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Virginia Tech researchers' work to help prevent balcony and deck collapses

Virginia Polytechnic Institute And State University : 15 October, 2003  (Technical Article)
A team of Virginia Tech researchers produced an inspection manual for residential wood decks and balconies in an effort to curb the numerous collapses that typically occur nationwide and prevent needless tragedies. The new Manual for the Inspection of Residential Wood Decks and Balconies will be available at the end of October from the Forest Products Society.
Cheryl Anderson, a graduate student in biological systems engineering from Bear, Del., investigated eight decks and balconies in the New River Valley as part of her study to produce an inspection manual. Not one of the eight decks and balconies studied met all code provisions based on the 2000 International Residential Code, which was adopted by the State of Virginia on Oct. 1, 2003 as the model code for residential construction.

According to Frank Woeste, a registered professional engineer and professor emeritus of BSE, the building code has specific load and construction requirements for decks and balconies. A lack of information about proper deck and balcony construction and failure to regularly inspect decks are the problems contributing to tragic collapses. Decks and balconies should be inspected on an annual basis since they degrade over time. Most decks do not collapse strictly from being crowded but because they were not properly built in the first place. 'I am not aware of any case when a 'code-conforming' structure failed,' Woeste said.

Woeste, of Blacksburg, Va., was consulted as an expert on deck safety in regards to the fatal porch collapse in Chicago last summer. The recent story, 'Fatal Chicago Porch Collapse Highlights Flimsy Connections' is on-line in the Journal of Light Construction: The code load requirement for residential decks is 40 pounds per square feet live load, which represents the weight of the people and moveable objects, such as planters and deck furniture, that use and occupy the deck. Assuming a 12 ft. by 20 ft. deck and that the average weight of the occupants is 160 lbs, the deck would safely support 60 adults.

'It is practically impossible to overload a deck designed and built to the 40 psf live load requirement,' Woeste said. The ever-present danger rests in the fact that the majority of decks and porches were not built correctly, making a crowded deck party a dangerous combination of these two factors.

While there are numerous code requirements for decks and balconies, Woeste and his associates are primarily concerned about the attachment to the house and the openings in the railings and stairs. The 40 psf load requirement produces a heavy shear type loading in a typical deck-ledger to house-band connection. Some deck and balcony ledgers are only nailed to the house band joist, and this approach may not be adequate depending upon the span of the deck joists. If proper flashing is not installed, the wall sheathing and band joist can rot, destroying the original capacity of the nail connection.

Both Woeste and Joe Loferski, faculty member in wood science and forest products, recommend designing decks using an independent and redundant support system where preservative pressure treated posts are embedded in the ground and placed next to the house to support the vertical loads and provide lateral stability. For more information, see Attaching Deck Ledgers at

The researchers are especially concerned with openings in railings and stairs as code violations creating severe hazards for small children. To be in code conformance, a four-inch diameter ball or sphere cannot pass through the railing at any location. This provision is intended to prevent a child from falling through the railing. Infill pickets that are vertical are preferred, as inclined or horizontal pickets can invite climbing by children. The railing system must also be sufficiently strong to prevent a collapse from forces created by adult occupants.

Stairs are also inviting to children so extra care needs to be taken to prevent serious injuries. To be safe, a four-inch diameter ball cannot pass through the infill pickets, a six-inch diameter ball cannot pass through the opening between the bottom rail and stair treads, and a 4-3/8-inch ball cannot pass through the spaces between the stair treads.
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