If I understand right, the stringers are straight and supported at either end, probably with bolted connections. And the curved platform wouldn't really enter into it.
I would approach it thus:
Assume zero vertical reaction at the upper support, and sum forces and moments to find the maximum horizontal reaction at top and both reactions at the bottom. Then assume zero horizontal load at the upper support, and sum moments to find the maximum vertical reaction at the top and vertical reaction at the bottom. The actual forces should lie somewhere in between these two cases.
To get any better results, you would need additional information not stated.
