By Paul Bianchina
Push hard enough against the side of an unsupported structure - a force called lateral or shear load - and you'll begin to deflect the structure to one side and eventually topple it over. The problem of lateral load effects on a building, be it from wind, mechanical force, occupant movement or other source, is one of the most basic situations in construction, and one to which a lot of thought and a lot of solutions have been applied over the years.
A shear load is not difficult to imagine. Look at the basic structure of a typical wall, which consists of two horizontal 2-by-4s nailed to the top and bottom of a series of vertical studs. If you push against one end of that wall framing, you can easily move it - all of the studs will simply pivot on the nails and lean over in unison.
Now take a piece of 1-by-4 and nail it diagonally across the face of those studs as a brace - push against the end of the wall, and you'll find that it has become considerably harder to move. And if, instead of the 1-by-4, you were to cover one entire face of the wall with plywood, nailed regularly to the face of each stud, you would find it virtually impossible to deflect that wall. This is what's known as a shear panel or a shear wall, and it's one of the simplest and most structurally solid designs in construction.
If you look at a house under construction, you will typically see that the outside of the exterior walls are covered with plywood or oriented strand board (OSB), sometimes in the form of finished sheet siding, and other times as wall sheathing that will be covered by another form of board siding. Those sheets of plywood or OSB essentially lock the vertical structure of the home in place and keep the problem of shear load from knocking things out of whack.
However, look a little more closely at that house - especially in the areas around large window and door openings - and you'll probably see some areas where all that plywood won't give the building as much support as it needs. Problems arise in the narrow walls - typically those about 24 inches or less, but sometimes wider - where there simply isn't enough width for the plywood covering to lend much additional strength to the wall structure. You'll typically see this situation arise in the narrow spaces between and adjacent to the garage doors, in short walls between large windows or sometimes between a large window and a wall.
In many of these areas, the building codes require the use of a specially designed shear wall. Shear walls are sometimes made on site, typically following the specific recommendations of an architect or structural engineer who has calculated the loads on the building and designed the wall accordingly. A shear wall may consist of an area where thicker plywood is used on one or even both sides of the wall, or where the plywood is glued to the studs and fastened in a specific pattern with nails or staples of a specific size - the details vary, depending on the situation and the forces and loads involved.
Another option may be the installation of a prefabricated section of wall that's been manufactured to meet specific load requirements. Pre-manufactured shear walls are offered by a couple of different companies and are brought to the site and installed in a specific manner according to the manufacturer's instructions and the provisions of the building codes. Shear wall sections of this type can offer some amazing strength characteristics in a very narrow space and may be just what's needed to solve a specific framing problem. In some instances, the use of these shear walls can allow the builder and the architect some design and construction freedom in the size and placement of openings that they wouldn't have had otherwise.
Shear walls can be ordered from most lumber yards, and the standard sizes typically only require a few days to get - custom sizes are also available. When ordering, you will need to know the size of the wall and also the design loads the wall needs to meet.
To be fully effective, the shear wall needs to be installed according to the manufacturer's specific requirements, which involves the use of steel connectors and reinforcements in key areas. Some of these connectors are actually imbedded in the concrete slab or stemwall at the time the foundation is poured, so you or your contractor needs to plan far enough in advance to make sure you get everything in the right place and accomplished in the proper sequence.
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