Art. YII. — Stability of Structures in regard to Wind- 

 Pressure. 



Paper 2. — Bridges. 

 By Fredk. a. Campbell, C.E. 



[Read lOth June, 1886.] ♦ 



Wind-pressure in connection with bridges assumes, for the 

 engineer, its most important aspect. This is especially the 

 case when the bridge is of large dimensions, and is used, for 

 railway purposes. Such a type will now be considered — 

 viz., a large iron railway bridge upon iron columns. 



Still following the method suggested in the first paper, it 

 will be necessary to fix the probable maximum for the 

 locality of the structures. As these are usually in exposed 

 situations, and as everything demands the highest state of 

 security, it would be wise to adopt a high maximum for the 

 purpose of calculation. 



From 50 to 70 lbs. per square foot, the latter where the 

 position is peculiarly exposed to high gales, would appear to 

 be suitable pressures to take for this purpose. 



The amount of force exerted upon one span of such a 

 bridge will depend upon the area of the surface of the 

 girders exposed, the area of the columns in the pier, the 

 vertical area of a train passing over, and the height of the 

 girders above the ground. 



If the girders be of web construction, the pressure can 

 readily be found ; but if, as will probably be the case in a 

 large structure, the girders l)e latticed, a little more difficulty 

 exists. 



After the failure of the Tay Railway Bridge a commission 

 of eminent engineers drew up a report upon the subject of 

 wind-pressure on bridges, and the following method was 

 recommended by them for determining the pressure upon 

 lattice girders : — 



