294 Improvement in the Construction of Bridges, &c. 
6th. String-plank are those pieces of which all the string-pieces 
are composed, and are half the thickness of the string-pieces, and 
are, in this case, 27 feet long. They break joints in their centres. 
7th. Floor-beams are those which rest upon the trusses, and 
support the flooring of the bridge. The size of them should be 
in suitable proportion to the distance between their bearings upon 
the trusses, or suspension-posts and trusses, when the former are 
introduced ; great depth in proportion to their thickness is im- 
portant. , 
Sth. Suspension-posts (see No. 1) are pieces 12 or 14 inches 
by 4 or S inches, firmly secured by tree-nails, locking, &c. to the 
top and bottom beams, and to each pair of principal rafters. They 
are introduced instead of a middle truss, for the support of the 
floor beams of a covered bridge, when the floor is on the bottom 
beams, and when the width of the bridge is so great that floor 
beams of sufficient strength cannot be procured. re 
9th. Floor-joists are pieces running lengthways of the bridge, 
and resting upon the top of the floor beams. They should be 43 
to 6 iiches square, according to the distance apart of the floor 
ams. ; 
10th. Floor-planks are the pieces resting on the floor-joists, of 
various widths, from 6 to 12 inches, and from 2$ to 4 inches 
thick, as may be required. In some instances, two thicknesses 
of planks are preferred; in which case, they cross each other at 
right angles or obliquely, and may be of less thickness, or such as 
their use may require. ; 
11th. Horizontal braces at the top are either framed in diago- 
nally, between the top beams, or consist of long planks of suitable 
dimensions, spiked and tree-nailed to the top of the beams, so aS 
to give the most secure support, to keep the trusses in a straight 
line at top. This is a very important support, and should be done 
in the most secure manner, in every respect. 
12th. Principal rafters are those which stand upon the top 
beams, and are thoroughly secured to them at their feet. They 
may be 10 to 12 inches deep, and 4 to 5 inches thick, according 
to their length, ‘ 
13th. Small rafters are those between the principal ones, to nail 
the roof-boarding to, for the purpose of shingling. 
14th. Floor-braces, or bottom-braces, are pieces about 44 by 
63 inches, as their length may require, framed in and keyed, be 
sme 
