A manual of the principles and practice of road-making: comprising the location, construction, and improvement of roads, (common, macadam, paved, plank, etc.) and rail-roads. By W. M. Gillespie ...

174 THE CONSTRUCTION OF ROADS
15 inches deep by 8 broad, and be placed at intervals of
about 2 feet.* The greatest weight which can come upon
them is when the surface of the bridge is covered with
men standing side by side, and is then equal to 120 lbs. per
square foot of surface, independently of the weight of the
materials. Recent experiments make this only 70 lbs.
This simple construction is only applicable to short
stretches. For spaces of greater width, supports from
the bottom of the opening may be placed at proper intervals. They may be piers of masonry, or upright props or
shores of timber, properly braced, and supported on piles,
if the foundation be insecure. They will divide the long
Fig. 89.
stretch into a number of shorter ones, and support the
ends of the timbers by which each of them is spanned.
But if the opening be deep, or occupied by a rapid
stream, it is very desirable to avoid the use of any such
obstructions. Means must therefore be devised for
strengthening the beams, so as to enable them to span
larger openings. This may be effected by supports from
below, or from above.
Of supports from below, the simplest are shorter tim
pers, (bolsters, or corbels) placed under the main ones
* Tredgold's Carpent y, p. 148. This gives a great surpli s of strength