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Hi Ness
We have finally had delivery of the balance steel landing balustrade and I think that we may be able to widen the landing at the point that it connects with flight 2, so could you hold off on producing the plans for the moment please? Would still be grateful for your feedback though as we are only talking a matter of a few centimetres …
Regards
Bob
Hi Ness
As a post script, the centre cut string follows the walk line (probably not the correct phraseology) rather than being in the actual centre of the stair – not sure whether this is correct but I can move it easily.
Regards
Bob
Hi Ness
So, some time later … I have been through lots of your recommended reading and videos and think I am clearer now about these stairs. I have settled on using a horizontal laminate central string at 144mm thick in pine (I’m fitting timber dimensions to fit with available PSE as I don’t own a face planer) which I hope will adequately overcome the narrowness of the string. The string board against the wall has 15mm outside of the stairwell as well as the 20mm that shows on the plan (we have dry lined walls) which neatly allows me to use your string splitting idea which will make life much easier. The lhs cut string would appear to be semi ornamental in reality so I have used 44mm oak board for the straight sections and plan to use your conventional laminate bending technique for the curved joins (the join at the base will form part of curved ended step for the volute). You will also see that the centre string has a crossover and will have a kicker at the base – I can see what you mean about the steps keeping everything in place but it makes me nervous nevertheless ……
Have I forgotten/overlooked anything that you can see? If not could you let me have a set of plans please, I think I am going to have to do the laminate templates by hand rather than learn a CAD programme!
Let me know what you think please
Regards
Bob
October 4, 2012
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Hi Bob,
To answer your questions:
StairDesigner will not draw the horizontal laminates, you have to draw these using the StairDesigner DXF output, see this article:
https://wooddesigner.org/how-t…..ved-stair/
If making by hand, you can print out the full size template for each laminate, if using CNC use the DXF file to cut the parts.
Alain Marin used StairDesigner DXf files and drew the laminates himself. The process is fairly simple if you have some 3D drafting experience and know how to use CAD or you can even draw by hand.
If you use oak for the laminates the string will be stronger and also much heavier. In general I have always used pine as this is light and as each element is only constrained in compression or extension there is little difference in the structural resistance.
If painted the best is to use flexi or bendy plywood for the outer laminates. This makes it easy and fast to glue up and I’d suggest 7mm bendy ply on each face glued with grain vertical to make it easy.
The larch veneers in this project are glued in a similar fashion:
https://wooddesigner.org/gluin…..ed-string/
Of course extending the string to the wall will add stability.
I personally wouldn’t consider even the central stringer absolutely necessary as this doesn’t add to the structural rigidity of the stair but would only be useful to give central support to the steps. But as the stair has risers, if steps and riser are jointed and glued the risers will stop the steps from bending over 1m20.
If built and jointed correctly and fixed firmly to the supporting walls, this type of stair is extremely rigid and requires minimal external support.
All the best,
Ness
Hi Ness
Not so much food for thought as a veritable feast! A few questions though:
Does sd7 provide drawings for the laminate sections – I noticed in the Alain Marin piece that he printed of full size templates of the laminate, not sure whether this was done from your plans or from his calculations from your plans.
Is there any advantage structurally in using oak for the laminate sections? Or will pine work as well?
You mention above the triangular structure of the wood fibres giving strength when the outer laminates; I will be having a painted finish on the cut string where visible – the central one being hidden by the plastered soffit – and am not sure what to use for this third element.
The central string on my current sd7 plan has a right angle join where the stair changes direction. I think that it would benefit the overall stability of the stairs if the central string carried through to the uncut strings that will be attached to the wall on the right of the stair, crossing where they currently join – how would I model this in SD7? is it just a case of doing one plan followed by another for these elements?
Look forward to your response, and thanks again
Regards
Bob
October 4, 2012
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Hi Bob,
Just to be clear, I would suggest that a stringer like this be built with horizontal laminates and not tradition laminates.
The difference is that not only is it much faster and easier to build but also that the resulting string is much stronger and more rigid.
This is due to the triangulation of the wood fibers, the horizontal laminates being crossed with the vertical dowels and the outer laminates crossing at an angle. This creates a triangular structure of wood fibers only work in compression / stretch and that is far stronger than tradition laminates that force the wood fibers to work all in flexion.
Here are a couple of our blog articles of stairs built using this technique.
https://wooddesigner.org/stair…..technique/
https://wooddesigner.org/makin…..-majestic/
You’ll also find a lot of other information here:
https://wooddesigner.org/suppo…../advanced/
hope this is helpful,
Good luck with the design,
Ness
Thank you Ness, the article is extremely illuminating and will help immensely in finessing the design. I had thought that the best way of engineering the curved elements would be laminating but hadn’t considered extending this to enable the strings to be beefed up – more thinking prior to building early next year!
Thanks again
Bob
October 4, 2012
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Hi Everyone,
IMHO I think this stair is a just a simple wooden stair.
In France this type of wreathed cut string is very common. I coached a UK company in building a similar stair, you can see the article on this here:
https://wooddesigner.org/tradi…..-low-tech/
The structural depth or width of the cut string under the steps is more than in your photo but could be reduced to the photo’s proportions simply by increasing the thickness.
For this type of stair built in the photo, the stringer is probably solid wood of around 150mm thick with a structural width of 120-140mm.
If this stair was built using horizontal laminates it would require even less width or thickness. I would consider 120mm structural width at 100 or 120mm thick would be enough for this stair.
All the best,
Ness
Sorry Michel, back again! In the US they use a “kicker” at the base of the staircase. It strikes me that this provides the third side of the triangle and therefore must stiffen the whole structure. Is this an option in Europe?
Regards
Bob
Thanks for coming back to me Michel. I appreciate the difficulty of building something this slender, my engineers brain has been working overtime to try to work out how the builders of the stair in the photo achieved it 200 years ago. I suspect that they framed the cut strings using steel which is an option I am considering. Sadly concrete is not an option as it is being built on an existing suspended wooden floor.
So, options:
1) Strengthen the carriers with steel – this would require both centre strings to be attached to the wall, crossing over at their junction, plus additional cross struts to increase stiffness. The staircase is being made from oak for additional strength and stability and the soffit will be boarded and plastered (SD7 refers to soffits but I can’t find a way of adding them!)
2) Support the junction of the centre strings where they cross, again reinforcing using steel.
3) Support the centre string by extending it downwards to the floor, giving the impression of slenderness.
I know that options 1 and 2 will need the input of a structural engineer. However in both cases I will need to have plans for the central strings that show each of them at full length – is this possible?
I have attached an update …..
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