Concrete design question 3

[AlmostPE]

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!

 

[apsix]

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.

 

[GoddyMgwasa]

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

 

[Lion06]

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.

 

[archeng59]

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.

 

[apsix]

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.

 

[BigswedeCO]

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.