494 Prof. C. A. Carus Wilson on the Influence of 



Curve. 



Spaa in millira. 



Ratio of span 

 to depth. 



1 



120 

 100 



88 

 80 

 78 



72 



6-31 

 5-26 

 4-63 

 4-21 

 4-10 

 379 



2 



3 



4 



5 



6 





This experiment shows that there are, generally, two points 

 of zero shear which close up as the span diminishes and then 

 open at right angles. 



The same phenomena may be observed by placing a beam 

 on a flat surface and loading it, and then placing over this 

 beam a second, which may be bent with a very long span, or 

 by two couples at the end ; the effect is the same for different 

 degrees of bending as for varying spans in the former 

 experiment. 



Thus for spans of four to five depths the normal under the 

 load is divided into three parts by two points of zero shear, 

 elements between these points being subject to shear (vertical 

 stretch), while elements above and below them are subject to 

 shear (vertical squeeze). 



When, however, the span is less than four depths, every 

 element in the cross section under the load is subject to shear 

 (vertical squeeze) and the greatest strained element is im- 

 mediately under the load. 



These results may be further checked and confirmed by 

 examining each part of the normal by placing over it a beam 

 bent in the hand ; if the part under examination is in shear, 

 say (vertical squeeze), darkness may be produced by super- 

 posing a part of the second beam oppositely strained; if the 

 strains were similar, increased brightness would result. 



I exhibit also the results of experiments made to determine 

 the position of the black bands for lower ratios of span to 

 depth. 



The dimensions of the beam were 124 millim. x 20 millim. 

 deep X 6'5 millim. thick, loaded on rollers like the others ; 

 nicols crossed and at 45° to the axis. 



Here the effect of the supports is very marked, so that 

 when p = 2 the black band only just touches the axis. 



It must be remembered that at the point where the black 

 band cuts the normal the shear is zero, but that everywhere 

 else on the band all that is indicated is that the directions of 

 resultant tension and compression are at 45° to the axis of the 

 beam. 



