GIRDERS AND TRUSSES 121 



in the strut by the safe unit compressive stress in the material 

 of which it is made. The area of the end of the strut against 

 the wood is found by dividing the compression in the struts by 

 the allowable safe unit compressive stress across the grain of the 

 wood. 



The size of the long compression member is obtained by design- 

 ing it as a column, plus the effect of bending caused by whatever 

 load it may carry as a beam, with spans figured between end sup- 

 ports and vertical struts, or ties. The unit compressive stress on 

 the end of wood may be used when the length of the member 

 between supports does not exceed 15 times the least thickness. 

 For longer pieces the unit compressive stress must be reduced by 

 an appropriate column formula. 



The sizes of metal tension members are fixed by dividing the 

 total tension by the allowable safe unit tensile stress in the metal 

 used. If threads are cut in a rod, this size must be at the root of 

 the threads. If the rod has upset threads the full area of the rod 

 may be used. The minimum size rod to use in any tie is f in. 

 diameter. 



The size of a tension member made of wood is obtained by 

 dividing the total tension by the allowable safe tensile stress in 

 the wood and adding thereto an area equal to that caused by 

 bolt holes and seating of other truss members. 



Trusses 



A truss is a system of framework forming a skeleton beam. The 

 top chord is in compression; the bottom chord is in tension; 

 the web members (interior braces and ties) carry the shear. The 

 parts must be in equilibrium, that is each push must be balanced 

 by a pull or a push from the opposite direction. This indicates 

 the triangle as the perfect truss, for it cannot be changed in shape 

 without breaking at the joints. 



Trusses are of two kinds, those with parallel chords and those 

 with nonparallel chords. The parallel chord truss will first be 

 considered. Fig. 78 shows the development of the Pratt truss 

 from two panels to six panels. Assume a load - 1 at the middle 

 vertical. Half the load goes to each support so the coefficient 

 for the two middle diagonal ties - J. If the load is applied at 

 the top, 1 is the coefficient for the middle vertical, but if the 

 load is suspended at d the coefficient - 0. 



