Article and photos by Gregory Havel
Earlier articles on flitch plate girders (in April and May 2008) discussed the original design that used a steel plate bolted between two planks, usually 2×10 or 2×12 inches (51mm × 254mm or 51mm x 305mm). They also discussed newer designs using plywood between the planks instead of steel and using laminated veneer lumber (LVL) or parallel strand lumber (PSL) instead of sawn lumber.
Not all of these girders use a single plate. The photos show a double flitch plate girder that uses two 0.5-inch (12.7mm) steel plates between three 2 x 12 (51mm x 305mm) LVLs. This is in a new home presently under construction in southeastern Wisconsin. It will carry a heavy load when the structure is complete--the second-story exterior wall plus the gable for the half-story above.
Photo 1 shows one end of this double flitch plate supported by a galvanized steel stirrup nailed to a large LVL girder. The bolts holding this structural member together are visible near the stirrup and at the upper right in the photo. On either side, are the I-joists that support the plywood subfloor above.
Photo 2 shows the other end of this double flitch plate supported on the top plate of a 2 x 6 wood-frame wall and carried by four studs.
A flitch plate can carry a larger load than a solid wood beam with the same depth and span. When a structure limits the depth of the girders and joists, they are often used because a comparatively shallow flitch plate girder can carry the same load as a truss or a solid wood beam of greater depth. It provides most of the strength of a steel I-beam at a lower cost and allows attachment of other structural members using ordinary wood-framing methods.
The heavy loads usually carried by flitch plate girders, the possibility of the steel plate being weakened by heat and failing in a structure fire, and the probability that fire will burrow between the wood joists and the steel plate should concern firefighters.
An additional concern for firefighters is that the flitch plate will be concealed between the ceiling and the subfloor above when the building is finished. It is visible only at this stage of construction; in the future, it will be visible only during remodeling and after a fire in the building has been overhauled.
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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor's degree from St. Norbert College; has more than 30 years of experience in facilities management and building construction; and has presented classes at FDIC.