COLUMNS AND STEUTS 111 



For posts 



' P I 



- = 7,000 - 40 - for live load stresses, 

 F t 



- = 14,000 - 80 - for dead load stresses, 

 F t 



P I 



- = 10,000 - 60 - for wind stresses. 



For lateral struts 



P I 



- = 9,000 - 50 - for initial stresses. 

 F t 



By initial stress in the last formula is meant the stress due to the 

 adjustment of the bridge members during construction. 



Problem 110. What must be the size of a square steel strut 8 ft. long, to trans- 

 mit a load of 5 tons with safety ? 



Problem 111. 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. 



Problem 112. 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 

 composed of four angles, a top plate, and two side plates. 



91. Beams of considerable depth. When narrow beams of consider- 

 able depth are subjected to compression, as, for example, in a deck 

 plate girder bridge, the strain is similar to that in a column. For a 

 narrow, deep beam the inertia ellipse is greatly elongated, and conse- 

 quently the radius of gyration relative to a line forming a small angle 

 with the horizontal is considerably less than the semi-major axis of 

 the ellipse. Therefore, if the beam is thrown slightly out of the ver- 

 tical by the unequal settling of its supports, or by any other cause, 

 such inclination results in a notable decrease in its resistance. Since 

 it is impossible to make allowances for such accidental reductions of 

 strength, beams of great depth or very thin web should be avoided. 



