150ft works out to be about 45m, In Australia we have requirements for rooftop helicopter landing sites (HLS), safety requirements, weight limits etc. i can track them down. But i think good information know is...
1. How high is the building (re: wind velocity)
2. Location (Sea side,inland,moutain,altitude etc.)
With regard to down wash, in my opinion i would say that would't be a factor...prob more static loads from the helicopters skids/landing gear. . on the HLS sureface, and the support systems that supports the load of 45m radius HLS, with helicopters from 1-2000KG coming and going...
And also by Failed do you mean - Cracking ? Callapsed etc ?
just my 2 cents.
JF