Flat slab in 0.3 PGA seismic zone
Flat slab in 0.3 PGA seismic zone
(OP)
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
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






RE: Flat slab in 0.3 PGA seismic zone
Dik
RE: Flat slab in 0.3 PGA seismic zone
RE: Flat slab in 0.3 PGA seismic zone
Dik
RE: Flat slab in 0.3 PGA seismic zone
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.
RE: Flat slab in 0.3 PGA seismic zone
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
RE: Flat slab in 0.3 PGA seismic zone
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.
RE: Flat slab in 0.3 PGA seismic zone
Dik
RE: Flat slab in 0.3 PGA seismic zone
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.
RE: Flat slab in 0.3 PGA seismic zone
RE: Flat slab in 0.3 PGA seismic zone
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.
RE: Flat slab in 0.3 PGA seismic zone
RE: Flat slab in 0.3 PGA seismic zone
RE: Flat slab in 0.3 PGA seismic zone
RE: Flat slab in 0.3 PGA seismic zone
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.
RE: Flat slab in 0.3 PGA seismic zone
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 ?
RE: Flat slab in 0.3 PGA seismic zone
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.
RE: Flat slab in 0.3 PGA seismic zone