WOOD AND CAST-IRON COLUMNS 183 



200,000 Ib. Therefore, a 7"X7" post is not strong enough. 

 Next, try a 10'XIO" post. Thus, 



5,000X15X12 



= 5,000 =4,100 Ib. per sq. in. 



100 X 10 



The ultimate strength of this post is therefore 4,100X10 

 X 10 = 410,000 Ib. Since this value is greater than 200,000, 

 the post is safe; but perhaps a smaller post would also fill the 

 requirements. Therefore, try an 8"X8" post. Thus, 



5,000X15X12 



= 5,000 =3,875 Ib. per sq. in. 



100 X 8 



The ultimate strength of this post is therefore 3,875 X 8 X 8 

 = 248,000 Ib. Since this result is greater than the required 

 ultimate strength, the post is strong enough. Moreover, 

 it has been found that a 7" X 7" post is not strong enough. 

 Therefore, unless the timber be cut to fractions of an inch, 

 the 8*X8" post is the smallest post that will fulfil the require- 

 ments. 



If the formula just given is transposed so as to read 



u = s ( 1 I , the factor ( 1 ) may be calculated 



\ 100 d/ \ 100 dj 



for various values of / and d and arranged in the form of a 

 table, as shown on pages 184-186, that will prove of great 

 assistance when using the formula. 



In this table, the first column contains various lengths 

 of posts, in inches, and in the top horizontal row are values 

 of the least thickness of a post, in inches. In order to find 

 the factor for a given post, first find the nearest length in 

 the first column; then proceed from this value to the right 

 until a point is reached that is below the least dimension 

 in width. The value found at this point is the factor desired. 



EXAMPLE. A 12"X14* Georgia-pine post is 18 ft. long. 

 What safe load will it carry? 



SOLUTION. According to the table, the ultimate strength 

 per square inch for Georgia pine is 5,000 Ib. The length of 

 the post, in inches, is 18 X 12 = 216, and the area of the post 

 is 12X14 = 168 sq. in. According to the table, the factor 



is .821. Inserting this value in the formula, u = s ( 1 



\ 100 a 



