Nailing Down Shear Walls
What is a Shear Wall?
A shear wall is a vertically built structure within a building that provides reinforcement from extreme winds and seismic activity. Load-bearing walls and columns keep structures standing up, supporting the structure’s compression load, while shear walls keep the structure from blowing over or collapsing outwardly. Almost all modern homes have external shear walls, and internal shear walls are found in large homes and high-rise buildings. Shear walls are usually built from braced light-framed plywood or OSB walls, reinforced concrete, concrete blocks reinforced with rebar, reinforced masonry walls or steel plates, or moment frames. With technological innovations, shear walls are available in prefab or kits as well as retrofit and the standard build-on-site option.
How Does A Shear Wall Work?
A shear wall is a vertically-built wood square frame made up of two columns and two beams. This frame supports compression weight as it bears down on the upper beam and is transferred down the columns to the base. If too much force is put on the sides (from lateral forces such as extreme winds or earthquakes), the frame will twist and collapse UNLESS the frame is braced by supports, providing the structure with strength and stiffness. Shear walls should be located on every level of a structure, and added to interior walls when the exterior walls don’t provide enough strength or stiffness. Shear walls should also be aligned vertically and supported on the foundation footing or walls. If the shear walls don’t align, other parts of the structure will need additional support.A well-engineered and well-built shear wall resolves the lateral wind and seismic forces by directing them to the foundation where they are resisted by the concrete and ground. While shear walls are usually recognized for preventing catastrophic collapses and fatalities, they also help protect against smaller damages such as cracked drywall and fractured tile.
Types of Shear Walls
There are six types of shear walls, and they may be retrofit for existing buildings, built on-site per the structural engineer’s specs, prefab, or assembled using a kit.Wooden framing with vertical studs and sheathing. A wood frame of vertical studs is covered in plywood or OSB to prevent deforming, and maintain rigidity under lateral pressure.
Reinforced concrete is widely used in homes. Reinforcement is provided in both horizontal and vertical directions, and at the end of each wall, bars are closely spaced and anchored.
Concrete block shear walls are built using hollow concrete blocks, reinforced with steel rebar arranged in horizontal and vertical directions, then concrete is poured into the hollow spaces. This method is used often to protect against seismic events.
Steel sheets are usually used in large buildings.
Diagonal braces, often made of steel, provide a high level of support.
Moment frames are rigid, rectilinear shear walls that allow window or door installation.
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Shapes of Shear Walls
Shear walls may be built in a variety of shapes to provide the most protection; however, most are built on a rectangular plane. Structural engineers will determine the most effective shape depending upon the needed protection for that locale. Here are the basic shapes:- Box shaped are square or rectangle shear walls built around a central core. They may contain an elevator, stairs or mechanical system.
- C-shaped walls have short extensions off each end of the main plane.
- L-shaped walls have a longer leg off one end of the plane.
- T-, U-, W-, and other shapes named for alphabet letters depending upon the letter that they resemble.
Installation Best Practices
Here are some basic installation tips for installing shear walls.
- Only install shear walls with the advice of an experienced engineer who will design the shear wall to meet the demands of a specific region, structure, and provides plan details regarding nail length and diameter, hardware placement, blocking size, and orientation.
- Select panels suitable for your sheathing project. Panels usually come in lengths up to 12’. The longer the panel you choose, the less blocking that will be required.
- Each panel has a strength axis. Even though installing panels vertically may require less labor, installing panels horizontally may provide greater wind resistance. However, some sources, like the Engineered Wood Association, state there is no difference between horizontal and vertical installation effectiveness.
- Fasten the sheathing to the frame using nails. Nail size and spacing should be specified in the plans from the designer or engineer.
- All sheathing must be attached mechanically with metal connectors to the foundation walls. The size and type of connectors should be specified in the plans by the designer or engineer.