PROBLEMS IN DESIGN OF BEAMS 



97 



a depth about twice the thickness of the bearing strip. The num- 

 ber of nails to use depends on the reaction and the thickness of the 

 nail. Divide the reaction in pounds by 100 to get the number of 

 20d. nails ; by 150 for 30d. nails ; by 175 for 40d. nails ; by 200 

 for 50d. nails; by 225 for 60d. nails. There is considerable dif- 

 ference in weight between nails and spikes having the same desig- 

 nation and the alx>ve figures refer to nails. The nails should be 

 spaced at least 3 ins. apart horizontally and this can be accom- 

 plished by putting half near the bottom of the strip and half near 

 the top, thus staggering them. The size of nail to use will therefore 

 be determined by the spacing when the reaction is considerable. 



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Fig. 63 Wood End Bearings for Joists 



The above described bearing strip support for joists is a cheap 

 method. Formerly it was customary to use girders considerably 

 larger than were necessary and seats were cut into them for the 

 joists. This increased the labor cost and when water settled into 

 the joints they rotted. The introduction of slow burning construc- 

 tion also acted to throw the gaining of joists into girders into 

 disrepute because of the increased fire risk in the joints. When 

 a nailed bearing strip is used it should be carefully computed. 

 The bearing should be at least half an inch wider than the com- 

 puted bearing. The ends of the joists should be carefully fitted. 

 It is advisable with thick joists to top nail them to the girder to 

 prevent twisting or winding. With thin joists a solid bridging 

 should be inserted at the ends, nailed to the girder. When this is 

 done many of the objections to the joiners pocket are introduced. 

 Therefore metal hangers are better when for any reason it is not 

 advisable to have the joists rest on top of a girder. In slow burn- 

 ing construction neither hangers or bearing strips are proper. 

 The thickness of joists in such construction should be not less 

 than half the depth and the minimum cross-sectional area should 

 be 72 inches. All joists should rest on top of girders. 



The maximum shear on a wooden beam is along the neutral 

 axis and season checks are apt to occur here, so nailing strips 



