192 TOWERS AND TANKS FOR WATER-WORKS. 



tance c having been determined, their summation is the moment 

 of inertia /, of the shape, and the moment of resistance R, of 



the girder, is . 

 c 



SUPPORTING TOWER. 



For small tanks with capacities of from 20,000 to 30,000 

 gallons, possibly a three-post tower is the most convenient and 

 economical type, and for such small structures presents a neat 

 and trim appearance. 



Larger tanks should be built of four, six, and eight columns, 

 but for capacities of from 30,000 to 90,000 gallons possibly the 

 four-post tower is more satisfactory on account of the material 

 saved in the design because the compression members are of 

 larger and more convenient dimensions than where the load is 

 distributed amongst a greater number of supporting points. 



The increase in the number of supports does not indicate a 

 corresponding security and strength, but unquestionably espe- 

 cially tall towers and capacious tanks equipped with more than 

 four legs produces a decidedly more stable as well as symmetrical 

 appearance than the four-post variety, and in such cases, the 

 loads will be large enough to require sections of economical 

 dimensions. 



Where a hemispherical-bottomed tank is specified and to be 

 riveted directly to the columns, increasing the points of support 

 allows a better distribution of the loads, and the likelihood of 

 unequal loading is thereby minimized. 



A majority of the towers are built with posts slightly in- 

 clined, and without change of inclination from the top to the 

 base, although a few towers have been constructed with vertical 

 legs, and of late several have been designed with change of 

 inclination in the batter posts at panel-points, producing a 

 pleasing curve in the tower outline. 



