282 



PRACTICAL STRUCTURAL DESIGN 



A concrete chimney is frequently more economical than a 

 brick chimney because the maximum compressive stress is greater 

 than for brick and the tension can be equal to nf c , in which 

 n = ratio of deformation between steel and concrete, 

 f c = maximum unit compressive stress in the concrete. 

 The wind may come from any direction therefore, the rein- 

 forcement must be equally spaced around the circumference of 

 i the shaft. The shaft 



may then be designed 

 by trial. Assume a 

 certain steel area and 

 assume it be in the 

 form of a thin ring of 

 steel. Find the mo- 

 ment of inertia and 

 ascertain how much 

 direct load and bend- 

 ing load it will carry 

 (see page 67). The 

 steel may be assumed 

 to have a value of 

 12,000 Ibs. per sq. in. 

 A concrete shell is de- 

 signed to carry the 

 direct load and wind 

 moment the steel can- 

 not carry, and when 

 the combined concrete 

 section and steel sec- 

 tion is found, which 

 will carry Uie direct 

 load and wind moment, the steel is placed in the middle of the 

 thickness of the concrete ring in the form of rods or bars. 



Each section is designed in this manner and when the base is 

 reached the vertical bars are run into it a sufficient length for 

 anchorage. 



Self-supporting steel stacks are often carried on columns and 

 the proper formulas to use for this condition are shown in Fig. 

 181. These stacks may be anchored to concrete foundations 

 by means of bolts, or they may be on girders and anchored by a 



Fig. 181 Formulae for Self-supporting Steel 

 Stacks 



