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.
Dear Sir,Hi Chris,
thanks alot of this interesting website.
I am currently studying to achieve a Builder's Licence.
Currently I am searching for ideas ,photos,explanation re formwork for a concrete Helical staircase. Any idea,have checked several books,the internet with very little success.
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.
" We are carrying out a shuttering job in France at the moment and we are doing a radius stairs and we would like some ideas about the under carriage The width of the stairs is 2 metres and it goes up 4 flights of floor each floor rises 3 metres the stairs has to be made in concrete"
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.
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.
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.
Single one off flights would be very expensive to form.
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.
More on concrete stairs at the end of this section where we tag onto the stair section.
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