QUESTIONS AND 



.EMS. CHAPTER i. 



19 



14. If E = 30000000, and the elastic limit of a steel bar 10 

 ft. long is 36000 Ibs. per sq. in., what will be the elongation per 

 inch at the elastic limit? What the total elongation? 



Ans., 0.0012" and 0.144". 



15. An iron bar 30 ft. long stretches 0.18 in. under a stress 

 per square inch of 14000 Ibs. ; what is the value of E? 28000000. 



16. The length of a steel compression member in a truss is 

 247 in., the area of its cross section is 60 sq. in., and its contrac- 

 tion of length during the passage of the live load is 0.0247 in. 

 What is the live-load stress, E being 29000000? Ans., 174000 Ibs. 



17. If a street railway rail be supposed to be rigidly held 

 at its ends, what kind of stress will be induced by a fall of 

 temperature of 60 F.? What will the intensity of stress be if 

 the modulus of elasticity is 30000000 and the coefficient of ex- 

 pansion 0.0000065? Ans., 11700 Ibs. per sq. in. 



18. If a plate girder be anchored at each end to rigid walls 

 so that it can not expand or bend, what stress per sq. in. will be 

 induced by a rise of temperature of 100 F.? E = 29000000 and 

 the coefficient of expansion = 0.0000065. 



Ans., 18850 Ibs. per sq. in. 



19. Is the stretch of a twisted wire hoisting cable elastic or 

 only partly elastic? Why? 



20. A steel column for an office building is made 250 ft. 

 long; how much will it be shortened when the average unit com- 

 pression in it is 10000 Ibs. per sq. in., E being 30000000? What 

 will be the contraction if the column is cast iron under an aver- 

 age stress of 8000 Ibs. per sq. in., E being 12000000? 



Ans. 1 in. 2 in. 



21. If a tension member be made of two bars of steel, each 

 6"Xl", and two bars of iron, each 6"Xf", so that the elongation 

 is the same for all when the total stress is 252000 Ibs.; what will 

 be the stress per square inch in each bar if E is the same for all? 

 What if E = 3 1000000 for steel and 27000000 for iron? 



Ans., 12000 Ibs. per sq. in. 11064 Ibs. per sq. in. for iron 

 and 12702 for steel. 



22. If E is 12000000 for cast iron, what should be the 

 elongations of a bar one square inch in cross section and 30 

 inches long, when the loads on it are 3000 Ibs., 6000 Ibs., and 

 9000 Ibs., if the metal is perfectly elastic under these loads? 



Ans., 0.0075", 0.015" and 0.0225". 



