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CHAPTER IX. 

 STRESSES IN BLOCKS AND COLUMNS. 



A piece of material in compression is a block when its ratio 

 of length to least width is small. When this ratio is large it is 

 called a column, a pillar, a post or a strut. The boundary be- 

 tween these two classes is not a definite one, as will appear from 

 the following discussion. 



Arch stones of stone bridges and ordinary masonry pillars 

 are examples of blocks. Compression members of trusses, and 

 posts in buildings are examples of columns. A vertical column 

 is usually called a post, and secondary compression members, 

 such as those belonging to the wind bracing, are called struts. 



108. Concentrated Loads on Blocks. When the load and 

 reaction are uniformly distributed as 



in Fig. 133 (a), the unit stress on any 



W 



square cross section is s c = T-. When 



A 



the load is not uniformly distributed, or j j t j j j 



is concentrated, the stress on any cross () 



section near the ends, can evidently not 



be uniformly distributed, but it may be at some intermediate 



cross section, provided the line of action of the resultants of the 



external forces is coincident with the axis of the block. 



Bauschinger's experiments on sandstone blocks show that 

 the smaller the surface of application of the load, that is, the 

 nearer the load approaches a concentrated one, the less the ulti- 

 mate strength will be. 1 Thus a block of stone will carry more 

 load when loaded as in Fig. 133 (a) than when loaded as in Fig. 

 133 (6), as would be expected by analogy with uniformly dis- 

 tributed and concentrated loads on beams. 



109. Eccentric Loads on Blocks. Fig. 134 shows a block 

 CDEF which is supposed to be loaded with a concentrated load 



1 See Johnson's Materials of Construction, Chapter III. 



175 



