210 



STRESSES IN ROOF TRUSSES. 



Art. 119. 



ported by the roof trusses, or any movable loads which might 

 come on the trusses. 



In adding the stresses found from the above loadings it 

 must be borne in mind that the maximum wind and snow could 

 not be acting on the same side of the roof at the same time. 

 The wind would blow the snow off. 



For the common types of trusses shown in Fig. 149, when 

 supported with one end free to move longitudinally, it will be 

 found that the combination of dead load and live load, with 

 snow over the whole roof, will usually give the maximum stresses 

 in all the members. 



When the trusses are supported on columns the stresses in 

 the trusses due to wind will be influenced by the height of the 

 columns and the position of the knee braces which give the 

 building lateral stability. Fig. 151 shows two types of trusses 

 supported by columns. In order to determine the stresses in 

 these trusses graphically, the imaginary members shown dotted 

 outside the columns are added and the stresses laid out in the 

 usual manner (54). The lower point of the imaginary truss 

 which replaces the column should be at the point of contra- 

 flexure in the column. If the columns are considered as pin- 

 ended at the bottom, this point will be at the bottom, This 

 assumption would give the greatest possible stresses in both 

 the trusses and the columns. The replacing of the columns by 

 the imaginary members will not affect the stresses in the truss. 

 The stresses in the columns must be analyzed algebraically (115). 

 (See Chapter XIV.) 



The stresses will now be determined for the truss shown 

 in Fig. 151 (a) with the data given in Fig. 152. The stress 



Fig. 151. 



diagrams for dead load and for full snow load are exactly similar 

 to that shown in Fig. 53. One diagram will answer for both 

 of these, the stresses being proportional to the panel loads. The 



