DESIGN OF COLUMNS 



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Alternate Tension and Compression. Where members are subject 

 to alternate tension and compression the specifications require that they 

 be designed to take each kind of stress, (paragraph 32). 



Member 4-^. Maximum stresses = ( 21,300 and + 2600 Ibs). 



Try two 3" x 3" x %" angles the same as member 1-2. The 

 net area required for tension is 21,300 -=- 16,000 = 1 .34 sq. in. 



The net area of two 3" x 3" x y^" angles is 2.88 0.38 = 2.50 

 sq. in. which is ample for tension. 



The least radius of gyration is r = 0.93 inches (page 185 Cam- 

 bria). Length = 1 08 inches, and / -=- r = 117. The allowable stress 

 per square inch = 16,000 70 x 117 = 8,810 Ibs. Required area = 

 0.30 sq. in. The section appears to be large, but it can not be made 

 much smaller without exceeding the maximum limit of 125 for / -=- r. 

 Two 3" x 2y 2 " x %" angles will be found by a similar calculation to be 

 sufficiently large, and will be used. 



Member 3-4. Maximum stresses = ( 10,900 and + 13,600 Ibs). 



Try two 2" x 2" x *4" angles. The area required to take the ten- 

 sion is 10,900 -=- 16,000 = 0.69 sq. in. The net area of the two 

 angles is 1 .88 0.38 = 1 .50 sq in., which is ample for tension. The 

 least radius of gyration is r = 0.61 inches (page 185 Cambria). The 

 length is 108 inches, and /-f-r = 177. This is greater than the max- 

 imum allowed of 125, and a larger section must be used. 



Try two 3" x 2" x %" angles, with short legs out. In this case 

 I -r- r equals 108 -r- 0.89 = 120. The allowable stress per sq. in. is 

 16,000 70 x 120 = 7,600 Ibs. The required area for compression is 

 13,000 -f- 7,600 =1.79 sq. in. The area of the two angles is 2.38 sq. 

 in., which is ample. The section is sufficiently large to take both ten- 

 sion and compression, and will be used. 



Design of Columns. Columns must be designed to take the 

 stress due to direct loading, to eccentric loading, and to wind moment. 

 The method of column design will be illustrated by the design of the 

 leeward column in the transverse bent shown in Fig. 56 ; the stresses for 



which are given in Table VI. 

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