242 PRACTICAL STRUCTURAL DESIGN 



buildings of a greater height than twice the width, for wooden 

 posts are not continuous and, therefore, cannot be relied upon 

 in case of heavy winds to stiffen the building. 



The allowable unit compressive stress for wood is for short 

 blocks only, in which the length does not exceed twice the di- 

 ameter. The unit stress on wooden posts is found by the follow- 

 ing formula: 



TO L \ 

 ~Cd)' 



in which/ = reduced fiber stress; 



c = unit compressive stress mentioned in ordinance; 

 L = length in feet; 



d = diameter, or least thickness, in feet; 

 C = a constant, which is 80 in Chicago and has values 

 ranging from 60 to 100 in other cities. 



The above formula is called in some places the "Straight-line 

 formula" and in other places the "Winslow formula," from 

 Benjamin Winslow, who is credited with being the originator. 



Wooden posts should be solid. A number of experiments 

 made on wooden posts built up of thick planks spiked side by side 

 showed that the strength of such posts is not the sum of the 

 strength of the planks. Each plank when loaded on the end 

 tends to deflect as though it were a long slender column. Some 

 experiments made for Mr. Dewell in California on short models 

 gave better results than any other recorded experiments, but 

 his built-up posts were better made than is apt to be the case 

 with full-size posts. If a wooden post is built up shear pins must 

 be put between the planks and bolts must go through the other 

 way. Sometimes plates on the edges securely screwed to each 

 of the planks will make them act together. 



Only very short posts fail by crushing under load. The usual 

 failure in posts and columns is caused by bending, which crushes 

 the fibers on the concave side and frequently causes tension in 

 the convex side. For long columns it is necessary to use a for- 

 mula for reducing the allowable compressive stress for short blocks. 

 The stress is progressively reduced as the length of the column 

 is increased until finally a point is reached beyond which the 

 "slenderness ratio" is so great that the column may be unsafe. 



The " slenderness ratio" is -5 for masonry piers and wooden posts 



