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Precast Concrete Framing Systems | |
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Precast Concrete Framing Systems Similar precast concrete systems that were in use during the same approximate time period included Tee joists and Keystone joists. Tee joists came in depths of 16 and 20 inches and were typically prestressed. Keystone joists were available in 8- and 12-inch depths and could be either conventionally reinforced or prestressed. Tee joists ultimately evolved into Single Tees, Quad Tees, and the Double Tee member that is still in common use today. Channel Slabs were another prevalent precast concrete member in the 1950s. These were typically used for roof construction between supporting precast beams or steel members. The slabs were typically 24 inches wide and one inch thick, with 3.5-inch-deep, 2-inch-wide, down-turned edge "flanges." This product was capable of spanning up to 9 feet and supporting up to 60 pounds per square foot of superimposed load. Examples of other proprietary precast systems that are no longer in use include: Gypsteel Floor and Ceiling Slabs The floor slabs of this system consisted of 24-inch-wide, 2.5-inch-thick molded precast gypsum, reinforced with cold-drawn wires that projected from the coped or rabbeted bearing ends of the panels. The wires were twisted together with the adjacent panel end and the slot was then filled with grout for a smooth top finish. These slabs were premanufactured to span both 24 inches and 30 inches between steel support framing members. Ceiling slabs were 24-inch-wide, 2-inch-thick molded precast gypsum, reinforced with flat steel bars that projected from the ends of the panels to act in conjunction with hangers suspended from the top flange of the supporting steel framing. The Gypsteel system was manufactured in New Jersey and used extensively in New York City. Waite's Concrete I Beams This system was used in a number of buildings constructed by The Standard Concrete Steel Company of New York City. The floor framing system consisted of precast concrete I-beams spaced at approximately 18 inches on center, of either 10-inch or 12-inch depth, which were supported from the bottom flange of steel beams that were spaced 5 to 7 feet apart. A field-cast concrete topping was then placed on top of the I-beams, while the spaces between the lower flanges were infilled as well to provide a flat ceiling surface. Watson Reinforced Concrete Floor System This type of precast construction was installed by the Unit Construction Company of St. Louis, Missouri, and included two types of framing. The first configuration was intended for long spans and heavy loads and involved the use of precast T-sections placed side by side. The T-sections were supported by steel beams that were encased in concrete. For shorter spans (less than 20 feet) and loads of 200 pounds per square foot or less, precast beams spaced at 5 feet on center were used to support precast channel slabs. The precast beams were in turn supported by steel beams encased in concrete. Miller Precast System This system was devised by the Precast Floors Corporation of New York in 1929. The precast units were shipped in three separate segments, which were aligned and supported on temporary shoring at the job site. Projecting reinforcement at the interior ends of the segments was embedded in a dry mix concrete, which was used to fill in the 9-to-10.5-inch-long gaps between the segments. The center segment was produced in a standard fixed length, while the end segments were produced in varying lengths to allow for adjustment to accommodate different span lengths. Negative moment reinforcement was then embedded in a field-cast topping for continuity across the supporting steel beams. The voided, 12-inch-wide, box-shaped units were produced in 6-, 8- and 10-inch depths. Lith-I-Bar System This system was developed in Michigan and involved a dry-mix, lightweight concrete that was placed in an I-shaped cross-section mold and compacted with cast-iron rollers. The units were typically spaced at 18 or 24 inches on center, with a 2-inch field topping cast on either removable or stay-in-place metal lath formwork for composite action with the members. Porete Floor System This system was manufactured in New Jersey and consisted of precast hollow formed units of 4 to 6 feet in length. This system was similar to the Miller system in that units were aligned and supported on temporary shoring at the job site. The gap between the abutting aligned units and the continuous pocket along the sides of each unit, in which field-positioned bottom reinforcement was first placed, were then filled in with a mortar/ grout. All of the units were closed at each end to prevent the mortar/ grout from flowing into the hollow voids of the precast member. >>> Discuss this article in the Architecture Forum...
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