Partially Connected Column & Punching Shear 2

[Trenno]

Hi all,

I'm trying to find ways of making the below detail work.

I'm using AS3600 currently, but am definitely open to other code's justification. I know we are pretty conservative when it comes to punching shear.

At this stage the Arch. is aiming for "X" to be 200mm, however I can only seem to squeeze 400kN punching shear capacity out of the configuration below.

Does anyone have any suggestions or point me towards material that can help me justify the smallest possible "X" dimension?

Thanks

"Shibby right..."

 

[Ingenuity]

What are the magnitude of spans and loadings?

My guess is your 275mm slab is based upon your shear issues, or are your spans large too that results in 275 (deflection and flexure)?

Also, be careful of using your average prestress values in your punching shear equations for edge (and corner) supported column where the P/A is not fully developed.

And 'kick' the architect in the head too...

 

[rapt]

I will go further than Ingenuity and suggest that you ignore the prestress completely. You will need sufficient reinforcement developed to the back of the column to transfer the moment to the column. With 300mm wide, I doubt that you can get it in there!

I would be insisting on an edge beam to tie the whole thing together.

PS you cannot "assume" no moment transfer to the columns. It is there and the only way it will go away is if the column connection fails and that is a brittle failure so no zero column stiffness assumptions are allowed!

 

[Trenno]

All fair points and duly noted.

Spans are roughly 8.5m each direction for an office environment.



"Shibby right..."

 

[asixth]

Damn architect's. That connection would feel better with a 300mm x 300mm bearing area as a minimum.

Have you looked at studrails. Reid and Ancon can supply and have good enough technical data regarding how to design them.

https://www.dropbox.com/s/mflb5saeb6tj767/Studrail Design Manual.pdf?dl=0

Other than that you could look at detailing a shear cage with normal reinforcement

https://www.dropbox.com/s/0k8weh7lkd1pr4y/Shear Cage Detail.jpg?dl=0

I also remember something that either rapt or hokie posted on the forums sometime ago regarding the use of bottom reinforcement for punching shear. Even though it's not explicitly written into the codes it does make the connection a lot more ductile. There's nothing like the feel of an N28 in your hands. That'll hold the crack from opening.

 

[hokie66]

This needs an edge beam, or a beam in the other direction. Architects can't dictate everything about our structures.

 

[Trenno]

I was trying to avoid the use of shear rails, as this project isn't in Australia thus there may be problems with procurement and supervised installation.

Will definitely be adopting integrity reinforcement as detailed in the Canadian code. I would rather it hang than fully punch, if worst comes to worst!

I think at this stage I really need to either thicken the slab and increase the "X" dimension.

"Shibby right..."

 

[Trenno]

Asixth, this is what the shear rail option would look like...

"Shibby right..."

 

[asixth]

Looks good. Just enough room to throw some heavy reo thru that column junction.

 

[hokie66]

It looks like the Americans are getting a bit nervous about how much good stud rails do.

thread507-378852

 

[asixth]

Has anyone done a comparison of AU practice against US in terms of punching shear capacity?

AU standard uses a concrete shear strength of 0.34*sqrt(fc) which is higher than the the 3*sqrt(fc) when all the conversions between SI and US units crunch out.

 

[hokie66]

asixth,

Haven't done it lately, but the last time I did, I remember coming to the conclusion that the overall factor of safety was intended to be about the same. In addition to the maximum shear stresses, you have to consider the different load factors and different strength reduction factors used by the two codes.

 

[rapt]

I still do not see how you can fit any decent top reinforcement into the column. The gap between the column bars is a maximum of 300 - 50-50-25-25 = 150. You can probably fit 2 bars between! And they have to develop fully so they cannot be large bars. For 8.5m slabs, that is not much tension force!

Also, your studs are at 154 spacing with a depth of 220. Hope someone tells the concrete the shear failure plane has to be at 30 degrees or less.

And you have ignored the applied moment from the analysis. That has to be included. Even your Mmin is wrong as it is based on a 200 long column, not 600.

I would be requiring a steel column with a full welded steel shear head.

 

[Trenno]

I think I need to go back and read about column design and analysis.

My current understanding - reduce column stiffness in floor structure analysis as this will result in a conservative floor design. Then when designing columns and things such as punching model floor with 100% column stiffness to see what moment shall be transferred into the columns?

In that case - how does one achieve a pinned-pinned RC column? Obviously it's all in the connection detail. I'll review this.

I'll look into the stud rail formulae and calcs to see if the design requires the shear crack to cross all four shear studs. Maybe there is some sort of assumption worked in that explains this. But I do agree with you, seems weird it would detail them like that.

 

[Trenno]

Follow up question... when would Clause 9.2.3 (AS3600) ever apply in practical problems?

 

[Trenno]

"Follow up question... when would Clause 9.2.3 (AS3600) ever apply in practical problems?"

Answer: Rarely at ULS, however Vuo is important in the overall punching shear picture.

 

[asixth]

Not for properly detailed insitu columns - floors but there would be other examples where it comes up. A concrete slab supported on a steel column or likewise a steel column transfer load on a concrete slab.

I'm quoting rapt again from a post many years back

Punching Shear is a Brittle Failure condition. The moment cannot redistribute away from it without a collapse occuring. Codes do NOT allow redistribution from a brittle failue condition for very good reason, they cause collapses. There is no elastic/plastic action, it is elastic/collapse for punching shear!

thread167-280869

Also can you post the provisions from the Canadian Code regarding integrity reinforcement? I'm in my own jam with a punching shear problem at the moment.

 

[rapt]

Trenno,

With your small connection to the column, just about the whole moment has to transfer through the front face (normally this is about 60%), so the full moment at the end has to be carried by reinforcement into the back face of the column.
.
The only way to get 0 moment is to detail it as a pin so that no moment can develop. As far as I am concerned, that Is just about impossible in this case, unless you have no top reinforcement and then you have no punching capacity - so collapse. Unless the pin is in the column below and above the slab - good luck with that.

We do not know if this is a braced or sway structure! if sway, it cannot be a pin anyway and must be designed for full moment no matter what you want to do!

Asixth, thanks for resurrecting that one, I was obviously more eloquent in my younger years!

 

[KootK]

My current understanding - reduce column stiffness in floor structure analysis as this will result in a conservative floor design. Then when designing columns and things such as punching model floor with 100% column stiffness to see what moment shall be transferred into the columns?

This is fairly sound and fairly common. The one chink in the armour is that, rationally, you need your slab top steel to be compatible with your punching shear assumptions. The codes that I work with decouple punching shear and slab moment which makes this invisible to designers. At edge columns and unbalanced columns, I would consider designing the slab top steel for moments reflecting column fixity.

I'll look into the stud rail formulae and calcs to see if the design requires the shear crack to cross all four shear studs. Maybe there is some sort of assumption worked in that explains this. But I do agree with you, seems weird it would detail them like that.

The crack only needs to cross one stud. I see no problem with 220 long studs spaced at 156 o/c.

Given your geometry, you might try this:

1) Abandon punching shear. You're not grabbing enough stuff.
2) Evaluate based on one way shear at the column face.
3) If shear works somehow, design your slab as pin supported.
4) Get as many top bars into your slab as will fit.
5) Design you columns for moments reflecting the capacity of the bars that you provide at over strength.
6) Use something akin to the integrity steel provisions that we use in Canada as a fail safe.

In Canada, the code allows a related procedure for the punching shear design of corner columns.


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.

 

[Trenno]

Can someone please link me to a good information source that shows good detailing practices for beam/slab/column connections.

I want to get a feel for how exactly how much moment can be transferred through these junctions, and then design accordingly.