Special How-to Feature

JAY CRANDELL WROTE the book on residential load bearing. Literally. It's called the Residential Structural Design Guide. About five years ago, Crandell and some of his associates at the NAHB Research Center in Upper Marlboro, Md., noticed a dearth of engineering information on modern housing. Most homes, they came to realize, are built from experience and instinct—not from a body of documented science.

“House construction has never fully been described in engineering terms,” says Crandell, now a private consultant with a firm called Applied Residential Engineering in West River, Md. “We just know what works. We wanted to develop a method describing why certain methods work.”

That method of looking at residential structures, he says, was modeled after what happened in Australia in the 1970s, when the government began to crack down on “under-designed” homes, ultimately leading to research on residential engineering.

In writing the book, Crandell discovered that one of the more challenging aspects of modern residential engineering is the hybrid nature of modern construction. It's not uncommon to combine insulated concrete forms, light-gauge steel framing, solid sawn lumber, and engineered I-joists in a single design. But for Dana Bres, a research engineer for the Partnership for Advancing Technology in Housing (PATH) in Washington, the complexity of connecting and applying different materials should not deter builders from using them.

“You're no longer forced to do 2x14 [dimensional lumber] headers,” Bres says. “You can do it with engineered products such as glulams, and we're seeing an incredible use of these alternatives. They give the builder a lot of flexibility.”

He adds that because new products are pushing their way into the mainstream of home construction, the manufacturers have a huge vested interest in seeing them installed properly. “The builder and the product manufacturer and the supplier all have skin in the game,” he explains. “They are truly partners in delivering a good product.” He adds that when dimensional lumber was the only material, the “guy with the hammer” was ultimately responsible for all load-bearing considerations. Now, with building so much more complex, framers need help to avoid mistakes.

Studies of homeowner warranty claims are one place to look for evidence of load-bearing mistakes. Considering their lack of formal engineering knowledge, home builders do surprisingly well, at building structurally sound homes. That's because margins of error are built into most building codes. A study by the Canada Mortgage and Housing Corp. of Quebec back in 1994 found that about6 percent of defect claims fall under the structural category.

The most dramatic result of load-bearing errors is not catastrophic collapse of a home under normal use but excessive damage to the home during a high-stress event, such as a hurricane or an earthquake.

Again, that's where close adherence to local codes and manufacturer specifications can save the day. This is where small mistakes that may seem unrelated to heavy matters like the building loads can be greatly magnified.

“Often, it's something as simple as whether or not nailing was done properly,” notes Bres. “I have seen cases where the roof is peeled back on a home, and you can see where the row of nails missed the rafter. That's something that goes all the way back to the worker. If you miss with a nail, you nail it again.”

Details such as fasteners highlight another poorly understood aspect of load bearing. Walls, floors, and roofs are not individual pieces of plywood nailed to individual joists—or at least they shouldn't be. In house design, they actually become an integral part of the load-bearing capacity. A floor, for example, takes much of its strength and stiffness from its combined bond with adjacent joists (see “Floors: Secret Strength,” below).

But the other reason for thinking of parts of a house as a system is long-term durability. As Alan Mooney of Criterium Engineers in Portland, Maine, points out, “Floors shouldn't move. The more something moves, the faster it's going to wear out.”

In load-bearing terms, the bending “movement” in something such as a floor joist is described as deflection. Not all deflection is destructive to the materials. And various materials respond differently to loads. Wood, for example, is especially good at bouncing back from short-term heavy loads. But it's not so good with long-term heavy loads. By the time a material gets to the point where fasteners have become loose, it's possible that it has exceeded its deflection capabilities—and is rapidly losing its integral strength. That doesn't necessarily mean that structural bearing points will be affected. According to Crandell, these areas are the least likely to show stress, even when fasteners fail at those points—because they are not fastener dependent.

Crandell says that when researchers from PATH studied damage from Hurricane Andrew, they found damage that engineers said shouldn't have happened—and some methods of construction that stood up stoically. For example, he says, he didn't see a single failure of a home built with standard masonry construction, yet the new codes demanded many changes. That's a disappointment, he says, because it drives up prices of new homes unnecessarily, without a significant gain in structural strength.

Bres notes that most builders have uncanny common sense about load bearing. “Many [homeowners] underestimate the complexity of their home,” he says. “The builders and subs bring a lot of wisdom to the project. Most builders have focused their product line to what they're comfortable building and what their core market wants.”

Because of this, Bres says, builders may pass up work that they can't easily get their head around in terms of structural loads—particularly production-oriented builders. And knowledge of loads has other perks as well, he says.

“A lot of builders try to move a large number of their load-bearing walls to the outside of the home,” he notes. “This saves costs on moving walls on the inside. It makes the floor plan more flexible and saves a lot of money on reengineering interior walls if a client wants a change.”

BOOK OF RECORD: The Residential Structural Design Guide has everything you need to know about residential engineering and is available for free digital download from HUD (see link). If you would rather own a desk copy, you can purchase one from from HUDUser ($30) or Amazon.com.

Included with this story are a few tips and tricks of load bearing to consider. If you want to delve more deeply into the art and science of home engineering, buy or download the Residential Structural Design Guide(download here).

The benefits of extended overhangs outweigh the negative impact on uplift loads, according to research by HUD back in 1978. It found that overhangs of 12 to 24 inches provide much-needed protection for exterior finishes, especially in humid climates. The NAHB Research Center notes that extra care has to be taken with these long overhangs to neutralize their uplift increase. It suggests, for example, the use of metal hurricane straps or other types of twisted metal tie-downs (already required by code in some areas). And engineered truss rafters with built-in soffit returns may be preferable to traditional 2x4 “outrigger” framing.

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).