146 PRACTICAL STRUCTURAL DESIGN 



usual methods for making joints in tension and compression 

 members and then design the joints. 



It is not always possible to get a single stick to use as a tension 

 or compression chord in a truss, and it is necessary to build them 

 up in nearly every instance. If a single stick is used the area is 

 increased to allow for the hole through which the largest vertical 

 rod passes. In the truss now being considered the sizes of the rods 

 are found as follows: 



The rod at f/ 8 will be 1 in. diameter if the threaded ends are 

 upset, and if the ends are not upset the diameter will be If ins. 

 This rod may be smaller as the stress is very low, but it is not 

 advisable to use a rod of smaller diameter for such a length. The 

 stress in rod at U 2 L 2 will be the same as the rod at U 3 , for the stress 

 is 15,000 Ibs. and the allowable stress in the steel is 16,000 Ibs. per 



square inch. The net area of the rod at U\L\ = r; ^ = 1.56 sq. ins., 



and we will use a rod 1| ins. diameter if the threaded ends are upset. 

 If upset ends are not used the loss due to cutting of the threads 

 will require the use of a rod If ins. diameter. The following table 

 gives the dimensions of rods with upset screw ends. To obtain 

 the diameter at the root of the thread cut in a bolt use the figures 

 in the second column for the outside diameter and the diameter 

 at the root of the thread will be found in the third column. The 

 screw threads in the table are Franklin Institute standards. In 

 the 1913 edition of the " Carnegie Pocket Companion " the tables 

 for upset screw threads are American Bridge Company standards. 



REMARKS. As upsetting reduces the strength of iron, bars 

 having the same diameter at root of thread as that of the bar 

 invariably break in the screw end, when tested to destruction, 

 without developing the full strength of the bar. It is therefore 

 necessary to make up for this loss in strength by an excess of 

 metal in the upset screw ends over that in the bar. 



To make one upset end for 5-inch length of thread, allow 6-inch 

 length of rod additional. 



