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StructuralEng, Consultant
Category: Structural Engineering
Satisfied Customers: 7265
Experience:  Structural Engineer
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I am planning a stairway up a garden slope. The sairs will

Customer Question

I am planning a stairway up a garden slope.
The sairs will be low to the ground, but not actually
touching it, so there is no natural support.
I am trying to estimate the number of supports I need
to install underneith, to support the stringers.
I have looked up physics forums, but with no prior knowledge
was not able to follow the formulas, or put them in context.
Hence I was hoping to find out if there are any rule-of-thumb
tips or guidelines.
(Of course I understand that it may not be that simple...)
Here are the details of the planned project:
The stairs will be 2 meters wide and covered with decking
boards (so quite a bit of dead weight).
Each of the 13 steps has a tread of 410mm (-> total lenght 5.3m)
The risers are 149mm heigh.
The 4 stringers will be 300mm x 50mm hardwood.
The base of the stringers will be mounted on concrete.
The top will be fixed to the joist of a small deck (landing).
Thanks for your help
Submitted: 2 years ago.
Category: Structural Engineering
Expert:  StructuralEng replied 2 years ago.

Can you post a sketch of how the stringer will be cut?

Customer: replied 2 years ago.
Hi StructuralEngSure. Attached here.
Two Views, one from the front/top and the other from the side.Let me know if you need anything else and thanks for checking
Expert:  StructuralEng replied 2 years ago.
Hello Rob. Thank you for the sketch. Can you show the dimensions of the stringer where you are cutting it to lay the treads on it? I need the minimum structural dimensions to check the reduced section.
Customer: replied 2 years ago.
Hi StructuralEng
I've attached a diagram with the dimensions, but I had these already listed in my question, so I'm not sure if I understand correctly what you need. So, if the attached is not what you needed to know, please let me know.
The dimensions for the 4 stringer timbers will be 300mm x 50mm hardwood.Thanks
Expert:  StructuralEng replied 2 years ago.

Hello. I would do the following...use a double stringer. One notched as you show it nailed to another on the outside that is not notched.

Customer: replied 2 years ago.
Hi StructuralEng
I don't understand what you mean. I am already using 4 stringers to span the 2 meter steps.My question was about how to estimate the number of supports I should include. To explain that better,
I have added another sketch. Here I show (in red) two cross supports I intend to put under the structure
to support the four stringers.
From my own estimation, I think 2 should be sufficient, but because I am not sure, I am wishing to learn
how to estimate something like that.
Hope that helps
Expert:  StructuralEng replied 2 years ago.

Two supports like you show will work. A more elegant solution would be for the two outside stringers to be doubled, but only one is notched. The outside one would be a full stringer, the inside one would be notched and they would be nailed together. That would require no intermediate supports.

Customer: replied 2 years ago.
I appreciate the alternative option, but I repeat that my question was about "how" to calculate or estimate the supports needed.
In other words, when you say that two supports will work, what was the methodology you took to derive that result?
thanks, Rob.
Expert:  StructuralEng replied 2 years ago.

Hello Rob. I appreciate your desire to learn, but I can't get into engineering lessons online or we'll be discussing this for weeks. I will opt out for you.

Customer: replied 2 years ago.
Hi StructuralEng
In that case I am curious as to why you opted to answer my question in the first place.
I never asked for engineering lessons, but simply how something like this is calculated or what guidelines can be applied.
Given that you told me that "two supports will work", I expect (and sincerely hope) that you didn't simply "assume" that it would be safe to do so, but that you used a formula (or guideline), and to state these here will not take weeks.
So please substantiate the advice you have provided by stating the approach you took.
Expert:  StructuralEng replied 2 years ago.

Of course I do the engineering. I take what I do seriously. First you determine the allowable capacity of the beam based on the notches (there is some judgment involved, because the NDS doesn't address the stress concentrations associated with abrupt changes in geometry) and then do analyses to determine what spacing of supports results in a moment demand that is lower than the moment capacity.

Customer: replied 2 years ago.
Ok, wonderful. So why don't you write how you determined the allowable capacity, the bases on which you applied judgement, and how you used these values to analyses the spacing required. I'd be more than happy to use that as a guideline, and which would answer my question.
Expert:  StructuralEng replied 2 years ago.

I don't provide calculations and the multiple equations as the basic service. Good luck with your project