Cantilevered Concrete Staircase Landing 3

[otto_eng]

Hi folks,

I have a relatively easy question for the more experienced structural engineers.. I have designed cantilevered balcony slabs for residential buildings in the past but have never done a cantilevered staircase landing for a public space, a school in this case. The location is not in the earthquake zone.
Due to the geometry of the space, there is no backspan of the landing so the plan is to have it directly cantilever it from a concrete wall. In my opinion, by providing sufficient reinforcement and the anchorage length within the wall, this should be doable; however, the dynamic approach of this landing is something that gets in between my progress. Considering this is a school, there will be kids jumping and running around the staircase area.. Would you run an eigenfrequency analysis in this case ?
Has anyone of you done such a similar type of a landing? This is nothing new in the game but I have not seen any references made on this one ? That would be nice if I can get some opinions or references on that !

Cheers!

 

[human909]

Deflection and vibration will make this extremely hard if no unworkable if than drawing is vaguely to scale.

Get your strength and deflection in order before you start considering vibration. The load and deflection requirements for schools will likely get you most of the way there for vibrations. But you should consider them.

 

[lexpatrie]

Is this exposed to weather?

 

[JLNJ]

Doable but…

I’m thinking your dowels would tear out of the wall. And why are the dowels shown inside the landing rebar top and bottom mats? Looks ugly.

 

[dik]

doable, but deflection and vibration may be an issue. Top dowels should be above longitudinal steel and should extend to outside of vertical wall reinforcing. Not a real problem...

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

-Dik

 

[BAretired]

I do not like that detail! I would prefer the detail below.

It relies on shear friction to carry the full weight of the landing - not a good idea.
Taper slab as necessary...it's not a diving board.
Bottom slab reinforcing is not required.

 

[hokie66]

Agree with BA. If not the full slab across the wall, provide for keying in the slab as done in high rise lift form construction. And the design of the wall will be affected. Consider the wall as well as the landing in your deflection/vibration assessment.

 

[HTURKAK]

I got trouble for visualizing the situation you're describing. It would be helpful if you could post a plan showing some dimensions , floor and landing if any .
Cantilevering the stair stringers from a RC wall is not impossible but not a good alternative.



He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[JoelTXCive]

Is there any option of increasing the wall thickness?

It would be good if you can get the full hooked development length (L_dh) on the dowel bars.

Also, I would draw the plans showing mechanical splicers. (Dayton Superior hooked dowel bar splicer or similar)

Having the dowel bars protrude out of the wall is sometimes difficult for the contractor. (I think it makes their formwork setup a lot more challenging.)

 

[milkshakelake]

In response to BAretired, I believe bottom reinforcement is necessary for controlling long-term deflection.

 

[BAretired]

In response to BAretired, I believe bottom reinforcement is necessary for controlling long-term deflection.

It can be used, but it is certainly not necessary if the designer selects a reasonable slab thickness and a reasonable wall thickness.

 

[otto_eng]

This is not the stair stringer being cantilevered. It it is the landing cantilevered out from the wall. The black lines with the arrows are just to indicate the reaction forces from two stairs running up and coming up- in plane..

 

[BAretired]

I would be hesitant to do that without a good deal more information. The slab (and possibly the wall) appears too thin if drawn to scale.

Please provide a plan view and some dimensions.

 

[SWComposites]

Well, I’m only an aircraft stress analyst,
but that design at the top look horrible. Two little bars to carry shear and moment? For that skinny slab and wall (assuming drawn to scale)? With that long cantilever? Just looks bad.
Needs some sort of gusset or corbel at the wall, and a thicker landing slab.

(If some aircraft designer gave me that sort of detail with a crummy load path, the response would be a big red X on the drawing and a “No” without bothering to do an analysis).

 

[otto_eng]

@SWComposites You should take a look at how retaining walls are reinforced then maybe it would make sense to you..No, this is not to scale.
My main question is the method of construction of such a landing but statically it is fine..
Here is what is going on in reality:

 

[BAretired]

Looks like it has a good chance of working, subject to strength and deflection compliance and non-reliance on shear friction.

 

[otto_eng]

Would you say that it would make sense the pour the landings with the conc. wall to achieve a better fixity ? Is there a reference about this you could forward me to ?
why there is no good reference r.c book for practical cases..

 

[BAretired]

No, loti eng, I believe that would be a bad idea. Concrete settles after it cures, so pouring a slab monolithically with a wall is the wrong thing to do as the slab hangs up on the forms while the wall settles. Stop the wall short and pour the slab over it as I showed in my earlier sketch. Then pour the upper wall separately. From a structural point of view, you could pour the upper wall section monolithically with the slab because in that way, settlement will do no harm; however I doubt that the contractor would be happy with that because of forming difficulty.

 

[hokie66]

It's a 12" wall. Should be enough, I think. But leave a 4" key, the depth and width of landing. And provide U-bars from the rear face of the wall, projecting into the stair landing.

 

[ALK2415]

Could you make the RC landing slab as L-shape cantilever ? to reduce the deflection ..