DESIGN OF TRUSSES 215 



where / = the length of the member in inches, and r radius of gyra- 

 tion of member in inches. 



Rivets and Pins, bearing 22,000 Ibs. per sq. in. 



Rivets and Pins, shear 11,000 " " " " 



Pins, bending on extreme fibre 24,000 " " " " 



Plate Girder webs, shear on net section 10,000 " " " " 



Compression Members. 



Piece #-2. Maximum stress = + 34,300 Ibs. 



The upper chord will be made of two angles with unequal legs 

 placed back to back, with the shorter legs turned out, and separated by 

 ^ -inch gusset or connection plates. 



Try two 4" x 3" x 5-16" angles. From table on page 187 Cambria, 

 the least radius of gyration, r, is 1.27 inches. The unsupported length 

 of the member is 8.5 feet, and l-~r = 102 -f- 1.27 = 80. The allow- 

 able stress per square inch = 16,000 70 / -f- r = 16,000 5, 600 = 

 10,400 Ibs. The area required will be 34,300 -f- 10,400 = 3.3 sq. in. 

 The combined area of the two angles is 4.18 sq. ins. (page 170 Cam- 

 bria), which is somewhat large. 



Try two 3^" x 3" x 5-16" angles. From the table on page 186 

 Cambria, r = 1 . 10 inches ; then / -f- r = 93, and allowable stress is 

 16,000 70 x 93 = 9,490 Ibs. per sq. in. Required area = 3.62 sq. in. 

 The area of the two angles is 3.88 sq. in., so the section is sufficient. 



To make the two angles act together as one piece it is necessary to 

 rivet them together at intervals, such that the two angles acting singly 

 will be stronger than the two angles acting together. On page 170 

 Cambria, the least radius of gyration of a 3^" x 3" x 5-16" angle about 

 a diagonal axis is 0.63 inches. The angles must therefore be riveted 

 at least every 0.63 x 93 = 58.6 inches. It is the common practice to 

 rivet angles in compression about every 2^2 to 3 feet. 



The truss will be shipped in two parts and in order to avoid a 

 splice, and because the difference in the stresses is small, the entire top 

 chord will be made of two 3^" x 3" x 5-16" angles. 



