Note that the picture above is not from the specifications for the Equitable Building. Rather, it’s from the National Fireproofing Company’s catalog, published just about the same time that the building was under construction. Drawings of that era typically did not provide much detail on terra cotta tile-arch floors. They were shown schematically in sections, and called out for the direction of span and the depth on plan. Here’s the entire spec description:
Floor Arches:
Floor arches shall be of the various thicknesses indicated by the drawings and unless otherwise shown shall be side or end construction flat tile arches. All floor arches shall be set so that the covering underneath the flanges of beams will in no case be less than two (2″) inches thick, and the arch skewbacks shall be so arranged as to closely fit the profile of beams, and to engage with the soffit protection of the beam flanges and securely hold it in place.
The form of construction employed shall be subject to the approval of the Architect, and in all cases the floors, after they are set in place, shall be capable of sustaining an equally distributed test load of 400 pounds to the square foot without deflecting.
Contractor must carefully examine the plans and various detail drawings to ascertain the arch thicknesses where special treatments are shown, necessitating varying thicknesses of arches, and shall carry out the work in accordance with details.
Three very short paragraphs, and the third is a just a statement of responsibility rather than having any technical meaning. Let’s start with some terminology clarification. Floor arches are not arches, they’re vaults. I’m going to go on the assumption that everyone involved knew the difference between 2D and 3D, so this is just a jargon problem. The earliest fireproof floor vaulting in New York used brick supported on iron beams; there were a few interesting but futile attempts to encourage the use of hollow terra cotta blocks (based on European precedents) in the mid-1800s in order to save on weight and span longer distances between beams. The field of hollow-terra-cotta vaulting developed very rapidly in the US after the Chicago fire of 1871 and the Boston fire the following year, and by the mid-1880s, various forms of terra cotta vault were in common use as “fireproof floors.” (For more on this topic, see The Structure of Skyscrapers.) The most common name for these floors was “tile arch floors” which made sense to those involved but can mislead people today as to what we’re talking about. The picture above shows a “side construction” tile arch, where the voids in the hollow blocks run parallel to the steel beams. In “end construction” the voids run perpendicular to the beams but the other details are similar.
The writers of the catalog were nice enough to include vocabulary diagrams:
The picture at the top is simply a section cut through the same floor as in the perspective drawing. The sides and bottoms of the blocks are corrugated to better hold mortar and plaster, which make them easy to see in the section. The “keys” at the bottom are familiar, as most people know what a keystone is. The “skew” is the end of the arch in contact with the steel beam support. The full name of the skews is “skewback blocks.” This is a carry-over of older masonry practice: in all-masonry construction, the end blocks on an arch or vault have to transition from the radial geometry of the arch to the straight geometry of the surrounding wall, and the end blocks literally have skewed backs. Here, the end blocks have vertical backs, to rest against the beam webs. The “protection lip” is one method of fireproofing the beams – I saw an old reference to those blocks as “lipped skews”, which is a great, if opaque, name – with the alternative being a small block of terra cotta that just covers the bottom flange (the “soffit protection” referred to in the spec) and is supported by the “skew[s] for soffit slabs.” You could, in theory, make a very short arch using just skews and keys, but usually you’d want a longer span, leading to the blocks between the skews and keys being called “lengtheners.”
Finally, the stated load capacity of 400 psf is, taken in isolation, ridiculous. Equitable is an office building, and even the 100 psf required by late-nineteenth-century building codes for that occupancy is too high. However, the New York City building department had been using 400 psf since the 1890s as part of a pass/fail fire-and-load test for floor systems. The result is that any tile arch floor that is not badly damaged (the most common cause of damage is people cutting holes in the tile because they don’t understand it is structural) is strong enough that it will never be the cause of failure in overload. The steel beams, which fail in a ductile manner, will be. So by making the arches too strong, which is not difficult to do, as short-span arches tend to be very strong, the brittleness of terra cotta is not a safety concern.
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