Now in its third decade, Thinshell building technology is rapidly gaining ground in an era that emphasizes sustainable construction. It starts with modified,
Now in its third decade, Thinshell building technology is rapidly gaining ground in an era that emphasizes sustainable construction. It starts with modified, light-gauge metal framing that bonds with thin exterior concrete to form precast or cast-in-place walls, floors and roofs. A physical and chemical union forming a composite connection between galvanized metal and concrete provides a synergy of metal’s tensile characteristics and concrete’s compression properties to create an assembly stronger than the sum of its parts. Thus, site-cast walls become a lighter, stronger, and less costly form of tilt-up; and, precast panels are easily lifted from trailers for placement.
Further validating the technology, the Construction Specifications Institute (CSI) recently revised its Master Format numbers and titles for the Thinshell building industry, notes SteelCrete, Inc. founder Tony Ruiz, who collaborated with CSI on the project. Accordingly, 03 48 33 Precast Pre-Framed Concrete Panels is applicable to Thinshell building systems, produced since approximately 1980 when Ruiz first parlayed the composite action of light-gauge metal framing bonded to conventional thin (1_-in.) concrete by fashioning a metal stud with a flange wrinkle to facilitate embedment. That innovation was followed by MetalCrete, now SteelCrete [SC I, also known as Hi-Tech Tilt].
More recently, SteelCrete advanced its Thinshell technology with development of a proprietary SteelCrete III strip that snaps in place from behind the flange, allowing rebar to be passed through for strongest embedment. Required wire mesh reinforcement can be tied directly to this rebar or heavy-gauge wire prior to pour.
Initially, metal studs were cold-rolled with a deformed flange lip that was wet-embedded face down in concrete to produce tilt-up on site or precast off site. Track was added later to increase shear values and remains as standard practice today. Reinforcing mesh was wired to the deformed lip to maintain its proper placement in the thin concrete. Codes subsequently mandated a change from the wet-embed method to concrete placement around the metal framing between studs and track.
Soon after, a major equipment manufacturer developed an energy-efficient metal stud with web punch-outs resulting in a truss design that is stronger and lighter than conventional models and typically allows a reduction in gauge (metal thickness). In 1991, a new technology branded MetalCrete was offered, comprising a punched metal strip with embedment fingers to be screwed on metal stud webs and track flanges. The first-generation system not only relied on screws and friction to secure its concrete connection, but reinforcing mesh had to be chaired up from the casting surface, since the fingers were not compatible with mesh spacing.
Later refinements included a punched tab method (circa 1999) that entailed shipping standard metal stud and track product to a shop where a punch-press stamped out L- or J-Bolt-shaped, 6-in.-on-center tabs for embedment purposes. Modified studs and track then were shipped to the job site, where Û again Û tab spacing incompatible with the wire mesh required that it be chaired up. When the system’s inventor discovered a 50 percent strength increase resulting from closer tab spacing, he filed for a patent. And, a former partner designed a trailered punch-press for transport to the job site, allowing standard studs and track to be punched on location.
The advantages were short-lived, as 2003 saw the invention of a V-stamped strip Û similar to MetalCrete Û that enabled perforated Vs on 4-ft. strips to be inserted through the flange of prepunched or site-punched metal studs and track, snapping in place without need for fasteners. An inexpensive, portable punch press sufficed to punch the slots. For added strength to anchor reinforcing mesh, a heavy-gauge wire or small-diameter rebar can be passed through the perforations.
Current Thinshell technology provides less costly walls that lift and install faster, using lighter equipment, to create a building envelope with smaller footings, pour-back strip elimination, architectural flexibility, insulation without furring, and convenient utilities installation. SteelCrete products, Ruiz affirms, can be used to erect a building enclosure with load-bearing walls or curtain-wall components in conjunction with a structural frame. In either case, floors and roof also can be constructed of SteelCrete members fabricated off site or on (tilt-up style). Floor, roof or wall panels may be designed with any arrangement of stud sizes, gauges and spacing to accommodate different load requirements.
Additional features favoring Thinshell construction include its meeting and exceeding Seismic Zone IV requirements; engineering for hurricane force winds; resistance to fire, termites and mildew; lowest strength-to-weight-ratio concrete mix designs; and, LEED credits for recycled steel, plus low maintenance. SteelCrete adds the benefits of the strongest metal-to-concrete connection, Ruiz emphasizes; and, any brand stud and track product suits the system.