JOINTS AND CONNECTIONS 



145 



will be shown, but, while it makes very little difference for tension 

 pieces, it is bad practice to use several pieces to form compression 

 members when single sticks can be obtained. 



In Fig. 90 is shown the truss illustrated in Fig. 80, the computa- 

 tions for which were made as an exercise. We will now proceed 

 to design some of the joints. Assume the wood to be Yellow 



Fig. 90. 



Pine, Grade 1 (Underwriter's Code), in which the allowable safe 

 stresses are as follows: 



Tension 1600 Ibs. per sq. in. 



Compression (end bearing) 1200 Ibs. per sq. in. 



Compression (side bearing) 350 Ibs. per sq. in. 



Shear (with the grain) 120 Ibs. per sq. in. 



The stresses in the steel are as follows: 



Tension 16,000 Ibs. per sq. in. 



Shear 10,000 Ibs. per sq. in. 



Bearing 20,000 Ibs. per sq. in. 



For the strength of bolts and lag screws in combined shear, 

 bearing and cross bending, see pages 138 and 139. 



The compression in L 9 U\ is 35,950 Ibs. and the tension in 

 Z/oLi 25,000 Ibs. The lower chord is always dimensioned to take 

 care of the maximum tension, which in this case is 45,000 Ibs. 

 This leaves considerable excess material in the end panels, which 

 is available for cutting to form connections. Similarly, the upper 

 chord is dimensioned for the maximum compression and the size is 

 uniform throughout. 



Before proceeding to design the connection joints at the ends 

 of a truss and at the ends of panels it is necessary to know the sizes 

 of the members. If material has to be cut out for the formation of 

 joints the required area can be added to the member provided 

 it has not enough excess material to provide the necessary area 

 for the details. Proceeding in this manner we will study all the 



