THE FORTH BRIDGE. 425 



drilled through the whole thickness of plates and angles after being put 

 together. 



Previous to the preparation of the designs and estimates, many con- 

 sultations were held with the Board of Trade officers with reference to 

 the maximum wind-pressure to be provided against, and the admissible 

 stress upon the metal. Existing rules limit the stress to 6^ tons per 

 square inch, and it was desired to get this limit extended to 7 J tons. This 

 was assented to by the Board, as the 6h tons working stress was based 

 upon the assumption of the steel having a minimum ultimate tensile 

 strength of 2G tons per square inch, whereas the Forth Bridge steel was to 

 have a strength of at least 30 tons. As regards wind-pressure, the present 

 Board of Trade provision of 56 lbs. per square foot has been adhered to, 

 and that pressure has been assumed to take effect upon a surface equiva- 

 lent to double the plane surface of the bridge, a deduction of 50 per cent, 

 being made in the instance of the cylindrical surfaces. Under the com- 

 bined action of the wind- pressure, estimated as above, and a rolling load 

 of two tons per foot run, or 3,400 tons on each span, the maximum stress 

 Avonld in no case exceed 7h tons per square inch, whilst upon the members 

 of the wind-bracing, subject to alternate compressive and tensile stresses, 

 it would not be greater than 5 tons per square inch. In ordinary work- 

 ing — that is to say, with heavy coal trains and light winds— the maximum 

 stresses would be about 6 tons in tension and 5 tons in compression, 

 which were about the same as the Saltash Bridge of 460-feet span would 

 be subject to under the same circumstances, and that bridge is of iron. 

 Even assuming that such a hurricane as 56 lbs. per square foot could ever 

 take effect over so large a surface as that offered by the 1,700-feet girders, 

 it was quite clear that no train could be on the bridge at the time, for a 

 pressure of 30 to 40 lbs. would certainly stop the progress of any train. 

 Without the rolling load the maximum stresses during the hurricane 

 would, however, be only about 4J tons in tension and 6 tons in compres- 

 sion. Indeed, if the Forth Bridge were made of iron insl^ead of steel, it 

 would be a relatively stronger structure than either the Britannia or Salt- 

 ash Bridge, so that the 50 per cent, extra strength dae to the adoption of 

 the steel may be regarded as an addition to the factor of safety, and not 

 as a necessity of the unprecedented length of span. 



Continental and American engineers at the present time almost univer- 

 sally take note of the vast differentie in the destructive effect of a live 

 load and a dead load ; but the Board of Trade entirely ignore this fact, 

 and adopt the Fame limiting stress in a main girder, where the greater 

 portion of the load may be dead, as in a cross girder, where it is prac- 

 tically all live, and where vibration is set up by every passing wdieel. It 

 is generally admitted — and the practice and experience of mechanical 

 eno-ineers confirm the conclusion — that metal of any class may be sub- 

 ject to a working stress twice as great under a dead as under a purely 

 live load. Some engineers make a compromise and take the ratio at 

 one and a half times. If this be done, and a factor of safety of three 

 be adopted in the instance of a purely dead load, the admissible working 

 stresses for iron having an ultimate strength of 20 tons per square inch 

 would be 6| tons for a dead load, 5^ tons for half dead and half live, 

 5 tons for quarter dead and three-quarters live (which is about the 

 proportion obtaining in railway girders of 100 feet span), 4^ tons for 

 all live load, and 2^ tons for members subject to alternate tension 

 and compression of equal intensity. With steel having a strength of 



