BENDING MOMENT 



291 



algebraic sum of the moments of all the external forces acting 

 on that portion of the beam to the right, or to the left, of that 

 section. The moment of resistance is the algebraic sum of the 

 moments of all the induced tensile and compressive stresses, 

 taken about the neutral axis. 



153. Let Fig. 97 represent a portion of a bent beam, so taken 

 that the form assumed is a circular arc of radius equal to the 

 radius of curvature. C is the centre of curvature. 







The top part of the beam is in tension and the lower part is 

 in compression, while there is one surface in an unstrained con- 

 dition, known as the Neutral Surface. The intersection of the 

 neutral surface with a certain section of the beam gives the neutral 

 axis of that section. 



Let ab be a fibre situated at a distance y from the neutral 

 surface. This fibre is in a strained condition, and 



The strain in ab 





increase in length 

 original length 



bc_ 

 ac 



be 



