Residential and light commercial construction generally use wood as the primary building material for studs, plates, joists, headers and trusses. However, all-wood construction has problems. The rapidly rising cost of raw wood supplies has in effect substantially raised the cost of these members. Further, the quality of available framing lumber continues to decline. Finally, wood is flammable and susceptible to insects and rot.
Due to these problems, many builders have been switching to using all steel framing. The costs between using wood or steel framing is getting closer. In January 1990, the cost of framing lumber was about $225 per thousand board feet, peaking to highs of $500 in both January, 1993 and January 1994. Since June 1995, the framing lumber composite price has been rising from $300 per thousand board feet Estimates from the AISI and NAHB Research Center state at a framing lumber cost of $340 to $385, there would be no difference between the cost of framing a house in steel as compared in wood. Thus, the break-even point between wood and steel framing is at about $360 per thousand board feet of framing lumber, and the lumber price has exceeded that point several times in recent years by as much as 40%, giving steel a competitive advantage.
Recycling has additionally helped the cost of steel to remain on a stable or downward trend. Steel costs have varied little in recent years. Traditionally variations can be correlated to steel demand by the automobile industry when demand is high, steel usually increases slightly in price. Consequently, the use of metal framing in residential and light commercial construction is increasing, a trend recognized and encouraged by the American Iron and Steel Institute (AISI).
All steel studs, tracks and trusses are being manufactured by Tri-Chord, HL Stud Corporation, Truswall Systems, Techbuilt Manufacturing, Knudson Manufacturing, John McDonald, and MiTek Ultra-Span Systems.
A problem with using all steel framing is its high thermal conductivity, leading to thermal bridging, "ghosting", and greater potential for water vapor condensation on interior wall surfaces. "Ghosting" is when an unsightly streak of dust accumulates on the interior wallboard, where the steel studs lie behind, due to an acceleration of dust particles toward the colder surface. Another problem of using all steel framing is the increased energy use for space conditioning (heating and cooling). Metal used for exterior framing members allows greater conduction heat transfer between the outside and inside surfaces of a wall, roof or floor. In colder climates, this increased conduction can cause condensation in interior surfaces, contributing to material degradation and mold and mildew growth. Metal framing also decreases the effectiveness of insulation installed in the cavity between the metal framing due to increased three dimensional thermal shorting effects. Higher sound transmission is another disadvantage of metal framing since sound conductivity is greater in metal than in wood. Electricians have more difficulty working with all steel framing when running holes for wiring since metal is more difficult to drill than wood, and grommets or conduits must be used to protect the wire.
U.S. Pat. No. 5,285,615 to Gilmour describes a thermal metallic building stud. However, the Gilmour member is entirely formed from metal. In Gilmour, the thermal conductivity is only partially reduced by having raised dimples on the ends contacting other building materials.
U.S. Pat. No. 3,960,637 to Ostrow describes impractical wood and metal composites. Ostrow requires each end flange have tapered channels, the end flanges being formed from extruded aluminum, molded plastic and fiberglass. Ends of the vertical wood web must be fit and pressed into a tapered channel. Besides the difficulty of aligning these parts together, other inherent problems exist. Extruding the channel flanges from aluminum or using molds, cuts and rolling to create the channelled plastic and fiberglass end flanges is expensive to manufacture. To stabilize the structures, Ostrow describes additional labor and manufacturing costs of gluing members together and sandwiching mounting blocks on the outsides of each channel.
Other metal and wood framing member patents of related but less significant interest include: U.S. Pat. Nos. 5,452,556 to Taylor; 5,440,848 to Deffet; 5,072,547 to DiFazio; 4,875,316 to Johnston; 4,301,635 to Neufeld; 4,274,241 to Lindal; 4,031,686 to Sanford; and 3,531,901 to Meechan.
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USPTO Backgrounds