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!.
Thanks!
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!.
Thanks!






RE: Concrete design question
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
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
RE: Concrete design question
Don Phillips
http://worthingtonengineering.com
RE: Concrete design question
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
RE: Concrete design question
RE: Concrete design question
http://www.cement.org/buildings/design_aids.asp
RE: Concrete design question
RE: Concrete design question
RE: Concrete design question
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
RE: Concrete design question
RE: Concrete design question
RE: Concrete design question
RE: Concrete design question
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
J
RE: Concrete design question
RE: Concrete design question
RE: Concrete design question
"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
RE: Concrete design question
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
RE: Concrete design question
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
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.