Behaviour and design of double-coped steel beams under combined bending, shear and axial force

Abstract

“Double-coping” is a common practice in steel construction for joining primary and secondary beams of similar section depths. Due to the partial removal of both top and bottom flanges, a double-coped secondary beam is susceptible to instability failures when subjected to vertical (gravity) loads. In certain circumstances, the existence of axial compressive forces in double-coped beams could further amplify the instability failure risk, and hence compromise the vertical load capacity. In order to gain a deeper insight into this effect, a thorough experimental and numerical study was carried out to investigate the behaviour and design of double-coped steel beams under combined bending, shear and axial force. Six double-coped beam tests were performed with zero, compressive or tensile axial forces, which were supplemented by a comprehensive numerical study based on validated finite element models. It was found that depending on different cope dimensions, lateral bracing conditions and axial force levels, a double-coped beam could fail in either local or sway buckling mode. With the increase of axial compressive force, the vertical load capacity was significantly reduced. On the other hand, the presence of an axial tensile force had a beneficial and relatively insignificant effect on the vertical load capacity. Based on comparisons with available test and numerical data, it was found that the existing design methods for double coped beams generally make conservative predictions for the cases with vertical load only. However, the existing methods cannot consider the effect of additional axial force. In this light, a new interactive design method was proposed to account for the negative effect of axial compressive force on the vertical load capacity of double-coped beams.