Resisting Wind and Seismic ForcesConcrete makes the job much easier

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Wind and seismic events both apply lateral loads to a home. Seismic design usually is based on forces that are related to some fraction of the weight of the building or its components. “For wind, the major complicating factor is that the roof uplift forces may be several times the weight of the roof,” says Reinhold. For seismic design, though, the dead loads, live loads, and wind loads are only a fraction of the maximum expected earthquake loads, so detailing the connections is critical to prevent collapse during a seismic event.

The 2x4 shot at the precast double wall panel at 115 mph shatters on impact leaving no scratch on the concrete.

Standards and guidelines for building wind-and seismic-resistant homes are covered by the IBC and the International Residential Code (IRC). The IRC covers one- and two-family buildings that are less than three stories high; other structures are covered by the IBC. Most local building codes in the United States today reference these codes. But each governing jurisdiction can choose to use whichever code it wants, can add modifications to the codes, or can even develop its own. Gowers says that parts of California, for example, still reference the old Uniform Building Code (UBC), which he refers to in his seismic designs. However, there are some states, like Alabama, where a unified state code hasn't been adopted and individual cities and counties decide which code to reference. The American Society of Civil Engineers (ASCE) Standard 7 (Minimum Design Loads for Buildings and Other Structures) includes a map describing wind speed in locations around the country. This map is referenced by the IBC and other modern building codes.

Most concrete homes built in the United States are constructed with concrete masonry blocks. According to Nick Lang, a research engineer for the National Concrete Masonry Association (NCMA), Herndon, Va., the typical residential exterior wall is constructed with 8-inch concrete block. Walls include bond beams at the bottom and top of the wall plus additional bond beams above and below each opening. The block cores are reinforced and grouted according to the engineered requirements for either wind or seismic at the location of the building. NCMA's Web site (www.ncma.org, click on Resources) provides an free extensive set of technical briefs (e-TEK) on the subject. Lang says that you also can call the NCMA hotline at 703-713-1900 for expert help.

Dramatizing the force of the impact, wood fibers from the impact of the 2x4 hitting the concrete tightly packed the grout joint of the brick formlined pattern of the concrete surface. Photos: Joe Nasvik

Concrete is the best building material for protection from wind and seismic events. Most new home construction in the southern part of Florida today uses either concrete or masonry block. Damage to reinforced masonry and concrete walls from wind events is less than with other building material and they provide life protection from wind-driven debris.

Source: NOAA, The Enhanced F Scale is a set of wind estimates (not measurements) based on damage caused by a tornado. It uses three-second gusts estimated at the point of damage based on a judgment of 8 levels of damage to 28 different damage indicators, which vary with height and exposure. For a list of the damage indicators, go to www.spc.noaa.gov/efscale/ef-scale.html.