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  1.  
    I've designed the following stair stringer. In hindsight it's not ideal structurally, but it's too late to change it significantly now. The span is 2.2m unsupported (diagonally). The problem is that the structural engineer has said that the bottom section of the beam (the bit that's not cut out to fit the treads) needs to be a minimum of 100mm x 125mm in C24 timber. This means we need to find C24 timber that is 100mm x 255mm, which can be a bit problematic as it's hard to dry. I went to a few timber yards yesterday who suggested that we might be able to do it in a D40 hardwood like Ash. The SE has said that he doesn't think this would make a difference. Can anybody point me in the direction of how to do a calculation like this? I'd like to understand / check it myself.
      20180613 Stringer Profile.jpg
    • CommentAuthortony
    • CommentTime4 days ago
     
    As an old fashioned builder I would say that the span structurally is 1.56m (horizontal span)

    Then in my book a stair string only needs to be 50mm thick and I agree with 125mm though 100mm deep would work for me and argue 50mm each side equates to 100mm thick.

    So just get a couple of carefully selected joists 225x50 ensuring no big knots or splits. Quick plane and sand and you are off.

    I have never seen a stair string 100mm thick, often they are 32mm thick.
    •  
      CommentAuthordjh
    • CommentTime4 days ago
     
    Posted By: ComeOnPilgrimThis means we need to find C24 timber that is 100mm x 255mm, which can be a bit problematic as it's hard to dry.


    100 mm wide sounds very wide. Are you having a single supporting beam rather than one at each edge? Have you considered laminating two 50x255 if so?

    One way to reduce the depth would be to laminate a steel flitch in between two pieces of timber.

    255 deep sounds an odd size. Did you mean 225? 250 would be easier to source than 255 I think. If you were able to slide the treads forward so the upstand was in front of the stringer rather than notched into it then there'd be more meat left in the stringer and presumably the depth could be reduced a bit.

    PS Surely you'd buy it kiln dried?
  2.  
    Posted By: djh100 mm wide sounds very wide. Are you having a single supporting beam rather than one at each edge? Have you considered laminating two 50x255 if so?

    We have a beam at each edge. The other beam is not a problem as it's screwed every 200 mm to a wall. It's just on the other side we have a 'flying' stringer like this that needs to be thicker.
  3.  
    Posted By: tonyThen in my book a stair string only needs to be 50mm thick and I agree with 125mm though 100mm deep would work for me and argue 50mm each side equates to 100mm thick.

    That's my feeling too, and also the view of those at the timber yards. However, given that a SE (or rather it seems a student working for the SE) has recommended these dimensions, is there a formula / means of calculating the required dimensions?
  4.  
    Posted By: djh255 deep sounds an odd size. Did you mean 225? 250 would be easier to source than 255 I think. If you were able to slide the treads forward so the upstand was in front of the stringer rather than notched into it then there'd be more meat left in the stringer and presumably the depth could be reduced a bit.

    255 is the effective size once the notches + 125mm are added up. I agree, it's a difficult size to source (particularly if it is also 100mm thick).
    Are you sure that the shifting the upstand forward would increase the depth? I though the upstand in the current position is actually increasing the depth already.
  5.  
    BS EN 16481:2014 Timber stairs — Structural design — Calculation methods seems to be relevant. I have a copy of it from my university, but it's going to take a while to digest!
    • CommentAuthortony
    • CommentTime3 days ago
     
    I sometimes used to get a 75x 200 nice bit of wood sawn into two 35x200’s and prepared all round and use that for stair strings lots of them are still there
    • CommentAuthormark_s
    • CommentTime3 days ago
     
    Tony, tell us more about the ones that aren't :)
    •  
      CommentAuthordjh
    • CommentTime3 days ago
     
    Posted By: ComeOnPilgrim
    Posted By: djh100 mm wide sounds very wide. Are you having a single supporting beam rather than one at each edge? Have you considered laminating two 50x255 if so?

    We have a beam at each edge. The other beam is not a problem as it's screwed every 200 mm to a wall. It's just on the other side we have a 'flying' stringer like this that needs to be thicker.

    In that case, I'm with Tony, 50 mm wide should be plenty and 32 mm is quite normal. Although that assumes routing for the treads and risers rather than notches cut out of the stringer. My stairs are like that and supported only at top and bottom both sides.

    Incidentally, I see that Wickes offer notched stringers fabricated from a straight beam and a bunch of triangular pieces.
  6.  
    Posted By: djhIncidentally, I see that Wickes offer notched stringers fabricated from a straight beam and a bunch of triangular pieces.


    Yes, Pears Stairs do a similar one. By my calculations, the effective depth is 95mm x 48mm. See: https://www.pearstairs.co.uk/richard-burbidge-ld403-treated-softwood-35-degrees-cut-string-for-5-steps-950mm-rise.html
    • CommentAuthortony
    • CommentTime3 days ago
     
    Outdoor stairs do start to go rotten after more than 25 years with no TLC
  7.  
    I now have the pages of calculations in support of the recommendation. Would any structural engineers like to have a look? I'm happy to pay.
  8.  
    It seems like the calculations have assumed that all the weight is on one stringer rather than spread between two stringers.
  9.  
    Can you arrange it so the ends of the treads butt up against the side of the stringer, instead of sitting on top of it?

    Then you can avoid all those notches which are structurally quite inefficient, as the remaining triangular bits of wood don't contribute to the stringer to resist sagging.

    Also the tread ends would stop the stinger from twisting (torsion) - in the current design the engineer may have needed to make the stringer extra fat to avoid it twisting length wise.

    Also the stringer could encase the end of the treads and stop feet sliding off.

    If architecturally the stringer has to go underneath the treads, then can you lose the triangle bits, and connect the treads to the stringer with something lighter, like a stainless strip?
    •  
      CommentAuthordjh
    • CommentTime1 day ago
     
    Posted By: ComeOnPilgrimIt seems like the calculations have assumed that all the weight is on one stringer rather than spread between two stringers.

    That seems fairly reasonable since people can stand on one foot at the side of the stairs. We frequently stand together at one side of our stairs to look out the window, for example. But our stringers are 32 mm IIRC.
  10.  
    Posted By: WillInAberdeenCan you arrange it so the ends of the treads butt up against the side of the stringer, instead of sitting on top of it?

    Then you can avoid all those notches which are structurally quite inefficient, as the remaining triangular bits of wood don't contribute to the stringer to resist sagging.

    Also the tread ends would stop the stinger from twisting (torsion) - in the current design the engineer may have needed to make the stringer extra fat to avoid it twisting length wise.

    Also the stringer could encase the end of the treads and stop feet sliding off.

    If architecturally the stringer has to go underneath the treads, then can you lose the triangle bits, and connect the treads to the stringer with something lighter, like a stainless strip?


    I agree, the design is very structurally inefficient (as I have discovered through thinking about this subsequently). However, that's the current design, and I'm loathe to go back to the drawing board now. I think I can reduce the width to at least 75mm by using Ash instead of softwood, but I'd hope I can reduce it a little further, to at least 65mm or so. A couple of SEs that I have spoken to informally have calculated it at approximately half the current cross-section (i.e. 63 x 100 mm).
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