42 The Stability of Structures 



Second. The nature of the situation as to shelter or other- 

 wise. 



This point may be dismissed in a few words. A tower or 

 chimney upon a high, bare hill, a lighthouse on the sea- 

 coast, a railway-bridge swung high across an open ravine, 

 will all require that the maximum be fixed higher than if 

 the structure were in a hollow, or sheltered by hills, timber, 

 or adjacent buildings. This point can only be settled from 

 an examination of the locality of the proposed structure. 



Third. The height of the structure above the ground and 

 the nature of its construction. 



As the wind approaches the ground it diminishes in 

 velocity; the more uneven the surface and the greater the 

 obstacles, the more is the velocity interfered with and 

 lessened. To what height and to what degfree this interfer- 

 ence extends is unknown. No experiments, I believe, have 

 ever been made for the purpose of ascertaining this. We 

 must conclude, however, that close to the ground this 

 interference is very considerable, and upon this supposition 

 we can explain the reason why chimneys and buildings 

 notoriously unfit to stand a pressure of 20 lbs. or less are 

 still standing, although gales of greater force than that have 

 occurred since their erection. 



Structures in connection with this part of the subject 

 may be conveniently divided into two classes — 



1. Those which extend from the ground continuously 

 upwards, as a factory chimney. 



2. Those which extend horizontally at a constant height 

 above the ground, as a girder bridge. 



If in the case of the first we assume 60 lbs. as the maximum 

 for the locality, it is evident that this will decrease as it 

 approaches the base, and that it would be permissible to 

 reduce this force ; or, if we maintain it for the purposes of 

 calculation, it should be recognised as allowing ample margin 

 as factor of safety. 



In the second class of structures, however, the maximum 

 would have to be taken as practically uniform all over the 

 surface exposed, allowing nothing as factor of safety. 



Fourth. The value and use of the structure, and whether 

 its failure would involve loss of life or damage to adjacent 

 property. 



The greater the value of the structure, the greater will be 

 the necessity of ensuring its safety. It evidently would not 

 be true economy to build an ordinary house-chimney to 



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