COLUMNS AND STRUCTURES 281 



The material is generally brick and the maximum allowable 

 compressive stress is fixed by building ordinances or in the speci- 

 fications governing the design. 

 For a rough check of a design 

 Let P - the total wind pressure in pounds, 



h = height in feet to the center of gravity of the shaft, 

 W weight of shaft above section, 

 D = outer dimension, 



WD 

 then, for round shafts hP = j i (Approx.) 



WD 

 and, for square shafts hP = ~ (Approx.) 



Having tested the shaft at a number of joints, approximately 

 twenty-five feet apart, redesigning the walls if found to be un- 

 stable, or if the maximum pressure exceeds the safe allowable 

 pressure the final test is made at the base, on top of the founda- 

 tion. 



The spread base must be designed to carry the shaft without 

 exceeding the safe allowable soil pressure. It may be square, 

 octagonal, or round. It is subjected to a direct load, which is the 

 weight of the shaft. The weight of the lining is neglected for 

 brick and concrete chimneys. It is subjected to an eccentric 

 load due to the moment caused by the wind. Then, 



W M 

 p = A 27' 



in which p = soil pressure in pounds per square foot, 

 M - moment in foot pounds, 

 7 = moment of inertia in square feet, 



and the distance to the most stressed fiber is one-half the length, 

 or widthy of the footing. 



W(D* - d 4 ) 

 7, for hollow circular shaft - - 7 - 



D* - d 4 

 7, for hollow square shaft - r^ 



LJ 



D 4 

 7, for square section - y^* 



WD* 

 I, for circular section - -vr~* 



