GIRDERS AND TRUSSES 



107 



The actual safe bearing value for any nail is about two-thirds 

 of the value as above computed. One reason is that there is no 

 common gauge used by nail makers, so that while tables may show 

 that nails are made of certain wire, not all tables give the diameter 

 of the wire in decimals of an inch, and 

 there being a number of wire and 

 metal gauges in use we do not know 

 the exact sizes of the nails used in the 

 published experiments. The experi- 

 ments referred to may have been 

 made with nails not quite so thick as 

 the nails used in computing the bear- 

 ing value. A second reason for the 



Fig. 66 Shear Diagram for 

 Original Beam 



actual bearing value being so small is that the nails push the fibers 

 of the wood aside and start a splitting action, which is increased 

 when the shearing action is set up. This second reason is no doubt 

 much more important than the first. The method of figuring 

 bearing value just illustrated is correct for bolts for which holes 

 must be bored, but gives a value about 50 per cent too large for 

 driven wire nails and for screws. The designer must not forget 

 this. Having settled on the size of nail and the bearing value of 

 each nail, the number and spacing must be determined. 



Fig. 66 is the shear diagram for the uniformly loaded beam. 

 At each end the shear = reaction = 10,450 + 2 = 5225 Ibs. The 



nails should be 

 closer together 

 near the ends, 

 where the shear 



Fig. 67 Shear Diagram for Reinforced Beam 



is a maximum, 

 so theoretically 

 the spacing 

 should vary 



from nail to nail. Practically the spacing can be maintained at 

 uniform intervals for each foot, which makes the diagram resemble 

 Fig. 67, the reinforcement ending 2.5 ft. from each end. 



The method to be described follows the common method for 

 sparing rivets in the flanges of plate girders. There is another 

 method which will later be illustrated, because it shows exactly how 

 the stresses in the top and bottom flanges of plate girders affect the 

 rivets used to connect the flanges to the web. In the present 



