162. Combination Method

Combination Method. In this system, the object is to utilize the superior strength of the end method, and at the same time provide greater bearing surface against the beams. Whenever an arch with side-method skewbacks has been tested to failure, it has almost invariably been these blocks that broke first; hence, it is very necessary to have them extra strong, so as to equalize the strength of the different parts forming the arch. Fig. 79 represents a form of combination construction; the inside blocks a are made similarly to those shown in Fig. 78; b is the skewback, having a flange extended under and protecting the beam, as indicated at c. These skewbacks have failed in some tests, and in later constructions have been replaced by those represented in Fig. 78.

162 Combination Method 206

Fig. 79.

163. Another combination arch, which is considered very strong and durable, is represented in Fig. 80; at a is shown the strongly braced skewback, having three or more webs; at b, the inside blocks, laid end to end; at c, the side-method key; and at d, the beam protection, formed in two pieces, and separate from the skewbacks. The side and end surfaces of the blocks are corrugated, permitting the insertion of more mortar, and preventing the pieces slipping by each other.

162 Combination Method 207

Fig. 80.

164. Weight

Weight. The weight, of course, varies with the depth and span of the arch. The depth ranges from 6 to 12 inches in arches constructed on the side method, and from 6 to 15 inches in those built on the end method. For office and store buildings, a common depth is 10 inches, the floor-beams being spaced from 5 to 7 feet apart. It is preferable to have the blocks equal in depth to the beams, as it makes a lighter floor than when much concrete filling is necessary. The weights and safe spans of both dense and porous tiling are given in Table 4:

Table 4. Weights And Spans Of Tile Arches

DENSE TILE.

Depth in Inches.

Span in Feet.

Weight in

Pounds per

Square Foot.

6

3 1/2 - 4

22-29

7

4 - 4 1/2

27-32

8

4 1/2 - 5 1/2

30-35

9

5 -5 3/4

32-37

10

5 3/4 - 6 1/2

34-41

12

6 1/2 - 7 1/2

37-42

POROUS TILE - END METHOD.

Depth in

Inches.

Span in Feet.

Weight in

Pounds per

Square Foot.

6

3-5

21

7

3 1/2 - 5 1/2

24

8

4-6

27

9

4 1/2 - 6 1/2

30

10

5-7

33

12

6-8

37

15

71-10

43

The construction shown in Fig. 78 weighs less than those given here, ranging from 22 pounds per square foot in the 8-inch size to 35 pounds in the 15-inch size.