INTERNAL FORCES 81 



Assuming common values : 



For steel, / = 16,000 Ibs. per square inch, D = - j-r- 



ou n 



For wood, / = 1300 Ibs. per square inch, D = TT-T- 

 For wood, / = 1000 Ibs. per square inch, D = 



44/i 

 For wood, / = 800 Ibs. per square inch, D = rz-r' 



'lO /I 



See page 98. 



Aids to Computation 



In addition to the handbooks of the steel companies and the 

 " Manual of Standard Wood Construction," designers use dia- 

 grams and slide rules to lighten their work on simple problems. 

 The following are suggested in this connection : 



The Wager timber scale for computing the strength of wooden 

 beams, $1. 



The Merritt beam scale for computing the strength of steel 

 beams, $1. 



Des Moines Bridge & Iron Company's calculator for steel 

 beams, channels, angles, and tees, 25 cents. 



The two first mentioned are made of heavy paper and the 

 third is of celluloid. The writer has used them daily in his work 

 for some years. 



The most complete rule for this work is one designed by Benja- 

 min Winslow. It enables one to design with any fiber stress, any 

 span, any spacing, any system of loading, etc. The rule is made of 

 German silver and costs $10. The size is 3J ins. x 10$ ins. x 16 ins. 

 Taking the place as it does of all pocket books, tables and diagrams, 

 the writer feels that to omit recommending it to structural drafts- 

 men and designers would be a neglect on his part of a plain duty. 

 Mr. \\ in slow has also placed on the market a similar slide rule 

 for reinforced-concrete design. 



Slide rules of the Mannheim type are used to-day by all engi- 

 neers, but a recent improvement is known as the Phillips slide 

 rule. This rule enables one to multiply three factors at one 

 setting and the arrangement of the graduations wonderfully in- 

 creases the value of the slide rule for all purposes. This new rule 

 sells for $5. 



Example. Determine the size of a wooden beam using a 



