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.
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.
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...
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.
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...
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.
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.
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.
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...
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.
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?
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...
