Refer SAE Paper 700222, Loading Conditions Measured During Aerobatic Maneuvers by Albert W. Hall from NASA Langley Research Center.
In this context, that work basically confirmed that the existing structural airworthiness requirements are satisfactory. i.e. an airplane must be in utility category to be approved for spinning and it will be strong enough.
There is a little bit of data in this Australian report and some-one has kindly made it available online (it was done by BASI and I have seen it on the ATSB website but they continually re-arrange the pages so this is a more reliable link).
http://www.8kcab.com/G-LOC%20Report.pdf