166 PRACTICAL STRUCTURAL DESIGN 



chapter dealing with column design. The straight-line formula 

 used for wooden columns in Chicago is as follows: 



in which/' = reduced fiber stress per sq. in. in compression. 



/ = fiber stress used for columns having a length less 



than 15 times the diameter or least thickness. 

 L = length. 



d = diameter or least thickness. 



When L is in feet, d is in feet; and when L is in inches, d 

 is hi inches. 



Using this formula for the case under consideration 



/' = 1200 (l - g^g 2 ) = 1200 X 0.65 = 780 Ibs. per sq. in. 



The area of the piece is 36 sq. ins. and the total working strength 

 = 36 x 780 = 28,000 Ibs. The actual load it must carry is only 

 7190 Ibs., but in ordinances and specifications the maximum ratio 

 for the length divided by least width (ratio of slenderness) is 

 30 and the 6 in. x 6 in. piece barely comes within the limit. This 

 ratio is for vertical posts, whereas sloping posts have a tendency 

 to bend under their own weight, so something must be added for 

 additional stiffness. If it were not for this the 4 in. x 6 in. piece 

 would be good, as it has a safe compressive strength of 18,240 

 ins. considered as a vertical post. The ratio of slenderness of 40, 

 however, is against it. 



The piece UzLi and the piece UiL will be made 8 in. x 8 in. 

 without computation, for the former has only a load of 21,750 Ibs. 

 to carry and as a vertical post a 6 in. x 6 in. can safely carry 

 28,000 Ibs. The additional stiffness secured by adding two inches 

 to the breadth and thickness saves it from bending. To compute 

 it we find that the safe fiber stress is 835 Ibs. and it can carry as 

 a vertical column 53,440 Ibs. ; therefore this size will do for the 

 end piece. 



When joint details are designed it may be discovered that some 

 of the members must be made larger to allow for bolt holes good 

 daps. This additional area may be added at the time, but the 

 detailing of the truss should proceed in the order here followed, so 

 the pieces used in the computations may be reasonably close to 

 the actual dimensions finally adopted. 



