JOINTS AND CONNECTIONS 



143 



tight so that washers should be designed with area enough to de- 

 velop the safe working stress in the bolts without exceeding the 

 safe bearing on the wood. If the safe bearing is exceeded, which 

 will be the case if 

 the washers are too 

 small, the fibers will 

 be cut around the 

 edges of the washers 

 and decay will set in. 

 At (d) is shown a 

 method sometimes 

 adopted for upper 

 chord joints and at 

 (e) is shown what 

 happens when the 

 wood shrinks. A 

 proper joint is shown 

 at (/) with the cen- 

 ter lines of all pieces 

 meeting at a com- 

 mon point. This 

 idea of all the center 

 lines meeting at a 

 common point is also 

 illustrated at (g) and 

 (h). This is neces- 

 sary to avoid rota- 



Fig. 89 



tion which would cause a bending moment. When part of a piece 

 of timber is cut out for a dap the stress is concentrated in the 

 rest of the piece, which alters the position of the center lines. 

 This must be considered in the design and will be discussed when 

 we take up the design of joints. 



At (i) is shown a brace abutting on a post. If a waling is spiked 

 along a line of posts, or blocks with the fibers in a horizontal posi- 

 tion are bolted to the posts, and the braces rest on the sides of the 

 fibers, there will be settlement when shrinkage occurs. When 

 blocks, or cleats, are used as shown they should have the fibers 

 vertical and the dap should be deep enough to transmit all the 

 load to the post. The bolts shown are used only to hold the cleats 

 in place. To design the bolts, however, it is well to have them 



