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 third in a series on antiquated structural systems that can be adapted or reanalyzed for safe reuse. —Editor

One-way and two-way clay-tile and unit-masonry joist systems — successors to one type of clay-tile arched floor system, and predecessors to the modern waffle slab — were used in the United States starting early in the 20th century.

In both clay-tile and masonry joist systems, the individual units were laid in such a way to form trenches that allowed reinforcing bars to be placed in the bottom of the resulting joist cross sections. This method of construction is very similar to the more recent pan joist system. However, unlike steel pans, the clay and masonry units were left in place for added strength, fireproofing, and to provide a flat ceiling surface.

The tile and masonry-unit joist systems were constructed as both one-way and two-way systems. Proprietary one-way floors included the Natcoflor and Republic Slagblock systems. Proprietary two-way floors included the Schuster, Smooth-Ceiling, Sandberg, and Republic Slagblock systems. All of the above systems employed regularly shaped units, available in a variety of sizes and depths, allowing a uniform modulation of joist sizes and spacings.

However, during the 1930s, a patented "wide-center" system was introduced for both one-way and two-way framing that allowed for wider clay-tile units to be placed within the center of the span and narrower units to be placed at the end of the span. This resulted in wider joists near the supports which in turn resulted in greater shear capacity at the end of the span. This system was similar to the more recent tapered-end pan joist system.   >>>

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SUBSCRIPTION SAMPLE The Marquette Building in St. Louis, Missouri — originally known as the Boatman's Bank Building — employs a concrete and clay-tile floor system. Photo: Benjamin Reed Extra Large Image A one-way concrete and hollow-clay-tile floor system. Photo: D. Matthew Stuart Extra Large Image   Click on thumbnail images to view full-size pictures.