# calculate the shear stress from surface traction

The problem is that I am simulating a hollow cylinder under torsion and I would like to compute the shear stress for it. The output of the simulation is the shear surface tractions. See the figure below

For each element in top cross-section, I have surface traction and the distance from the center of the element to the center of the hollow cylinder.

In order to calculate the shear stress I have to use

$$T = \frac{1}{n}\Sigma \tau_i r_i$$

where $$\tau_i$$ is the surface traction for element $$i$$ and $$r_i$$ is the distance.

Can anyone explain why I should multiply the surface traction by the relative distance? I couldn't find the theory behind it.