90 PRACTICAL STRUCTURAL DESIGN 



per square foot, including weight of beam and floor, is 132 Ibs. per 

 square foot. Material yellow pine with an allowable fiber stress 

 of 1300 Ibs. per square inch. Deflection ignored. 



Answer. The total load on the panel is 132 x 4.5 X 18 

 = 10,700 Ibs. 



M = 1.5 x 10,700 X 18 = 288,900 in. Ibs. 



Try an 8-in. x 14-in. beam (7.5 ins. x 13.5 ins.) 



1300 x 7.5 x 13.5 2 



Mr = ? = 296, Io6 in. Ibs. 



o 



Try for shear 



T , 7 46/is 4 x 7.5 x 13.5 x 130 



W = -5- = 5 = 18,550 Ibs. 



o o 



W. N. Twelvetrees, a British engineer, developed the following 

 method for designing a beam in which the breadth is to be some 

 definite proportion of the depth. 



Let n = r , then 6 = 

 b n 



m j i. h ^ 1 o 



To design so o = - W = TT = TT? = 2 



- 



,. D ,,, ''*"- u ^ 3 



M = Rbh? = R x - X h 2 = 



n n 



Applying the method to the example under consideration : 



Let B .f 1300, 217 . 

 6 6 



First. Design so the breadth equals one-half the depth. 



\s OGG QOO 



/C OOt/VJ\/ -| r> Ct A * 



= 13.84 ins. 



6 _ _ 6 . 92 ;,. 



ft 



Use commercial size 7.5 ins. X 14.5 ins. 



Second. Design so the breadth equals two-thirds the depth. 



