332 Ohscrvatio.'cs on Sas-pension Bridges. [OcTc 



parts of 1" diameter and bolted together at the ends as sliewvi m the 

 plan. The suspension piers may of course be made of any material ac- 

 cording to circumstances — if of wood, it would be advisable to furnish 

 the top of the pier Vv'ith an iron plate, to prevent its destruction by the 

 friction of the chains. 



Tlie rnetiiod of fastening the ends of the chains will be seen by refer- 

 ence to the plans, but the strength of the masonry will of course depend 

 entirely on the nature of the ground, as also of the bridges, whether for 

 bandies or foot passengers only. The bolt connecting pins should be 2" 

 in diameter and the vertical suspending rods may be of |" iron 

 as the strain on them will be very slight. The method of fastening the 

 chains as shewn in fig. 5 is preferable to that shewn in fig. 4, as the 

 strain upon the connecting bolt will be more even — or perhaps that shewn 

 in fig. 7 will be the best, for should the chains stretch they will only 

 tighten the iron binding straps, and avoid the necessity of welding which 

 is very indifferently done by native workmen, unless well superintended. 



The platform consisting of 2" planking is laid upon and fastened to 

 crossbeams, 10 feet x i x i- supported by the vertical rods which pass 

 through them, and are secured with bolt pins of % inch diameter — a few 

 inches of earth may be laid on the planking. 



To determine the transverse strength for the wrought iron retaining bolts, 

 cube root of strain in lbs. x length of bolt in feet= cube root of 



' * -. — ~~ c^^^^ I'^ot of 5.9 = ii inches nearly. 

 1,600 ^ 



A good plan for the road-vray of a light bridge is (earth being very 

 heavy) to fasten down iron plates for the wheels to run on, as shewn in 



From numerous experiments made by Mr. Drewry and others, it has 

 been found that malleable iron bars will stretch slightly at 10 tons per 

 square inch. Although the stretching is almost imperceptible, however, 



9 tons per square inch is allowed as the strain that the chains of suspen- 

 gion bridges will bear continually without injury, and if made of wire, 



10 tons per square inch. I should say 12 or 14 tons may be safely al- 

 lowed for the chains of suspension bridges in this country. 



Estimate for Bridges. 



Allowing 30 feet for the length of each back stays, 

 then (7i X .00148 x 60) = .531 = 11S9.4 lbs. 

 Chain between piers = .101 = 2262.4 

 Vertical suspending rods = 51.36 lbs. at 



20-lbs. per rupee— 175 about 



