Stability of Structures in regard to Wind- Pressure. 59 



The maximum pressure for all railway bridges to be 56 lbs. 

 per square foot, but this to be multiplied by four as a 

 factor of safety ; where stability alone is concerned, to be 

 multiplied by two. 



The full pressure to be taken from the level of the rails 

 to the top of a train passing over, and the full pressure 

 for all the area of girder exposed below and above 

 this. 



For all the vertical surface of the leeward girder below 

 level of rails and above top of train the pressure to be taken 

 as follows : — If the area of the open spaces does not exceed 

 two-thirds of the whole area within outline of orirder, the 

 pressure to be taken at 28 lbs. per square foot. If such area 

 be between two-thirds and three-fourths, the pressure to be 

 taken at 42 lbs.; and if over three -fourths, to be taken in 

 full. 



The members of the commission were Messrs. Hawkshaw, 

 Barlow, Armstrong, Stokes, and Yolland. 



It is not stated whether the method adopted of determin- 

 ing the pressure upon the leeward giider is obtained from 

 the results of experiments or not, but it is evident that 

 without such data it is impossible to say in what manner 

 and to what extent the strength of the wind is interfered 

 with on passing through orifices. This question requires 

 further elucidation. 



In event of the spans being of greater length than the 

 passing train^ the area of exposed surface of the girders 

 should be taken all round the area presented by the train, 

 not only above and below. 



The effect of the wind-pressure upon the columns in low- 

 level bridges is not of great consequence. Where the piers 

 are high, however, it would not be advisable to neglect this 

 force ; and in determining it the area should be taken of all 

 the columns in the pier, even although they are placed in a 

 single line at right angles to the bridge, for a very slight 

 deviation of the wind from that line would expose all the 

 columns to its force. 



When a bridge of the construction named is destroyed by 

 wind-pressure, the failure is due to one of the following four 

 causes : — 



Instability of pier. 

 Lateral weakness of pier. 

 Instability of superstructure. 

 Lateral weakness of superstructure. 



