When calculating the maximum diaphragm deflection, should engineers assume that the diaphragm is rigid or flexible?
In the January 2015 SE University session, Load Transfer Through Diaphragms, Terry Malone, PE, SE, from WoodWorks, was asked if engineers are to assume rigid or flexible diaphragms when calculating the diaphragm deflection. Terry indicated that if a diaphragm cannot be idealized as flexible or rigid, then the flexibility can be determined by calculation.
This requires determining the maximum deflection of the diaphragm and comparing that to the average deflection of the supporting walls or frames. Diaphragms with horizontal offsets will have varying moments of inertia and varying uniform lateral loading. It becomes necessary to modify the diaphragm deflection Equation 23-1 of the 2012 IBC to match these conditions.
In answering the question, Terry said that the first step is to determine the diaphragm stiffness based on the varying moments of inertia. The maximum diaphragm deflection is then calculated using a modified version of IBC 2012, Equation 23-1. If the maximum diaphragm deflection is greater than two times the average story drift of vertical elements, then the diaphragm can be considered flexible.
In this video clip from the January 2015 SE University webinar, Terry steps through this process of determining if a diaphragm is flexible based on the diaphragm deflection calculation.