Lateral Stability of the Victoria-street' Bridge. 283 



4. It was proposed to increase the width of the piers 

 threefold in order to give the requisite resistance to wind 

 pressure. 



In conclusion, the case of the Tay Bridge was cited as an 

 example confirming the preceding recommendation. 



Let us consider the above investigation in detail. 



1. The assumption that the bridge was liable to be exposed 

 to a wind pressure of 35 lbs. per square foot is erroneous. 

 No doubt such a pressure was once recorded at the Williams- 

 town Observatory, which is excessively exposed. The bridge, 

 however, is quite differently situated, and is protected on the 

 north by a high range of hills. Whatever may have 

 happened at Williamstown, the Victoria-street bridge is not 

 likely to be exposed to a wind pressure of above 25 lbs., 

 either from the north or the south. 



2. The multiplying of the wind pressure by 3 involves a 

 confusion between stability and strength. In a question of 

 strength we need to allow a large factor of safety to cover 

 the gradually weakening effect of a series of strains, each of 

 which may be considerably less than what would be needed 

 to cause immediate fracture. In the case of stability no 

 such factor is needed, or as yet been proposed. If it takes a 

 pressure of 35 lbs. to overturn a given object, a pressure of 

 34 lbs. may be allowed to act for ever, or may be exerted 

 and removed a million times with perfect safety. 



3. The calculation that makes the overturning wind- 

 pressure of the structure only 56 lbs. per square foot is not 

 a fair one. It arises from taking the distance between the 

 centres of the cylinders (16 feet) as the effective base of the 

 structure. As the cylinders are 3 feet in diameter the 

 extreme width of base is 19 feet, and the effective width in 

 view of overturning at least 18 instead of 16. Taking this 

 into account, and calculating the weight and the area exposed 

 to the wind with extreme care, I come to the conclusion the 

 resistance to wind pressure of the highest pier is 69 lbs. per 

 square foot, or 2*7 times the greatest possible wind pressure. 

 Nor is this all. The adhesion of the concrete filling of the 

 cylinders to the bed rock, the friction of the soil in which 

 they are imbedded, and the assistance derived from the ends 

 of the bridge through the medium of a wide and well-braced 

 platform, constitute additional sources of stability, the effect 

 of which cannot be exactly calculated, but which may at the 

 most moderate estimate be taken as increasing the resistance 

 to wind pressure to at least 100 lbs. per square foot. Thus 



