AS3600 CL 9.1.2 (25% rule) 2

[rowingengineer]

How does the reinforcement Ratio affect Punching shear? I do not hold all/any the answers but the following is some references and a little of what I understand:
Concentrating the reinforcement over the columns as per AS3600 CL 9.1.2 will increase the stiffness of the slab. This will improve the column to slab connection, as moments are generally concentrated around the columns until redistribution occurs. This re-disputation is caused by cracking of the concrete; this cracking can affect the shear capacity. Hence the reinforcement reduces the crack widths such that the shear interlock between the aggregate is maintained.
References:
Elstner and Hognestad (1956), Regan (1974) and Zaghlool and de Paiva (1973) showed that increasing reinforcement ratio increases the punching shear strength Alexander and Simmonds (1992), from the test of eight-isolated interior Column-flat plate connections, found that decreasing the bar spacing at a certain steel ratio increase the punching capacity.

My Personal favourite book: Warner, Rangan et al. Chapter 19.5.


When in doubt, just take the next small step.

 

[hokie66]

The tensile reinforcement has been shown to improve punching shear capacity, but AS3600 does not recognize this slight benefit. The Australian approach is slightly more conservative than the ACI code, particularly in the strength reduction factor, which is 0.70 in AS3600, 0.75 in ACI318. I think the more conservative approach is warranted for punching shear.

 

[rowingengineer]

I agree AS3600 does not recognise this fact, It only provides detailing requirements such that the punching shear is conservative.

I would like to see a relationship between the amount of tensile steel and the distance you assume for punching shear (currently dom/2 for AS3600)be created. However, limited to about 1.5-2dom. Like the CEB-FIP code, but a little different.

As for Dowel action of the steel, i think is a minor increase in stength that really hasn't been shown in testing to helf that much, hence i wouldn't want this to be included in my designs.






When in doubt, just take the next small step.

 

[asixth]

I agree with hokie and yourself in saying that tensile reinforcement increases punching shear reinforcement but isn't capture in the set of unified rules that we know as codes. I have a paper that was supervised by my old professors at the University of Queensland. I use to talk with the phD student who authored this paper and I can say he is a reputable source.

 

[rowingengineer]

Maybe they will look at this for the AS3600-2013 concrete code, if they ever get around to publishing the 2009.
But in saying that I think this needs to be included in the commentary asap (I am hoping they update the commentary) thus that people have a fair idea on what they are trying to archive in design and detailing.


When in doubt, just take the next small step.

 

[OzEng80]

I have also found this a difficult code issue - particularly when trying to explain to a builder (who has stuffed up his column heights) that putting extra steel into the slab doesn't fix the problem. It seems counterintuitive and inconsistent given that beam shear (Vuc) includes the effects of the tensile reinforcing - however minimal. The 'catastrophic' nature of failure has always had me treading carefully and conservatively around this.

 

[rowingengineer]

I feel your pain Ozeng80, over poring of columns seems to be happening more and more these days.
I now make it standard practice when i get to site to ask if the top of the column form have been levelled with a dumpy, and give foreman the run down if he over pours.
But give the catastrophic nature I also tend to tread carefully, however I think with better numerical formula’s to work from, I would feel more comfortable.


When in doubt, just take the next small step.

 

[asixth]

OzEng80

How did you go about rectifying the situation with the columns poured at the wrong levels, did they just jackhammer away the concrete that was too high.

I have a paper from the March-April 2008 ACI Journal written by Canadian academics saying that the British/Euro standard takes the reinforcement ratio into account for the shear resistance in the form of ?^(1/3), similar to concrete beam shear capacity equations.

An alternate formula which I do not recommend using is also presented which I have checked using one of my designs (designed to AS3600) and have found the formulas to give a significantly reduced punching shear resistance. The formula presented was:

v=16*(fc*?/d)^(1/3)

Compared to the present AS3600 formula:

v=0.17*(1+2/?h)*fc^(1/2)

I'm not saying that the codes should be revised, it is just the more that I look into punching shear capacity, the more I realise that people are agreeing that flexural reinforcement should be taken into consideration.

 

[rowingengineer]

asixth,
can you please supply the authors of the ACI article you mention. I am also currently comparing AS3600 code to a few others that include the reinforcement ratio, trying to work out what AS3600 is based upon. I have a feeling that is based on about 1.0%, but I am yet to find a reliable calculation.


When in doubt, just take the next small step.

 

[asixth]

Influence of Slab Thickness on Punching Shear Strength by Birkle and Dilger.

 

[rowingengineer]

Thanks aixth.

When in doubt, just take the next small step.

 

[OzEng80]

Asixth

Yes – I have had builders jackhammer back in the past – they love it! I guess the only other question would be how much is required? Theoretically 50mm around the perimeter of the column would adequately engage the design shear perimeter?

The actual project that I had in mind involved steel end plates to columns (that were supposed to be retrofitted) that got cast-into a thin (150mm) slab. This was enough to push the design over the limit for punching. We ended up with the rather unusual solution of load limiting the slab – it was a balcony for a school and was designed for 4kPa. I think the load was limited to 2-3kPa and this went on the design certificate and the building and was accepted by the building certifier. There was redundancy in the design (the slab could cantilever from the classroom line for strength) so the outcome wasn’t quite as dodgy as it sounds…..

 

[hokie66]

As to how much has to be removed, I wouldn't compromise. Take it all down to the correct level. If columns are found to be cast too high before the floor formwork is in place, a neat job can be done by sawing around and chipping away the rest. But usually the problem is only discovered late in process, maybe on the morning just before the concrete arrives. There is no option other than knocking the whole thing down with a chipping hammer. Then you invariably have broken corners below the form, and those have to be repaired. Morons!

 

[degenn]

Here are my thoughts on the square and cube root shear relations ships to f'c.

Shear capacity is strictly based on a square root rule (if you assume tensile capacity is based on a square root rule) and you apply a Mohr circle approach. If however the section is cracked, there is only a limited depth of section in compression that can transfer shear. The compression depth, dn (if linear) is based on steel proportion and E values using

dn=(sqr((np2+2np)-np)d eq 5.17 of Rangan.

Note maximum shear and moment don't typically occur together, so a linear approach is OK.

If you apply the shear capacity based on the square root relationship (say 0.34sqr(f'c)) to the reduced compression area you end up with a shear capacity closer to a cube root relationship that is also dependant on steel proportion and Ec.

If you set this up as a spreadsheet, you find the capacity is quite like the AS3600 beam shear (cube root of f'c) (8.2.7.1) over quite a range of beam sizes, steel proprtions and concrete strengths. I haven't been able to derive the equation purely algebraically.

If you also allow for the effect of aggregate interlock effectively increasing dn, by a small constant amount (presumably based on aggregate size), then the beta1 effect on shallow beams is explained.

If there is sufficient steel over the column (25% rule), there is effective confinement for a strut-tie to work in the vicinity of the column, allowing a square root rule for shear to apply.

 

[asixth]

degenn,

Saying maximum and shear can be a bit misleading, high moment and shear occurs over supports.

What is meant by 25% rule, when I started work as a graduate it was drilled into me that I should have 25% of the bars in the column strip centered over the column at reduced centers. Is this the same approach with other consultants.

AS3600 says 25% of the total design moment needs to be designed at the face of the column.