When.

As'  

= area of the compression steel.

As''

= area of the tension steel.

d'  

= depth of the beam measured from the top of section beam to the centroid of compression steel area.

d''

= depth of the beam measured from the button of section beam to the centroid of tension steel area.

fc'

= compressive strength of  the concrete.

fy

= stress in the tension and comprssion steel.

h

= total depth of the beam.

w

= width of the beam.

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