132 TOWERS AND TANKS FOR WATER-WORKS. 



that the table just referred to can be used for determining the 

 stresses in a horizontal curved girder. 



In using this table for determining the stresses in a horizontal 

 curved girder, the value of H to be used is the horizontal thrust 

 in pounds at the top of each post, and the value of r to be used 

 is r 4 , as shown in Fig. 31, i.e., the radius of the neutral axis 

 of the girder in inches. The value of r 4 is approximately r, 

 the radius of the tank in inches, plus ^d, d being the width of 

 the girder in inches. This approximate value is sufficiently 

 accurate for determining stresses. 



The flange stress in the girder resulting from the bending 

 moment can be determined by dividing the bending mom rut 

 by the depth, d, centre to centre of flanges in inches. To this 

 flange stress must be added algebraically the comprcssicn at 

 the point under consideration. It is proper to consider that 

 the compression is resisted by the flanges of the girder and divided 

 equally between them. As in ordinary plate-girder design, the 

 shear may be considered to be resisted by the web plate. 



If a lattice girder is used instead of a plate girder, the bending 

 moment at the abutments of the arch may be replaced by a couple 

 whose lever-arm is the depth of the girder centre to centre of 

 flanges. From these reactions and the loads the stresses in 

 the members of the lattice girder may be determined by graphical 

 methods. 



Horizontal Stresses at Plane of Change of Inclination of 

 Posts. The amount of the horizontal thrust at each post at the 

 point of change of inclination has been determined. Let H r 

 represent this thrust (Fig. 32). Then if the thrust is resisted 

 by struts between diagonally opposite posts, as AD, the com- 

 pression in each strut equals H'. If the thrust is resisted by 

 struts between adjacent posts, as AB, the compression in each 

 strut equals 



H' 



