Art. III. — The Stability of Structures in Regard to 

 Wind Pressures. 



Paper No. 1. 



By Fred. A. Campbell, C.E. 



[Read 8th April, 1886.] 



There is no force upon earth more variable and uncertain 

 than that of the wind. It approaches from every quarter ; 

 it moves in circles as well as straight lines, and vertically as 

 well as horizontally, Its force may be as steady and con- 

 tinuous as that of gravitation, or as intermittent and 

 impulsive as the blows of a battering-ram ; whilst in strength 

 it varies from the gentle zephyr that will hardly lift a 

 feather, to the mighty hurricane which levels every obstacle 

 to the ground. 



Fully recognising then the existence of this invisible, omni- 

 present, ever-varying force, to which every structure is more or 

 less exposed, it becomes a question of serious import to the 

 engineer and architect as to what is the best — that is, the 

 safest and most economical way of providing for it. This 

 question is continually pressed upon our notice by the failure 

 of structures around us during heavy gales. It was 

 especially brought home to us in a very startling manner 

 by the destruction in 1879 of the great Tay railway bridge. 



Having to erect any structure which shall be exposed to 

 certain forces, the designer naturally ascertains in the first 

 place as nearly as possible the extent of such forces. If 

 these can be determined with tolerable accuracy, as in the 

 case of a dam to retain water, or a column to uphold a 

 known dead- weight, the way is clear, and the designer has 

 simply to apply known laws, stresses which are calculable 

 being provided for by the disposal of material whose 

 resistance is calculable, But if, on the other hand, the force 

 is one which is beyond the range of exact knowledge, the 

 task is of a totally different kind. 



To this latter class the pressure due to wind undoubtedly 

 belongs, as will be evident from what follows. 



It might be urged that if we have not precise data to start 

 with, any attempt to solve the problem scientifically must 

 be useless, and that the best method must be to work from 



