268 W. P. Trowhridge — Design for a Bridge. 



learn. The roadway is supported upon light trussed beams thrown 

 across between the chords about nine feet apart. These beams have 

 a depth of four feet, and a span of about 23 feet, and are built of T 

 and angle iron. Upon these the string pieces of the roadways are 

 laid. It is unnecessary to describe the manner of building the ap- 

 proaches, as they are independent of the spans. To recapitulate the 

 advantages of this construction. They are — 



1. Simplicity. 



2. The avoidance of cross strains in all pieces of the bridge. 



3. Freedom of expansion and contraction from change of tempe- 

 rature, by which deflections and sliiing motions are avoided. 



4. Stability and strength, with the least amount of material. 



5. Freedom from vertical oscillations from passing loads or high 

 winds. 



6. Lateral stiffness from the horizontal diagonal bracing of the 

 towers and chords. 



7. The distribution of strains among a large number of stay-rods, 

 and the parallelism and independent connections of these rods. 



8. The avoidance of separate anchoring abutments distinct from 

 the piers. 



9. Facility of construction and facilities for testing the strength as 

 the work progresses. 



An application of this system of construction might be made with 

 great advantage at the crossing of the Niagara River, where the pres- 

 ent suspension bridge is built. Sketch 3 represents the valley or 

 gorge of the Niagara River at this point spanned by such a struc- 

 ture. The present suspension bridge is thrown across between two 

 points of the crest B B, distant from each other about 800 feet, while 

 the width of the river at the water level, 250 feet below, is only 382 

 feet. If from the water's edge piers were erected of masonry to the 

 height of the crests, on each side, giving the proper batter, these piers 

 would be about 400 feet apart at the top. This is not a long span. 

 The longest tube of the Menai bridge is 460 feet, and trains cross that 

 at full speed. It would be very easy to construct each half span 

 from A to B on the land, in the prolongation of the bridge, and when 

 these half spans should be completed to push them out until they 

 should meet in mid-channel ; then to unite them firmly as a single 

 girder. This girder might have the tubular form, and the bridge 

 would then possess all the elements of strength and stiffness of the 

 Menai bridge, with the additional security of the counterbalanced 

 half spans. 



