138 PRACTICAL STRUCTURAL DESIGN 



A more logical rule given by Mr. H. D. Dewell, in Western 

 Engineering, Vol. 7, page 291, is as follows for Douglas Fir, 



P = 4000 d 2 , 



in which d - diameter of nail in inches. 



For other woods multiply the result by the following coefficients: 



Long-leaf yellow pine 1.05 



White pine 78 



Norway pine 65 



White oak 78 



For common wood screws use the constant 4375 instead of the 

 constant 4000 used for nails. 



Nails or common wood screws are generally thought of first 

 for fastening timber because they are cheap and the labor cost of 

 driving them is low. Their usefulness, however, is limited to thin 

 pieces carrying little stress. When the load is too great to be 

 transmitted properly by nails or common screws, or the pieces 

 are too thick, lag screws may be used on account of the low 

 labor cost as compared with that required for bolts, for which 

 holes must first be bored. Mr. H. D. Dewell, in Engineering 

 News, Vol. 76, page 797, gives the following recommended working 

 values for lag screws. 



Description Pounds per screw 



Metal plate lagged to timber, % X 43^-in. screw 1030 



Metal plate lagged to timber, % x 5-in. screw 1200 



Timber planking lagged to timber, % x 4J^-in. screw 900 



Timber planking lagged to timber, % x 5-in. screw/ 1050 



Generally speaking the resistance of lag screws varies with the 

 ratio of their diameters, so the values above given may be used 

 as a basis for other sizes. 



The strength of nails, lag screws, and bolts in wood cannot be 

 computed the same as rivets in metal, for the rivets may shear, 

 but this is impossible with joints in wood. Nails, screws, or bolts 

 will bend, for the wood will crush long before the shearing strength 

 of the metal is reached. It is necessary, therefore, to use bearing 

 values obtained by experiments. Mr. H. D. Dewell, in Engineer- 

 ing News, Vol. 76, page 115, described the result of tests made 

 with bolts. Two thin pieces of timber were fastened to a thick 

 piece, by bolts passing through with washers on the ends. The 



