20 QUESTIONS AND PROBLEMS. CHAPTER I. 



23. Which is the stiffer, steel with a unit stress of 12000 Ibs. 

 per sq. in. and E = 30000000, or iron with a unit stress of 10000 

 Ibs. and E = 27000000? How do they compare? Ans. As 4 to 3.7. 



24. A steel piano wire 4250 ft. long with an 8 Ib. plumb- 

 bob was let down a mine shaft; when the 8 Ib. bob was exchanged 

 for a 50 Ib. bob, the observed elongation was 15 ft. What was the 

 change of stress per square inch, the cross-sectional area of the 

 wire being 0.00038 sq. in.? What was the modulus of elasticity 

 of the stcjel? Was the elastic limit exceeded? 



Ans., 110500 Ibs. per sq. in. 31,330,000. 



25. If in Fig. 5, Art. 12, the section is 2"x3" and P = 72000 

 Ibs., what is the maximum principal stress per sq. in.? What 

 will be the shearing stress per sq. in. on planes making angles 

 of 45 with the axis? On what planes is the unit shearing stress 

 a maximum? How does it compare with the maximum unit ten- 

 sile stress? 



Ans. 12000 Ibs. per sq. in. max. tension. 6000 Ibs. per sq. 

 in. max. shear. 



26. What kinds of stresses are found in simple beams? 



27. May a combination of bending and tensile stresses re- 

 sult in tensile stress over the entire section? Illustrate. 



28. What is the maximum unit stress in a steel bar 7 ft. 

 long, whose section is 3"x6", when the compressive load on it 

 is 180000 Ibs.? 



29. What is the difference between shear and shearing 

 stress? 



30. Will a notched bar elongate more or less than a plain 

 one? 



31. What is the shear in the middle panel of a truss of five 

 equal panels, and carrying equal panel loads at each panel point? 



32. What is the shear at the middle of a beam, supported 

 at its ends, and carrying a uniformly distributed load over its 

 entire length? 



33. Prove that the intensity of shearing stress on the plane 

 AC, Fig. 12, Art. 17, is equal to s. 



34. What is the deflection at the middle of an I beam, sup- 

 ported at its ends, due to shear only, for a load at the middle 

 such that the shearing stress per square inch (which is assumed 

 uniformly distributed over the web) is 4000 Ibs.? The span 

 is 20 ft. and E s = 12000000. Ans., 0.04". 



