what problems in slabs with too much service holes? 3

[IJR]

I have a couple of slabs typical dimensions 25ft by 25ft supported on RC walls and interrupted intensively by service holes. And I have a limited depth of 1ft.

Running numbers I can easily find that with no beams a thickness of 1ft will be more than enough.

Total area of holes is about 30% of total slab area and that sort of makes me nervous.

I will appreciate experience shared and info exchanged on this subject.

Thanks

IJR

 

[Riz]

IJR,

Is it a 2 way spanning slab. If so 300mm thick slab is very conservative. Are the ducts BIG?
The area of rebars cut need to be replaced at respective sides and corners for ensuring 2 way action.

If u r still worried than redesign it as a 1 way span slab with smaller connected ducts.

 

[14243]

Hi IJR:
what is the loading detail? Is there any point loads on the slab next to the holes? what is the size of the hole?I am sure that you checked the slab for shear. The slab may be ok for shear consideration. What we have to worry about more is , cutting the main reinforcement in the field. The slab is double reinforced both top and bottom?
One suggestion is to add diagonal reinforcements around the openings.

Hope this will help .

JeyR

 

[IJR]

Thanks Riz and thanks JeyR.

Riz: 1) Yes it is a 2 way slab and I have modelled it accordingly using finite elements(shells) with 1 item 2mx2m hole and 3 others 0.65m*1.3m each of holes.

2) If you feel it is OK, then I am going to detail zones between holes as hidden beams by providing larger reinforcement and tying with shear reinforcement.

3) How do you feel about beams the size of slab only provided with larger reinforcement and tied up?. That is what I want to know.

JeyR: 1) Hole sizes as above and no point loads. Only standard platform loads.

2) I agree with your detailing and I am not going to cut rebars.


Thanks once more and again.

 

[Ginger]

IJR

I believe that there are probably too many holes to allow the slab to act as 2-way spanning. I would agree that the solution most likely to work is to incorporate beams within the depth of the slab.

The usual way to design this is to mark out your beams around your openings and then design the remainder of the slab as 1-way or 2-way spanning panels in-between the beams. The beams are then designed to carry the support loads from your slab panels. If the entire opening is framed on all sides by the hidden beams then there is no requirement for diagonal "cracker" bars.

Regards Andy Machon

 

[IJR]

Thanks Ginger

I think that is what I will probably do: provide as many hidden beams as is practically acceptable.

Next time you visit, do you want to tell me how comfortable you are with within-slab beams(hidden beams)

 

[Riz]

IJR,

A 1 way slab in your case is a beam 1m wide.
Ginger has the right solution. U just span the slab bits to transfer loads on to your 1m wide beams.
Corners bars need to be in place as well.

With a FE analysis u actually get stress contours. The resulting moments are good enough for designing a 2 way spanning slab. Additional steel on the duct sides & corners are required. This option is more economical.

Try this site :

http://www.prokon.com

& download a demo version. U may be able to run your slab!
Riz

 

[IJR]

Thanks Riz

I have done my FEM, and have modelled as many beams as could be conveniently detailed.

Take care and "asante sana" (You are in Dar es Salaam arent you"- and I speak original Swahili)

 

[Ginger]

IJR

I have no problem at all with using hidden beams within the depth of the slab. So long as they are designed as beams in their own right and you follow the rules of your design code for beam design then all will be well.

I don't speak Swahili so "Good Luck" is about all I can wish you.

Best Regards
Andy Machon

 

[IJR]

Thanks Ginger for keeping the promise.

Can you imagine, how great it is all my worries cleared off in a single day.

Thanks guys