Design & Tools

Diaphragms

Diaphragms are a roof, floor or other system transferring lateral forces applied to a building to the vertical elements, such as the shear walls. For wood structures, diaphragms are commonly constructed of wood structural panel (WSP) sheathing or decking applied to the upper face of regularly spaced floor or roof wood framing members such as joists, trusses, purlins or sub-purlins. The sheathing transfers both vertical gravity load to the regularly spaced framing members and shear loads horizontally in diaphragm action. The construction requirements and load capacities of WSP sheathed diaphragms are found in the American Wood Council’s Special Design Provisions for Wind and Seismic (SDPWS).

Two idealized diaphragm behaviors are commonly used during design:  flexible diaphragm modeling and rigid diaphragm modeling. For seismic design, ASCE 7-10 section 12.3.1 gives conditions under which the flexible diaphragm condition can be automatically used as well as additional requirements to justify the use of flexible diaphragms by calculation. A similar, but often overlooked justification of the use of rigid diaphragms by calculation is given in the Chapter 2 Definitions of the IBC.

WSP sheathed diaphragms can be constructed as either blocked or unblocked. In blocked diaphragms, 2x framing blocks or similar are installed between the common framing members to support all the edges of the sheathing panels. Unblocked diaphragms do not use these extra blocks and some edges of the sheathing are not supported by framing underneath in the direction perpendicular to the framing. Blocked diaphragms require more material and connections, but are stronger and stiffer than unblocked diaphragms.