The following is an excerpt from an article published in the August 2007 issue of Construction Specifier magazine. We've republished it here with Simpson Strong-Tie's permission.

Due in part to the more liberal height and area restrictions of the International Building Code, the use of wood-framed Type V construction for mid-rise multifamily and mixed-use buildings has greatly increased in recent years. Constructing multi-story buildings with wood has several advantages. Wood framing and sheathing are readily available and economical. There are countless construction crews that have the necessary skills to construct wood-frame buildings. The use of wood framing also results in a clear load path that is easily understood by both designers and contractors.

However, with the prevalence of wood-frame multi-story buildings, it is important that designers understand how to design these structures to resist high wind forces. Some of the fastest growing markets in the U.S. are areas along the Gulf Coast and Eastern Seaboard, which are more susceptible to hurricanes and high wind events.

Understanding high wind forces
Regardless of the type of construction, buildings are subjected to two basic types of loads under high winds. Uplift loads result from air flowing over the roof causing a suction force. Lateral loads result from wind blowing on the windward wall as well as wind blowing past the leeward wall. These two lateral forces act in the same direction and combine together to create a force that tries to push the building over or slide it in the direction of the wind. Lateral loads can also result from wind blowing on a steep pitched roof.

These loads and the requirements for their application are described in the International Building Code (IBC), or in the reference document ASCE 7, Minimum Design Loads for Buildings and Other Structures. In fact, with the removal of the simplified wind loading requirements from the 2006 IBC, ASCE 7 is now the single source for designers in calculating wind loads on a structure. The wind loading section of the 2006 IBC only includes such basic information as wind speed maps, exposure classification, opening protection requirements and some roofing requirements.

In large, multi-story wood-frame construction, the shear forces can become quite high, especially on the lower stories. This is intensified by the large number of openings for windows and doors that building owners often demand. When wood shear walls are loaded, they must resist two separate forces: shear and overturning.

Shear force
Shear force must be transferred from the top of the wall to the bottom of the wall. It’s resisted by nailing panels at their edges to the wall framing (reduced nailing at the panel interior is also provided to resist panel buckling and wind suction loads). By varying the nailing, and the thickness of the panel, different levels of shear resistance can be achieved. The amount of shear in each panel depends on the wind speed, exposure, size of the building in the perpendicular direction, height of the building, and the amount of sheathing on the shear wall. The more sheathing on the wall, the less each panel has to work. Since plans today often require many openings in the walls, there is less and less sheathing on the walls to act as shear walls, so each panel tends to have to resist more shear.

Overturning force
Overturning force is the tendency for a wood panel to tip over (overturn) when shear force is applied to it. Overturning is resisted by anchoring the end of the panel to the foundation. These forces depend on the amount of shear in the panel, the shear wall aspect ratio, and any uplift from shear walls above. Again, as the shear forces increase due to modern wall configurations, the overturning force also increases.

Traditional holdown methods
In wood frame construction, shear wall overturning is resisted by hardware known as holdowns. The goal is to anchor the edge of the shear wall so that it stays upright, limiting the horizontal wall movement, or deflection. The allowable wall deflection is specified in the building code for seismic resistant construction, and is typically not permitted to be more than 0.025 times the wall height for residential structures. The code does require structural systems to be designed to limit deflections and lateral drift, but there are currently no deflection limits for wind-induced in-plane shear deformations.

It is prudent for designers to evaluate, with the building owner, acceptable levels of architectural damage and user discomfort, and to limit the lateral drifts of shear walls when loaded during a wind event. Designing shear walls with limited anchorage deflection is one way to reduce wall lateral drift.

In one-story buildings, the holdown is often attached directly to a post, which then connects to the foundation with an anchor bolt. Straps which are embedded into the foundation and then attached to the post above are another popular solution.

For multi-story buildings, the traditional way to anchor an upper story shear wall against overturning has been to install two holdown anchors (one on the upper story studs and the other inverted on the lower story studs), which are then connected with a threaded rod through the floor. This ‘floor-to-floor’ holdown system effectively transfers the overturning force from the upper story to the story below. For upper-story shear walls where the overturning force does not exceed 4,000 to 5,000 pounds, a strap from the upper story studs to the lower story studs can be a cost-effective option.

For typical multi-story applications as described above, the holdown at each floor level must resist the overturning at that level plus the overturning from the shear walls above. Generally speaking, floor-to-floor holdown systems work well for structures three stories and under. For buildings higher than that, it’s possible for holdown forces at the lower floors to exceed the capacity of readily available holdown anchors.

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To read more about the design solutions for resisting uplift and lateral forces, download the full article or view the article on Simpson Strong-Tie’s website.