Square opening in a solid slab

[VTAdam]

I am designing a 25cm (~10") two way solid slab. The dimensions of the slab are 300cm x 300cm (120" x 120"). The slab contains a square opening; 70cm x 70cm (28" x 28"). My question is what design considerations do I need to consider when designing this slab? Do I need to provide beams around the edges of the opening or is it small enough that it can be disregarded?

Thanks!

 

[hokie66]

I assume you mean a slab which is supported at the corners by columns, or at the perimeter by beams and/or walls. And further that the opening is in about the centre of the slab. A lot of assumption on my part, but you didn't make it clear. If my assumptions are correct, the opening won't make much difference. I would add some diagonal bars across the corners to restrict the cracks which tend to form there.

 

[Trenno]

How many bars will be 'cut' if the void weren't there? Are there alternative load paths? Is the void in the middle of the bay? How thick is the slab? Loading?

Also - What's this cm business!?!

 

[VTAdam]

The slab is supported at the perimeter by a load-bearing wall. The opening is near one of the corners; it's 30 cm (~12") away from the walls in both directions. The slab thickness is 25cm (10"). The service loads are about 15 kN/m[sup]2[/sup] (~300 lb/ft[sup]2[/sup]) for the dead load and 2 kN/m[sup]2[/sup] (~40 lb/ft[sup]2[/sup]) for the live load.

I don't know what the spacing is yet. Probably 20 cm or so (~6").

We use the metric system around here .

 

[hokie66]

You don't disregard the opening, but neither would you provide "beams" around it.

Converting to proper SI dimensions, you have a 3000 x 3000 slab supported on walls all around. There is a 700 x 700 opening about 300 away from a corner in each direction. So I would model that as a 1000 x 1000 opening, leaving you with an uninterrupted 2000 width in each direction. I would put all the required bottom reinforcement in those two bands, then use the same reinforcement in the interrupted strips. You will have twisting moments in the corners without the hole, so make sure to reinforce those adequately in the top.

 

[Trenno]

Sorry VTAdam, just having a dig... engineers rarely use cm...

 

[canwesteng]

I've used cm in south america, but in canada it's always been mm. Where are you?

 

[TehMightyEngineer]

Yeah, what little metric work I see in the states is almost always mm.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[BAretired]

Why is the dead load so high? A 10" slab weighs only 125 psf. I'm guessing you have about two feet of soil above the slab. Do you have dead load over the opening?

BA

 

[VTAdam]

Thanks for the help. I work in Israel at the moment. I was educated in the United States and used the Imperial system of course. I am fairly new practicing engineer and I am not sure whether they use the cm or not around here.

 

[sponton]

@Trenno

actually countries that use mks system always use cm, it's kind of funny how the world wants the SI system but it is unnatural to work with [as much as imperial in my opinion], it's not like I buy 10 newtons of chicken in the supermarket, we buy stuff in kilograms. Plus to those familiar with cm, it's just a 10^1 conversion to mm and numbers are smaller to work with.

 

[hokie66]

sponton,
To the contrary, for engineers it is unnatural to work in centimetres, unless you are a dressmaker. Engineers don't worry about chickens. The beauty of SI units is that every thing is in multiples of 10^3. If you ever use it, you will love it. The first 20 years of my career were in the US, the last 20 in Australia. I wouldn't want to work in kips, feet and inches again.

 

[jayrod12]

I'm only going to chime in on the cm/mm argument. The reason no one here uses cm (as far as I know) is none of the stresses you will use are ever in cm, there's no N/cm^2 or kN/cm^2. Pa, kPa, MPa, GPa. all in newtons/kilonewtons per mm^2/m^2