Column effective lengths in sway-permitted modular steel-frame buildings

Modular steel-frame buildings (MSFBs) are a fast-evolving alternative to traditional on-site construction, providing factory-level quality and significant time savings. The main difference between sway-permitted MSFBs and traditional sway-permitted steel frames, which always have continuous columns in adjacent storeys, is that in the former the connections between the designed column and adjacent columns are semi-rigid. This paper reports on how governing equations on the effective length factor (K) of columns for sway-permitted MSFBs can be derived using a three-column sub-assemblage model. A simplified method is then proposed to determine K-factors. Through comparing governing equations and finite-element simulations of a six-storey building, the accuracy and effectiveness of the method were verified. The accuracy of the alignment chart method and the ‘French rule’ method were also studied and assessed. The results show that these traditional approaches are not applicable and may provide unconservative values. The influencing factors on the K-factors for sway-permitted MSFBs were studied. It was found that the boundary restraint parameters and their relative values will significantly affect the K-factor. The semi-rigid connections in sway-permitted MSFBs should be double-checked when the relative beam-to-column stiffness is either small or large.