continued

With the exception of the Smooth-Ceiling and Sandberg systems, the tile and unit-masonry joist systems could be constructed to span between steel beams, concrete beams, or load-bearing walls. In addition, most of the systems could be placed with or without a concrete topping. When a monolithic concrete topping was used, the thickness typically varied from 1.5 inches to 3 inches. Joists were typically analyzed as T-beam sections when a monolithic topping was used. With the exception of the Natcoflor system, joist widths typically varied from 4 to 6 inches.

Typically 0.75-inch clear cover was provided between the square or round deformed reinforcing bars and the adjacent tile or masonry units or the top and bottom of the exposed concrete surface of the joist. It was typical to use straight bottom bars and trussed top bars bent down to align with the bottom bars near the center of the span.

When a concrete topping was used it was also typical for temperature/ shrinkage reinforcement to be used orthogonal to the joist span. The amount of temperature steel provided was typically .0025 times the gross cross sectional area of the topping and was spaced at no more than 18 inches on center.

One-way systems were very efficient for spans over 12 feet and were used very frequently for spans up to 24 feet for loadings that ranged from 40 to 125 pounds per square foot and up to 18- and 20-foot spans for heavier loadings.

For two-way systems, and at the end of the span for one-way systems, it was common for the open webbed ends of clay tiles (or masonry units) to be filled with cardboard or metal inserts to prevent concrete from flowing into the voids in order to minimize the dead load of the slab.

Specifics of Proprietary Systems

The Natcoflor system used specially manufactured clay tiles with curved flanges that allowed only the bottom of the tiles to be exposed as the ceiling soffit. Other one- and two-way clay-tile systems could either be formed and cast with the bottom of the concrete joist exposed or with tile soffit pieces along the bottom of the trenches that resulted in a uniform tile ceiling soffit.

The Natcoflor joists were no more than 2 inches in width, spaced at 13 inches on center and varied in depth from 4 to 12 inches. The joists were typically cast using cement grout (one part cement to two-and-a-half parts sand). A composite concrete topping was not required above the tiles in order to obtain the maximum load-carrying capacity of the system.

The Schuster two-way system, which was patented in 1915, used clay tiles 12 by 12 inches and 16 by 16 inches, in depths of 4, 6, 8, 10, and 12 inches. The joists were typically spaced at 16 inches on center and 20 inches on center, respectively; however, tiles could be doubled up to allow for joist spacings of 28 and 30 inches on center. This two-way system was typically used in square bays or rectangular bays in which the longer span was not more than 50 percent greater than the shorter span.

The Republic Slagblok system could be installed in either a one-way or two-way configuration. The Slagblok unit measured 8 by 16 inches and came with one open and one closed end. Each unit was placed in combination with another Slagblok to form closed 16-by-16-inch cells. Slagbloks came in 3-, 4.5-, 6-, 7-, and 8-inch depths. The concrete ribs or joists were typically 4 inches in width and spaced at 20 inches on center. Typical spans for this system varied from 15 to 25 feet. I have seen similar one-way joist systems constructed as recently as the 1970s using regular concrete masonry units.

The Smooth-Ceiling system, which was patented in the 1930s, and the similar Sandberg system, both eliminated the need for beams or drop panels by employing embedded internal steel shear reinforcement around either structural steel or reinforced concrete columns. As with other two-way systems, standard tile units were placed in a modular layout in order to establish a uniform two-way grid of concrete joists. Typically both systems eliminated all tiles from around the column to enable the area around the top of the column to be cast as solid concrete.

Although load tables (which included considerable factors of safety) for most of the above systems were provided by the manufacturers for use in determining safe superimposed load-carrying capacities of various concrete joist systems, the actual design of the joists was accomplished using conventional working stress methods of analysis that were available at the time. Moment and shear coefficients were typically employed to establish the maximum positive and negative moment and end shear design envelopes.

However, continuous beam analysis was also used to establish the required design parameters. Moment and shear coefficients were also used for two-way analysis.

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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 March 2008 issue of STRUCTURE magazine, with permission of the publisher, the National Council of Structural Engineers Associations (NCSEA).

References

Kidder, Frank E., and Harry Parker. Architects' and Builders' Handbook, 18th ed. John Wiley & Sons, Inc., 1956.

Plummer, Harry C., and Edwin F. Wanner. Principles of Tile Engineering: Handbook of Design. Structural Clay Products Institute, 1947.

Stecich, John P. "Analysis and Testing of Archaic Floor Construction." Standards for Preservation and Rehabilitation, ASTM STP 1258. ASTM International, 1996. DOI: 10.1520/STP15437S; DOI: 10.1520/STP1258-EB.

SUBSCRIPTION SAMPLE Axonometric drawing of a Natcoflor one-way concrete and hollow-clay-tile floor system. Image: Courtesy Brick Institute of America Extra Large Image A two-way concrete and hollow-clay-tile floor system. Photo: D. Matthew Stuart Axonometric drawing of a typical two-way concrete and hollow-clay-tile floor system. Image: Courtesy Brick Institute of America Extra Large Image The two-way cast-in-place concrete waffle slab can be seen as the next step in a structural evolution that includes the two-way hollow-clay-tile and concrete-slab system. Photo: Scott Snider Extra Large Image Ralph Rapson's Southeast Branch of the Minneapolis Public Library explored the architectural possibilities of the two-way waffle slab. Photo: Kevin Matthews/ Artifice Images Extra Large Image The Yale University Art Center designed by Louis Kahn employs a triangular-waffle-slab floor system. Photo: Elizabeth Felicella   Click on thumbnail images to view full-size pictures.