Making decks safer
by Tina Hilding | © Washington State University
Most of us don't lie awake at night worrying about our decks. But we should.
The deck is the most dangerous part of the house, says Don Bender, director of the Wood Materials and Engineering Laboratory at Washington State University.
Decks cause more injuries and loss of life than any other part of the home structure. Except for hurricanes and tornadoes, more injuries may be connected to deck failures than all other wood building components and loading cases combined.
News stories often report that decks fail because of being overloaded with people. Bender, however, disagrees. "Many decks collapse before they reach their code-required design load," he says. "In fact, we're lucky that we don't load the decks to their capacity."
For his part, Bender, a structural engineer, likes to look under a deck before he goes onto it.
Working with colleagues Frank Woeste and Joe Loferski at Virginia Tech University and David Carradine at the Wood Materials and Engineering Laboratory, Bender is developing design guidelines for making residential decks safer. "It is appropriate that we have two universities from the East and West to attack a national problem of this magnitude," he says.
The engineers recently published an article in International Code Council's Building Safety Journal, read by approximately 40,000 building inspectors and officials throughout the United States, disclosing their findings about deck failure.
The national building codes offer little coverage on residential decks, says Bender. And, because decks look relatively easy to build, homeowners attempt to add on decks themselves rather than work with a professional.
Decks most commonly fail at the ledger when they are connected to the house by insufficient fasteners. Compounding the problem, most decks don't have a back-up structure, such as a support under the deck next to the house. Without this redundancy, when the deck ledger fails, the deck collapses catastrophically—without warning.
Furthermore, metal fasteners can corrode, and wood can rot under a deck, where most people don't bother to look. New materials, like chemicals used to preserve lumber, contain high levels of copper. That copper interacts with steel fasteners and can accelerate corrosion. Fastener manufacturers, such as Simpson Strong-Tie, provide information on selecting corrosion-resistant fasteners.
Finally, deck railings are rarely constructed to withstand code-prescribed loads. Although the rails and posts may be strong enough, the connection between the deck and the railing post consistently fails at low load levels. Many deck-railing constructions are "grossly inadequate," says Bender.
Based on experiments conducted at WSU and Virginia Tech University, the researchers say builders should carefully stagger lag screws or bolts to attach a deck to the house and follow exact distances for the bolt spacing. They also recommend structural supports near the house. Using hardware similar to a "seismic tie down," they also tested ways to make deck railings safer.
The group's recommendations on deck-ledger connections were recently incorporated into the Virginia and Indiana state building codes. They were also submitted for inclusion in the International Residential Code at a hearing in March 2006 and will be voted on at the fall 2006 meeting.
As director of the WSU wood lab, Bender has researched a myriad of issues related to wood materials and design. In addition, he has taught hundreds of college students and building professionals about design of timber structures and advanced wood engineering.
Still, he says, "this project is probably going to have more of a positive impact on public safety than any other thing I've done in my career."
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