Construction Concerns: Flitch-plate Girders of Parallel Strand Lumber

Article and photos by Gregory Havel

Flitch-plate girders of sawn lumber and of laminated veneer lumber (LVL) were discussed in an earlier article. A third variation now in use is made of a steel plate sandwiched between two pieces of parallel strand lumber (PSL). Usually these are held together with bolts arranged in a triangular pattern on the face of the girder. They can also be assembled using a powder-actuated tool that uses blank pistol cartridges to drive steel pins with heads like nails through the PSL and the steel plate, from both sides.

The flitch-plate girder was first used in the 1880s. It can carry a larger load than a solid wood beam with the same depth and span; provides much of the strength of a steel I-beam at a lower cost; and allows attachment of other structural members using simple wood framing methods.

The flitch-plate girder shown in Photo 1 is made of a 3/8-inch steel plate sandwiched between two 2 × 12 PSLs. It is held together by bolts with washers and nuts, arranged in a triangular pattern whose dimensions were specified by the architect. It has a 36-foot span; is located in a 15,000-square-foot lake home that is under construction; and will support a wall that will be built directly above on the second floor.


Photo 1. Click to enlarge

Photo 2 (below) shows one end of this flitch-plate girder, and the ends of the doubled wood I-beams that are parallel to it. The girder and the wood I-beams bear on, and are attached to, a 2 × 10 board bolted to the top of the steel I-beam, which is supported by square tubular steel columns on a concrete foundation. Although this large residence is essentially of lightweight wood-frame construction–including manufactured wood materials–some parts are supported by structural steel girders and columns that will be concealed inside wood frame walls covered with gypsum board.


Photo 2. Click to enlarge

The underside of the floor deck shown in the photos is of ¾-inch plywood with tongue-and-groove edges. This will support a radiant floor heating panel made of several inches of lightweight concrete with plastic tubing embedded in it to circulate hot water from the heating boiler. Parts of this floor will be finished with ceramic tile, and other parts with carpeting. The bottoms of the flitch-plate girder and the wood I-beams, and the wires, cables, pipes, and ducts between them, will be covered with a ½-inch gypsum board ceiling.

While preparing prefire plans and during structure fires, firefighters should be concerned by the long spans and heavy loads carried by flitch-plate girders; by the possibility of the steel plate failing in a structure fire; the probability that fire will burrow between the wood and steel components of the girder; by the weakening of the glue bonds in the manufactured lumber from the heat of a fire; and by the behavior of the structural steel that can be concealed inside wood frame walls.

For additional information on the materials discussed above, visit the website of the Engineered Wood Association (formerly the American Plywood Association) at http://www.apawood.org. Also, try Internet searches for “flitch plate girder”, “parallel strand lumber”, “laminated veneer lumber”, “oriented strand board”, and “engineered lumber”.

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 and fire officer II, an adjunct instructor in fire service programs at Gateway Technical College, and safety director for Scherrer Construction Co., Inc. He has a bachelor’s degree from St. Norbert College. He has more than 30 years of experience in facilities management and building construction.

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