80
MODERN GASWORKS PRACTICE
brickwork required for filling up the spandrils, etc., may be of ordinary stock-work or, if preferred, old firebricks well cleaned. A reveal (as shown in Fig. 29) should be left in the pier walls and arches so that a tight joint is obtained when the front wall goes up.
The semicircular main arch. is the ideal, as it accounts for practically no hori-
zontal thrust on the side walls.. With settings of two vertical rows, the semicircle should alwaysbe em-ployed ; but the greater width neces-saiy with three or four vertical rows makes the use of such an arcli somewhat inconvenient. In such, cases elliptical or segmental arclies-are often. erected, as in this way excessive headroom above tlie top tier of retorts is avoided. Gare must
Fig. 29.—Main Arches tob Retobt Settings. The blacked foetion indicatbs fibebrick.
be taken, however, to see that
decidedly flat arch.es, liable to failure, are abstained from. A generally recog-nized rule is to the cffect that no arch should have a radius of more than 7 feet 6
inches. Many designers will hear of nothing eise but a semicircular arch, whatever the type of setting. It is maintained that the excessive headroom does not matter in the least, and, if necessary, can be tiled in and isolated.
All types of arches are generally built from concentric rings of brickwork,
Fig. 31.—Support for Soffit of Main Arch.
although a growing (but more expensive) practice is to employ special radius blocks. When the brick arch. is used, however, there should never be less than three rings, as it is almost impossible to prevent some sagging of at least the lowest ring after the arch. has been in operation for some time. To overcome this drawback the barrel-arch with a camber in both directions (Fig. 30) has been introduced with marked success, and in times such as the present, when firebricks have reached an abnormal