Precast Concrete Framing Systems
by D. Matthew Stuart
Focusing on structural engineering issues involved in the repair, restoration, or adaptive reuse of older buildings for which drawings no longer exist, this article is the fifth in a series about antiquated structural systems that can be adapted or reanalyzed for safe reuse. —Editor
As previously discussed, many contractors during the early-to-mid-1900s prefabricated modular clay-tile and masonry units offsite into beams and slabs that could be delivered to the job site. This method of construction ultimately progressed to solid precast concrete units.
In the 1950s, one of the most prevalent precast concrete systems in general use was the F&A System. This system used conventionally reinforced precast concrete inverted T-joists spaced at 28 inches on center, which supported concrete block filler slabs. The entire assembly then received a 2-inch, cast-in-place concrete topping, which acted compositely with the precast joists. Prior to the F&A System, Peter Rutten developed and patented a similar system in the 1930s
The F&A precast joists were available in depths of 6, 8, 10, and 12 inches and were capable of spanning anywhere from 6 feet to 36 feet for load capacities from 30 to 900 pounds per square foot, depending on the span, depth of joist, and reinforcement. The ends of the precast joists could be cast integral with a site-cast concrete beam or bear directly on either precast concrete girders or steel beams. The F&A System included filler blocks that could be either placed flush with the bottom of the joist or recessed at the same level as the bearing ledge of the joist.
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This article is reprinted from the September 2008 issue of STRUCTURE magazine, with permission of the publisher, the National Council of Structural Engineers Associations (NCSEA).