COLUMNS AND STRUCTURES 283 



number of bolts to rings riveted to the girders. To determine 

 the <i/e of the bolts the chimney is assumed as tending to over- 

 turn about one edge of the bolt circle. The principle is that used 

 in (lie case of bolts fastening column bases to footings. 



Brick stacks may usually start with a thickness of nine inches 

 for the top twenty-five feet and increase half a brick in each 

 twenty-five feet down. This is merely a rule by which to deter- 

 mine trial thicknesses. A general rule for the top thickness is 

 as follows 



t = 3 + 0.4 + 0.005 H, 



in \vliich t = thickness in inches of upper course (neglecting 

 ornamentation). 



d = clear inside diameter at top in feet, 

 H = height of stack in feet. 



The thickness of metal in steel stacks is governed by dura- 

 bility as \\<-ll as by strength. The stack is a hollow circular can- 

 tilever beam (see page 67) in which the weight of the metal is of 

 relatively small importance, the wind being the largest force. 

 It is usual to start with plates f in. thick at the top of the stack 

 if not lined and some designers use J in. plates at the top for 

 lined stacks. Some designers increase the thickness by T ' ff in. 

 each 30 or 40 feet, while others increase by | in. At each 30 or 

 40 feet the section is investigated and the thickness of the plate 



fixed by the fiber Stress. 



To insure tight joints the rivet spacing is not less than 2.5 

 times the rivet diameter, or more-than 16 times the thickness of 

 the plate. Usually the rivet spacing is investigated and deter- 

 mined only for the lowest tier of plates of any thickness. The 

 rivets are in shear due to bending moment as well as ordinary 

 shear. 



Not enough data is available for reinforced concrete chimneys 

 to fix trial thicknesses, as for brick and steel stacks. The least 

 thickness should be six inches. The average increase in thick- 

 ness is approximately at the rate of one inch in 50 ft. for trial 

 sections. 



Tanks and Retaining Walls 



Fig. 182 shows curves for ascertaining the pressure in bins 

 and tanks. They were computed by the author when he was 

 Chief Engineer of the Fireproof Construction Bureau, Portland 



