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Concrete design question
3

Concrete design question

Concrete design question

(OP)
I have not been exposed to concrete design too much in the real world and I have general questions:

1. If I have a structural slab that is suppoted by 4 walls.  The slab sits on the top of the walls and dowelled into the wall with bent rebars (one leg into the slab and one leg into the wall).  So is this fixed or pinned?

2. Do you guys have any pointers on how to design the slab?  I was going to design it as if it were 1 way slab and design it to span the short direction and then put the rebars the same way both ways.  Shall I follow ACI and make the slab thick enough so I dont have to worry about deflection? Some of you may say to ask my supervisor to train me but I dont think he knows any better than I do and he has a PE stamp!.  sad

Thanks!

RE: Concrete design question

1) You have to consider the joints as pinned connected, because, the walls may not have been designed to carry fixed end moments and the required amount of reinforcement for considering the joint fixed will be too large and may not be economical.

2) For deflection control, you can use ACI-318 suggested thickness.
For designing reinforcement, yield line theory is most applicable for this kind of situtation.

RE: Concrete design question

1. From what information you have provided, it seems that the slab would be considered simply supported (pinned) by the walls.

2. ACI is likely the correct code document, but it would depend on where the structure is located and who has code juristiction. If you are in the US this should be the case.
You need to look at the ratio of the span lengths to determine if the slab is a one-way or a two-way slab. You'll need to evaluate how much deflection is acceptable and verify that you meet that requirement. ACI gives some guidance on minimum slab thickness.

Good Luck!

RE: Concrete design question

(OP)
Well, do you think my approach of designing it as one way slab (pin pin) on the short direction will be a satisfactory design?  Then Ill just put the same rebar going the longer direction.  This project is not very big so I do not have the time right now to really learn it to save amount of steel and concrete.  I have a feeling the PE will love and trust whatever design I come up with (which is scary).  So I want to make sure I do it right in so little of time.

RE: Concrete design question

From what I remember of concrete design, you will have a different d in one direction compared with the other since the bars in one direction will likely sit atop the bars in the pther direction.  This difference would need to be checked to ensure the slab can support the design loads.

Don Phillips
http://worthingtonengineering.com

RE: Concrete design question

Designing as a 1-way slab will be safe and conservative. The rebar spanning the short direction should be in the lower layer.
You should also have top steel at the supporting walls, especially at the corners, to avoid cracking due to negative bending moment. However, I would still design it for pinned supports.

RE: Concrete design question

(OP)
yeah, thanks apsix.  That is what I was thinking too.  Thank you everyone!

RE: Concrete design question

you could design as two way (put the short direction steel in bottom) by use of tables. In the british standard theres a simple table to use not sure if equivilent in US. Should be considered simply supported. If you design as one way spanning design will be conservative so be ok, check deflection as one way spanning as well, (short span). effect of four support sides will mean deflection will be less than you allowed for. Generally if the ratio of spans > 2 then one way spanning.

RE: Concrete design question

When dealing with a single slab bearing on 4 walls, you will also have to account for Mxy and Myx moments.  You can look at PCA Design of Rectangular Tanks for the design procedure.  I would definitely buy this book (it's only like $40) and use this design methodology.  They have an example in the book for a tank design with a roof slab, which is basically what you are describing.  Is the wall you are supporting below ground?  If so, the walls will have to be designed like a concrete tank.  They will not be braced until the top slab is poured.  If you do not account for this in your design of the wall, you could have a failure or at least a wall with large deflections or large cracks.  Also, be careful in your detailing of the wall.  You should look in the ACI Detailing Manual for help in detailing a concrete wall or "tank".

RE: Concrete design question

You could certainly design it as a one way slab and it would be safe (i.e. it won't collapse), but depending on the aspect ratio (I didn't see it mentioned above, just that one side is longer than the other.  there is a difference between a ratio of long to short of 1.25 and 4) it really wants to behave as a two way slab.  It won't collapse designing it as a one way slab, but you may have some serviceability issues.

RE: Concrete design question

AlmostPE,

These guys are just confusing you (they are almost confusing me and I have done heaps of this stuff!). Look up section 13 of ACI318 and it covers 2 way slabs. There is a chart that gives moments in each direction depending on the span.

These charts are based on yield line theory as mentioned by shin25.

csd

RE: Concrete design question

The Direct Design ACI Chapter 13 requires multiple bays.  The Equivalent Frame method might be able to be adapted to a One-Bay situation but I don't know how accurate it will be.  If you try to use the Equivalent Frame Method of Chapter 13, you will need PCA Notes on ACI 318 to get the factors to solve the problem.  PCA Notes on ACI 318 also has a design example which is incorrect.  The easiest solution is to use the method in PCA Rectangular Tanks. Or model it in a finite element or 3-d frame analysis program.  I would use at least 8 plates elements per side (as a minimum) for a computer analysis or 1 to 2 foot wide plate elements.

RE: Concrete design question

Sorry I am looking at the 95 version and assumed the latest would be the the same.

RE: Concrete design question

(OP)
vincent.. hmm I do have PCA rectangular tanks.  I didnt think about that.  I will analyze with both pca rect tanks table and 1 way slab and see how close they are.

RE: Concrete design question

I have also seen it done but setting the deflection equitions for each direction equal to each other to determine the load used for each direction (for simple beams in each direction).  Essentially determines the ratio of load that is applied to each span direction.  Has anyone ever seen it done this way?

RE: Concrete design question

(OP)
twinnell, I do remember learning something like that.


So here is what I come up with, the slab is about 16x22 ft.  Based on L/20 I made the slab 10" thick.

DL = 10/12 X 150 = 125 psf
LL= 125 psf (IBC light storage)

Mu = 387.5 psf

I came up with #5 @ 10" o.c. bottom reinf. ea. way.
This is also good enough for shrinkage.
Now can I just stop right here and not worry about deflection since I used L/20?

Now my next question is, if I design this slab as pinned, can I use bent rebar into the wall?  Wouldnt that create a fixed connection?

Since I designed it as pinned, how shall I dowel the slab into the wall?  What design value shall I use since the moment is 0?  Shall I just space it @ 10" o.c. also like the slab?  Maybe embed it development length distance?  I am just afraid doing this will create fixed connection then I have to redesign the walls to take moment.  Sorry with all of these questions.  I just wish I had a really good engineer working with me so I can ask him all of these questions.

RE: Concrete design question

I would put top steel around the perimeter and extend it into the slab to at least the quarter point maybe even the third point of the span.  I would provide an L-bar that ties the negative steel (top) in the slab to the wall.  As far as the bottom steel just extend straight into the wall.  You might want to increase your dead load some, for ceilings, plumbing, mechanical units, etc.

J

RE: Concrete design question

(OP)
Im sorry.. I meant Mu=387.5 * 16^2 /8 = 13150 lbs ft.

RE: Concrete design question

I just realised what you guys are on about - ACI have dropped the 4 sided slab charts. Whats the story with that!?

RE: Concrete design question

From above:
"it really wants to behave as a two way slab.  It won't collapse designing it as a one way slab, but you may have some serviceability issues."

The only serviceability issue that I can see is that it will deflect less than a one way slab of the same span.

RE: Concrete design question

using British standard you just take span ratio, if its >2 then design as one way otherwise 2way slab. Main bars span short lengh.

RE: Concrete design question

apsix-
If you don't design it the way it wants to behave you could (and probably will) get cracking that isn't expected and can damage floor finishes.
If you fail to design for moments that are there, you will get a greater degree of cracking than the simple cracked section that we assume in concrete design.

RE: Concrete design question

I assume from the description the four walls are the edges of the slab and there is no continuity of the slab over the walls?  It's a single room with a concrete lid?  Design the slab with pinned supports and as a one-way slab in the short direction with the bottom layer of steel for the shorter span.  If both spans are close to the same in each direction, use the longer of the two as the span for the one-way slab.  In my opinion, even though the slab might be the same dimension in both directions, you won't get the benefits of a two way slab without the continuity of adjacent spans.  

RE: Concrete design question

StructuralEIT
You're right, for 'serviceability' I read 'deflection'. Top steel is required in the corners of 2-way slabs, and also along the edges if there is some rotational restraint.

By the British & Australian concrete standards the midspan moment is reduced by about 50% for a pin supported 2-way slab with equal spans.

RE: Concrete design question

I have a little spin to add to this type of design.  With the underground concrete tank I am working on, the contractor/timeline requires to form and pour the lid on the ground while working on the floor and walls.  When the walls are complete and adequate cure time, the lid(s) would be craned on to the tank.  
Can anyone recommend information on specifying lift loops and guidance on spacing and associated reinforcing details required for the lift?
The lid(s) also have openings of various sizes and arrangements to allow future mechanical replacement, ie pumps, etc.  
Thanks.

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