Forced one way slab

[dgkhan]

I have a situation here where slab is spanning 15 feet. Width is 12 feet. As aspect ratio is < 2. Can I still design it as one way slab spanning 15 feet and place supporting beams at two short ends? OR I have to design it as two way slab and provide support to at all four sides.

 

[ali07]

You can consider designing on 3 sides also, if you can't support on one 15 ft edge. Is it not possible to add beam/beams on 15 ft side and make it one way slab?

 

[JedClampett]

If you provide supports on the short side the load will eventually get there. IMHO, if you provide a reasonable load path and member capacity to achieve that load path, your design should work. Make sure your long bars are clearly shown as closest to the slab faces. In any event, I wouldn't design an individual slab as two way. It's too much trouble. And then you need to provide minimum bending reinforcement in both directions.

 

[dgkhan]

JedClampett
I wrote one span dimensions. In reality it will be continuous for four spans.

 

[kslee1000]

I don't see the reason you cannot design it as one way slab.

 

[dgkhan]

Thanks kslee1000. I was wondering if I am violating <2 aspect ratio rule, no matter what I do slab will span and /or distribute moment in transverse direction also. It might cause excessive cracks if not failure.

 

[apsix]

"OR I have to design it as two way slab and provide support to at all four sides."

If you don't provide supports at the long sides then it can only act as a 1-way slab in the long direction.

 

[kslee1000]

dgkhan:

Small experiment to make you more comfortable in on way behavior:

Take a 8"x11" cardboard, and lay it on two engineering scales (triangle shape), one on each end. Now load it with your finger, then check the deflect shape, isn't that looks like the smily? Or a dish?

You need end beams, but not edge beams as pointed out by apsix. You will need to consider two way action if the slab is enclosed by beams with similar rigidity, or columns at 4 corners with beams in between.

Either way, you will need to provide rebars transverse to the main reinforcement, the matter is the amount, which is pending upon the choice of 1 or 2 way design.

 

[frv]

Cracks, absolutely.

failure? NO!

This is actually a very important concept. If you provide sufficient reinforcement so that the slab can withstand the moment in one direction, then that slab WILL NOT fail. It may, however, develop enormous cracks.

 

[kslee1000]

For crack concern, follow ACI crack control criteria for one way structures (mainly force one to select smaller main reinf with closer spacing). Provide adequate transverse reinf will further reduce the likelihood of unsightly cracks. Please many bridge decks are 1 way slabs.

 

[ali07]

Where you are using this slab which is unsupported at long sides with 4 spans.

Slab thickness appears to be same in both directions either you go one way in 12ft direction, or for one side continous span in 15ft direction.

So it's your choice to go in either way and design it is as oneway slab.

 

[rapt]

dgkhan,

Several pople above have convered it reasonably well. My opinion has always been that, if the slab wants to act as 2way (elastically) because of the positions of the supports, then that is the best way to design it. For it to work any other way, elastic stresses will cause cracking in unreinforced stressed areas which will allow redistribution of those stresses to areas where there is reinforcement, as long as there is a load path (if not it will fall down!).

However cracking and redistribution cause deflections and wider cracks.

If you do not reinforce a major stressed area for strength, allowing redistribution to other areas to make the design work, to get reasonable crack control in that area, nominal reinforcement will not be enough. You will find that you need the reinforcement that was originally required for strength to give good crack control. So you are better off designing that way in the first place. I have done the nuimbers on this previously.

So you end up reinforcing it twice. Just do it as 2way like the slab wants to work in the first place.

 

[csd72]

You can design it any way you want really as long as the structure matches the analysis assumptions.