134 PRACTICAL STRUCTURAL DESIGN 



In Fig. 85A (c) the rafter is divided into three equal parts. Each 



P 3P 



joint carries a load, P. The load on DC = -~ ; on DE = -^ ; 



A z 



5P 

 on EF = -^-- The vertical FF carries 3P. The rafter AF carries 



m 



3P 5P 



-^-; AD carries r-; A B carries 3 P. The tie rod carries at the 



2i i 



opr 



end a stress = -^-j The stress in the tie rod on the section 



TE= , In actual practice the tie rod is uniform in size 



throughout the span. 



With the examples given the student should have no trouble 

 tracing the loads on the members of the truss shown at (d). 



Each vertical is in tension by an amount equal to the load it 

 carries. Each diagonal member is in compression by an amount 



= - in which x = amount of load on the member. 

 a 



L = length of member, 



d = the vertical height from the bottom to the 

 top of the member. 



All measurements are on center lines. The slope of the rafter 

 is constant so the ratio is obtained once by dividing the slant 

 length of the rafter by the height of the truss. The slopes change 

 at each panel for the interior braces, so a ratio must be found for 

 each separately. 



Coefficients for Fink trusses, Fan trusses, and Pratt trusses with 

 inclined rafters have been calculated for different degrees of slope 

 and for varying numbers of panels, based on uniform symmetrical 

 loads. Tables of these coefficients are given on pages 309-311, of 

 the 1913 edition of the " Carnegie Pocket Companion " for trusses 

 to be made of steel or wrought iron. Steel is commonly used except 

 when corrosion is a grave danger, in which case wrought iron is 

 preferred. All metal trusses are made of rolled shapes with 

 riveted connections. The trusses illustrated may be a combina- 

 tion of steel rods for tension members and wood for compression 

 members. Fink trusses are very generally used because most of 

 the members are in tension and the struts are short. Partial 

 loading can never cause maximum stresses in the parts of Fink 

 trusses as they may in other forms of trusses. 



