86 PRACTICAL STRUCTURAL DESIGN 



The factor of strength is obtained as follows : 



The fiber stress is in pounds per square inch and the section 

 modulus is in square inches, therefore the resisting moment is in 

 inch pounds, or 12 times the bending moment in foot pounds. 

 Two-thirds the moment in inch pounds is equal to 8 times the 

 bending moment in foot pounds; which, in turn is equal to the 

 total uniform load in pounds times the span in feet, for a freely 

 supported beam. 



Let S - section modulus in inches. 



/ = maximum fiber stress in Ib. per sq. in. 

 Then C = F = f/S. 



Let M = bending moment in foot pounds. 



Then C = F = 8M. 



Having computed the bending moment in foot pounds, multiply 

 by 8 and in the table of properties of beams look for this value, 

 or the nearest higher value, of F (or of C) in the Cambria or Beth- 

 lehem book. Following the line to the right, the beam is found 

 which has this factor of strength. Each of the books mentioned 

 contains a separate table of bending moments in foot pounds for 

 each beam, so the designer has his choice of methods to use in 

 obtaining a beam size when he has the bending moment instead 

 of the uniformly distributed load. 



Example. A beam carrying several concentrated loads must 

 resist a bending moment of 46,680 ft. Ibs. What is the best size 

 and weight of beam to use? 



Carnegie (1913 edition) : On page 184 it is shown that the 

 resisting moment of a 12-in. I-beam weighing 31.5 Ibs. per lin. ft. 

 = 47,960 Ibs., so this beam will be used. 



Page 182 contains a description of all the factors shown on 

 page 184, relating to the properties of beams. The student is now 

 prepared to study pages 133, 140, 141, 164, 167 to 171 inclusive, 

 176 to 182 inclusive. 



Cambria (1913 edition) : On page 118 it is shown that a 12-in. 

 I-beam weighing 31.5 Ibs. per lin. ft. has a resisting moment of 

 48,000 ft. Ibs. 



The following pages should be studied by the student, 76, 77, 

 80 to 89 inclusive, 142 to 147 inclusive, 158 to 163 inclusive. 



"Lackawanna Hand Book" (1915 edition): This book does 

 not contain a table of bending moments for standard beams so 



