Dear anurag,
if the explanation in ATENA Theory, 2.1.7 Shear Stress and Stiffness in Cracked Concrete and 2.2.3 Rankine-Fracturing Model for Concrete Cracking, around Equation (2.47) is not illustrative enough, I recommend to look in ATENA Troubleshooting, 2.2.4 I want to use the user-defined stress-strain law of concrete to replace that used in ATENA program. How can I do it? + the there referred spreadsheet. There, you can play with the parameters of the spreadsheet to see the influence.

The shear factor coefficient value of 20 is based on experimental work by Walraven, (1981).

Regards.