RECTANGULAR BEAMS 



115 



3.0 in., allowing 100 per cent for unequal distribution of the reactions over 



the bearing surface. A practical limit is about 6 in. on concrete supports. 



Let us now review the beam we have designed. It is not necessary to do 



this unless the values of bd 2 and a a are made considerably different than the 



la "round ,. 



rods 



.,,, equal 

 ^spaces 

 -c- 18" 



FIG. 53. 



calculated values. The method will be shown here, however, for the 

 benefit of the student. 



a a 3.98 



495 



= 0.0080 







fc=-V(2) (0.0080) (15) + (0.0080) 2 (15) 2 -(0.0080)(15) 

 = 0.384 

 7 = 1-1/3(0.384) =0.872 



1,440,000 

 /= /a ogx/n g7ox^-y ^ -15,100 Ib. per square inch. 



fc 



= 630 Ib. per square inch. 



(3. 98) (0.872) (27.5) 

 (2) (15, 100) (0.0080) 



0.384 



In long beams, it frequently occurs that stock bars are too short to extend 

 the full length of the beam. In such cases welding may be resorted to. 

 Since the maximum moment is at or near the middle of the span in most 

 beams, the welding should be near the ends and not near the middle of the 

 beam. This will require two welds instead of one, but welding near the 

 section of maximum moment should never be permitted. 



Illustrative Problem. It is not always possible to commence the design 

 of a beam with an accurate enough idea of its weight to proceed as in the 

 previous problem. The usual way is to make an intelligent guess of the 



