166. Hollow Blocks For Arches

These blocks are made of hard-burned fire-clay, hollow, of equal vertical thicknesses. The number of blocks varies according to size and distance between the iron beams. The two outer blocks are called skew-backs, and are made with a shoulder formed to fit the flange (Fig. 6, opposite), and extend three quarters to one inch below, completely covering the beam. The centre block is the key, and the intermediate blocks are so placed, when laid with cement, as to form a self-supporting arch. All are dovetailed, as shown in Fig. 5, when burnt, to receive the plastering.

Width of Span.

Depth of Arch.

Weight per square foot.

Safe Load in pounds per square foot.

3 ft. 6 in. to 4 ft.

6"

29 lbs.

1,000

4 ft. to 4 ft. 6 in.

7"

33 "

1,200

4 ft. 6 in. to 5 ft.

8"

37 "

1,400

5 ft. 6 in. to 6 ft.

9"

40 "

1,500

6 ft. o in. to 6 ft. 6 in.

10"

43 "

1,500

6 ft. 6 in. to 7 ft.

12"

48 "

1,800

167. Porous Terra Cotta

Porous Terra Cotta is a mixture of clay and sawdust; or any other combustible matter may be substituted, such as shavings, tanbark, and charcoal. After the compound is properly mixed the blocks are moulded and, when sufficiently dry, placed in a kiln prepared for the purpose, and subjected to an intense heat adequate to consume all the combustible matter, leaving the blocks porous. For hanging ceilings, etc., these blocks are made in different sizes: 25 inches between centres is a good spacing of T irons, the blocks being 24 inches wide and allowance being made for cement.

167 Porous Terra Cotta images/ArchitecturalIronAndSteel01 63

Blocks 2 inches thick weigh 11 pounds per square foot; 3 inches thick, 15 pounds. Fig. 3 shows blocks as flat arches. Fig. 5 is a segment arch with a space allowed for filling of concrete or other incombustible material, and timbers to which flooring is to be nailed.

Fig. 4 is a column protected by a casing of ribbed blocks, with an air space between. The blocks are dovetailed on the outside for holding the plastering, and set with cement and firmly secured with copper wire bound on the outside.

Partitions of hollow clay

3" thick, 14 lbs.

per sq. ft.

" " "

4" " 18 1/2 "

" "

" " "

5" " 23 "

" "

" " "

6" " 25 "

" "

" " "

7" " 31 "

" "

" " "

8" " 34 "

" "

Partitions of porous terra cotta

3" " 12 "

" "

" " " "

4" " 17 "

" "

" " " "

5" " 23 "

" "

" " " "

6" " 27 "

" "

" " " "

7" " 31 "

" "

" " " "

8" " 36 "

" "

Furring, Roofing And Ceiling

Hollow clay furring

2" thick, 12 lbs.

per sq. ft.

Porous terra-cotta furring

2" " 8 "

" "

" " " roofing

2" " 12 "

" "

" " " "

3" " 16 "

" "

" " " ceiling

2" " 11 "

" "

" " " "

3" " 15 "

" "