140 PRACTICAL STRUCTURAL DESIGN 



Let n = the bearing per sq. in. on the diametrical area of the 



pin, having arriving fit. 



p = the allowable bearing per sq. in. on the ends of the fibers, 

 q = the allowable bearing per sq. in. perpendicular to the 



direction of the fibers. 

 n = IP + f<7- (Dewell formula) 



Example. A brace rests against a 

 2-inch pipe driven through the bottom 

 chord of a truss to act as a pin joint. 



_* -P/N / 7 The allowable bearing on the ends of 



/ \ the fibers is 1200 Ibs. per square inch 



1 r 



and the allowable cross bearing is 300 

 ,11 Ibs. per square inch. 



*T T * i 



-s-s^ f What bearing can be used on the 



W))\ diametral area of the pin? 



= J Answer. (| X 1200) + (f x 300) = 



Fi 8- 86 - 600 Ibs. per sq. in. 



Mr. H. D. Dewell was Chief Structural Engineer, Panama- 

 Pacific International Exposition, San Francisco, 1915. The 

 possibilities of timber construction were probably never better 

 treated than in the work under his charge, and a great many 

 experiments and detailed studies were made in order to design 

 properly. The results were given by him in a series of articles 

 in the June to December issues (inclusive), 1916, of Western Engi- 

 neering, San Francisco, Cal. The author has made free use of the 

 articles by Mr. Dewell and discarded much material prepared for 

 this chapter, based on older writings, 

 some of which were speculative and 

 some of which were founded on experi- 

 mental work conducted by men not so 

 skilled as are the modern experimenters. 



Standards are well enough in their 

 places, but men should not blindly use . g _ 



standards without knowing all reasons 



and the authority. Standard washers should not be used, merely 

 on the advice of an advertisement writer, to carry certain loads 

 until their sufficiency has been checked by computations for the 

 wood with which they are to be used. If a washer is too small it 

 will be drawn into, and crush the fibers of, the wood. 



Bolts placed through the joints of trusses will have washers as 



