JOINTS AND CONNECTIONS 



203 



author never uses a plate as thin as J in. The least thickness of 

 any member or plate in a truss should be & in., for some metal 

 should be provided to offset the wasting effect of rust. It is well 

 enough to consider that the metal will always be inspected and 

 kept painted, but we know that painting is neglected for shame- 

 fully long periods of time. 



In Fig. 120 (a) is shown a joint in a riveted Pratt truss that is 

 of common occurrence. Here the axes of the members are con- 



Fig. 120 



current, but the rivet connections through the chord are eccentric 

 to the intersection of the lines of stress, and a bending moment 

 results. The proper construction of this joint is shown at (6) 

 and the student is advised to perform the calculations for the 

 two joints and determine for himself the amounts of the eccentric 

 stresses. The truss for which joints were designed in wood can 

 be designed now for steel and the two details in Fig. 120 can be 

 assumed for this truss. 



Fig. 121 (a) shows the heel of a roof truss. It is a common 

 detail, but that does not make it desirable or proper. It merely 

 shows the power of example and illustrates the proneness of 

 draftsmen to copy blindly. The three forces acting at the heel, 

 namely, the compression in the rafter, the tension in the bottom 

 chord, and the column, or wall, reaction are non-concurrent. A 

 bending moment results which induces large fiber stresses in the 

 members. The method to follow in determining the amounts of 

 the stresses was illustrated in the case of the Warren truss. 



Fig. 121 (6) is, likewise, an improper detail unless the heel 

 plate is thick enough to resist the bending moment between the 

 point of intersection of the three forces and its attachment to 

 the members. The plate should also be planed or chipped flush 

 with the backs of the angles of the bottom chord when it is not 

 possible to get sufficient rivets immediately over the column to 

 transmit the total reaction into the plate. 



