Draw a spiral staircase with a central stringer

Hello,

I would like to draw a spiral staircase with a central stringer.

But I would like the central stringer is made of rectangular tube.

How is it?

Bonjour,

Je voudrais dessiner un escalier hélicoïdale à limon centrale.

Mais je voudrais que le limon central soit fait en tube rectangulaire.

Comment fait on?

SolidworksWeldments