GIRDERS AND TRUSSES 109 



Shear carried by each plank = - - = 67.5 Ibs. 



A 



145 x 12 6 _ . 



***-&*- = 25 ' 8m - 



Nails should be driven not more than 12 ins., on centers, so, 

 beginning at the end of the fourth foot from the end of the beam 

 drive nails on 12-in. centers top and bottom. Along the neutral 

 axis drive nails on 18-in. centers. The completed work is shown 

 in Fig. 68. 



No reduction in area was figured, as nails merely push wood 

 fibers aside, but when bolts are used the effective depth of the 

 beam is reduced by the thickness of each line of bolts. If bolts 



Bottom 



Fig. 68 Beam Reinforced by Planks on the Sides 



1 in. in diameter are used in two lines, and the hole for each bolt 

 is I in., the effective depth is reduced by 2 x | = 1$ ins. This is 

 serious, for the strength of beams varies with the squares of the 

 respective depths. 



Sometimes beams are reinforced by nailing a plank or strip of 

 steel along the bottom. Assume the same conditions as in the 

 last example, and use a thin white pine plank on the bottom. 

 Maintaining the breadth the problem is to obtain a new depth. 



The fiber stress for white pine is 800 Ibs., so R = 800/6 = 167 



t /.!/ . / 300,000 



and & = V/TTZ = V T^T * - 16 ins. The original depth is 14 

 f Kb T Io7 X 7 



ins., so a plank 2 ins. thick by 7 ins. wide must be spiked or 

 bolted to the bottom. A thick plank like this must be fastened 

 with bolts, and the holes will reduce the area, which will make 

 necessary an increase in thickness. Methods for finding the length 

 of the reinforcing plank and the pitch of the bolts have been given, 

 the depth used being the full depth of the original beam plus half 

 the thickness of the reinforcing plank. 



