W. p. Iroichridge — Design for a Bridge. 267 



pendent of the pier. No deflections and no hurtful sliding move- 

 ments are therefore possible from change of temperature. In the sus- 

 pension bridge both of these consequences follow a change of tempe- 

 rature. To secure this important condition, perfectly, the deflections 

 of the stay-rods by their own weights are prevented by a system of 

 braces shown in sketch 2, the only object of which is to keep the 

 stay-rods in right lines, and thus preserve the true triangular struct- 

 ure. The weight of these supporting braces adds only about 12 tons 

 to each span of 600 feet. 



4. The lateral stability of the structure is provided for by diagonal 

 bracing between the horizontal chords and between the three iron 

 towers ; and also by light ties of wire rope between the stay- rods. 

 The above description refers to a half-span of 300 feet. To complete 

 the span another similar structure is erected on the opposite side, as 

 shown in sketch 1, the ends of the half chords at A not being joined 

 together., l)ut an opening of 4i incites being left for the free movement 

 from expansion. This opening is covered by the string pieces of the 

 road-way and by light slip joints along the sides, which act merely as 

 guards. 



To erect this bridge the opposite piers are first built up, during the 

 erection of which the materials for the superstructure are made ready. 

 These will be in duplicate, as the half spans are precisely similar. 

 When the piers are completed, the half spans are built by first erect- 

 ing about ten, twenty or thirty feet of the towers. Proportionate 

 lengths of the chords are then bviilt outward, overhanging the 

 river, and the suspension and stay-rods attached. Another sec- 

 tion of the tower is then built up, and a second section of the 

 chords added. By this process the successive sections may be 

 tested as the work progresses, and the lines of the structure perfectly 

 adjusted. The stay-rods are made in sections or parts, united by 

 screw turn buckles for this purpose, and thus the whole of a half 

 span may be built out until the two half spans meet. An important 

 feature in this process is that the strains encountered in the erection 

 are precisely those Avhich the structure will afterwards be subjected 

 to, and no abnormal strains are brought to bear by uniting the half 

 spans. 



It will be seen on inspection that the complete structure is analo- 

 gous to a combination of two large fixed derricks or cranes, exam- 

 ples of which have been so thoroughly tested in this country in the 

 use of the famous Bishop's Derrick, which has been subjected to the 

 most severe tests. In this bridge, however, there are arrangements 

 of detail which do not occur in any existing structure, as far as I can 



