592 



THE DESIGN OF STEEL DETAILS. 



CHAP. XVII. 



TABLES OF AREAS, MOMENTS AND MOMENTS OF INERTIA. 



The bending moment of this section, from Fig. 14 is 



M = 189,980 X 27 = 5,130,000 in.-lb. 

 or 



M = 173,000 X 29.5 = 5,130,000 in.-lb 

 The direct tension is 80,000 Ib. 

 The shear on the section -is 173,000 Ib. 

 Compression in extreme fiber due to moment 



Si = M-c' -f- I = (5,130,000 X 16.65) -* 10,802 = + 7,850 Ib. per sq. in. 

 Tension in extreme fiber due to moment is 



Si = M-c" '/I = 5,130,000 X 18.85 -* 10,802 = 8,950 Ib. per sq. in. 

 Tension on whole section due to direct stress 



S 2 = P/a = 80,000 -f- 97.2 = 820 Ib. per sq. in. 

 Total compression in extreme fiber 



S = Si + Sz = 7,850 820 = + 7,030 Ib. per sq. in. 

 Total tension in extreme fiber 



S = Si + S z = 8,950 - 820 = 9,770 Ib. per sq. in. 

 Unit shear is approximately 



S = 173,000 -r 97.2 = 1,780 Ib. per sq. in. 



The allowable unit stress in compression = 16,000 Ib. per sq. in. (Spec. 16). 

 The allowable unit stress in tension = 16,000 Ib. per sq. in. (Spec. 15). 

 The allowable unit stress in shear = 10,000 Ib. per sq. in. (Spec. 19). 

 END CONNECTIONS FOR TENSION AND COMPRESSION MEMBERS. For simple 

 connections with concentric stresses the number of rivets in riveted end connections may be taken 

 as equal to the total stress in the member divided by the allowable stress on one rivet for bear- 

 ing or for shear, Table 114, whichever gives the larger number of rivets. Specifications uni- 

 formly require that the connections of members be designed to develop the full strength of the 

 member. The minimum number of rivets in shop connections should be two rivets, except for 

 lacing bars; while the minimum number of rivets in field connections should be three rivets, 

 except for lacing bars. In lateral bracing or stiff bracing or struts the actual number of rivets 

 required to develop the full strength of the member should be increased by two rivets, for the 

 reason that two rivet holes are almost certain to be badly distorted by the drift pins in draw- 

 ing the member up. Rivets should be grouped symmetrically about the neutral axis of the 

 member or the eccentric stresses should be calculated and provided for. The strength of a struc- 

 ture depends very much upon the strength of the connections, and the details of the joints and 

 connections should be worked out with great care. 



