There are several styles of combination arches now manufactured. The object in making this shape of arch is to obtain the strength of the endmethod construction and at the same time get a flat bearing for the skewbacks. In order, however, to develop the full strength of the interior blocks the skewbacks should be made very strong and with several partitions, as they are generally the weakest portion of the arch.
Fig. 181 illustrates the "Excelsior" dense tile arch made by Henry Maurer & Son. This arch was patented by Mr. E. V. Johnson, formerly general manager of the Pioneer Company, and was formerly made also by that company. The shape of the interior blocks undoubtedly gives great strength with the minimum amount of material.
This arch has been quite extensively used in Chicago and also in Eastern cities, and apparently has given general satisfaction.
An end-method, dense tile flat arch, with side-method skewbacks,. is also made by the Empire Fireproofing Company. The interior blocks have vertical and horizontal partitions similar to the Lee tile, but the sides of the tile, instead of being a true plane, have an offset at the middle of the tile, so that one course laps over the other, thereby preventing any possibility of the tiles slipping down, which sometimes occurs in the ordinary end-method arch.
Fig. 182 shows a triple web combination flat arch made by the Hocking Clay Manufacturing Company and also their patent flange cover for beams.
The depth of arch most frequently used for office buildings and retail stores is 10 inches, the girders being spaced so as to use 10-inch steel floor beams spaced from 5 to 6 feet apart. As a rule the depth of the arch should be about equal to the depth of the beam, as it is just about as cheap and much better construction to use deeper tiling and less concrete filling.
The following tables give the published weights and safe span for both dense and porous tiling :
Depth of Arch.
Span between Beams.
Weight per sq. ft.
3 feet 6 inches to 4 feet.
22 - 29 pounds.
4 feet to 4 feet 6 inches.
27 - 32 pounds.
4 feet 6 inches to 5 feet 6 inches.
30 - 35 pounds.
5 feet to 5 feet 9 inches.
32 - 37 pounds.
5 feet 9 inches to 6 feet 6 inches.
34 - 41 pounds.
6 feet 6 inches to 7 feet 6 inches.
37 - 38 pounds.
POROUS TILE - END METHOD.
3 feet to 5 feet.
3 feet 6 inches to 5 feet 6 inches.
4 feet to 6 feet.
4 feet 6 inches to 6 feet 6 inches.
5 feet to 7 feet.
6 feet to 8 feet.
7 feet 6 inches to 10 feet.
The weight of the Pioneer Company's transverse arches (Fig. 180), as given by the manufacturers, is as follows:
DEPTH OF ARCH.
WEIGHT PER SQ. FT.
DEPTH OF ARCH.
WEIGHT PER SQ. FT.
From a few tests of the weight of blocks, as they were being delivered at the building, the author is inclined to believe that the actual weights of both dense and hollow tile will generally run at least 10 per cent, over those given in manufacturers' catalogues. (See Section 312.)
The strength of hollow tile floors can only be determined by actual experiment.
At the tests made at Denver,* December, 1890, two 10-inch dense tile arches (5-foot span), with one horizontal web and built on the side method, failed under distributed loads of 271 and 428 pounds per square foot, respectively. A porous tile end-method arch, 10 inches deep, with two horizontal webs, sustained 757 pounds per square foot for two hours without breaking.
* See full account in American Architect and Building News, March 28, 1891.
Tests made at Richmond, Va., in 1891 of 6-inch and 12-inch side-method arches made by the Empire Fireproofing Company, showed a variation of from 288 to 579 pounds per square foot for the 6-inch arches and from 554 to 1,057 pounds per square foot for the 12-inch arches, the average strength of the nine 12-inch arches being 858 pounds per square foot.
The Pioneer Company describe a test of a 15-inch flat arch similar to that shown in Fig. 181, in which the arch sustained 3,287 pounds per square foot (over an area 4x4 feet) before breaking.
The average breaking weight of five arches of 10-inch tile, with spans varying from 4 feet 11 inches to 5 feet 6 inches, tested by the Metropolitan Company, was 519 pounds per square foot.
It is generally considered by engineers that a tile arch should not fail under a load less than five times that which it is intended to carry. Arches of the types shown in Figs. 176-182, inclusive, if properly set and built of sound blocks, should be abundantly safe for office floors and light stores when proportioned according to the table. The instances where tile arches have failed when in actual use are very few indeed.
The cost of hollow tile arches of either kind, set in place ready for plastering in lots of 20,000 square feet, ranges from 14 cents to 25 cents per square foot, according to size and weight of the tile. In Chicago the average price is 20 cents.