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Stair with integral landing
4

Stair with integral landing

Stair with integral landing

(OP)
when doing a concrete stair with a integral landing, do you guys just deisgn as a continuous beam, or do you have other methods for deisgning?

RE: Stair with integral landing

Stairs with cross landings are usually designed with the landings spanning across, and the flights spanning between the landings.  But you can also model them with the entire stair including both landings and the flight as one slab span.

RE: Stair with integral landing

I usually design slab stairs as one span from support to support (the landing being part of that).

RE: Stair with integral landing

3
As long as you can model the behaviour and see the constructability, there is no reason you can't approach this with almost any system.

Is this a precast stair?  If it works well for your situation you can span this as one long span beam, but need to be very careful about the compression forces in the angle changes.  There have been very spectacular low load failures of precast stairs where the entire compressive block blew out.  You should always provide a spiral in the change of angle between stair and landing where compression forces occur against "empty" space.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Stair with integral landing

A "spiral"?

RE: Stair with integral landing

(OP)
a "spiral" Do you mean a cage, or something to take torsion? I cant see the mode of failure any different than a beam at the compression top

RE: Stair with integral landing

I'll post a copy of a very good article on the issue Monday,however the basic problem occurs in long staircases when the landing is near centre-span.

If you think about walking up the stairs and arriving at the landing, that last stair is poking "out" from the landing.  Think about what happens to your top and bottom bars here.  They cross over, and good detailing demand you add a bent bar to reinforce the outter face.  The area becomes similar to a beam column joint and in particularly long spans the forces can be so severe that they exceed the shear stress capacity of this joint.  The problem relates to the change of direction of the tension and compression forces, causing a localised area of high shear.  A spiral stirrup accross the length of this top stair (R10, 100 dia, 75 pitch | #3, 4" dia, 3" pitch) confines this zone and prevents this failure mode.

Hope that's a bit more clear as to what I was talking about.  If anyone's interested I'll post the article on Monday.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Stair with integral landing

I do detail the bars here so that I have top and bottom bars. The bars that terminate here get extended to the other face. I assume that you are talking about a stair going up and reaching a level landing where the componenets of the compressive forces combine to want to push upward.

In 38 years, I have never seen a failure of this type but stairs are rarely loaded to design load except perhaps when people assemble to have a photo taken.

RE: Stair with integral landing

I have never seen this failure either... Not that I have seen 99 percent of the failures I routinely design against.  However, this failure is possible, and at quite LOW load levels.  I'll post the article; It's an excellent read.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Stair with integral landing

Ali:

Is that staircase meant to span from left to right, as a long beam, with the landing in the middle as an unsupported flexural section?  Or is this side to side supported?

IF that is a long span staircase I think you should have a careful read of the article I posted.  I am not sure you are adequately guarding against the compression block blow out failure which is possible.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Stair with integral landing

This staircase is spanning from padestal foundation to slab shown first floor above. I don't think compression block failure would initiate unless you are treating stair waist slab as minimum to behave section in a way your comprssive stress at top exceed the allowable limit. Make sure your allowabel reinforcement ratio does not exceed the maximum. I believe the collapse which is attached is due to that reason that waist slab was kept minimum alongwith poor rebar detailing.

RE: Stair with integral landing

Ali:

Agreed; However if you continued your down-turned bar at the nose of the mid-way landing (currently terminating in a simple hook) into the body of the stair, you would further increase the strength of the staircase at negligible cost.  Something to think about...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Stair with integral landing

If you consider a strut and tie model for the change in angle in a stair with a landing at midspan, the tension in the bottom reinforcement, acting in two different directions, produces a resultant force tending to cause the bottom reinforcement to burst downward out of the concrete.  

In the top, the two compression components do just the opposite, creating a resultant tending to force the concrete up and away from the bottom reinforcement.

The two resultant forces are equal and opposite.  If the bottom reinforcement is not tied to the upper concrete, the only thing resisting these opposing forces is tension in the concrete.  Concrete is not reliable in tension, so steel ties are required at each bend which is concave on the bottom.

When the bend is concave on the top, there is no problem.  The resultant force from the two tensile forces oppose the resultant force from the compressive forces, producing a compressive force which concrete can easily resist without additional reinforcement.

RE: Stair with integral landing

show in sketch your wording guys

RE: Stair with integral landing

with respect to YS's article, not sure I understand what the author means when he says "the compression block stress needs a tension normal to the neutral plane to get around the corner." Can someone please explain?

RE: Stair with integral landing

BAretired: Good summary of the mechanism involved.

Ali & Engn555:  This is just like a beam column joint in concrete.  Or like any force interaction at a bend.  I'll post a sketch when I get to the office.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Stair with integral landing

Please see attached.

These forces become non-trivial with long span staircases.  They also become quite high with dead load alone, which is what occured in the failed staircase.  Until end restraint is in place (by pouring the precast stairs into the floor slabs) the mid-span moment can cause high enough break out loads to fail the staircase under very minor live loads.

It would be rare for this failure mode to govern once a staircase is fixed into the structure, however not unthinkable.

I look forward to your thoughts.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Stair with integral landing

Thanks the article - never seen this type of failure, but I will keep the detailing change in mind in the future.  

Mike McCann
MMC Engineering

RE: Stair with integral landing

I believe that you have raised a very important issue regarding this type of stair.  The fact that it has not been broadcast throughout the structural engineering community is surprising to me.  Hopefully that will change now.  

It is clear from your first page that the two compressive forces and the two tensile forces meeting at the junction of the landing and the stair slab require a tie force to prevent them from moving apart.

If the tensile strength of the concrete is adequate to resist the tie force there is no problem, but obviously, in the case you have brought to our attention, that was not the case and the stair failed.  Luckily, the man on the stair was able to escape before it collapsed.  The two stairs below collapsed with the weight of the falling concrete and no workers were on the stairs or the result would have been much worse.  Fortune smiled!

In fifty three years of practice, I have not witnessed this type of failure.  On one occasion, I saw a failure where the tension steel burst out of the concrete because it was not properly anchored to the opposite face in accordance with accepted practice but never have I seen a compression "blowout".  

Being a conservative type of guy, I guess I would tend to avoid this type of stair altogether.  But if I was forced into designing a stair like this, I would want to include something a little more positive than circular stirrups to provide the necessary tension tie.  

I have not thought it through, but further discussion will no doubt be forthcoming.

RE: Stair with integral landing

After reading the article, I believe this failure is a result of poor detailing practice. I have always detailed these bends in the stairs differently than my collegues recognizing that there exists an upward component due to the compressive forces.  

RE: Stair with integral landing

Gentlemen:

This failure mode is rare, particularly in traditional cast insitu staircases.  It most often results in cracking that is not noticed or is considered minor, and once the structure is finished (and thus the staircase is locked in) the forces are no longer large enough to cause problems.  Ergo the incidence of collapse is very low.  Like most rare failure modes, this is something to watch out for when "outside the norm".

Personally I start thinking about the deflectioni first whenever I see a span to depth ratio over 25, and start thinking about the abnormal when my span to depth ratio exceeds 40.  At that point I start looking at everything from first principles; An approach that should catch this issue out.

BAretired:

I am curious, Sir, where you practice?  If there is seismic detailing (ie: stairs discontinuous between levels), the lock-in forces are not always present to shift the bending moment envelope, and thus this cracking (Serviceability Failure) is something I have seen fairly often.  Often, perhaps because the possibility of failure is not known, this cracking is overlooked as shrinkage... It looks nearly identical; It is the location that gives it away.

Ali:

I am no expert on the matter, nor do I pretend to be.  That asside, looking at your detailing options, I would use B.  There is no reason to be discontinuous with your top steel if you can avoid it.  Stress reversals happen for all kinds of reasons, and I would want some tension steel continuous if this were to occur.

I believe your detail would address 99% of situations, with only the longest of staircases exceeding the strength of the now fully reinforced "knee" or beam-column joint area.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Stair with integral landing

YS,

In June 2008 I retired from active practice.  From 1955 to 1960, I practiced in Toronto, Ontario.  Since then I have practiced in and around Edmonton, Alberta.

The Alberta Building Code is virtually identical to the National Building Code of Canada insofar as seismic effects are concerned.  

The province of Alberta is not considered a high hazard area for earthquakes by seismologists. In an average year, Alberta will experience approximately 150 seismic events ranging to a maximum intensity of around 3.0 to 3.5 on the Richter Scale.   

RE: Stair with integral landing

BAretired,

A very impressive and substantial experience: You are exactly the kind of engineer I seek out to work under, listen attentively to, and learn from.  There is no question who of the two of us is more experienced...

I doubt you would have needed to worry about this detailing at all during your career, but would have picked up thousands of other useful detailing, design, and analysis points/tricks/knowledge.  Though rare, I still believe it is something that all structural engineers should be area of, and think about from time to time.  I wish there were some way to learn everything you have... I'll just have to satisfy myself with getting your help from time to time online, if you're willing.

Congratulations on your retirement;  Always a melancholy moment for the profession.  I have several goals in my career, but above all else I want to be the best engineer I can be, and finish my career as the office's senior engineer at the end of the hall.

Respectfully,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Stair with integral landing

Hello YS,

Thank you for your kind remarks.  From what I have seen so far, I have no doubt that your goals will be attained long before you expect them to be.   

Best regards,

BA

RE: Stair with integral landing

If a riser of 7" and a tread of 11" is assumed, the slope of the stair is about 32.5 degrees.  The "blowout" force, V which 'youngstructural' showed on a recent post would be 2*C*sin(32.5/2) = 0.56*C directed at an angle of 106 degrees clockwise from the landing.

If the tensile capacity of concrete is to be neglected, stirrups of adequate size and in the correct orientation are needed to provide the required tensile resistance.  Continuous top bars such as detail (b) by 'ali07' do not prevent blowout.

When dealing with a long span and a critical landing/stair junction occurring near midspan, some very careful thought is required to prevent this type of failure.
 

Best regards,

BA

RE: Stair with integral landing

Just curious BAretired, have anybody provide this type of stirrup in his career for the stair. When we talked about lond span stairs, how limit is defined for long span.

RE: Stair with integral landing

Hello ali07,

I do not know if anybody has provided this type of stirrup in a concrete stair.  However, it would not constitute a significant  cost item, so perhaps it should be considered in the future.

The article "A precasting mishap" acknowledges that an ordinarily competent designer would not likely have recognized this particular stair design as a hazard prior to its  collapse.  The date on the article is June, 1988.  That was twenty years ago.  For some reason, the message does not seem to have been heard by the structural engineering community.  Perhaps it is time it was.

What is your opinion?

Best regards,

BA

RE: Stair with integral landing

Hello All;

This is a typical detail for New Zealand, and from that I have been told that this detail is used in California as well.

Again, I must emphasize that this detail is needed most when a staircase is to be pinned at supports, not just pin assumption.  Fixity at the support shifts the bending moment up and reduces the required strength.

Again, this is a common detail, at least where I practice.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Stair with integral landing

This is true, this detail required for precast construction. Code cannot ignore the safety concern for over 20 years. In most of the situation we are dealing with monolithic construction. But definately the force need to be reviewed not taken lightly in design for all situations.

RE: Stair with integral landing

Is this type of failure applicable for steps/folds in slabs? With a typical 'Z' type fold thru a slab you are guaranteed to have a compressive stress at a convex corner regardless of the loading.

I have never fully understood how a strut-tie model works around a 'fold' and have always relied on ensuring minimum thicknesses and tensile development.

I would really appreciate it if someone could flesh this out for me?

Thanks  
 

RE: Stair with integral landing

OzEng80:

Yes, these forces will exist in a typical fold in a slab, however it will normally not be as serious a concern, as the slab thickness is greater, there are often walls and other distributed supports breaking the spans, and you nearly never have truly pin connections at the edges.

That said, if I needed a fold near the middle of a long span, I would definately be checking for this failure.

Strut and tie is not easy to master, and I still have a lot to learn about it, however I believe very strongly in the method.  It is worth the time and effort, and is something you should keep learning more about it...  There are some pretty good STM threads on eng-tips too!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

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