Spiral concrete stairs or helical stairs.

I have had two questions recently about spiral concrete stairs, here is the last one from Chris A. who lives in Haz-Zebbug, Malta. In it I quote from a previous question and answer.

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Hi Chris,
There is not much on the web that's for sure.   I had a bit of correspondence a few weeks ago, with an English contractor doing a job in France, here it is below.

Here's my reply to Steven's original spiral concrete stairs question.

"Hi Steven, Shuttering job in Nice, good for you. Soffit on a spiral stair, now that's a tough one.  For the most part people use either full steel construction or precast units for spirals.

I am guessing a bit here, but let's say you have a 4m diameter outer wall and you were doing one full 360deg turn with each flight.
4 x pi(3.14) will give you your outer perimeter of 12.57M
So at the outside you have a go of 12.57M and a rise of 3M.
So if you draw that to scale you will get your angle of rise at the wall.
( put this another way, if you imagine a piece of cardboard cut to a triangle with a 12.57m base and a 3m height, then you would be able to roll it inside the outer wall to mark a soffit line on that wall).

At the center you will have some sort of column I guess, either permanent or temporary. Let's say that is 300mm dia. That's a perimeter of only 940mm with a rise of still 3M.

So you have a go of 940 on the inside and 12570 on the outside.   This means that any given section of the soffit has a curving twist in it which is just too much for all but the thinnest ply.

I have only ever done one spiral concrete stair, and It was less than perfect.   From memory we had a center column that was 300 dia.  We put horizontal spokes off it to the circular outer wall (blockwork) and then we laid thin strips, say 25 x 25 at the outer and 12 x 25 at the inner edge where the twist gets severe.  The same method that the boatbuilder's call strip planking.

It worked OK for a one off and we had heaps of cheap timber for making the strips, but it was not reusable.  We had to plaster the soffit after.

For reusable formwork for spiral concrete stairs I would be thinking of making up bolt together steel panels with say 1mm sheet steel faces.  It depends on what sort of finish you have to provide too.
Another possibility is a set of fibreglass moulds".

Back to Chris

The risers would be formed in the normal way, but if the stair was wide, it would be impossible to support the middle of them the normal way, (with a strongback square off the riser) so I'd be thinking of stiffening them with steel and a strong anchor at each end.
When it gets to jobs like these I think structural engineers do the design of the formwork at the same time as the rest of the design.

Single one off flights would be very expensive to form.
One of these days I will get down to doing something on the site, or even an E-Book, as it is very hard to explain the concepts without drawings and photos.
Best of luck with your studies
cheers
Bill

A few extra notes on spiral concrete stairs.

The soffit of one of these stairs has an incredible amount of twist in it, and the people that work all the time with curved shapes are boat builders.  It seems to me that we could steal a few of their ideas.

• I have already mentioned the strip planking soffit that I built.  With a little more thought I could have made that set of formwork reusable.
• We could have set up on site say enough soffit for four steps, plus overlaps.
• Fiberglass the surface with mesh or chopped strand mat.  strip it and fiberglass the underside.
• Then we could have used that one panel to pour four steps a time, or just made a few more panels the same way, so that we could pour a full flight.
• Boat builders have a technique called tortured ply.  They laminate two or three layers of ply that is very thin to create a far stiffer structure than the equivalent thickness of flat play.
• Again, the technique of foam sandwich construction could be used to make strong stiff forms for spiral concrete stairs.

More on concrete stairs at the end of this section where we tag onto the stair section.

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