108 PRACTICAL STRUCTURAL DESIGN 



example the original beam may be said to represent the web, 

 the reinforcing pieces the flange members, and the nails the rivets 

 used in plate girders. The spacing of stirrups in a reinforced-con- 

 crete beam is another illustration of the transmission of stresses 

 from one part of a beam to another by designing to resist shear. 

 In a truss the sizes of the members are varied, for the panel lengths 

 are equal. When the nails in a reinforced wooden beam are of the 

 same size, the rivets in a plate girder are the same size and the 

 stirrups in a reinforced-concrete beam are the same size, the 

 variation in shear is taken care of by varying the spacing. 



5225 x 8 

 Two feet from the end of the beam the shear is = 4180 



Ibs. The width of the original beam is 7 ins., and the two planks 



4180 x 7 



increase the width to 9.75 ins., or - ^-^ = 3000 Ibs., which will 



y. / o 



. 4180 - 3000 _ ft . 

 be carried by the original beam, leaving - = - = 590 Ibs. to 



m 



be carried by each plank. The nails must transfer this from the 

 beam to the plank and they should be driven 1 in. from the edge, 

 both top and bottom. Let 

 V = total vertical shear at the point considered, 

 r = resistance of one nail (bearing value), 

 d = distance in inches between lines of nails (in the present 



example d = 12 ins. vertically), 



p = pitch of nails in inches (the horizontal distance center to 

 center between nails) ; 



rd 145 x 12 ft _ . 

 then p = -y - 59Q = 2.95 ins. 



Space the nails 2f ins. center to center along the upper and lower 

 edge for at least 6 ins. at each end, the first nail being driven 1 in. 

 from the end of the plank. 



v 7 

 Shear 3 ft. from end = - -= 3660 Ibs. 



Shear carried by each plank = - - = 330 Ibs. 



A 



p = = 3.92 ins. Space the nails 3f ins. center to center 



ooU 



along the upper and lower edge of the plank for 1 ft. 

 Shear 4 ft. from end = 522 ^ Q X - = 3135 Ibs. 



