Aluminum Stair Stringers 1

[Jerehmy]

I have aluminum stair stringers of 16.5ft horizontal length and 11.5ft rise which comes out to ~ 20ft actual length.

The problem is Ohio Grating doesn't want/like us to use the planking as bracing and neither does the engineer who we're subbed out from. From my understanding they're attached with a clip that can't support much load. Without lateral bracing no 12" deep size works. I've attached my Mathcad in case I made a calculation error.

Does x-bracing between the two stringers seem feasible? It doesn't to me. Any ideas/advice?


Thanks

https://www.dropbox.com/sh/jfrolsdh5qwkqoj/Y99KXhbmVm

 

[msquared48]

Any chance of welding on top and bottom flange plates to increase the lateral stiffness?

You can always spec a larger section too - a built-up section - the cost of which will go thru the roof. Frequently that begets a more reasonable Architectural design.

Mike McCann
MMC Engineering

 

[MikeHalloran]

A few pans totaling ~20 ft x (width between stringers) + bent up flanges on all four sides could be bolted between the stringers flush with their the bottom face, providing some lateral bracing, and keeping debris from falling on whatever's below.

just a thought




Mike Halloran
Pembroke Pines, FL, USA

 

[Jerehmy]

msquared48 - if we had to I'm sure we could. I'm just not sure if I want to open that can of worms with aluminum because welded sections have reduced strength and it wouldn't be something that the manufacturer typically does.

Mike - if I understand correctly, that may work. unfortunately I don't think it would fly architecturally.


I emailed Ohio Grating just to double check their stance.

 

[Ron]

I use X-bracing on steel stairs routinely.

 

[IFRs]

I second Ron's notion of x-bracing. Flat bar is inexpensive but the architect may not like it visually. Otherwise, find an unobtrusive way of beefing up the requisite number of treads to satisfy the need.

 

[IFRs]

On page 11 how is the M < Ma and OK ?

 

[Jerehmy]

They aren't ok, it's a standard spreadsheet. I adjust the factors til it is OK.

How much does x bracing the bottom flange help in lateral torsional buckling? Whatever we're x-bracing with would stop the bottom flange from moving laterally, but what would stop the top flange from moving laterally? It'd have to be a moment connection, no?

 

[Jerehmy]

"Jerehmy,
Thanks for contacting me. Neither Ohio Gratings nor the other members
of NAAMM Metal Bar Grating Division rate the grating stair treads as
lateral braces for the stringers. Other structural members should be
used for this purpose.

Please let me know if I can be of additional assistance. "

 

[Bagman2524]

There is a previous thread that discuss that the stair treads do provide lateral bracing for the stringers. And NAAMM AMP-510 does state that the threads laterally brace the stringers. See table on page 5-10

 

[msquared48]

Does that refer to solid treads or open grate treads?

Mike McCann
MMC Engineering

 

[Jerehmy]

I have the NAAMM book, they're solid, welded treads. These are gratings with two clips. I'm sure it provides some, but the guy from Ohio said neither him nor the NAAMM guys recommend it.

I think I'm going to do welded cross bracing at mid-length. I want it welded to the web below the treads. I just have to make sure it's rigid enough. I just don't think they like to do field welding, and a bolted connection wouldn't be rigid enough... Meh

 

[structSU10]

Yea, you can design the bracing using AISC appendix 6 strength and stiffness equations, and do the stress checks with the aluminum code.