146 Mr. Benjamin Baker [May 20, 



Superstructure. 



I must now say a few words respecting the design, manufacture, 

 and erection of the superstructure. 



Design. — I have already illustrated the principle of the canti- 

 lever bridge, and need only deal with the details. At the Forth, 

 owing to the unprecedented span and the weight of the structure 

 itself, the dead load is far in excess of any number of railway trains 

 w^hich could be brought upon it. Thus the weight of one of the 

 1700 feet spans is about 16,000 tons, and the heaviest rolling load 

 would not be more than a couple of coal trains w^eighing say 800 

 tons together, or only 5 per cent, of the dead weight. It is hardly 

 necessary therefore to say that the bridge will be as stiff as a rock 

 under the passage of a train. Wind even is a more important element 

 than train weight, as with the assumed pressure of 56 lbs. per square 

 foot the estimated lateral pressure on each 1700 ft. span is 2000 

 tons, or two and a half times as much as the rolling load. To 

 resist wind the structure is " straddle legged," that is, the lofty 

 columns over the piers are 120 ft. apart at the base, and 33 ft. at 

 the top. Similarly, the cantilever bottom members widen out at 

 the piers. All of the main compression members are tubes, because 

 that is the form which with the least weight gives the greatest 

 strength. The tube of the cantilever is, at the piers, 12 ft. in 

 diameter and 11 in. thick, and it is subject to an end pressure of 

 2282 tons from the dead load, 1022 tons from the trains, and 2920 

 tons from the wind ; total, 6224 tons, which is the weight of one 

 of the largest transatlantic steamers with all her cargo on board. 

 The vertical tube is 343 ft. high, 12 ft. in diameter, and about % in. 

 thick, and is liable to a load of 8279 tons. The tension members 

 are of lattice construction, and the heaviest-stressed one is subject 

 to a pull of 3794 tons. All of the structure is thoroughly braced 

 together by " wind bracing " of lattice girders, so that a hurricane 

 or cyclone storm may blow in any direction up or down the Forth 

 v^ithout affecting the stability of the bridge. Indeed, even if a 

 hurricane were blowing up one side of the Forth and down the 

 other, tending to rotate the cantilevers on the piers, the bridge has 

 the strength to resist such a contingency. We have had wind 

 gauges on Inch Garvie since the commencement of the works, and 

 know, therefore, the character of the storms the bridge will encounter. 

 The two heaviest gales were on December 12th, 1883, and January 

 26th, 1884. On the latter occasion much damage was done through- 

 out the country. At Inch Garvie the small fixed gauge w\as reported 

 to have registered 65 lbs. per square foot, but I found on inspection 

 that the pointer could not travel further, or it might have indicated 

 even higher. I did not believe this result, and attributed it to the 

 joint action of the momentum of the instrument, and a high local 

 pressure of wind too instantaneous in duration to take effect upon 



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