters. Concrete strength ranged from 5.9 to 6.8 ksi. Results of the HT beam tests compared well with the predicted ultimate capacity of the sections. Results for the CFT sections ere presented as moment-curvature plots. These plots showed a region of linear behavior followed by gradual yielding which approached a limiting ultimate moment value. When compared to the HT tests, the CFT tests showed a moderate increase in strength, but a dramatic increase in ductility due the prevention of local buckling by the concrete core.

*Slip.* The slip recorded at the steel-concrete interface was observed to be very small up to the point of sudden concrete crushing which accompanied the local buckling of the steel compression flange. For small z values, the author suggests shear transfer through a tied-arch model, whereby a compression strut develops in the concrete to transfer the applied load to the support. For larger z values, shear transfer occurs through bond between the steel and concrete. In no case did the z value adversely affect the ultimate moment capacity of a CFT section.

## References

Lu, Y. and Kennedy, D. (1994). “Flexural Behavior of Concrete-Filled Hollow Structural Sections,” Canadian Journal of Civil Engineering, Vol. 21, No. 1, February, pp. 11-130.