Stability of Structures in regard to Wind-Pressure. 61 



extend is another point, regarding which information would 

 be of service to the engineer. 



In connection with the sustaining powers of the girders, 

 it may be recollected that the portion of the Tay Bridge 

 which fell was deprived of this support ; for its girders were 

 above the rail level, and disconnected with those of the 

 remainder of the bridge, which were below rail level. Had 

 the whole of the bridge been of the continuous girder type 

 it might not perhaps have stood, indeed, but it would have 

 been better able to resist the force of the gale which, 

 destroyed it. 



2. Lateral Weakness of Pier. — In this case the pier will 

 fail by each column turning upon its base independently. 

 The overturning moment may be determined in the same 

 way as for the first case. The resisting force is provided 

 chiefly by the bracing between the columns. When these 

 are sunk in the ground additional resistance will be obtained 

 from their strength as semi-beams; when they are bolted to 

 brickwork or masonry, from the strength of the fastenings. 

 In long columns, however, and especially in soft ground, such 

 resistance will be but slight, and as well as that due to the 

 attachments to masonry, might be neglected, &c ; but where 

 the piers are short, and the ground strong, this resistance 

 must receive due attention. 



It is usual to connect columns at intervals by somewhat 

 massive horizontal strutts. These are intended to give 

 the structure the necessary rigidity under passing loads. 

 These braces are usually of sufficient strength to keep the 

 columns perfectly parallel to one another, but to prevent 

 them turning each on its own base diagonal bracing is 

 requisite. Practice varies as to the design of this bracing. 

 The method most in vogue appears to be two tension bars 

 crossing in the centre of each square formed by the horizontal 

 strutts. 



This form is not the best, as from whichever side the^ 

 wind blows, one-half of the bars — those which slope down 

 from the wind — do no work, or, what is worse perhaps, are 

 exposed to compressive strains. 



Again, unless the tension-bars are fixed in position with 

 some initial strain, it is quite possible that a certain distortion 

 of the pier might occur before these tension-bars would exert 

 the necessary resisting force. 



A single diagonal strutt might be used where the distance 



