Dear Pravin,
The spanwise locations of the nodes where the local blade loads (Spn1MLxb1 etc.) are output are described in the old FAST User’s Guide–see the description of BldGagNd. I don’t have a great figure to share, but a description of the local blade coordinate system where the local blade loads are output is given in my Mar 13, 2015 post in the following forum topic: Discrepancy in Loads Between Blade Root and Local Spans.
While a detailed description has not been written, there is some discussion about post-processing FAST results for structural analysis of stress, etc. through FEA in the following forum topics:
Stress analysis of blade.
Best regards,