208 TOWERS AND TANKS fOR WATER-WORKS. 



This stress must be overcome by the resistance offered by 

 the girder section, whose unit stress and modulus must be 

 found. 



In the discussion of the girder, Carnegie's formula for shear 

 strains was given, but ordinarily it is usual to take 10,000 Ibs. 

 per square inch of metal as an allowable unit stress, as it is con- 

 sidered good practice to allow flange strains of 15,000 Ibs. per 

 square inch and as much as 11,000 Ibs. of net section for the 

 vertical shear of the web. The modulus of resistance, R, of a 

 shape being its moment of inertia, /, divided by c, the depth of 

 its neutral axis, the safe resistance moment is RXS, or modulus 

 multiplied by the unit stress. 



The web of the girder having been previously found, the 

 other elements must be obtained by trial. 



Deciding upon one top and two bottom flange angles of di- 

 mensions and placed as shown in Fig. 48, their elementary areas, 

 a, would be as follows: 



Area. 



2 bottom angles, 6X4X7/16 8.42 



5/i6-in. X 36-in. web ir - 2 5 



Top angle, 6X4X7/16 4.21 



Total section area, or la . = A = 23.88 



The distance, z, from an axis to the centre of gravity of each 

 elementary area must next be found. The various handbooks 

 give this for numerous shapes, some one of which can generally 

 be used; otherwise special calculation is necessary. The quantity 

 given must be taken from the length of the lever arm of the com 

 pound shape as measured from an axis at one end of the web, 

 or in other words, from the depth of the girder; in this case from 

 36 inches. The elementary areas, in the present case, multiplied 

 by their leverage is found to be 



8.42X34.03+ ii. 25X18 + 4.21X4-03 = 506.8, 



