Prediction of axial load capacity of stub circular concrete-filled steel tube using fuzzy logic

A circular concrete-filled steel tube (CFST) has several advantages compared with the conventional reinforced concrete member or hollow steel tube, since the tri-axial state of compression of the concrete infill increases its strength and strain capacity. Extensive studies have been conducted on the CFST and several researchers suggested empirical and theoretical formulas to predict the confinement effect and confined compressive strength of the concrete infill of the CFST. However, the previously proposed equations vary significantly because of the nature of complexity and uncertainty of the tri-axial stress state in the concrete infill. This study presented an alternative method to determine the confinement effect of the concrete infill and the axial load capacity of the stub CFST by using fuzzy logic. The focus was made on the accurate estimation of the confinement effect of the CFST by using a fuzzy-based model that makes it possible to evaluate the interaction between various parameters that affect the confinement effect. The proposed fuzzy-based model for the confinement effect and axial load capacity of the stub CFST was compared with current deign codes and the results of previous studies. It was found that the proposed fuzzy-based model provides a good prediction of the confinement effect and axial load capacity of the stub CFST. Finally, the design chart to estimate the confinement effect of the stub CFST was provided for practical design purpose.