Ribbed Slab

[perfectaccess]

Hi,

I hear that if ribbed slab is resting on shear wall, it is a must to provide perpendicular rib parallel to shear wall,and note to let the block in touch with shear wall,since it is not safe for shear.

But I dont buy this,block is not structural element,and if rib is design for shear ,why I shall do this?

Any reference in ACI 318

 

[hokie66]

I have no idea what you are asking. Any chance of a sketch of your situation?

 

[KootK]

The "rib" in line with the shear wall is pretty common I think. It distributes the reactions delivered by the joust ribs and provides a capable collector element for diaphragm shear.

I don't understand what the "block" represents here so I'm unable to speak to that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

I think he's saying that at every shear wall (I'm assuming they're using concrete block) you should provide a rib parallel to the wall. This is regardless of which direction your ribs are typically spanning.

The concrete block is a structural element if it is a shear wall. it is possible to have a shear wall that doesn't take vertical load, but it seems like a waste of block then.

 

[KootK]

Ok. I'm going to hazard a guess that the block of concern is the infill bits that a cooperative mason might frame up to the deck in between the joist ribs were there no CIP distribution/collector rib. In that case, I would not rely on them for shear as a consequence of their inherent lack of robustness.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[perfectaccess]

sorry for make things not clear,the block I mentioned is the filling material between ribs, on of the codes are saying that for any ribs laying directly on shear wall or drop beam ,there should be a parallel concrete rib(about 15 -20 cm width) to shear wall and in same time it will be perpendicular to main ribs.

So i am wondering if such statement in ACI exist,also he said the reason behind this ,is that filling block could fail under shear and separate form ribs(but I don't buy this,however there should be a reason ).

I will attach sketch after couple of hours

 

[KootK]

No worries, it sounds as though we figured it out after all. That being the case, I stand by my previous responses. The rib parallel to the wall is prudent for both vertical load distribution and shear transfer. Unless exceptional care is taken in the construction of the infill between ribs, which I assume are fairly tights spaced, I would not rely on them for shear transfer. They simply do not tend to be built robustly.

I know of nothing in ACI that prescribes/proscribes any of this explicitly. But then not all sound engineering practices are codified.

I think that the important part of this is the understanding that there should be competent load transfer between floor diaphragm and wall. Whether or not the infill bits touch the wall is immaterial.

In open webbed steel joist systems, we sometimes transfer shear through joist seat rollover rather than shear lugs installed between the joists. Similarly, with your rib slab may be able to transfer shear to the wall via local weak axis bending of the ribs rather than the use of a distribution rib running parallel with the wall. If you ran the numbers and it worked, I wouldn't much care if the "non-structural" infill block between ribs was in contact with the ribs/floor. I suppose this infill bits might be exposed to a very small risk of inconsequential cracking under the application of lateral load.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[perfectaccess]

Sorry for late respond,it has been while ,without internet .

Kootk, it does make logic now.so if the weak axis is capable to transfer load to shear wall,then no need to put crossing rib.

But my question ,how can i check this by numbers?equations?

 

[KootK]

But my question ,how can i check this by numbers?equations?

With considerable difficulty which is why I'd probably stick with the distribution rib. I know of no established procedure to check this but we could probably make something up:

Take a 1' strip of slab parallel to the wall and treat it like a pinned based, multi bay moment frame of unreinforced concrete. You could use inflection point assumptions similar to those used in the portal frame method to simplify the problem.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.