Diaphragms


Diaphragm Loads

Lateral forces are typically applied to structures either as:

  • loads distributed over the outer surfaces of the structure (e.g.- wind loads distributed over wall surfaces and inclined roof surfaces), or

  • inertial forces that resist dynamic motion in proportion to the distributed mass of the structure (e.g.- inertial seismic forces applied at floor and ceiling/roof levels that resist seismic ground motion at the base of a structure)


In both cases, the lateral loads acting normal to the face of a an exterior wall can be idealized in plan view as uniformly distributed loads applied to one edge of a horizontal diaphragm. Thus, in plan view a horizontal diaphragm is frequently modeled as a simply-supported “deep beam” carrying a uniformly distributed load along its span (diaphragm length perpendicular to the applied load is analogous to “beam span”). The “pinned supports” for a horizontal diaphragm(“deep beam”) are typically vertical shear walls aligned parallel to the direction of the applied lateral load. Since horizontal diaphragms are typically supported along their entire boundary, the magnitude of the reaction force at each support is divided by the length of the diaphragm parallel to the applied load (“beam depth”)to determine the “unit shear” load on the diaphragm (typically in units of lb/ft). Thus, the magnitude of unit shear load is typically calculated as:

v = wL/2b

where w = magnitude of uniformly distributed lateral load applied to edge of diaphragm
        L = diaphragm length perpendicular to applied lateral load (“beam span”)
        b = diaphragm length parallel to applied lateral load (“beam depth”)



Both the horizontal diaphragm and the supporting shear walls must be designed to resist the “unit shear load” associated with applied lateral loads.




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