Source Newsroom: Virginia Tech (Virginia Polytechnic Institute and State University)
Newswise — Within the United States, millions of homes are equipped with exterior decks and balconies made from treated wood products, which frequently are the preferred location for a variety of enjoyable outdoor activities. However, many accidents also occur on decks and balconies causing severe injuries and fatalities.
In an effort to improve deck safety, Virginia Tech wood science and forest products professor Joe Loferski and biological systems engineering professor emeritus Frank Woeste have been conducting research for nearly six years on the structural engineering of decks and balconies. Their research and subsequent recommendations have directly led to changes in the Virginia Uniform Statewide Building Code.
Loferski and Woeste have identified two major points of failure for decks and balconies. Decks commonly fail due to the connection between the deck and the house, which typically causes a sudden and total collapse of the deck. Research has shown that many cases of deck collapse occur at loads that are much smaller than what the design should have supported. Loferski said, "If a deck is designed and constructed to carry the intended loads, very few failures would occur. Decks can be built to handle many people, but past design and construction standards weren't always adequate."
Until recently, building codes have offered little guidance on how to produce connections that are capable of safely supporting the deck's design load. By reconstructing various types of deck to house connections, using the resources in the Virginia Tech College of Natural Resources' Wood Engineering Laboratory at the Brooks Forest Products Center, Loferski and Woeste were able to measure the load needed to break the connections. They also identified safe connections using bolts and lag screws for a variety of joist spans. The Virginia Building Code has adopted new standards based on the results of Loferski's and Woeste's testing.
An even safer option than using bolts and lag screws to connect the deck to the house is using what Loferski called "a free standing deck," which uses columns near the house as well as away from the house so that the bulk of the deck's load is transferred directly through the columns to the ground instead of through the deck-to-house connection.
The second major area of concern in deck safety identified by Loferski and Woeste is the deck guardrail system. The building code requirement for a residential deck guardrail is 200 pounds of force applied 36 inches above the deck's surface in any direction. "We tested numerous commonly used post-to-deck connection designs and didn't find any that could safely carry the design load," said Loferski.
After months of research, the two Virginia Tech faculty members developed a connection that uses a hot-dipped galvanized steel connector to attach the post to the deck. These connections were the only ones that were strong enough to carry the required loads.
In addition to making recommendations that have been adopted by the Virginia Building Code, Loferski and Woeste are also working to have the results of their research included in the International Residential Building Code. To further the work of their safety research, Loferski and Woeste teach many short courses on the structural design and inspection of decks and balconies for builders, engineers, architects, and building inspectors.
"Virginia is the leader in code development because folks like Woeste and Loferski care about the life safety issues that safeguard our buildings," said Roger Robertson, chief of inspections with the Chesterfield County, Virginia, Department of Building Inspections.
"This is one more example of how Virginia Tech's College of Natural Resources not only has significant impact on the state, but also leads the nation in important research," said wood science department head Paul Winistorfer.
WHAT HOMEOWNERS CAN DO TO DETERMINE IF THEIR DECK IS SAFE
Determining whether or not a deck is safe involves many factors. Virginia Tech wood science and forest products professor Joe Loferski recommends that in many cases a qualified inspector should be used. The local building inspection department can be contacted to arrange an inspection. Some things to examine in any inspection include:
* Wood species and condition: Approved preservative pressure treated wood should be used for the structural components of the deck. Building code approved heartwood of naturally durable wood species (redwood, cedars, and black locust) can also be used. If any decay is found, the deck components and possibly the entire deck should be replaced because decayed wood is extremely weak and unpredictable.
* Connections: All structural connections should be inspected for proper size and spacing of fasteners. Nails alone should not be used to attach a deck or balcony to a house because they are simply not strong enough. Use fastener schedules found in the Virginia Building Code for bolts and lag screws. Critical connections include deck-to-house, girder-to-column, and post-to-deck, rail-to-post, and picket-to-rail.
* Condition of fasteners: Corrosion of metal fasteners is another possible problem. Some preservative chemicals have been found to be highly corrosive to steel and aluminum. Aluminum flashing, which traditionally was commonly used, should not be used in contact with the new preservative chemicals. Steel fasteners should be hot-dipped galvanized or 316 stainless steel.
Steps should have a graspable handrail.
A guardrail is required if the deck is higher than 30 inches above the ground.
* Guardrails should be at least 36 inches high above the deck surface and should be able to safely resist 200 pounds of force applied 36 inches above the surface. It is not useful to simply wiggle a deck guardrail to determine if it safe. The entire guardrail system must be evaluated to determine if each and every connection is adequate.
* The picket spacing should be no wider than 4 inches to keep children from falling through.
* Notched guardrail posts are not safe and can break at the notch with little warning. Surprisingly, notched posts are very common and are found on many decks, but they are very weak when horizontal loads are applied to the top of the rail.
Other inspection points should also be examined and can be found in the book, A Manual for the Inspection of Residential Decks and Balconies written by former master's student in biological systems engineering Cheryl Anderson under the guidance of Loferski and wood science and forest products adjunct professor and biological systems engineering professor emeritus Frank Woeste.