Prefab Clay-Tile and Concrete-Block Framing Systems
The one-way slab system involved prefabricated slab units that were placed directly adjacent to each other, spanning between previously constructed load-bearing walls, steel beams, or joists.
Both the beam and slab systems included a site-cast concrete topping, which was poured over the beams and filler tiles or one-way slabs. The prefabricated beam-and-slab systems offered the advantage of not having to construct supporting formwork before the framing could be erected; however, shoring in the center of the span was sometimes employed to increase the clear span capability of the members through composite action with the site-cast topping.
Examples of the clay-tile systems included the "T" Beam Floor, the "U" Beam System, the Joistile System, the Sheffield Floor System, the Adel Joistile System, the Kalex Floor System, the United Floor System, and the Tilecrete Floor System.
Some of these systems required filling the joints between the adjacent ends of the clay tile units with mortar, while other systems allowed the ends of adjacent tiles to butt up against each other. All of the prefabricated beam-and-slab systems, except for the infill tiles, included internal longitudinal flexural reinforcement for positive moment resistance. Negative moment reinforcement for continuity across a supporting wall, beam, or joist was also sometimes placed in the site-cast topping. None of the units included shear reinforcement.
Similar prefabricated beam-and-slab systems were also developed from modular concrete block. Most of these systems used conventional internal reinforcement for flexural strength; however, a few were developed using prestressed bars and strands.
Probably the most widely used masonry block product in the eastern United States during the 1950s was the Dox Plank system developed by NABCO in Washington, D.C. This product was manufactured with recessed slots in the bottom of the block to allow for the flexural reinforcement to be grouted into the bottom of the plank. There was no mortar required between the adjacent ends of each block.
Other cross-sectional variations of the Dox Plank were developed by members of the Dox Plank Manufacturers Association. In one case, the internal reinforcement was completely encapsulated by the block by means of a continuous sleeve. It is not clear whether the continuous reinforcement in the sleeves was grouted in place, or if the bars were threaded at each end of the plank so that the modules could be precompressed together via tensioning of the bar as it was tightened against each end of the member using a nut.
Flexicore, a product similar to the NABCO Dox Plank, was also available in the 1950s. A hollow-core plank is still manufactured today under this same name; however, the current product is a true precast, prestressed concrete member.
In the 1950s, the consulting firm of Bryan and Dozier and the Nashville Breeko Block Company designed and constructed prefabricated, post-tensioned concrete block beams. This method of construction resulted in the first linear prestressed structure to be built in the United States (the Fayetteville Tennessee High School Stadium) and the first prestressed bridge to be built in the United States (in Madison County, Tennessee). This method of construction was made practical and economical by the Roebling Company through the development of high-quality tendons that could be bonded without expensive end anchorages.
The Breeko Block system was further refined through the use of external, deflected tendons. However, by the late 1950s, this system was replaced by precast, pretensioned concrete members.
Other, more obscure examples of prefabricated modular concrete block beams and slabs include a prestressed bar system developed by P.H. Jackson of California in 1872; a prestressed wire system developed by C.W. Doering in 1888; a system patented by K.E.W. Jagdmann in 1919; a horizontally draped stressed reinforcement system patented by Albert Stewing and Stefan Polonyi in 1967; and a tensioned, Y-shaped block system patented by Hossein Azimi in 1987.
All of the above tile and concrete block systems were designed based on the basic reinforced masonry and concrete beam analysis theories of their era. Load tables were also commonly developed and published by most of the manufacturers.
The problem with all of the above systems, when one encounters them in a building, is that in the absence of existing drawings, it is difficult to determine the internal reinforcement and, subsequently, the load-carrying capacity of the system. However, it is hoped that this article, by identifying the many different types of products that were in use at one time or another, will facilitate research of antiquated or archaic systems when they are encountered in existing structures.
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D. Matthew Stuart, P.E., S.E., F.ASCE, SECB, is a licensed structural engineer in 20 states. He currently works as a senior project manager at the main office of CMX, located in New Jersey, and also serves as an adjunct professor for the master's of structural engineering program at Lehigh University in Bethlehem, Pennsylvania. More by D. Matthew Stuart
This article is reprinted from the June 2008 issue of STRUCTURE magazine, with permission of the publisher, the National Council of Structural Engineers Associations (NCSEA).
Bryan, Ross H. "Prestressed Concrete Innovations in Tennessee." PCI Journal, January-February 1979.
NABCO Plank Company. Dox Plank for High Speed Floor and Roof Construction. Publication date unknown. Made available by the National Concrete Masonry Association (NCMA), Accession No. TF02657.
Plummer, Harry C., and Edwin F. Wanner. Principles of Tile Engineering: Handbook of Design. Structural Clay Products Institute, 1947.