'60 Siahiliiy of Structures in regard to Wind- Pressure. 



1. Instability of Pier. — In this case, whilst the various 

 parts maintain their respective positions, the whole pier 

 overturns upon the base of the leeward column. 



The overturning moment is the pressure upon the girders, 

 upon a passing train, and upon the piers, multiplied by the 

 distance of the centres of pressure from the foot of the 

 leeward column. The moment of stability is the weight of 

 the portion of the bridge considered, superstructure and 

 piers, and the weight of the passing train, multiplied by the 

 distance at which it acts from the leeward column — that is, 

 half the width of the base of pier. The point in the 

 leeward column to which the width should be measured^ lies 

 somewhere between the centre and the outside of that 

 •column. It would probably be advisable, however, in all 

 calculations for the purpose of determining the width of 

 pier in any proposed structure, to take the measurement to 

 the centre of the column, except where these are of great 

 dimensions compared to the width of pier. 



In connection with the stability of the pier, no allowance 

 need be made for factor of safety; there can be no imperfec- 

 tion of workmanship in the power of gravitation, nor any 

 weakening of material by successive strains. If 60 lbs., 

 then, be considered as the greatest wind-pressure the bridge 

 will be called upon to bear, there is no necessity to make the 

 pier stable under a pressure of 120 lbs. or 180 lbs. There 

 •are, however, other forces tending to keep the pier in 

 position, of which no mention has yet been made, and 

 which might act as a set-off to the chance, which is always 

 within the range of possibility, of the structure meeting a 

 gale of greater force than that for which it has been designed. 

 These forces, though not as a rule calculable, are un- 

 doubtedly of considerable extent, varying according to the 

 size and construction of the bridge, and the nature of the 

 ground in which the piers have been sunk. First, there is 

 the hold which the columns on the windward side -of the 

 centre have upon the ground, or if bolted to brickwork 

 or masonry, the strength of such attachments; and, 

 second, there is the sustaining power of the girders acting 

 as a tie throughout the length of the bridge, for in a 

 structure of some length those portions struck by the 

 most violent gusts will be supported by the whole of 

 the structure experiencing a force less than that necessary 

 to overturn it. 



The area over which the very high pressures of wind 



