Stair Stringer - engineered timber and torsion?

[Backcheckrage]

Hi All, I am working on some architecturally designed stairs with an architect. The brief is to rely on 1 giant stair stringer to support vertical and torsional loading (from rail and imbalanced live load). It's about 4m long, about 1m wide. I really want to just make it a steel tube but the theme is exposed timber so that's what the Arch wants... I am really hesitant to call-out a glulam or LVL stringer/girder because it seems that the laminations is a weak point for torsion... also I have never seen a timber stringer used in such a way before!

What do you think? Do you have comments on the torsion+vertical loading on say an LVL or Glulam? (obviously steel is the answer but I would like to explore other options....)

Thanks!

 

[msquared48]

I've used stair stringers to support concrete stair treads that were 4X12's with side mount hand rails, and 6X12's for top mount hand rails with no problem for years. Torsion from the stair tread loading never really was an issue.

PT if they are exterior.

Mike McCann, PE, SE (WA)

 

[jayrod12]

Mike. I believe he's talking about a single stringer which I would be worried about as well. What about a wood wrapped steel tube?

 

[msquared48]

Yea, you're right. Sorry, I missed the "1".

Then I agree with the steel tube and furring out wood to simulate the wood look.

Mike McCann, PE, SE (WA)

 

[Ron]

Agree....steel tube for torsion, wood cladding for looks.

 

[BUGGAR]

I saw one with stainless steel thru-bolts through the stringer and the stair brackets above it. With a bolt at each step holding the glu lam together, it seemed to work.

 

[EngineeringEric]

Try looking at ArcWays Stairs and see what they do... I've worked with them in the past on a few projects.

Otherwise, steel it is!

 

[KootK]

I agree that, with an intelligent connection, one should be able to deliver torsion from the treads to the stringer successfully. I'd mostly be worried about the corresponding torsion connections between the stringer and whatever is supporting it torsionally at the ends. Perhaps not a deal breaker but something to keep an eye on for certain.

I'm not sure if the stairs below are wrapped steel but, if they are, they are very convincingly constructed.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.