44 The Stability of Structures 



figures, taken from Ranldne's Applied Mechanics, will 

 show that this is not the case : — 



Height above ground. 



Eesistance of joint. 



435 ft. 





850 ft. 



77 lbs. 



210 ft. 



55 lbs. 



114 ft. 



57 lbs. 



54 ft. 



63 lbs. 







71 lbs. 



55 lbs. is therefore the resisting strength of that structure, 

 and all the material going to raise the strength of the other 

 parts beyond this might have been saved. 



The most satisfactory way to design a building of this 

 kind is by a diagram. I have prepared one for a round 

 brick chimney, 200 feet high, 20 feet diameter at bottom, 

 and 10 feet diameter at top, by way of illustration. Figures 

 have been adopted throughout to facilitate computation. 

 The weight of brickwork is taken as 112 lbs. per cubic foot, 

 the maximum wind pressure at 56 lbs. per square foot on a 

 flat surface giving 28 lbs. per foot for the surface of the 

 chimney. The tensile strength of the brickwork has been 

 neglected, and the effective width has been taken from the 

 centre of the chimney to the centre of the wall. The 

 stresses and resistance have been calculated at intervals of 

 20 feet, and plotted to a scale of 20 feet to an inch vertical, 

 and 800 foot tons to an inch horizontal. 



The red line, showing a curve almost parabolic, is the curve 

 of moments due to wind pressure. Had the building been 

 of uniform diameter the curve would have been truly para- 

 bolic. 



Then, starting from the top with a 9 -inch wall, the blue 

 line represents the resistance due to that, crossing the curve 

 of moments about half way. The orange line shows the 

 resistance of a 14-inch wall starting at 120 feet elevation, 

 and crossing the curve of moments at about 20 feet from 

 the ground ; and the green line shows the resistance of an 

 18-inch wall starting at 40 feet elevation and running to 

 the surface outside of the curve of moments altogether. 



The compound curve due to the use of these three thick- 

 nesses of brickwork gives a line of resistance which is equal 

 to a wind pressure of 76 lbs. per square foot, or 20 lbs. 

 beyond the maximum assumed, and varying from this at no 



