294 Improvement in the Construction of Bridges, Sfc. 



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. 



8th. Suspension-posts (see No. 1) are pieces 12 or 14 inches 

 by 4 or 5 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. 



9th. Floor-joists are pieces running lengthways of the bridge, 

 and resting upon the top of the floor beams. They should be 4 J 

 to 6 inches square, according to the distance apart of the floor 

 beams. 



lOih. Floor-planks are the pieces resting on the floor-joists, of 

 various widths, from 6 to 12 inches, and from 2 J 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 4^ by 

 6^ inches, as their length may require, framed in and keyed, be- 



