Stair stringer with multiple flights and landings

[TedEP]

I have an architecturally driven stair stringer design challenge. The stringer spans 2 flights of stairs and 3 landings, see attached sketch. As such, there are 4 "kinks" in the beam. I know that the beam has to be designed for the global moment WL*L/8, with full strength welds at the kinks to develop the member capacity.

What about the local affects at the kinks? I believe there needs to be a stiffener at the joint, designed for axial load. The axial load, C or T, is derived from breaking the flange forces down into components, see sketch.

Comments? Is it really this straight forward?

 

[mijowe]

how worried are you that vibration of the stair is going to be a problem?

 

[TedEP]

Not concerned about vibration so much. The steps and landings are concrete filled pans, which should dampen vibration. Static deflection is ok, about L/400.

 

[BAretired]

For strength, it really is that straightforward.

BA

 

[hokie66]

Yep, straightforward. And you are right about the stiffener. Put it in.

 

[msquared48]

No worries.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

The long stairs will suffer from impact vibrations (of people bouncing down them) and a static stiffness may not be appropriate.

Dik

 

[Ron]

I agree with the others. I would provide X-bracing on the bottom of the stringers as sidesway will be a bit of an issue. I have found X- or W-bracing to be necessary on anything over about 10 treads.

I'm surprised that your deflection is so low...are you sure? It looks like your span is somewhat over 20 feet.

 

[PicoStruc]

Not concerned about vibration so much. The steps and landings are concrete filled pans, which should dampen vibration. Static deflection is ok, about L/400.

Concrete filled pans or thick stone for step increase global mass of the stair and then can lower the frequencies of vibration (of the stringer) enough to be really sensitive to vibration. Seriously. More than one mode shape must be accound for too!

The parameter to check for stair vibration is the acceleration response felt by walkers or worst felt by people wating on mid-height landing.

With this kind of stair, you can go up to the fourth walking/running sub-harmonic. Then all stair with vibration frequency lower that 10-12 hz (walking freq. * 4) can be sensitive and further analysis are required by specialized structural engineer.

 

[JacksPanic]

Hi there,

I am stuck with a similar stair design problem. Can somebody guide me as to how to design the connection as shown in the attached?
the reason for the bolts being off-centre is: I have to use chemical anchors as it is an existing slab i am fixing to. Many thanks.

 

[Ron]

JacksPanic...you should start a new thread rather than hijack this one!

However, for your application, you should provide a "ledge" overrun off the top flange of your stringer channel that will extend over the edge of the concrete. This will reduce some of the shear load on your fasteners and make stair installation much easier. Another way to do this is to create an "L" shaped base plate with a 50 to 75mm overhang onto the concrete. Again, this will significantly reduce the shear load on the anchors.

Pay close attention to the shear reduction values for your anchors as the anchors are quite close together. You will need to reduce the allowables for both spacing and edge distance to the bottom edge of the concrete. Check the anchor manufacturer's tables.

 

[JacksPanic]

My apologies! didn't mean to highjack. i thought it was relevnt to the topic.

Thanks for the tip. I agree, an L-shaped plate is a good idea!

(Can the moderators please move the post?)

 

[dik]

Is it possible to provide a 'lip' on the stringer so it bears on the existing concrete and reduces/eliminates the shear connection. Anchorage only holds it in place.

Dik

 

[Ron]

dik...exactly

 

[BAretired]

JacksPanic,

I am not sure how you calculated your reactions at the top of the stair.

If the right hand support is regarded as a pin and the left hand support is a vertical roller, the only reaction at the left end is horizontal while the reaction at the right end is both horizontal and vertical. It would be similar to a ladder pinned at the bottom and leaning against a frictionless wall at the top.

Alternatively, if the stair is supported with a pin at one end and a horizontal roller at the other, the only reactions are vertical.

How did you arrive at Rv and Rh?

BA

 

[PicoStruc]

Of course there will be horizontal reactions on all supports. The bending of the stringer pull on the support (like an arch).

Note : If your support are pin, the stair is more flexible and more suceptible to vibration !

 

[BAretired]

PicoStruc,

Not sure whose post you are responding to, but if it is mine, you are way off base. The bending of the stringer does not "pull on the support (like an arch)" as you stated. Kindly defend your comment so that I can respond to something semi-sensible.


BA

 

[JacksPanic]

You are correct. I agree, roller at top, i.e. no Rh reaction.

 

[hetgen]

TedEP

What is the span? Foot fall induced vibration could be limiting criteria.

 

[jdgengineer]

If you have it bolted at both the top and the bottom I don't see how that will react as a roller on top. In order for the end connection to act as a roller the stringer needs to be able to deflect on that end when subjected to a force. If its restrained by bolts on both ends I would think it would have a thrust force. Maybe I am missing something.