Openings in wood diaphragms are unavoidable but can cause significant changes to the shear distribution in the diaphragms.  Openings complicate the design procedure, however, there is a simplified method available when certain conditions are met.

In the April 2025 SEU Session, Kimberly Kramer, PhD, PE, SE, from Kansas State University, presented Designing for Openings in Wood Diaphragms.  Kimberly walked through how to calculate wood-frame diaphragm deflections to classify diaphragms as flexible or rigid.   She identified the mechanisms of the concentration of shear around openings and reviewed designs for openings in flexible diaphragms using the Truss Frame Analogy.

Kimberly noted that a simplified analysis criteria was developed by the Canadian Wood Council after extensive research on shear forces in flexible wood diaphragms with openings.  This criteria allows for the diaphragm to be designed for a 10% increase in the maximum shear force if all four criteria are met.  The four criteria are included on the slide below:

The maximum aspect ratio requirement in the fourth criteria can be found in SDPWS Table 4.2.2.  If these four criteria are met, the maximum shear in the diaphragm must be calculated, as demonstrated below:

The shear is calculated along the edge of the diaphragm as well as each edge of the opening, and the largest shear is selected. Then, it is increased by 10% and then the entire diaphragm is designed for this increased shear force.  

This useful design procedure can save time when the diaphragm opening meets these basic four conditions.  This procedure eliminates the need to complete a detailed Truss Frame Analysis of the diaphragm and provides a simplified design for the entire diaphragm based on an increased shear force throughout.  Although this procedure does not currently appear in any US building codes, the research is useful to determine when a more detailed analysis should be performed. To learn more about this research performed by FP Innovations and the Canadian Wood Council, you can read the full report here.