330 



Observations on Suspension Bridges. 



[Oct. 



a bridge of 90 feet span and 8 feet breadth of road-way, and the strength 

 proportioned to the greatest weight that could be brought on the bridge 

 at one time, i. e. supposing it to be crowded with people, and allowing 2 

 square feet of platform for each person, or 70-lbs. per square foot. This is 

 far beyond the average weight of people in this country, and the bridge 

 is never likely to be so laden. 



Calculation for an Iron Suspension Bridge. 



Span 90 feet — deflection of main-chain l-I5th of span = 6 feet then 



length of curve chain 



2 2 



* square root of (deflec. x ^leflec.) ^ semi-chord square root of 2089 



3 



= 45.7 X 2 = 91.4 feet. 

 Tension at suspension piers. 



. 5 



square root of (semi-cord^ 4- ^ deflec. ) x suspd> wt. 24 square 



4 deflection, 

 root of 2169 = 1.93 times wt. 



Sine of the angle of direction of the chains. 



2 deflec. _ 12 



Square root of (2 deflec 2-j- semi-chord) ^~ square root of 2169 = 

 .257 and cosine = 0.966. 



Do. of back stays do. do. do. nearly. 



Tension at the middle in parts of suspended weight 



tension of extremity x cos. of angle of direction = 1.864. 

 Suspended Weight. 



Planking for road- way 



90' X 8' X ^ = 120 cub. feet at 40-lb. per cub. foot = 4800 

 earth for road-way * 



90' X 8' X i = 180 do. at 120 do. = 21.600 

 bridge crowded with people. 



90' X 8 X = 720 square feet at 70 do. = 50.400 

 15 cross beams to support planks. 



each 10' X 1 ^ i. = 8| cub. feet at 40 do. == 333.3 

 30,2 inch diameter vertical suspending rods mean length 3 feet. 

 .7854 



90 X -24^^ =107 square feet at 480 do. =51.36 



77184.66 or 



say 34 tons for the suspended weight exclusive of the main chains then 



weight exclusive of main chain x tension due to deflec. 34 x 1.93 



. = 7.5 



9 (length of chains x 0.00148 x tension 91.4 x 0.00148 x 1.93) 



square inches of section for main chains. 



The weight of 7.5 square inches of iron 91 .4 feet 

 long = (7.5 X 00148 x 91.4) = 



* See Drewry on Suspension Bridges. 



