Concrete flat slab sway frame

[blihpandgeorge]

Hi

Looking for a sanity check. I am designing a flat slab sway frame and I cant find too much in the way of design guidance, particularly for punching shear but perhaps i am over thinking it and the normal (ie as per the AS3600) punching provisions are all that is needed?

If using the normal approach, the punching calc would be based on:
- the axial load would be from that floor transferred to the column
- the moment transferred between the slab and columns - taken as as the difference in moment from the columns above and below the slab.

Have i missed anything?

Thanks

 

[hokie66]

How many stories? Flat slab rigid frames are not a good idea, so you will have to search hard for design guidance.

 

[rapt]

It is not the difference in the moments in the columns above and below. It is the difference in the moments in the slab either side of the column, which would normally be the sum of the column moments!

There is no special guidance in AS3600 on this. I would check the NZ and ACI codes to see if they have anything special.

Plus make sure you provide the bottom reinforcement that AS3600-2018 now requires.

 

[Agent666]

Look into ACI421 series of standards. NZS3101 kind of follows ACI codes, and refers you to ACI421 for the design of shear stud reinforcement. There is a procedure in ACI421.2 to investigate the out of balance moment developed due to accommodating lateral drifts.

I believe in Australia ancon have some software that works out punching shear.

However NZS3101 all but prohibits the use of flat plates as the primary lateral load resisting system. It's meant to be paired with another stiff lateral load resisting system that limits the drift, the flat slab is designed to accommodate the same level of drift.

 

[blihpandgeorge]

to pick up a few comments made:
- The design is for the upper 2 storeys on top of a shear wall stabilised system (so not i dont feel it is pushing the boat out to far). Loads are coming are from a 3D spectral analysis
- Agree it is the sum of the column moments (bad phrasing by myself when i said difference). I can see how it is the moment in the slab either side of the column, i guess my head went to how to pull results out of the analysis model easiest,and from the columns seemed easier than from slab contours.
- Similarly agree on detailing as per latest AS3600-2018 with robustness type reo
- or punching shear we made early decision to use the ancon stud rail and yes their software is very helpful. With the sizes we have (550 cols / 275 slabs) we have punching covered so far.

Thanks all, appreciate your comments

 

[Agent666]

To be honest I cannot think of a worse place to have a flat slab/column lateral system extending/projecting above a stiffer walled system for several storeys. Considering the difference in stiffness and the fact that to some degree it's going to respond out of phase with the remainder below. This could/would lead to some significant accelerations and hence peak drifts in excess of what your elastic modal analysis is telling you. I'd reconsider or add some lateral structure above the walls to more dependably control the drifts and higher mode behaviour, or verify performance using time history analyses.

 

[rowingengineer]

Stud rails are an interesting topic, not a fan myself. I agree with agent666 changing stiffness up a building is not a great idea.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Toby43]

Agree that changing in stiffness not the best idea.
Search for CUREE publication "Seismic performance assessment of flat plate structures". I haven't read the entire thing, but may be useful to you.