Numerical study on the effects of diaphragm stiffness and strength on the seismic response of multi-story modular buildings

Modular building construction relies on prefabricated modules which are assembled onsite to form complete buildings. The assembly requires modules to be connected at discrete locations and results in the formation of discontinuous diaphragms. Diaphragm discontinuities could potentially lead to structural instability or possible diaphragm failure if unaccounted for. Therefore, the primary objective of this study is to evaluate the influence of in-plane diaphragm stiffness and strength on the seismic performance of multi-story modular buildings. A simplified method is presented to establish diaphragm service stiffness considering shear deformation of individual module diaphragms as well as shear and axial deformation of diaphragm connections. This method is used to construct numerical models of a four-by-four bay four-story modular steel building. Three diaphragm stiffness levels, namely rigid, stiff and flexible, are considered for these models and were each subjected to 44 horizontal ground motions relating to seismic events having a return period of 500 years. The results show that increased diaphragm flexibility leads to inter-story drifts that are dramatically large and inertial forces that are considerably different from calculated values using the equivalent lateral force procedure described in current seismic codes. This study is extended further to evaluate performance targets for both elastic and inelastic diaphragm response under a seismic event having a return period of 2500 years. The results are used to propose new seismic design factors, which include force and ductility amplification, and could be implemented for the design of diaphragm connections in multi-story modular buildings.