Flat slab in 0.3 PGA seismic zone 2

[Akeee]

Hello,
There is a new trend in my country "everything with flat slab".. I want to ask you guys what's your opinion about making a flat slab 13th story height with shear walls (not columns) in a seismic area with 0.30 PGA acceleration. I will limit the compressive strut at 0.4 of capacity, limit the ultimate drift at 1.5% and of course punching shear reinforcement (a lot) but with all i dont have a good vibe about this. The core idea about seismic design is that you are counting on ductility to dissipate a large part of earthquake induced energy by ciclic deformation of the rebars in the main plastic zone's you define (and most of them are at the beams ends and base of the columns/walls). Here there is only one option, the base of the shear walls, of course that the slab will bend and the reinforcement will yield but on what i read hysteretic behavior is not near the beam like, compression concrete strut degenerates fast and so there is a real danger of crussing the concrete and failure even with all the punching shear reinforcement you want. So i think i cant consider plastic dissipative zones at the flat slab... remanins only the base of shear walls. I can take a reduced R factor to 4 but even with this i cant be confident that is ok. Any opinions/advices please ? Thank you

 

[dik]

For the seismic information, I see no problem with a flat slab (slab w/ drop panels, I assume) and it should work well. The shear walls, due to their stiffness will take a fairly large lateral load, and if this can be accommodated, then don't see any problems.

Dik

 

[Akeee]

Thank you Dik for response.

 

[dik]

There are likely others on eng-tips that are far more qualified...

Dik

 

[KootK]

First, we need some terminology clarification. In North America:

1) Flat slab = slab with drop panels. The slab soffit is flat except for local thickenings at the columns.

2) Flat plate = truly flat slab with no drop panels at the columns. The slab soffit is truly flat which, of course, is what makes form work cheap and contractors happy.

Based on your repeatedly mentioning punching shear reinforcement, I suspect that you have a flat plate.

Codes around the world very a bit but, for high seismic work, I believe that the general consensus is:

1) Flat plate slab to column joints should not be used to dissipate energy as part of the lateral system.

2) Flat plates may be used in conjunction with other lateral load resisting systems so long as slab to column joints are verified to be able to handle the anticipated joint rotations.

Shear walls work well with flat plates, like Dik said.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

KootK:
I was thinkin' flat slab w/drops... for seismic. Slab shear becomes a real issue, even, with shear walls, and a bigger problem if a plate.

Dik

 

[KootK]

Yeah, I figured you were thinking flat slab. I really wish that the nomenclature would switch over to something less confusing. Flat slabs really aren't all that flat now that beam & slab is pretty much a dinosaur in most markets.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

The distinction between flat slabs and plates has been around for over 40 years, that I'm aware of.

Dik

 

[KootK]

Agreed. I think that "flat slab" originally came about because the system was much flatter than the original beam and column framing system. 'Aint so flat no more compared to modern systems.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Akeee]

Sorry guys, i meant flat plate, 20-25cm thickness (8 to 10 inch). Yes no way i will make column - slab connection because of large unbalanced moment from seismic lateral forces that increase a lot the punching shear force. But my fear is with shear walls too that on a seismic large number of cycles the compresive strut will fail and the reinforcement for shear is nothing without a concrete diagonal to transfer. Shear walls have a large moment at the base but because of thick plate it will appear some momente in the plate too. I trust a lot more a beam to carry cyclic forces vs a plate mainly because the high height of the section and so large concrete strut and concrete 45 degree shear crack that loads the strirrups. Maybe its just a unjustified fear of mine...

 

[KootK]

the reinforcement for shear is nothing without a concrete diagonal to transfer

I agree completely but why do you fear the strut not remaining in tact? We check our designs for that. In concrete, the wall is pretty much the gold standard for shear resistance. If you're not going to trust that what are you going to trust without switching to steel?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Akeee]

In essence a flat plate can be symplified to a bream column/wall frame too, a beam for example with the 2m width and 0.20 m height. I will be very flexible so it will rotate a lot with columns and less with wall but it will rotate some amount. The main issue of mine is that NONE of the design code cover flat slab or flat plate and by none i mean really none. There are some guides (unapproved by goverment) and some articles on the web written by some teachers and thats it... It's like a ghost, in the design codes they dont say "dont use flat ...", just dont exist, not even a word on the subject, but they all build it. So with this new trend here "all flat plates" if i design such system i am on my own,i am not covered by any design code/requirement. I can apply only what i read online and what others suggest me and i think that this is not ok at all. They should be banned until a design code is published oficial in which is clearly specify the limitation for example of the compressive strut to a precent value from the capacity to count the cycle of seismic attack. I read online (i think is in ACI to) that is recommended limit of 40% when calculate the reinforcement... If something happens with the structure i cannot say that i respected the design procedure because the is none so that scare my a lot... Maybe that why i have this repulsion with flat plates...

 

[Akeee]

I agree completely but why do you fear the strut not remaining in tact? We check our designs for that. In concrete, the wall is pretty much the gold standard for shear resistance. If you're not going to trust that what are you going to trust without switching to steel?

Yes indeed the best example of shear resistance is a RC wall but there is implemented by the design code the limit of the concrete strut capacity,for example its 15% of area*compressive resistance ( 0.15*Area*fc) and i assume that this value didnt come from lottery and is based on a lot of experiemental work combined with a probabilistic aproach, research that i dont have acces to analyze. So how can i know that this can be apply to slabs too? I cant... I trust the mechanism of the shear resistance but because it's not an analytical one and its based a lot on empiric design there has to be a lot of tests, tests that i cant do on paper on my own so i need this calibration values (for example on the same formula for column is 0.28) to calculte a proper capacity and to make a good design. All of that need a code... I can use the top building codes there is, for example ACI, for guidance but that is it....

 

[Akeee]

So KootK for example say you design a building for seismic attack and the building code in that contry does not mention anything about frame (column-beam) lateral resistance system, no provision, no formulas, nothing. But you know from other similar codes from other regions (say for example Europe) what to do, what to limit and how to do it, you still design it and assumed by signature ? Learning from other codes and books how to treat plastic zones at the end of beam, how to calculate the nodes and how to prevent shear failure of the columns and beams but is one thing but it not the same as doing it for real blindly. For example on flat plates there are mainly 5-6 codes in the world that treat this problem different. Luckly i found a study that tested on real experiments the differences between these codes and which is more conservative and which is not, but say you dont have this, which design method you will be taken ?

 

[KootK]

Your frustration is coming through loud and clear Aketr. I'll try to answer that with as much honesty and practicality as I can muster. Here's what I've got:

1) Unless some real world failures occur, the flat plate + shear wall system is here to stay. And that's as true in seismic locations like California as it is everywhere else. It's a matter of construction economics. If you force the universe to choose between keeping flat plate around and keeping you around, you won't like the answer.

2) Look around at the work of your local competitors and get a sense for the range of technical rigor being exercised. What are they allowing and what are they not for systems? How detailed is their checking with regard to the issues that concern you? Once you have a good feel for that, position yourself in the upper third with regard to the rigor that you exercise yourself, and no higher. The truth of the matter is that, if you get driven out of the marketplace for being too good of an engineer, then there will be only bad engineers left designing the infrastructure in your community. And that helps nothing.

3) Realize that codes are not intended to be complete lists of everything that an engineer needs to consider. That which is contained within your local codes must be complied with. Beyond that, it's up to your judgment. It's both terrifying and liberating. Pick the provisions that you find most rationale, and will keep you competitive, and try not to worry over the fact that codes differ and your local code may not be the greatest. Where Dik and I practice, the concrete code suggests that we do this:

a) Work out drift assuming no lateral participation of the slab/column system.

b) Check our slab/column connections for deformation compatibility after the fact.

That's not as rigorous as actually knowing the force demand at the joints but it acknowledges the problem and attempts to address it in a manner that is relatively friendly to production efficiency. That said, I doubt that this method is adequate in all instances. If you've got a column 6' from a shear wall, you may well fail the concrete in punching shear under high drift. Fortunately, we also have code provisions for column integrity steel which mean that a punching shear failure would be ugly but not catastrophic. Adding column integrity steel might be an effective way to reduce your worry about punching shear.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Akeee]

Hehe, thank you;

1. I dont expect they stop building flat plates/slabs because i dont like them, i am aware that is imposible and that the price controls almost everything but my frustration is about lacking of support material for designing it.

2. I do look and always try ways to improve my techniques and when i see something that makes sense and appears to me is a good thing for safety, sure i adapt it on my designs, but from what i observed so fare there are 2 kinds of designers: first kind so afriad that almost double all the steel/concrete quantities just to be sure and the second one that really dont stress about it, the design is made from previous buildings that are made and the argument is that "they still holding our will be ok" with almost 0 calculations, not an engineering approach keeping in mind that we are a country in development so every couple of years the seismic hazard is risen so that stept by stept not to produce an economical impact we want to achive like US level of design forces. Thats why i read a lot of other designing codes to try get the optimum equilibrium between safety and feasible so that i dont run out of business.

3. Yes that is terrifying and that's why i dont jump on the optimum/at the limit design. Step by step when i become more confident (mostly from reading books and talking with other pro's) about a subject i start making more efficient reinforcement and designs. Best example is the balance between force design and ductility design/concept; that is more important to treat very good the plastic zones and dont obsses so much about the values of the forces from static analysis because making for example a good horizontal reinforcement that help to confine the concrete and so increase the rotation capacity of the section is a lot more important that a 100% check with forces. After some pushovers and advanced analysis slowly i become confidend about things but is a long demanding (and here i mean the time) process. But the fear that something i miss that is not ritten in the codes it is still there especially with the short time framing that the design have to be made..

Yes the integrity reinforcement is a very very good thing, i was very glad when first i learn about it, that you can prevent the collapse of slab with the shearing (i dont know in english what is called) capacity of the rebars; that is why i never overlap the bottom bars (of course the top one's neither) in the column/wall section, i really want to make sure that i dont risk this sharing effect. This is one good example of thing that i learned and adopted it in my design (what i was talking in 2.).

What is you opinion about using steel profiles like HEA100 for example ( I steel profile) for punching shear capacity ? You thing is better then rebars ?

 

[KootK]

What is you opinion about using steel profiles like HEA100 for example ( I steel profile) for punching shear capacity ? You thing is better then rebars ?

I'm not a big fan. I worry that they make it more difficult to have well consolidated concrete at the joints. And I think that having well compacted concrete at the joint is far and away the most important thing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Akeee]

Yes you are right this errors in concrete pouring might go to fail bond of rebars or fail compression zone, both as bad as it sounds.