Editor’s note: In article published in the August, 2012 issue of Structural Engineer, Nicholas Mannix, P.E., senior structural engineer and project manager at Miller-Remick in Cherry Hill, N.J., detailed how a Philadelphia elementary school renovation took a detour after engineers found an archaic tile arch floor system and had to address the structural challenges it presented. D. Matthew Stuart, P.E., S.E., F.ASCE, SECB, MgtEng structural division manager at Pennoni Associates in Philadelphia, elaborates on the subject further with the following article.
Although the referenced article includes practical solutions to the challenge of constructing small mechanical openings in an existing clay tile arched floor system, there is a considerable amount of additional technical information concerning this type of antiquated structural framing, listed below:
1. Originally there were two types of clay tile arches: segmental and flat. The segmental system is constructed as an arc segment and functions like a traditional radiused arch, as opposed to the voussoir action of a flat or jack arched floor.
2. Flat arches were further subdivided into two groups; end construction and side construction. End construction consisted of laying the axis of the tiles’ hollow cells parallel to the direction of the span. Side construction consisted of laying the axis of the hollow cells perpendicular to the span. Combination side and end construction can also be encountered.
Hollow clay tile arch construction was originally developed as a lighter framing alternative to the popular solid brick arched floor construction that was used predominately in the 19th century.
3. If a single tile of an end-constructed flat arch was removed in a row, then the remaining tiles became unsupported unless the scored sides of the tile were mortared in with the adjacent rows of tiles. Because side-constructed arches (in which the scored sides of the tiles were placed adjacent to one another, transverse to the arch span) were more conducive to placing mortar between the tiles, this type of construction had an advantage over end construction.
4. A third type of flat clay tile arch was the reinforced system. This system was also referred to as the Natco “New York” reinforced flat arch and served as a precursor to one and two-way tile joist framing systems.
5. A fourth type of clay tile arch construction includes the Guastavino timbrel arch. However, this type of construction was not typically used in conjunction with steel floor framing.
6. Hollow clay tile arch construction was originally developed as a lighter framing alternative to the popular solid brick arched floor construction that was used predominately in the 19th century.
7. Typically tie rods were 3/4 inch in diameter and were spaced as required to resist the specific thrust of the given arch span, however, a minimum spacing of 15 times the width or eight times the depth of the supporting steel beam was recommended. Tie rods were generally placed approximately 3 inches from the bottom of the beams in a flat arch.
8. The total arch thrust, net area of the tie rods and maximum spacing for both a flat and segmental arch can be found as indicated below:
Total Thrust (in pounds) per Arch Panel: T = (3wD2/2R)L
Where; w = uniform dead + live load on arch in PSF
D = arch span in feet
R = effective rise of arch in inches (typically 2.4 inches less than the depth of the clay tile units for flat arches)
L = length of the floor beam supporting the arch in feet
The total net area of tie rods per panel (square inches): A = T/f
Where; f = allowable unit stress (typically 18,000 psi)
Maximum spacing of tie rods (feet): S = (af)/ (3wD2/2R)
Where; a = net area (square inches) of tie rod
|Rod Diameter||5/8 inch||3/4 inch||7/8 inch||1 inch|
|Net Area (a)||.202 in2||.302 in2||.420 in2||.550 in2|
9. The principal disadvantage of tile arch floor construction was the difficultly of adapting standard sizes to irregularly shaped spaces. In addition, tile arches are more easily weakened by holes and penetrations than a monolithic floor system. Clay tile arch construction was also susceptible to poor workmanship because the quality of the work could only be observed from the top and not from below during construction until after the formwork was removed.
10. Another issue that structural engineers need to be concerned about when dealing with clay tile arched floors is the fire rating that is provided by the system. While this is a relatively straightforward process with concrete structures where the fire rating can be determined based on the rebar cover and material properties of the concrete, this can be a more challenging undertaking with hollow clay tile and other similar systems. For these types of systems, in which virtually no historical data is available, it is possible to construct computer models that can very accurately determine the fire rating of any given type of construction. This approach to determining the fire rating of a structure is recognized by the NIST and has been readily accepted by most Fire and Building Code Officials because the computer modeling has been verified by actual tests conducted at nationally recognized testing labs. The computer modeling should be performed by a licensed fire engineer.
11. Tie rods were required only temporarily during the original construction to resist the outward thrust imposed by the arch on the steel beams at interior spans, but had to remain permanently in place at end spans. Nevertheless, tie rods at interior spans were typically left in place even though the thrust from adjacent interior spans served to offset the thrust of neighboring spans. Nevertheless, when modifying an existing building constructed with clay tile arches that involves the removal of an interior span or portion thereof, the capacity of the remaining adjacent span’s tie rods should be verified to assure that the equivalent end span conditions created on either side of the new opening will remain stable.
12. When large openings are installed, temporary shoring of the adjacent arch construction should be employed, particularly if the floor is an end constructed flat arch as opposed to a side constructed system.
The principal disadvantage of tile arch floor construction was the difficultly of adapting standard sizes to irregularly shaped spaces. In addition, tile arches are more easily weakened by holes and penetrations than a monolithic floor system.
13. The most reliable reference book for clay tile arch construction is the Principles of Tile Engineering Handbook of Design. The load tables included in this reference book include a safety factor of 7.