Tension and the Shear Capacity of a Wall

Function

• Buildings must be designed to adequately support lateral loads such as ground movement and high wind force. As these loads are applied to the roof down, shear walls apply tension vertically and transfer loads to the building’s foundation. Shear walls act in concert with horizontal structures such as floors and rooftops to resist the shear forces from the applied loads. As shear capacity is reached, the walls gradually yield and absorb the tension throughout their length to prevent the building from collapsing.

Composition

• Shear walls consist of cords, metal connectors and the web. The web and cords are held together and attached to the unit via metal connections. Connections play a crucial role in panel design, since loads must find a pathway to the foundation, roof and other elements of the building.

Types

• Shear walls are installed as sheathed elements in buildings. These elements consist of wood or lumber, fastener size and the sheathing material itself. Sheathing can be made of cement plaster, fiberboard, plywood or wood lath. The type of sheathing material used for shear walls will impact its shear capacity per foot of length in each segment. Moreover, lumber grades and sheathing materials provide varying strengths. For example, the density of the lumber affects fastener strength. Dense wood species such as Douglas-fir provide fasteners with a stronger hold than less dense wood species such as redwood.

Limitations

• Certain shear wall sheathing requires coupling with other materials to sufficiently resist building loads. The Canadian Sheet Steel Building Institute states that gypsum shear walls must be used in conjunction with wood walls in buildings with height limitations due to seismic activity such as earthquakes. This ensures that the building shear capacity can resist tension caused by ground movement and other lateral loads.

Location

• To increase tensile strength in the building, shear walls are installed on each floor including the crawl space. Shear walls of equal length should also be positioned along all exterior walls to provide buildings with enough strength and stiffness to support the interior structure, according to the Engineered Wood Association. As long as the strength and stiffness of exterior shear walls are adequate, interior shear walls are not required in buildings where the subfloor's width is approximately one-third its entire length. For instance, a structure with a width of 25 feet does not require interior shear walls unless its subfloor's length exceeds 75 feet.