164 PRACTICAL STRUCTURAL DESIGN 



For many years it has been accepted as true that if a piece 

 under compression is made of a number of smaller pieces it will 

 not act as a solid piece. A number of experiments were made to 

 determine this and it was discovered that the secret lay in the 

 connections. Pieces as ordinarily made were found to be very 

 deficient in strength. Thin planks spiked together as thoroughly 

 as the ingenuity of the experimenter could devise proved to be 

 almost as strong in small specimens as solid pieces. It is not 

 likely, however, that in actual work this amount of nailing will 

 be done. Experiments made on rather large columns did not show 

 up so well as experiments on smaller columns. 



It is considered to be not the best practice to build up com- 

 pression members of thin pieces, and when slender pieces are used 

 they should be as few hi number as possible and shear pins should 

 be used as shown in Fig. 97 (d). If a number of thin planks must 

 be used they should be spiked together by gradually building up, 

 no expense for spikes being spared. After the piece is built up 

 lay wide pieces across the edges and spike these pieces to the edge 

 of each plank. These cross pieces will cover the two sides from 

 one end to the other and serve to call each plank to the assistance 

 of all the others in case there is any bending. 



It has been stated that the compressive fiber stress given in 

 specifications for wood is based on pieces having a length not 

 greater than 15 times the diameter or least thickness. When 

 the proportions adopted provide for a more slender column the 

 following formula is used to ascertain the reduced fiber stress to 

 be used. 



SOdJ 

 in which 



f* = reduced unit fiber stress. 



/ = unit fiber stress for pieces having a length not exceeding 

 L = length of post. 



d = diameter of round post or least dimension of rectangular post. 



This formula is used in Chicago for wooden posts. There are 

 several formulas in common use for finding the reduced stress to 

 use for slender posts and these will be discussed in the chapter 

 dealing with columns. 

 L The reason it is difficult to get several pieces to work properly 



