Concrete Column - Development Length 1

[slickdeals]

Folks,
How do you typically handle the column reinforcing at the last lift (roof)?

If you have a 8" flat plate with columns at 28'-30' on center, I assume it is impossible to develop reinforcing inside the 8" slab with hooks unless you are using very small bars (#5 or #6).

In that case, do you analyze the building with a slab supported on hinged columns at the top?

I am curious as to how you detail/analyze/design them?

 

[JAE]

If it is not developed into the slab then I would suppose you really can't transfer any significant moment...maybe some but not all.

I recall my mentor many years ago - we were driving somewhere to a meeting and we drove past a multi-story concrete building. I noted to him that there were 90 degree hoooked bars at the top floor columns. We typically didn't do that - just extended the column bars as far into the roof structure as possible and called it good.

He stated to me that day, "Well, where would that floor ever go? The wind certainly can't lift the concrete floor system off the columns."

I suppose that in a seismic area you'd have to deal with some shear or something possibly. But if you have a pinned condition at the top of your columns, and you've taken story shear out some other way, there really shouldn't have to be a full blown developed connection there.

 

[hokie66]

Agree with JAE. Pin the roof slab to column connections in the analysis. There will be some fixity, but no reason you can't ignore it.

30' is too big a span for an 8" flat plate.

 

[asixth]

I have a flat slab which has noticeable deflection problems with 28' spans and 10" thick slab.

 

[slickdeals]

I have successfully designed 8" post tensioned slabs spanning 28' - 30' with no deflection problems.

asixth: was your slab post-tensioned?

 

[asixth]

no, just reinforced

 

[JAE]

28 ft. doesn't seem inappropriate for a two way flat slab. I once investigated a building with 10" slabs at 30 feet typically and one interior span at 40 feet. The 40 ft. span had long term sagging problems - no good way to repair it - strength was OK...just long term creep in the concrete - 3" sag with stud walls pulling away from ceilings.

 

[rowingengineer]

asixth,
flat slab or flat plate? flate plate would be struggling.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[hokie66]

Quite a variance in opinion on span/depth. Just thought I would share the guidelines I use as a starting point for slab thickness:

Reinforced flat plate, L/26 end, L/30 internal
Posttensioned flat plate, L/36 end, L/42 internal

Reinforced flat slab, L/33 end, L/37 internal
Posttensioned flat slab, L/37 end, L/46 internal

 

[asixth]

RE

flat slab, the same one I posted in the AS/NZS forum a few months back.

 

[rowingengineer]

Hokie those numbers sound good however I'm a bit more conservative on the flat plate try to limit to L/28 and for a heavy load would start about L/24.

Aixth,
I will have to go back and review because I don't remember.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[hokie66]

I can't remember ever doing a flat plate with more than 8 metre spans except for really thick transfer plates. Just doesn't seem the right solution. Flat slabs or band beams with one way slabs make me feel a lot better about deflections when the spans get up there, and you don't have to worry as much about punching shear.

I think this is the thread asixth is talking about.

thread744-265955