Response Modification (R) for Structural Aluminum

[STR04]

I'm reviewing an aluminum bleacher design (low seismic area) and they are using R>3. Whoa! So I look at ASCE 7-05 and cannot find any reference to aluminum except for mechanical piping. I also looked at the aluminum design manual and it references ASCE 7. I want to say nonbuilding structures per Table 15.4-2 under All other self-supporting structures, tanks, or vessels not
covered above or by reference standards that are
similar to buildings for R=1.25. Any thoughts?

TIA