Design of Plate Stair Stringers 4

[dik]

Can anyone recommend a source for designing stair stringers that are constructed using plate and not C Sections? On this latest project, the architect wants to use 1/4" plate for the stringers with the stair riser and concrete filled tread providing the stability to keep the 1/4" plate from buckling. Does anyone have a source for this design other than using FEM? Thanks, Dik

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[driftLimiter]

AISC Design Guide 34 mentions plate stringers in passing. Then it has commentary on the unbraced length related to treads. Check out Section 4.4 of DG 34

 

[dik]

Thanks drift... I'll take a gander. I've only encountered them a couple of times, not designed them. They have a slight 'sway' to them and have only been used in a single 'flight'. This stair is for high rise and it's been my experience that the only times stairs in a high rise are used is during an emergency. I'll see what DG says.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JStephen]

That construction is common on tanks, based on ease of fabrication. Typically, I'll have 3/8" plate and supports about every 5 treads. I'm not aware of any design guides. Design is usually based on fairly approximate analyses.
I would think on a straight stairway, lateral stiffness might be an issue even if strength was adequate.

 

[dik]

They don't deal with plate stringers directly... but consider fully braced...

I'll use the depth that is fully braced, ignoring the little strip at the top and the bottom where the plate isn't braced by the stair treads. thanks

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

That's been my experience with them... they sway a bit.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[patswfc]

Dik, heres a link that provides some info of buckling of the stringers

https://files.engineering.com/downl...e=AD_310_-_Staircases_With_Flat_Stringers.pdf

 

[dik]

Thanks Pat... I'll take a gander.

I've decided that I will do this. I'll treat the stringer as simply supported between landings and determine the factored design moment. I'll then use the depth of plate equal to the depth of the rise and run, excluding the inch or so beyond. I'll then design a 1" strip of the plate in compression, with a length from nosing to nosing and see if this is greater than the flexural stress. The tread and riser will have to be fully welded to the stringers and the stringers will have to be fully welded at the landings. The proposed stair has 1/4" stringers and if these work, I'll use them, else I'll look at 3/8, and try it all over again. I forgot to add, if there is the possibility of any lateral motion I'll secure the stringers to the stair shaft wall. It will be one of the conditions for me to do it.

It's the first interesting thing I've done in weeks... now to draw a stringer and get some dimensions...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

I've checked the design and the 1/4" plate stringer works from a strength condition. I was planning to weld the pans to the stringer with 1/8" welds all along the length of the connection. I was thinking that this might be excessive and have decided to do the following. Does anyone have information on the reqirements required to prevent the stringer from buckling?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[GC_Hopi]

Here is a snip from AISC design guide 34

 

[GC_Hopi]

Also a snip from NAMM Metal Stairs Manual.

 

[dik]

I've limited the strength of the stringer to the buckling capacity of the plate between the two adjacent nosings. I think the fully enclosed pan provides a very rigid 'diaphragm'. I think the proposed welding makes the connection of the pan to the stringer very rigid. since most of the weld is in the areas where it is likely needed. The buckling strength resistance between the nosings is about 7.5ksi.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

Thanks GC... The NAMM one is common with channel stringers, but I'm not comfortable with it for plate stringers. This is the first plate stringer I've done... and just figguring out how to do it. I thought for intermittent welds, the length had to be 1-1/2" min. The spacing can be 12" for compression members and 18" for tension members.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik