124 QUESTIONS AND PROBLEMS. CHAPTER VI. 



size of I beam should be used as a floor joist, in which the 

 maximum moment is 93300 ft. Ibs. Ans. 18"x55.0 Ibs. 



17. Where is the neutral axis located in the sections of 

 Fig. 70. 



18. Prove that the neutral axis in the beam of problem 11 

 is located as given in " Cambria." 



19. Find the location of the neutral axis of a section 

 composed of an 18"x60.0 Ib. I beam, having a 12"x20.5 Ib. 

 channel riveted to its upper flange, through the web of the 

 channel (flanges down). Make a complete sketch. 



Ans. 11.19 in. from lower edge. 



20. What is the moment of inertia of the section given in 

 problem 19? Ans. 1221.5 in. 4 



21. What is the extreme fiber stress in the beam whose 

 section is given in problem 19 if the maximum is moment 105000 

 ft. Ibs.? Ans. 11460 Ibs. per sq. in. 



22. What is the moment of inertia of a triangular area of 

 height h and base b about its base? 



23. In the fourth section of Fig. 70, what is the moment 

 of inertia of one of the angles about the axis NN, through the 

 center of gravity of the section? Of the four angles? Of the 

 plate? Of the entire section? 



24. What is the moment of resistance of the web of a plate 

 girder which is 48" deep and f" thick, when the extreme fiber 

 stress is 10000 Ibs. per sq. in.? 



25. What will be the maximum moment allowed in the 

 beam of problem 23 if the working stress is 12000 Ibs. per sq. in.? 

 What in a beam whose cross section is shown in Fig. 73? 



26. In what cases is the neutral axis perpendicular to the 

 plane of the bending moment? 



27. What is the axis of a beam? The neutral axis? The 

 neutral plane? 



28. What is the location of the principal axes of a 6"x4"x 

 I" angle? 



29. If in the problem illustrated in Fig. 80, a timber 

 purlin 4" wide by 10" deep is substituted, what will be the 

 extreme fiber stresses? 



30. What is the location of the neutral axis in problem 29? 



31. Find where the shear passes through zero in Fig. 81 



