106 PRACTICAL STRUCTURAL DESIGN 



The reinforcing planks (theoretically) should have a length 

 of 20 - (2 x 3.55) = 13.90 ft. Practically it will be best to make 

 them 15 ft. long, which leaves 2 ft. 6 ins. without reinforcement at 

 each end of the beam. 



The planks must be attached to the beam by screws or nails, 

 the latter being the cheaper. To get the best results the length 

 must be not less than three times the thickness of the plank, in 

 order that the nail may be embedded in the beam a depth at least 

 twice the thickness of the plank. From a table of sizes of standard 

 steel wire nails and spikes (in the steel manufacturers' handbooks) 

 we find a 30d. nail is 4.5 ins. long, the length required. There must 

 be enough nails used so the beam and planks will act together 

 and the force to be resisted is shear, for if the beam bends and the 

 planks do not bend there will be a sliding movement between 

 them. 



When a nail resists a shearing force three actions are set up: 

 1. A bending caused by the pull of one piece against the nail em- 

 bedded in the other piece. 2. A shear in the nail which is caused 

 if the nail is so stiff that it will not bend. 3. Bearing against the 

 wood in which the nail is embedded. The size of the nail must be 

 proportioned to care for the action most likely to cause a failure. 

 When the material to be held is wood the bearing action of the nail 

 against the wood is the only one to be considered, for if the nail 

 furnishes area enough to transmit the shear it will be thick enough 

 to resist bending and also thick enough not to shear across. Rivets 

 in metal have to be similarly proportioned, but bending is seldom 

 feared while failure by shear of the rivet or by insufficient bear- 

 ing against the metal is practically, and usually, of equal impor- 

 tance. Both must be figured, whereas in the case of wood only 

 the bearing value is considered. The bearing value is computed 

 as follows: 



The 30d. nail (not spike) is made from No. 5 wire, the diameter 

 being 0.207 in. The cross-sectional area through a 1^-in. plank 

 is 0.207 x 1.5 = 0.311 sq. ins. The compressive value of the softer 

 wood must be used, which is 700 Ibs. per square inch with the grain, 

 assuming the nail to bear on the end of the wood where it enters. 

 The bearing value for one nail is found by multiplying the bearing 

 area by the allowable fiber stress in compression with the grain, 

 0.311 x 700 = 218 Ibs. This is the method to be used when no 

 data is at hand giving the actual safe bearing values. 



