Transfer column on a thickned slab
Transfer column on a thickned slab
(OP)
Folks,
One of my colleagues is designing a building where a column is offset by 24" from the column below (architects !!!!). This is an edge condition. They don't want a transfer beam and will only allow a thickened slab beam (20" thick).
Column below is a rectangular column (24x48) and column above is circular 18" diameter.
I was suggesting the use of an embedded steel beam to transfer the shear at the interface. The axial load from the column above is of the magnitude of 700-750K (ultimate).
Has anyone had a similar situation to deal with and if so, what details did you use?
One of my colleagues is designing a building where a column is offset by 24" from the column below (architects !!!!). This is an edge condition. They don't want a transfer beam and will only allow a thickened slab beam (20" thick).
Column below is a rectangular column (24x48) and column above is circular 18" diameter.
I was suggesting the use of an embedded steel beam to transfer the shear at the interface. The axial load from the column above is of the magnitude of 700-750K (ultimate).
Has anyone had a similar situation to deal with and if so, what details did you use?






RE: Transfer column on a thickned slab
Sorry, but I have seen this too many times to be considerate with the client. The Architect can change the design, in spite of what he claims. Therefore, specify a transfer beam and let him deal with it.
Mike McCann
MMC Engineering
RE: Transfer column on a thickned slab
Although I totally agree with you, I am sure it won't fly with our bosses. In their opinion, you are making a client very "unhappy".
RE: Transfer column on a thickned slab
I, as the engineer of record, have the structural liability, and, ultimately, the final call here. If they cannot understand that simple principle, I show them the door.
I can live with one less client, but not the loss of my reputation after the loss of life having compromised my position to the whim of a client or Architect.
I suggest your boss consider that, not that it will make a difference here. I know that doesn't help much, but I'm just being honest.
Mike McCann
MMC Engineering
RE: Transfer column on a thickned slab
Mike McCann
MMC Engineering
RE: Transfer column on a thickned slab
If it works, press on. If it doesn't work, tell the client to take a giant leap. Making the client unhappy is the least of our worries. Making the right decision is paramount.
BA
RE: Transfer column on a thickned slab
Mike McCann
MMC Engineering
RE: Transfer column on a thickned slab
BA
RE: Transfer column on a thickned slab
If an embedded steel beam or two does work within the 20" or so, then well and good.
Detailing may be messy, using steel support columns could help.
RE: Transfer column on a thickned slab
I agree with everyone else, you must take a stand as the engineer. If the numbers don't work to a degree that you feel comfortable with, give the architect a choice: move the column or allow a deeper beam. If the architect/client is not amenable to that, tell them to find another structural engineer.
RE: Transfer column on a thickned slab
I have transferred column loads on slabs like this several times. Lots of judgement calls. How wide an area can you use to resist the shear? How wide is the slab beam? If the next column is fairly close it probaby will work in bending. Shear can be a problem. You can add shear reinforcement and increase concrete strength. When you reach the code limits then something has to change. I would recommend adding plenty of reinforcement to be sure you mobilize whatever you assume and then add some more because a crack in a transfer condition is the scariest thing I have ever seen.
RE: Transfer column on a thickned slab
Do you think I could discuss this issue with you outside the forum. I know forum rules are stringent on personal information, but is there a way to reach you?
RE: Transfer column on a thickned slab
In this case, as long as the depth is available for direct shear (and I would not go to more than about half of the code maximum limit on this) then it is a strut-tie problem. The problem with a thicvkening in the slab, is where the bottom of the 2 horizontal thrusts goes to. It can only be in bending in the column below which I would not recommend. So your thickened slab must extend back to the main framing members to allow you to transfer the thrusts back to the bracing members.
The easiest solution for a strut tie model would be if there was an overlap in the column over 1 level, so that the horizontal thrusts can be taken out in slabs above and below back to the main framing/bracing members. Then you need to put in enough reinforcement to take that force.
But this requires the architect to accept a larger column over 1 level of the building.
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
BA
RE: Transfer column on a thickned slab
That said, if you cant make it work you cant make it work and you should not compromise on that. When I have reached an issue like this I have learnt that giving the client options shows that you have thought through the issue and it also gives them the power of choice.
I always like to thank that I am in the business of providing solutions, not of providing problems.
Now, on to the problem. When I have come across situations like this I have used strut and tie analysis to make it work. Remember that your push and pull continue past the and of the column so continue your rebar accordingly.
Also make sure you allow for the extra bending induced in the slab.
Good luck.
RE: Transfer column on a thickned slab
You the the picture correct. There is a 60" wide by 20" deep slab beam. The rest of the slab is a 10" post-tensioned two-way slab system
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
For punching shear, part of the critical section overlaps the rectangular column, lessening the probability of punching shear failure but definitely muddying the waters of understanding.
Perhaps the strut and tie model is the best way to deal with the problem.
I think I would insist on some changes in geometry as suggested in earlier posts. The consequences of being wrong are simply too great.
BA
RE: Transfer column on a thickned slab
I would tell the architect to use an extended circular column about, or a blade column, to get a reasonable overlap and take out the thrusts in the 2 floors as I mentioned above is more depth is not available.
RE: Transfer column on a thickned slab
I agree that it is a strut and tie problem, but believe it can be resolved in one level if the architect comes to the party with a bit more depth in the band.
RE: Transfer column on a thickned slab
I can't see that shear over the whole beam width is relevant. I also don't think that it's a classic punching shear problem as about half the upper column lies within the critical shear perimeter.
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
That is your prerogative. I would agree for a normal slab/band without the transfer.
However, with the face of the column above only 3" from the face of the column below, the stresses do not have sufficent time/distance to distribute sideways to make the full width effective. The only way they will distribute significantly sideways is if there is a shear failure at the column which defeats the purcpose!
RE: Transfer column on a thickned slab
BA
RE: Transfer column on a thickned slab
The Avf bars need to be fully developed at where they cross the assumed crack line but since you are using the top and bottom mat of reinforcement that is already there this is not usually a problem.
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
20" thick slab with only 5" between column faces. This will Not fail in shear!
Strut and tie is the correct way to analyse this and also leads to the simplest solution.
Now you can get onto the hardest bit of figuring out how you develop those column bars.
RE: Transfer column on a thickned slab
If this wouldn't fail in shear, how do you think it would fail? It might not be typical beam shear or typical punching shear, but shear is the problem. Strut and tie is just a design model for dealing with shear by truss analogy.
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
RE: Transfer column on a thickned slab
Anchorage of the upper tie at points 'a' and 'b' are essential.
BA
RE: Transfer column on a thickned slab
You did not nominate the width of the struts. Under the 18" column above where the compression strut starts, it will be 18", and this controls the strut design.
The compression over this width and the splitting forces/stresses in this strut will govern the whole design, not the compression over a 60" width which is the width of the band beam.
RE: Transfer column on a thickned slab
I am not entirely sure of your meaning, so why don't you tell us how the strut and tie model should be drawn? It is not entirely clear to me and perhaps we can all benefit from your insight.
BA
RE: Transfer column on a thickned slab
With the assumed strut dimensions, it seems to me that the structure is quite doable.
BA
RE: Transfer column on a thickned slab
Trusses dont fail in shear they fail in axial load.
at a macro level you may think of it as shear but at a detailing level it is axial compression strut.
RE: Transfer column on a thickned slab
I wonder if all of the moment has to be taken at this junction or if the adjacent columns could be designed to prop/tie the top of the 18" dia. column in place.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Transfer column on a thickned slab
If you treat it as a beam there will be a small uplift on the next column. The column load is vertical so there is no need for a horizontal component (except locally with the strut and tie)
RE: Transfer column on a thickned slab
Beam shear, better described as diagonal tension, is always dealt with by a combination of compression struts and tension ties which can be thought of as analogous to a truss, where the tension is taken by stirrups.
others,
The strut under the 18" column is confined by a 60" wide beam. It's not going to fail by splitting or crushing.
BA,
There is not a unique strut-tie model for this, but I disagree that the top corner is critical. I would just use the column to column strut, and provide bottom ties to resist the horizonal component. Anchorage of bottom bars at the lower right corner is the most important issue, IMHO.
RE: Transfer column on a thickned slab
That is the way I first looked at it too, hence why I initially showed bottom bars anchored at the right end of the beam. But the problem is the 'L' shaped bars from top into the column have to be anchored at the bend point and this means we need a second compression strut to handle the anchor force.
I should say that strut and tie model is not something I have used extensively in my practice, so I may be missing something.
BA
RE: Transfer column on a thickned slab
Well said.
I think you should still check strut stresses and check they are below code requirements.
RE: Transfer column on a thickned slab
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Transfer column on a thickned slab
I agree with you, but to consider the eccentric column acting on the column without benefit of the beam is conservative. The beam will carry some of the eccentric moment, but we don't know its span, so cannot determine relative stiffness of the beam vs the column.
Even in the absence of the beam, the strut and tie model can be made to work.
BA
RE: Transfer column on a thickned slab
I was not disagreeing with your model, only pointing out that there is a 3rd dimension to it, in relation to my disagreement with Hokie66.
The struts have a width and this is dependant on the applied load width and the support width. My commnet was that the width of the compression strut from the column above will be controlled by the width of the column above and is not related to the width of the band beam. This means that compression stress in the strut cannot be based on the 60" width, tather it is based on the width of the applied load above and that this will control the strut tie design.
RE: Transfer column on a thickned slab
Remember, though, that we have only talked about transferring the column load. The band has other loading, but we have not been advised how much.
RE: Transfer column on a thickned slab
As hokie stated in an earlier post, the beam reaction should be included. And as paddingtongreen suggested, the fixed end moment for the beam, including the upper column load should be used in the frame analysis.
BA
RE: Transfer column on a thickned slab