When working with circular stairs it can be difficult to draw a wall that exactly matches the stair. This method makes it easy.
First make sure that your stairs are complete and that they meet code. You want to make sure that they are the correct height, width, radius, etc. Also make sure that you have the stairs positioned correctly on the plan.
Start out by drawing a circle at the very center of the stairs. Rounded objects such as stairs include a center point that is automatically created. You can't dimension from this point but you can snap objects such as circles and lines to the center point.
Since the center point does not print I will typically create a circle and crossed lines centered on the stair center point. With snap on (F11) use Draw--> Shape--> Circle--> Circle--> Center and Draw--> Shape--> Line. This gives you the ability to place dimensions to walls in order to designate the center point for the stairs.
Next use the Tools--> Offset tool to offset the center circle some arbitrary distance. Check the radius of the stairs by editing the stairs and noting the radius dimension. Edit the circle you created using Offset and set the radius to the stair radius + 1/2 the width of the wall that will be curved to the stairs. In the example I am using I have a 10' radius stair with a 3 1/2" wall. So I would make the circle radius 10'-1 3/4".
If applicable, draw tangent lines from the circle for the straight walls that will be coming off of the curved walls.
Next use Tools--> Convert Shape to Wall and select the circle and any line extensions from the circle. Select the appropriate wall that you want to create, and then select OK. Select the circle and lines. They will become walls.
Circular walls created in this manner are actually 4 walls, one for each quadrant. Erase the walls you do not need and use Adjust if necessary to adjust the wall you want to use.
In this example I used an interior wall that was defined as having a center trace line position. This wall is created exactly centered on the circle and lines when using Tools--> Convert Shape to Wall. If you are placing your stairs on an exterior wall you may need to do some experimentation to determine where the walls will be created relative to the shapes.
To determine the correct circle size for a given wall start out by drawing a circle and edit to the desired radius, say 10'. Use Tools--> Convert Shape to Wall. Draw another circle, snapping it to the center point of the walls just created. Edit the circle to be the same radius as the previous circle, in this case 10'. Compare this circle to the walls you created. Measure from the inside of the wall to the circle. Edit the circle and add this dimension to the circle radius. Delete the previously created walls and use Tools--> Convert Shape to Wall to make the walls again. The inside of the round walls should now be at the desired radius.
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