Have been toiling away on this for an hour or so.

It's part of a 26M spiral walkway. 2 sides of mold outer and inner.... and yes there are more columns under ... it's not superstrength concrete 

SPIRAL-OUTER SEGMENT SURFACE.SLDPRT. The first feature in tree is the outer dia surface of a 300mm thick walkway.

A bit of history...

When constructing the raw spiral, made the OD a little larger and ID a bit smaller and then trimmed outer and inner to correct dia's to ensure a flat side perpendicular to ground plane. The idea was to avoid any odd twisting of side surfaces  when sweeping main shape out.

End result hopefully is a 6mm thick steel plate that can be flattened into pattern to fabricate outer & inner sides for mold.

As there is no flatten surface feature available to my Office version of SWX I tried to sweep a 6mm thick section to create sheet metal down the line. Using a path and guide curve that are the top and bottom edges of swept surface. bottom guide would not  work, so just did a regular sweep with normal constant. which is probably good enough for something this big .

Sheet metal now fails 

Any suggestions, trickery or magic........  are you out there !

Both outer and inner attached 

The outer is the only on I've made my shaky start on.

"The last yard is the hardest"