Random analysis: RMS value of contact force

Hello all,

I'm doing a random response analysis with application of a PSD acceleration function, that follows a frequency step to calculate the modal basis. 

In the random step, we can output the RMS value of stress or displacement. But I couldn't find how to output the RMS value of contact normal force or contact shear force at specific interfaces. Is there a way to do that, either as a field or history output? 

If not, can I compute the RMS value in another way, from the curve of contact force in N²/Hz through the frequency range considered?  

Thank you,

Regards,

Jimmy