 David Wheeler
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
(www.strongtie.com/ftp/bulletins/T-PTWOOD06.pdf).
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.”
For more information….
Fairfax County, Virginia, has an excellent Website that gives recommended
deck construction details.
The WMEL is a leader in the development and testing of
wood-plastic composite materials, which are gaining popularity for
residential decks and rails. Some 40 percent of all commercial WPC
products in North America use formulations developed at WSU. The
lab has developed an excellent source of information.
The North American Deck and Railing Association is another good
source
of information on decks.
--Tina Hilding
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