A well-known arrangement for forming beams of greater strength than could be obtained by single pieces of timber was adopted by Mr. Brunel after a careful investigation of its merits. This arrangement consists in joining together two beams of timber placed one above the other, by means of bolts and joggles, so as to form a beam nearly equivalent in strength to a single piece of timber of the same depth as the two pieces united.[86] By this plan, the length which could be spanned by simple beams, without the introduction of trussed framework, was nearly doubled.
The distance between the piers of railway bridges is generally too great to allow of the superstructure being constructed of simple beams, and in such cases Mr. Brunel adopted forms of framing similar in the arrangement of their parts to the common designs of king and queen trusses employed in roofs.
One of Mr. Brunel’s early timber viaducts was that erected in 1842 at Stonehouse, on the Bristol and Gloucester Railway. It consisted of a series of five openings of queen trusses 50 feet span, resting on piers formed of timber trestles.
In the Bourne viaduct, across the Stroudwater Canal, on the Cheltenham and Great Western Union Railway, there was a span of 66 feet, with three timber trusses, for the two lines of way. Each of these trusses may be described as a king truss with an internal queen truss. The inclined timbers or principals rested in iron shoes upon the piers, and were connected together by bolts and joggles.
The upper horizontal or collar beam of the queen truss carried the roadway planking, which was continued upon beams supported by the principals. The timbers carrying the roadway received support from struts radiating from the feet of the queen posts, which were connected with the apex of the king truss by iron ties. The horizontal tie bars were of wrought iron. The arrangement of the truss is shown in the woodcut (fig. 2).
The side openings consisted of four spans of 30 feet, with trusses of the Stonehouse viaduct type, of one span of 25 feet and ten spans of 20 feet, with double beams.
The St. Mary’s viaduct, across the canal in the Stroud Valley, was constructed with one span of 74 feet, with trusses similar to those at the Bourne viaduct.[87]
In the year 1846 Mr. Brunel made an elaborate series of experiments on the strength of large timber. Some account of these is given in the note to this chapter.