Need some guide on irregular 2 way slab design 2

[BeckSt]

It is my first time to design irregular 2 way slab and I need some help from you.
I have used FEA for the slab analysis and below is the shell stress. Most of the stress is less than 3 MPa. but I have noticed that the stress in some mesh is greater than 35MPa (the green ones). Why is that? Is it more because of the model error? (There is only UDL on the slab. no point load added)
Also. what books do you guys recommend for slab design? I am not very confident in arranging the reinforcement correctly. Can anyone provide some resource for irregular slab design example including the reo arrangement after analysis? Thank you very much.

 

[WARose]

I would never use a general FEA package for concrete slab design. You need specialist software such as RAM Concept that is able to make allowance for Mxy stresses, check punching shear and (most importantly) calculate long-term deflections including creep and shrinkage effects.

You can do that with a "general" FEA package......you just have to know what you are doing. (I've got a bunch of RC buildings sitting out there right now done that way.)

Anytime I've left the design to the software.....I've wound up questioning/tweaking the results. I'd rather have the raw forces/stresses and sort it out myself.

 

[r13]

I also have a simplified question regarding this, how do you analysis the following simplified slab?

You can design this slab in many ways. By visual measuring, the aspect ratio (short/long) is less than 1:4, so first design it as two way slab without the opening. Once the bar size and spacing are determined, bundle the bars, that are to be placed in the opening in the long direction, to the long edge of the opening to create an embedded beam strip. You may then want to add a few bars along the short edges of the opening to stiffen the edges. The last thing is I would add diagonal bars at the corners to minimize crack length and width.

 

[Guest090822]

Buy the concrete engineering textbook by McGregor - great resource.

 

[KootK]

1) I wouldn't give up on FEM for slab design. Nowadays everybody wants a flat, beam-less sofit and there's no such thing as a regular column layout. In my opinion, that means that;

a) we just barely have reasonable, hand design methods to cover us for strength which, frankly, rarely governs.

b) we don't have reasonable, hand design methods to cover us for deflection which usually governs.

As such, if you restrict yourself to hand methods, you're likely to find yourself in one of these undesirable positions:

c) you're too conservative relative to your competition and lose work as a result.

d) you're not conservative enough and in-service deflection issues are landing your in court regularly.

2) I have found it frustratingly difficult to find a decent reference on practical, modern slab design using modern tools. The classical strip methods that you find in most university textbooks are useless to me other than for providing the historical context for how rebar detailing typically needs to be handled. The one decent reference that I now of is shown in the first clip below. Besides that, I've found that books on post-tensioned slab design tend to be better at covering suspended flat-work than generic concrete texts. One good example is shown in the second clip below.

3) As you've rightly anticipated, practical slab design requires one to know how to take complex FEM results and translate them into simple rebar layouts that really reflect how things have traditionally been done, pre-FEM. For this, I recommend the third book below. More on that reference next.

4) It is indeed very difficult to hand-evaluate concrete slab design situations of any complexity without resorting to the rather brutish "I'll pretend there's little beams in the slab". That method is valid but pretty much tosses any two-way plate benefit out of the window. And, as I mentioned above, it's just for deflection estimates except, possibly, to provide very conservative upper bound estimates. For this, again, I recommend the third book below. Tackling complex situations efficiently by hand really requires the use of methods like the Hillerborg strip method and the yield line method which seem to get little, if any, coverage in university nowadays (if they ever did). That's all covered in that book. It's a bit frustrating to read the Park & Gamble book because it's long, tedious, and does next to nothing to help you actually complete your assignment right out of the gate. That said, over the long haul, it will pay dividends with respect to your fundamental understanding and your confidence with two way slab design. The book also gets into the theoretical basis for FEM design as it pertains to slab design which is helpful.

5) Appreciating the economic imperative to rationalize your reinforcement layout such that no contractor would even suspect that you used FEM is a great first step for a beginner. To that end: Rationalization of Flat Slab Reinforcement

 

[Retrograde]

You can do that with a "general" FEA package......you just have to know what you are doing.

I know you can do it. I'm sure I could do it myself given enough time (I would need to make some broad assumptions regarding long-term deflection multipliers). I'm just saying "I" would never do it given the availability of specialist applications.

 

[JLNJ]

I have used the "pretend there's little beams in there" method and modeled individual beams running the same directions as the concrete reinforcement. Small-ish beams close together.

You have to be thoughtful how you tie them together at the nodes, but you don't have to lose all the two-way action.

 

[KootK]

I have used the "pretend there's little beams in there" method and modeled individual beams running the same directions as the concrete reinforcement.

#MeToo. There's a time and place for everything, even the horribly inelegant.

Small-ish beams close together.

...but you don't have to lose all the two-way action.

Do tell. Basically grillage analysis by hand?

 

[KootK]

You do gotta be a little careful those big openings though. The richy-rich penthouse of my first PT building had all kinds of massive two story openings up to the loft. I designed the snot out of everything so that the balancing forces had the slab weightless! Then the contractor went and stacked a zillion pounds of cold formed studs on one of my stupid bridge things, using it as a staging area. You know, because I didn't explicitly say that he couldn't do that. It opened up a crack in the bottom big enough for me to count the strands passing through. Pretty great.

 

[KootK]

There might be another way to make a beam-ish solution work if, in fact, you have head room available for that.

 

[Blackstar123]

Thats what i would do if i was designing this slab on Etabs.
1. I would not use automesh to mesh this slab since it has some irregular openings and software will mesh the slab as the picture you have attached. I will do the meshing manually in orthogonal direction of the building and keep the meshing as simple as possible such as avoid triangular meshes and keep the aspect ratio less than and equals to 1.5.
2. Even though slab has some irregular openings (such as trapezoidal going on rectangular in your case), i would simply make them rectangular with an average width (only if the difference in the width is not be very large)
3. I will keep the mesh size such that within the width i have at least 3 mesh areas so i can get some reasonable stress output at these location as well. But you need to be careful here because smaller mesh will increase the analysis time.

I usually design reinforcement for slabs-beam system using FEA software(Sap2000 to be precise). Till now it has given me very reasonable results.

Euphoria is when you learn something new.

 

[Trenno]

Have a read of this, if you haven't already.