Art. 109. CONCENTRATED LOADS ON BLOCKS. 



177 



Both extreme fiber stresses are compressive in this case. The 

 average unit sitress is the mean of these two and is 27.8 Ibs. per 

 sq. inch, or the same as the uniform unit stress, which is also 

 apparent from an inspection of the diagrams Fig. 134. 



The extreme fiber stress at B is zero when 



or when e=-r- 

 I Avi 



For a rectangular cross section this becomes 



-? 



<5 



as is evident from the stress diagram ABk whose center of grav- 

 ity is y G AB from o or y 3 AB from A. (In this case the dia- 

 gram for the total stress on a cross section is similar to that for 

 unit stress). If the eccentricity of the load is greater than one- 

 sixth of the total width of the section either way from the center, 

 the stress at the further edge becomes tensile. Then to avoid the 

 tensile stresses in masonry, as is usually required, the resultant 

 of the loads must not fall outside of the middle third of the 

 width, or more definitely, it must not fall outside of the shaded 

 zone in Fig. 135 



(). 



For a circular ,^ 

 cross section the i 

 above equation L _ 

 will give y s d as 

 the limit for e, 

 that is the result- Fig. 135. 



ant must not fall outside a central zone whose diameter is equal 

 to % the diameter of the cross section. Fig. 135 (b). 



When in a rectangular cross section, no tension is possible, 

 the stress diagram is still a triangle in the second case, as Apo 

 for example, Fig. 134. The distance of the resultant from A is 

 V v Ap~y 2 he. 



Total stress=^ip X & X %Smax-=3 ( %& e ) & X % s max.=-P- 



Smax 



(-i h e) b 

 As the eccentricity e, increases, the denominator diminishes 



