I'm new here, so I don't know how to attach a document, so I'll have to post some limited text on the subject.
You have to first calculate the areas encompassed by each individual "bin" on your PSD profile.
A "bin" is defined from a bandwidth (Bw) of frequencies (e.g. PSD = .1 g2/Hz from 20 thru 60 Hz [Bw = 60 Hz - 20 Hz = 40 Hz) containing the desired PSD level(s).
Therefore, a "bin"'s area would be calculated by multiplying it's bandwidth by it's corresponding PSD value.
So in the above example, the bin's area would be:
Individual_Bin_Area = .1 * 40 Hz = 4
To calculate total Grms for a PSD profiles entire bandwidth, you take the square root of the [SUM OF ALL individual bin areas].
Triangular shaped bin areas may be calculated, by calculating a rectangular area that the triangle area resides and then simply dividing the result in half.
