QUESTIONS AND PROBLEMS. CHAPTER II. 



33 



1. Which is the more important to the designer, the elastic 

 limit or ultimate strength? 



2. If a floor carrying a constant load were supported by 

 steel rods, what objection would there be to having the stress in 

 these rods exceed the elastic limit? What objection would there 

 be to having the stress in a member of a truss exceed the elastic 

 limit? 



3. What is the difference between natural elastic limit, true 

 elastic limit, limit of proportionality, yield point, and commercial 

 elastic limit? 



4. Name three materials that do not strictly follow Hooke's 

 Law and one that does, but does not have a yield point. 



5. Why does a piece of steel pulled in a testing machine 

 break, if the load does not increase up to the time of rupture? 



6. What is the total resilience of a piece of steel 8" long 

 and ^ sq. in. in cross section, if its ultimate strength is 60000 

 Ibs. per sq. in., its elastic limit 30000 Ibs. per sq. in., and its 

 elongation 2"? Ans., 50000 in. Ibs. 



7. Up to what point are deformations practically perfectly 

 elastic? Between what points are they wholly permanent? 



8. Name four substances that will show definite failures 

 when tested in compression, and five that will not. 



9. What conditions have a marked influence upon the value 

 of the ultimate strength in compression? 



10. Why is the yield point the limit of strength for the 

 purposes of the engineer? 



11. What experiments upon the strength of American tim- 

 bers have been the most extensive? 



12. What values for the ultimate strength in tension, com- 

 pression and shear does Rankine give for wrought iron? 



13. What material has an ultimate strength in compression 

 greater than that in tension? And in what material is this 

 relation reversed? 



14. When may the factor of safety for dead and, live load 

 stresses be the same? 



15. Why are factors of safety for stone, timber and cast 

 iron taken greater than for steel? 



16. Why should not a highway bridge have as large a factor 

 of safety as a railway bridge? 



17. Why is the factor of safety for wind stresses taken 

 smaller than for other stresses? 



