Franklin County Cooperative Inspection Program
Shear Wall Requirements
780 CMR, Section 5301 requires all buildings and structures to safely support all loads, including wind loads. For the average house, the common practice of sheathing all exterior walls fulfills the requirements of this section and, until now, much thought has not been required in design or plan review. When it comes to massive window walls and garage door openings, wind loads must be considered in both the design and the plan review of the building inspector. The following information should be used as a guide only. Consult 780 CMR 53 for appropriate regulations.
A few definitions:
A "shear wall" is an interior or exterior wall, engineered to resist lateral loads, such as winds.
A "braced wall panel" is the prescriptive equivalent to a shear wall. Section 5602.10.4 requires that each braced wall panel be at least 48" long.
A "braced wall line" is a series of braced wall panels in a single story designed in accordance with 780 cmr to resist racking from...wind forces.
"Aspect ratio" is the ratio of the height to width of a shear wall. A garage door opening that has an 8ft ceiling and a 2ft corner wall, would have an aspect ration of 4:1.
How many braced wall lines are required in a building?
Section 5602.10.1 prescribes that:
- They begin no more than 12.5 ft from each end
- They are spaced no further than 35ft o.c.
In a structure that is longer or wider than 35', besides the exterior walls, there must be interior braced wall lines.
What is the amount and location of braced wall panels in a braced wall line?
Table 5602.10.1 prescribes the amount and location of braced wall panels in a braced wall line, for different methods of wall bracing. Below are the requirements from that table as they pertain to Method 3(typical of this area, as mentioned above):
- One story or the top of a 2 or 3 story:
- Located at each end
- And at least every 25ft on center
- First story of a 2 story, or 2nd story of a 3 story:
- Located at each end
- And at least every 25ft on center
- But not less than 16% of braced wall line
- First story of a 3 story:
- Minimum of 48" wide panels located at each end
- And at least every 25ft on center
- But not less than 25% of braced wall line
Example:
In the first floor of a 30 x 50 colonial, there should be 4ft sheathing at each corner of every exterior wall and 25' on center. This means no windows or doors within 4ft of corners. Is there a way to avoid this limitation?
Exception 1: Length of braced wall panels for continuous wood structural sheathing(typical of this area) shall be in accordance with 5602.10.5.
This exception allows the designer to use Table 5602.10.5 as long as exterior corners are nailed per figure 5602.10.5 Assuming that the example above had 8ft ceiling height, this table would allow a window as tall as 62" to be installed 24" from the corner. A 6'8" door could be placed as close as 32" to the corner. Is there a way to get closer to the corner?
Engineered products and designs
Besides highering an engineer to design a shear wall, there are several engineered products such as Simpson Strong Wall and Hardywall that can be used if installed per manufacturer's specifications.
Additionally, the American Plywood Association's Portal Frame Bracing designs can be used. Garage walls at the corner of a door opening can be as small as 16" for an 8' ceiling. See: http://www.wallbracing.org
This page was last updated on 01/05/2009 .