WOOD AND CAST-IRON COLUMNS 181 



compression equally, and not to crush at one place before 

 the remainder of the section can be brought under compres- 

 sion, as would be the case if the bearing surfaces were uneven 

 and rough. 



LONG POSTS 



If the length of a post is over ten times its diameter or 

 the width of its narrowest side, it is termed a long post. A 

 post of this length, if not secured against 

 yielding sidewise, is liable to bend before 

 breaking, as shown in Fig. 2 (a). In 

 this case, the compressive stress is not 

 uniformly distributed over the cross- 

 sectional area of the post, but will de- 

 crease from a maximum value at the 

 concave side of the post to a minimum 

 value at the convex side. Or, if the 

 bending proceed far enough, the com- 

 pressive stress at the convex side may 

 change into one of tension. In some 

 cases, the post will split, as shown in 

 Fig. 2 (6), the two halves bending inde- 

 pendently. 



The formula generally used for long 

 square or rectangular wooden columns 

 with flat ends, deducted from elaborate 

 tests made on full-sized specimens at the ( a ) 

 Watertown Arsenal, is: FIG. 2 



si 



in which u is the ultimate strength of post per square inch of 

 sectional area; s, the ultimate compressive strength of mate- 

 rial, in pounds per square inch; /, the length of post, in 

 inches; and d, the dimensions of least side of post, in inches. 



EXAMPLE. A white-pine post with flat ends is 10 in. 

 square and 20 ft. long. Using a factor of safety of 6, what 

 safe load will the post support? 



SOLUTION. The ultimate compressive strength of white 

 pine parallel to the grain is 3,500 Ib. per sq. in. Inserting 

 the several values in the formula, 



