The following is an excerpt from an article published in the October 2007 issue of Structural Engineer magazine. We've republished it here with Simpson Strong-Tie's permission.
We all know that structures are subject to lateral forces. We’ve seen the awesome power of Mother Nature at work and learned valuable lessons from natural events, such as the Great San Francisco Earthquake of 1906; the Northridge, California, earthquake in 1994; and the destructive forces of wind during Hurricane Katrina in 2005. In wood-frame construction, building codes throughout the country either require structural engineers to calculate and design for wind or seismic forces or require contractors to build by a prescriptive method of resisting lateral forces.
Wood has become one of the most prolific building materials in the construction industry because of its versatility, availability, load capacity and economical value. As such, a variety of design methods have been developed to resist lateral loads on wood structures. Within the past half century, the use of site-built, wood-framed shear walls has gin leaps and bounds as a means of resisting lateral forces. A relatively new design method also has arrived in the past decade—prefabricated shear walls, built out of wood and/or steel.
Wood structures today are one to six stories tall and include some new design features, such as higher floor-to-floor heights, taller doors and larger window openings, typically along the front and rear exterior walls. Consequently, finding available wall space to meet the minimum load demands and wall lengths prescribed by the code is far more challenging when using site-built shear walls. The invention of the prefabricated shear wall was created out of the necessity to provide narrow shear wall solutions. These walls are designed to meet the high load and extreme aspect ratio demands. They also are being used in stacked configurations in multi-story structures, introducing cumulative overturning into the designer’s calculations. Because of this, designers need to revisit the general mechanics of shear wall design as it applies to both site-built and prefabricated shear walls.
Fundamentals of shear wall design
When lateral (wind or seismic) forces engage the diaphragm of a structure, it’s easy to visualize the force pushing horizontally on the top of the shear wall. Sheathing then transfers the shear from the top of the wall to the bottom of the wall while holding the wall together to resist racking. If the bottom plate of the wall is anchored to the foundation to resist sliding; the far end of the wall will press down (compression force) and the near side of the wall will lift up (tension force) – this is overturning.
Racking: When wind blows against the side of the structure exerting a lateral force that causes it to lean over (rack) to one side.
Sliding: When wind blows against the side of the structure exerting a lateral force, causing it to slide off of its foundation.\
Overturning: When the structure is anchored in place to limit racking or sliding, the lateral force of the wind causes the structure to rotate or overturn.
The members at each end of a shear wall (typically wood studs or posts) are designed so that the capacity of these members will withstand the compression demand caused by overturning and gravity forces. The International Building Code (IBC), Section 2305.3.7 states “where the dead load stabilizing moment…is not sufficient to prevent uplift due to overturning moments on the wall, an anchoring device shall be provided. Anchoring devices shall maintain a continuous load path to the foundation.” These components of the shear wall required to resist overturning are fundamental to the wall’s design, and proper calculation of the demand forces are important in single-story buildings and absolutely essential in multi-story structures where forces are amplified and cumulative.
About the author
Bryan D. Wert, M.S., P.E., is a branch engineer for Simpson Strong-Tie. He works in McKinney, Texas, and services the southeast region.
Learn more
To learn more about shear walls and cumulative overturning, download the full article or view the article on Simpson Strong-Tie’s website.
