DESIGNING. 



Since the width of the girder is small compared with its radius, 

 the solution of the stresses in a circular hoop may be applied, 

 and the girder designed to resist the whole weight of the tank, 

 including its own weight and the contents of the tank; a radial 

 inward thrust at the top of each of the pests, and reactions due 

 to wind stress. 



In the riveted plate girder it is usual to assume that the flange 

 sustains the horizontal arid the web all of the vertical strains 

 due to the load, the flange acting under tension and the web 

 being subject to shear. The tank plate is usually taken as the 

 web of the curved girder; angles are riveted to the web as flanges, 

 and stiffeners are introduced at proper intervals consisting 

 of one or more angles or channel shapes. Such a continuous 

 girder, when properly proportioned, provides an economical 

 and effective support for the gravity and wind stresses to which 

 the tank is subject. To determine the safe load for the girder, 

 its elements must first be found. 



The principles of moments are applicable to areas as well as 

 to weights, and from such application an equation is obtained 

 from which the value of c, or the distance from the neutral axis, 

 passing through the centre of gravity of the shape to the most 

 remote fibres can be determined. 



If a be any area and z the distance from its centre cf gravity 

 from an axis, the product, az, is called the static moment of the 

 area. The sum of the static moments of all parts of the figure 

 is represented by 2az, and if A be the total section area, then 



_Iaz 



C A 



Since the moment of inertia of a plane surface with respect to an 

 axis is the sum of the products obtained by multiplying each 

 elementary area by the square of its distance from the neutrcl 

 axis, the elementary areas cf the compound shape and the dis- 



