Checks Required at Concrete Column Transfer Slabs (Walking Columns) 5

[KootK]

Please review the sketch below (and attached). I am curious what others do to check slabs at column transfer locations. I'm thinking of "walking" columns in thinner slabs but the discussion is equally germane to full blown podium style transfer slabs as well.

The options:

1) Individual column punching shear with small moment.
2) Combined column punching shear with large moment.
3) Shear friction in the slab between columns.
4) Bearing stresses.
5) P x e moment in slab.
6) Vertical seismic acceleration effects.
7) One way shear between columns, similar to #3 (forgot to draw this)

I believe that the following checks are fairly standard: 1,4,5,6. Combined punching shear (#2) is a KootK invention as far as I know. Shear friction (#3) is standard practice at one of North Americas top tier structural firms. I feel that it's unnecessary as I can't imagine that ever governing over the diagonal shear failure modes (punching & potentially one way).

So, what do others check? And I'm not looking for FEM solutions. And I know it's awful.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[hokie66]

Late to this one, but agree with rapt. Short offset transfers are strut and tie, and long offset transfers can be done that way as well, but are usually treated as conventional flexural problems.

As to studrails, we had a discussion some time ago, which has never been resolved to my satisfaction.

thread507-306324

 

[KootK]

With regard to the stud rail concerns, I wonder if one is any better off with conventional ties or if similar issues crop up there too. The insufficient flexural issue ought to apply in both scenarios.

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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[hokie66]

My understanding is that stud rails are used mostly in flat plates, which are generally too thin for conventional stirrups. I think the better solution for punching shear is more depth of concrete, maybe flat slabs or banded slabs.

 

[CELinOttawa]

We've debated this before... Every engineer wants a system other than flat plate, but Kootk's right - Economic drivers are pushing towards flat plate to make it very cheap to do flying formwork jobs... Faster, and dirty as all get out.

I don't like it, but it is where we're seeing the industry go. Thank God I've managed to avoid involvement with any of these minimal, awfully small fee, monstrocities so far...

 

[KootK]

I'd be perfectly happy if the codes mandated that all two way shear be dealt with using concrete strength alone. For better or worse, that cat is very much out of its bag. I just assume flat plate and studrails nowadays. From a construction economics standpoint, that's usually the right answer.

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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[rapt]

Koot,

Being an older engineer, that was always out preference. If punching shear did not work, add a column capital to increase the shear perimeter of a drop panel to increase both the perimeter and the effective depth until it worked. Never use shear reinforcement except possible in very deep transfer slabs.

I still do not accept that you can justify punching shear reinforcement in thinner slabs. BS and EC codes limit the slab thickness to a minimum 200mm for punching shear reinforcement, but I think this is still too thin.

And there should be a rule like the CSA one requiring a minimum area of bottom reinforcement continuous through the punching zone in the bottom of the slab in both directions in all cases. This provides an enormous increase in the FOS against brittle collapse in a punching shear failure situation for nominal cost. This actually came out of the Studrail testing, where they tested different slabs with terminating rails and continuous rails and the ones with continuous rail had FOS against collapse of 2-3 times those with terminated rails.

 

[KootK]

Impressive how you were able to anticipate my response before I posted it CEL. I must be getting repetitive.

I believe that the next edition of Canada's concrete code will contain new provisions related to column walking. Apparently, transfer slabs are going to get penalized a bit in acknowledgement of the impact of vertical seismic effects which don't get much attention in NBCC. I'm hoping that will improve the margin of safety for non-seismic load cases.

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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

It likely will... It would be hard to make a slab more robust for seismic while leaving the gravity capacity unchanged. It would, frankly, probably cost more and take specific effort! lol...