COLUMNS AND STRUTS 



105 





173. A column 18 ft. long is formed by joining the legs of two 10-in. steel chan- 

 nels, weighing 30 Ib./f t., by two plates each 10 in. wide and in. thick, as shown 

 in Fig. 85. Find the safe load for this column by Johnson's straight-line formula, 

 ^^^ / ^ N using a factor of safety of 4. 



174. A wrought-iron pipe 10 ft. long, and of inter- 

 nal and external diameter 3 in. and 4 in. respectively, 

 bears a load of 7 tons. What is the factor of safety ? 



175. What must be the size of a square steel strut 

 8 ft. long, to transmit a load of 5 tons with safety ? 



176. Design a column 16 ft. long to be formed of 

 two channels joined by two plates and to support a 

 load of 20 tons with safety. 



177. Using Cooper's formula for live load, design 

 the inclined end post of a bridge which is 25 ft. long 

 and bears a load of 30 tons, the end post to be com- 



FIG. 85 posed of four angles, a top plate, and two side plates. 



178. A strut 16 ft. long, fixed rigidly at both ends, 



is needed to support a load of 80,000 Ib. It is to be composed of two pairs of angles 

 united with a single line of l-in. lattice bars along the central plane. Determine 

 the size of the angles for a factor of safety of 5. (Note that the angles must be 

 spread | in. to admit the latticing.) 



179. For short posts or struts, such as are ordinarily used in building construc- 

 tion, it is customary to figure the safe load as 12,000 lb./in. 2 of cross-section area 



for lengths up to 90 times the radius of gyration ; that is, for-^ 90. To what factor 



of safety does this correspond, by Johnson's straight-line formula ? 



180. The posts used to support a girder in a building are 8 in. x 8 in. timbers 

 8 ft. long. Find the diameter of a solid cast-iron column of equal strength. 



If a wrought-iron pipe 4 in. in external diameter is used, what must be its thick- 

 ness to be equally safe ? 



181. At what ratio of diameter to length would 

 a round mild-steel strut have the same tendency 

 to crush as to buckle ? 



182. A load of 100 tons is carried jointly by 

 three cast-iron columns 20 ft. long. What saving 

 in material will be effected by using a single 

 column instead of three, the factor of safety to 

 be 15 in both cases ? 



183. Determine the proper size of a hard- 

 steel piston rod 48 in. long for a piston 18 in. 



in diameter and a steam pressure of 80 lb./in. 2 _ 

 Consult table for proper factor of safety. 



184. The side rod of a locomotive is 9ft. long between centers, 4 in. deep, and 

 2 in. wide. The estimated thrust in the rod is 12 tons, and the transverse inertia 

 and gravity load 20 Ib. per inch of length. Determine the factor of safety. 



185. The vertical post of a crane (Fig. 86) is to be made of a single I-beam. The 

 post is pivoted at both ends so as to revolve about its axis. Find the size of I-beam 

 required for factor of safety of 4 and for dimensions and loading, as shown. 



-H- 



8 Tons 



FIG. 86 



