Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Checks Required at Concrete Column Transfer Slabs (Walking Columns)
(OP)
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 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.






RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
By no FEM do you mean that you are getting all the demands by hand? Are you again assuming all pins? It seems that in some cases this would be unconservative.
#3 goes back to your does shear friction have to be satisfied discussion. I am on board that it always needs to be satisfied across any section that you can draw, it's just that at some point you can dismiss it by inspection.
Curious who the top tier firm is. My experience is that rating top tier by size/projects is not a good measure. I've peer reviewed a lot of jobs by those firms and often they are rubbish.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
I think that this is a completely different animal. In the other discussion, the effect was secondary and not required for equilibrium. Here the effect is primary and absolutely required for equilibrium. In the section view of #2 above, I showed a section of the slab rotating under the effect of P x e. Something needs to prevent that from happening. As I see it, the choices are:
1) Fixity between the columns and the slab. Due to detailing limitations, it seems sketchy to count on that.
2) Bending forces applied to the perimeter of the slab section. Due to the small area involved, flexural capacity will be insufficient.
3) Eccentric punching shear. I believe this to be the dominant contributor.
I've already rationalized it into something that can be put on paper. In the plan view of #2 above, I've defined the punching shear perimeter explicitly. Other than the odd shape of the punching perimeter, this would be the same as a regular eccentric punching shear check.
As I mentioned, I'm considering a case where the columns are in close proximity and the chunk of slab that punches would be fairly rigid. Once the the distance between columns becomes large, slab flexibility would kick in and a punching shear model would no longer make sense. Is that distance 8 x slab thickness? More? Less? Dunno. We're off the reservation here.
All that I meant is that I don't want anyone answering my question with "just run it in SAFE". I'm looking for fundamental conceptual understanding here, not pass/fail answers for a particular project.
I agree that big <> good. I think that big firms often become cultish in terms of their engineering dogma. That can lead to a lot of blind procedure following and not a lot of critical thinking. That being said, I was formerly an employee of the company that I mentioned. Their bread and butter is high-rise residential condos. They'll do a few hundred each year and almost all have transfer slabs near grade and near the penthouses. I can say with confidence that;
1) There have been no reported failures. Normally I don't put much stock in that argument. However, in this case, the predominant load is self weight. These column transfers are effectively load tested to a degree that most things aren't.
2) People at this company have spent a lot of time thinking about this. I think that their out to lunch about the shear friction business for reasons that you and I have discussed at length in the past. However, I definitely think that they're are on to something with the check. I just think that the check should be one way Vc, not shear friction. I'll elaborate on that in my next comment.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
One thing about the loading that should be used for design: is watch out for pattern loading. I remember checking one some years back where a pattern loading (for the live load) actually controlled for design.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
It is actually #7 that worries me the most. It's the same as #3 but one-way diagonal tension shear (Vc) rather than shear friction. See the attached sketch. When the columns are spaced further apart, the shear stresses have time/space to shift around. In that case, I think that punching checks on the individual columns makes sense. When the columns are close, however, my gut tells me that the bulk of the shear makes its way across the slab via localized one way shear.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Well... yes. What I have suggested is actually quite unconventional however. In fact, I've never seen it proposed anywhere. It's a punching shear perimeter that encompasses two, offset columns and transfers P x e moment into the slab. Procedures for checking eccentric punching shear on a given perimeter are fairly well established. The big question here is whether or not the check is required.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
A few hundred high rise flat plate residential in N. America per year? Not many people doing that. Cantor/wsp, tt...?
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
In a real design of a thick transfer slab, I generally would consider fixity. For this discussion, however, I have assumed pinned connections for the sake of simplicity. Assuming fixity in this discussion introduces new forces which muddle things and don't materially affect the arguments being proposed. That's my opinion at least. If I'm wrong about that, I'm interested to hear more.
I shall not tell as I have questioned their methods and do not wish to be guilty of sullying their reputation when they're not here to defend themselves. The hundreds of high rises they do aren't Burj Dubai's. The overwhelming majority would fall in the 15-30 story range with the odd 60+ thrown in there. Maybe mid-rise would have been more apt. Regardless, oodles of multi-story concrete with transfer slabs in the mix.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
This has me worried WARose. At minimum, the upper column needs to have an individual column check to ensure that it won't punch downards through the slab. Take my sketch, flip it upside down, and take another look at the situation. I'll think that you'll come to see that both columns require punching shear checks.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
That calc can't take much more than 5 minutes and would put you at ease (or on alert)
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
My gut impression is that you can not assume pins and ignore the fixity. There is a lot going on from slab moments + transfer moment and the unbalanced portion will eat up a good portion of your capacity.
The check should be for max stress occurring anywhere and verifying that vu < phi*vc (little v's). If you take your original #1 and #2 plan diagram and look for example at the far left perimeter (up/down on page line) the stress is whatever the stress is, it doesn't change by how you draw your critical sections. By checking the two different sections as you've shown my guess is that you'll end with quite different answers, which means that one is not correct. It seems like this is taking a complicated indeterminate system and drawing two simplified determinate systems and saying which one is correct, I'm not sure either one is. I think you need to include the unbalanced moments and then look at the stresses d/2 away.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
The stress in your check would also need to include something to reflect that plane's contribution to resisting the axial load "P" though, right? P/4L if it were a square column?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
I'd still like to get some thoughts on #7 and my last sketch. If that's a valid failure mode, the rest is just window dressing.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
I agree with one way shear entirely and that it's a little bit fuzzy how far away you have to be before you spread that out. It is not the same but similar to a question I asked recently about 1 way shear in a mat foundation. Several responses opined that it's impossible and only the full mat width needs to be checked but I disagree.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
As for number 7. How do you determine the length of the one way shear failure plane? I think it is likely that it may start failing in one way shear directly between the columns, but once the cracking reaches the edges of the columns it would need to turn into two way shear. There is no way it is going to cause one-way shear for the whole length of your slab.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
To me, this sounds like simply evaluating shear stresses at all locations and designing for them. And, whether the stresses come from FEM or some old school hand method, we're talking about one-way shear stresses. I believe that two way shear is fundamentally different. From a code perspective, we don't just check one way shear on a square shaped perimeter around our columns. We actually check for a different stress.
My gut tells me that the difference is about proportion. Two way shear makes sense for stocky things that will move as rigid bodies and do a lousy job of redistributing shear. One way shear is for bendier stuff that can redistribute shear stress a bit. That's just my theory though. I've never heard any explanation for the difference in stress other than "it tests out differently".
My big versus little argument is about where the transition ought to be between where a one-way shear stress check is appropriate and where a two-way check makes more sense. My concern with mode #2 is that there may be a section of slab encompassing both the upper and lower columns that might still be deserving of a two-way check.
I'm glad that we agree on this and am simultaneously terrified. This would govern thickness in the vast majority of cases.
I remember that thread as I was the primary responder. Only one commenter suggested that only a full width shear failure was possible. And, at the risk of offending that commenter, I still find that notion implausible. Suppose I build a suspended slab the size of the USA spanning between the eastern and western sea boards. If I drop twenty foot long a N-S running shear wall over Nebraska, am I supposed to believe that they'll be feeling that as one way shear in Fargo and Houston? Fat chance. There's a limit.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Does this clear it up Jayrod? The diagram at the right. You could have a chunk of concrete loaded entirely in flexure, with no axial load, and still have a punching shear failure. In fact, the situation that we're discussing is probably getting close to that.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Precisely. I've seen no guidance on this. I would think some multiple of the slab thickness would be a good rule. If only I knew what that multiple was... I know Bookowski would like to know too.
Agreed. In my original sketch, I mentioned that I thought that a shear failure initiated as one way might morph into an unzipping style punching shear failure around the perimeter of the column. Whether or not that's a possible failure mode is an important part of that discussion.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
But that's just my thought.
As for the diagram you posted, that is what I envisioned however, your punching shear perimeter on the original problem is not only unbalanced moments, you have punching shear stresses in both up and down orientations from moment and column loads. So when I imagined the total shear stress diagram there would still be an inflection point where the total shear stress is practically zero.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
It's an extreme example in all forms of work other than residential towers. Many of those will have this condition for light loads up at the penthouse and for heavy loads at the transfer slab near grade. Having checked these conditions numerous times I can tell you that the one way shear check, as proposed, always governs until you get some meaningful separation between columns.
This sort of depends on how you see things. As I mentioned above, I've been assuming that the difference between a one way shear appropriate situation and a two way shear appropriate situation to be whether or not I expect that the shearing block will behave more or less rigidly. If it is rigid, then you can aggregate the effect of all the moments and axial loads into a single, representative moment and axial load (like we do with eccentric footings).
With a single moment and axial load on an assumed rigid body, I feel that you'll get diagrams like I've posted above. Whether or not the shear crosses zero and flips sign would depend on the ratio of moment to axial force. In these transfer situations, the moments tend to be rather high and the only net axial force is the contribution from normal slab support loads. As a result, I usually do see a punching shear stress diagram that crosses the zero line.
Thanks for your endurance.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
1) do we want the spacing to be field adjustable? That makes me a bit nervous.
2) how do you mean pre-engineered? You supply just forces to supplier?
3) how do we feel about top side install? At first, I thought it would just be another cause of top steel -- and KootK -- depression. Now, I wonder if it might actually be better from the top. From a mechanics standpoint you want the stud head at the top of the tension steel. And having it higher is really of no benefit.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
two way check (#1) or the one way (#7)? I've never heard of the latter. And it would take a TON of studs in many cases.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
What I do is show a line with a light plot style and dashed over a width and actually tag it with a beam tag. I usually also provide a section because I know it's likely to get missed/confused, I figure that beam tag + section mark should make it obvious. Basically I build a beam in slab.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Also, did you us Vc I your stud design or just Vs alone?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
What have I missed? Why is this causing a wide-reaching debate with some of the smartest folks around here.
Note: It is late, I am tired, and I just would rather ask and feel stupid in the morning than not ask and potentially miss an opportunity to learn.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
1) I wanted to take a survey of what folks are checking in this situation and, hopefully, get some new ideas. While the group here seems to be pretty in tune with one another, I know that there is considerable variation in practice. My impression is that most folks just do checks one and five. Obviously, I seem to be a lone wolf when it comes to check number two.
2) In this situation, it is not at all clear to me whether one way or two way shear checks are appropriate. I wanted to get some input on that. My gut says -- as you seem to -- that it's predominantly a one way situation. However, I still "feel" the need for check number two. And that strikes me a permutation of punching shear.
So, now that you're here, which of the seven proposed checks do you support? Any additional ones? Why do we use different stresses for one and two way shear? Just different test results? I feel that it has something to do with the rigidity of the section that punches through and the capacity for shear redistribution in plan.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
With all of those studs, the rail must be part of the reinforcing? it would seem so unless it can be rationalized away. If it is visible it can't be relied upon because the MEP butchers will surely chop some of it away. However, it seems to me that it must be included in the 75% of balanced reinforcing because it most will be undamaged.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
As.for the checks, I also don't see the combined column punching shear. This feels a bit like a doubling of forces - Personally I think you've muddled the source of a load with the design of how to resist said load. I stand ready to be corrected...
The check of the shear through the slab is the core requirement. All else is secondary, even the flexural case. I doubt flexure would govern many of these, and even if it did, it is unlikely to be more than theoretically governing.
I'd be a 1-5-6&7 guy... With a long term creep check thrown in as well if the propodtions of the spans made me feel it was warranted.
Lots of engineers don't think of creep often enough; I've seen some very odd situtions with creep, and have learnt enough respect to keep it in mind.
I also think I'd be likely to request a drop beam to I make this all easier to do...
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
That's how it was explained to me, but it could of just been the guy blowing smoke.
I agree with your new understanding, I think it's better from the top.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
As other thread participants have rightly noted, there are several sources of moment at the joint: 1) the column axial load eccentricity 2) the column end moments 3) the slab moments. These all exist and, in my opinion, are very difficult to estimate accurately. For the sake of this discussion, I've been trying to focus on the P x e moment as I believe that dominates the situation.
Interesting. Can you elaborate with reference to this particular situation? What do you see taking place on the creep front? Progressive rotation of the joint and exacerbated punching shear?
No doubt we'd all want this. However, construction economics and intra-firm competition steer things towards a flat plate solution.
Yeah, with respect to check number two, either you're all right and I'm crazy... or the reverse. Probably the former. I'll take one last stab at trying to sell the combined punching shear check. Consider the sketch below which depicts two situations:
1) The offset columns where the moment is P x e and it is dubious whether or not a punching shear check is required. Mode number two above.
2) A stacked column situation where the columns and slabs form moment frames for lateral resistance. Moment is still P x e but, here the need for a predominantly rotational punching shear check is obvious.
How about that? Anything? Does this resonate with anyone else???
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Well now if you're just going to be Mr. Know-it-all and guess correctly every time I bring up something new in a thread, well, well, well than I'm just going to run away and have my Mom make me some coco!
On the serious end of things, the sustained high-shear-load situation at the slab is potentially going to mean aggressive creep conditions - Creep is at its worst when dealing with the "over one-third" stress case. Unfortunately that rule of thumb is from the UK, so it applied to cube strength. It is more like the fifty-precent rule for cylinder strengths... Point being you're likely in the high-stress range where your creep is very bad.
In the case of our slab with serious point load, we also have to worry about the possibility of additional cracking under creep if we load too early... And most construction sites aren't looking to sit around waiting for 28 or more days before building the next floor.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
If the column above is relatively close to the column below, it is not a flexure/shear situation, it is a deep beam/strut and tie situation and should be designed and detailed accordingly. In that case, the columns are close together but there is no "combined punching shear".
If the column above is further away, the slab needs to be checked for flexure and shear as it would normally. Plus each column has to be checked individually for punching shear.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
EIT
www.HowToEngineer.com
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Keen observation. Maybe it would be prudent to run shoring from the center line of the bottom of the upper column to the side of the bottom of the lower column for just this reason. That was a mouthful.
Not so. I'm seeking generally applicable advice based on general principles. I don't want "I would do X". I want "I would do X in this this situation and Y in that situation". Precisely the kind of advice that you have supplied above.
At a lower level floor slab, where the thickness might be 3', you may indeed get some STM appropriate situations. At a penthouse level transfer it would be pretty rare. There, you've really got two scenarios:
1) Overlap, which is essentially poor man's STM. This was discussed recently at length here:Link
2) A clear separation in excess of 1xD for which STM will no longer be applicable.
The zone of most interest to me is the transition from overlap to where one could confidently say that it truly is just slab flexure and single column punching checks. While I might be out to lunch with my combined punching check, I very much contend that individual punching checks are insufficient. Per my middle left sketch on 11 Dec 14 18:33, I don't see the shear "making its way around" fast enough when columns are tightly spaced. Everything that I know of the logical flow of stresses leads me to believe that model is flawed.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
The shear between the columns is covered by the "beam shear" calculation. It is not a punching shear situation.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
I think that we still will need a punching shear check, just a three sided one.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
1) How wide of a "beam" should one assume for the one way check and;
2) At what magnitude of column separation does it switch back to two way punching?
What does the term "direct shear" mean to you in this context? I'm not familiar with that terminology.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Indirect shear is where you have an adjacent area, not necessarily engaged in resisting a load, which becomes loaded through a P-Delta effect of gradual yielding/plastic behaviour. That one is a short hand I picked up while working in NZ; You'd occassionally see a notation in another engineer's calculations to the effect of "as adjacent area reaches plastic design strength, additional capacity is available through indirect mobilization of XXX".
I think I just started talking about them in this way as I found it helpful for imagining the real-world behaviour we need to deal with. I tend to use the french "cisaillement" for direct shear, at least in my notes and in my own head.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
In my younger days I would often check this as a shear friction case as well ensuring that the reinforcement supplied for the tension strut also satisfied the shear friction rules for monolithic concrete. And put some horizontal reinforcement in the top half of the member for this and also some extra ties to give the same area vertically but I think this was probably overkill.
If the gap between the columns is > 1.5D, it is flexure shear and use the appropriate code equations, preferably from a more modern shear design method like the Canadian or the Eurocode which address the strut angle requirements better!
Between D and 2D, I would check it both ways!
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Kootk - I fell behind here, just saw your question about a screen shot - will do that. Yes, the steel can get confusing if you have rails as well but not that tricky really - it just needs to be scheduled and detailed that the rails are only on the non "beam" sides. For a calc I just assume the stirrups replace the rails, it might not be completely accurate but it's as close as I can wrap my head around. On the type of mats you are talking about I don't feel too bad about wasting a little steel in the form of stirrups (if it is a waste).
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
I definitely believe the 1-way shear model should be checked. Then the question becomes how long of a line of the slab should be used for the check? I would use the distance between your 2-way shear lines that you drew at the beginning of the post. T would add the capacity from a 3-sided 2-way shear check to the capacity for the 1-way shear between the columns. Check whichever column has the higher 2-way shear stresses and place the 1-way shear failure line half-way between the columns. With this method you would need to use capital Vu and Vc since you need the whole perimeter to breakout for a failure. Based on this, you no longer need to check 1-way shear when the capacity of your Vc from the 3-sided 2-way shear plus 1 way shear capacity exceed the shear capacity of the 2-way shear for a single column.
I would check situation 2 as well since adding a beam within the slab between the columns would be of no benefit in this situation. The failure planes are entirely outside the columns. You could theoretically run a beam between the columns and have very thin slabs outside of the columns. However, in this case the shear has to change signs so that part of the slab can punch out down and the other part up. Therefore, the parts of the slab that are parallel to the elevation views you have drawn will need to fail by torsion. I'd grab the torsional components of the slabs from ACI 421.1R-99. If both the torsional and 2-way shear are greater than allowable, the section fails. Now here's where it gets a bit convoluted. If either the 2-way shear capacity or the end faces or the torsional capacity of the side faces are greater than the allowable, "borrow" some of the orthogonal faces. Eg. if the torsional side face resistance of the slab fails use enough of the end face until the torsion is satisfied. Then recheck the 2-way shear with the "borrowed" portion of the end faces no longer included in the 2-way shear check. If you can get both torsion and 2-way shear to pass then the section will be okay. If you can not get both to pass the section fails.
A word of note in ACI 318 Art. 13.5 you can use Direct Design of slabs if columns are offset by up to .1L. I believe that is more of a rule of thumb than justification to say all your "close column" concerns can be ignored because by the time you get an offset of .1L you are in the realm of flexure/shear controlling.
For #4, I have never seen bearing stresses control unless the column is a different concrete strength than the slab.
I can dream up any inventive ways for the slab to fail, I think you have it pretty well covered.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
I still wholeheartedly believe in the combined punching shear check (#2). I only stopped selling it because it became clear that others were not being persuaded by my arguments.
Thanks for your excellent comments Rapt. This is very interesting as you are the first poster to validate check #3 at the top. I agree. Where the shear becomes a strut and tie situation rather than a diagonal tension situation, I think that shear friction warrants attention. In one of my previous threads, I pitched a theory that shear friction is self-satisfying in these scenarios without the need for rebar (Link).
This touches on an important point that I've been wondering about as well. We all seem to agree that it's not appropriate to perform the one way shear check using the entire width of slab. However, does one way shear failure over the shorter width proposed constitute failure on its own? Or can that one way capacity be combined with the three sided punching shear capacity as you have proposed? I'm really not sure. I worry that the one way shear failure would initiate an unzipping failure around the two way perimeter and, thus, the one and two way shear capacities would not be additive. Unzipping aside, is it even correct to combine one and two way shear capacities? Allowable stresses for two way shear are in excess of twice the values for one way shear. This may be because, as Rapt suggested above, punching shear is more of a "direct shear" situation (i.e. shear through a compression strut).
Yes! Finally, an advocate for the combined punching shear check (#2)! I agree completely and should have thought of this argument myself. The one way shear resisting beam element envisioned, on it's own, does nothing to resolve the P x e rotation problem. The rotation of the combined perimeter encompassing both columns can only be addressed through slab moments and/or eccentric shear around the combined shear perimeter (effectively torsion when the stresses are taken in aggregate).
I believe that provision pertains to column offsets in plan rather than elevation.
Agreed. My concern was that, in addition to the column f'c potentially being higher, the column may also use the rebar to transmit compression. I believe that it breaks down into two cases:
1) At thick lower level transfer slabs, column bars can be developed in compression and bearing isn't generally an issue.
2) At thin upper level slabs, column rebar likely cannot be developed in compression. However, were a column loaded heavily enough for this to be a concern at a thin upper floor slab, other failure modes would surely precede a bearing failure. As such, a bearing check is moot.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
I have always thought of shear friction as a poor mans strut and tie anyway! Just gives a way of allowing for a construction joint rather than monolithic!
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
I've met with a professor by the name of Amin Ghali on several occasions. He did most of the research that got us using stud rails in North America. He tells me that people in the know don't consider two way stresses, as calculated, to be "real" stresses. At least not with respect to magnitude. He said that it was better to consider the calculated stresses as "indexes". You can compare them to code limits to see if you're in trouble or not but that's about it. So... take that as you will.
Since this thread seems to be winding down, I'd like to thank everyone for their participation. I've learned a good deal with this.
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.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
Good thread koot. I have never had the chance to work on a structure with this condition, but it has been an interesting read.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
RE: Checks Required at Concrete Column Transfer Slabs (Walking 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 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.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
As to studrails, we had a discussion some time ago, which has never been resolved to my satisfaction.
thread507-306324: Concerns about stud rail punching shear reinforcement
RE: Checks Required at Concrete Column Transfer Slabs (Walking 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 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.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
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...
RE: Checks Required at Concrete Column Transfer Slabs (Walking 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 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.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
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.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)
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.
RE: Checks Required at Concrete Column Transfer Slabs (Walking Columns)