Special How-to Feature

A home's wall and roofing design and construction method, if undertaken properly, ultimately creates a shell that is stronger than the sum of its parts, when it comes to load resistance. Here again, fasteners and adhesives play a key role. Experts at the NAHB Research Center observed the following about the lateral pressures exerted by strong winds:

Studies by the NAHB Research Center and others have found that modern floor systems often test out two to three times as strong as individual joists, because all of the structural components are joined together via nails and adhesives. This construction technique—particularly the addition of adhesives—has been shown to create a floor system that reduces the bend in any single joist by up to 60 percent. Another interesting finding was that neither cross bracing nor bridging adds significantly to the load-bearing strength of dimensional lumber joists (2x6- to 2x12-inch). Other floor systems have not yet been similarly tested.

As this chart shows, the size, length, and spacing of nails make a big difference in terms of the shear resistance of wood-framed walls. For example, when nailed at 6-inch intervals along the panel edge, a 3/8-inch wood panel has only about one-third of the shear resistance of a slightly thicker 15/32-inch panel attached at 2-inch intervals. Of course, using that many nails means a lot of labor costs. A better use of time might be to apply adhesives. Canadian researchers found that nailed and glued shear walls gain a shear capacity of 45 percent to 70 percent over walls that are just nailed.

WHAT THE NUMBERS MEAN: The shear strength figures under nail spacing represent the “racking strength” of a panel nailed in that fashion. You multiply that number times the length in feet of the panel. For example, an 8-foot panel with a shear strength of 821 could transfer 6,568 pounds of wind/seismic pressure without failure.

Because so many structural components are interdependent, best practices with regard to load bearing are almost always delineated by the manufacturer. For example, here's how Georgia-Pacific graphically identifies do's and don'ts for installing its 16-inch Wide Open Engineered I-Beam system. The manufacturer's instructions become even more critical with precision-engineered products such as this, which has holes cut out of the webbing for mechanicals. There's less room for error in load-bearing calculations.

Cracks in foundation walls are one of the most costly construction defects, and a poor understanding of lateral soil loads is often at the root of the problem. The pressure exerted by backfill soils is measured using something called the equivalent fluid density. In layman's terms, this measurement says that the wetter the soil, the heavier the potential lateral load. The chart at right shows the pounds per cubic foot imposed by various types of soil. The trouble is that backfill soils are rarely homogeneous, so loads on different parts of the below-grade wall often vary widely. Studies by Jay Crandell, of Applied Residential Engineering, have found a major cause of lateral concrete wall failure: excessive compaction by the excavator (residential backfill should not be heavily compacted or poor drainage resulting in excessive hydrostatic pressure will occur.) Best solution: Backfill with loose, granular fill to allow best drainage, and stick to reinforced concrete walls. Window and door openings should include, at minimum, ¾-inch rebar close to each side of the opening.

WET WEIGHT: Soils such as clay and silt exert much more lateral pressure on below-grade concrete walls than less dense soils do. Experts recommend that concrete walls in flood zones be built to withstand 90 to 120 pounds per cubic foot of equivalent fluid density.

No doubt you're familiar with terms such as “dead load” and “live load,” but what do they really mean?

There are two basic types of loads that affect homes: vertical and horizontal. Within the category of vertical loads there are several types: dead loads, live loads, snow loads, wind (uplift), and seismic. Dead loads and live loads probably cause the most confusion. A dead load is a permanent load—created by the building materials themselves. A live load measures the concentrated load created by people, furniture, and other objects that aren't built into the structure—things that might change over time. Homes are built to adequately manage the combination of live and dead loads, over a given time period.

To give you an idea of just how much weight a home (built using typical wood-frame construction methods) is designed to handle, here's a chart from the Residential Structural Design Guide showing long-term uniform and short-term concentrated loads. The “typical” vertical that each surface is designed to sustain is shown in pounds per square foot (psf).