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Transfer column on a thickned slab
2

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?

RE: Transfer column on a thickned slab

Whomever is the Structural Engineer here has the call on the solution, not the Architect, not the client.  As the Engineer of record, it has to be comfortable with you, and YOU only.

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

(OP)
Mike,
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

Probably true.  Nevertheless, I stand with my opinion.  

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

As an alternative here, overdesign the thickened slab to protect your liability.  You will have to draw the line somewhere soon, or the control of the design will not be yours.  That's dangerous.

Mike McCann
MMC Engineering

RE: Transfer column on a thickned slab

slickdeals,

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

Thanks BA...  This is a sensitive subject for me.

Mike McCann
MMC Engineering

RE: Transfer column on a thickned slab

Any engineer who finds himself confronted with a million dollar lawsuit because he tried to keep the client happy will come to understand the utter futility of that route.

BA

RE: Transfer column on a thickned slab

That's all true, but if the Architect digs in we can only insist on recommending what he doesn't want when we are certain that what he does want won't work.

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

Ahh the classic case of the architect wanting to have their have their cake and eat it too.  They want a transfer girder, but oh by the way it can only be but so deep.

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 agree with both msquared48 and the boss. The architects are the client and they may have a valid reason for doing what they propose (may not also). We are the ones trained to design solutions for the architect/owner's design. However it obviously has to work.

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

(OP)
Ron:
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

I agree that you cannot give in to anything that the architect/client wishes, but if it can be made to work structurally, then there is no problem.

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

(OP)
I am not designing the building. But I think the shear is high enough that it exceeds 10 sqrt(fc') and an embedded steel beam is being used to transfer the entire shear back to the column, which will then be in bending. The 800 kip column reaction is about 18" from the supporting column's centerline

RE: Transfer column on a thickned slab

I agree with slickdeals. Ultimate shear appears to be a problem. Hence S+T probably wont work either.

RE: Transfer column on a thickned slab

I would design the steel beam(s) to take the shear and moment and then secondly check a direct compression strut since it is a deep beam compared to the length. My next idea would be to embed a steel column within the adjacent concrete supporting column, creating a steel-to-steel connection encased inside the concrete column. The steel column embedment would vertically extend down until the forces (including vertical) could be assumed as transferred from the embedded steel column into the entire concrete column.   

RE: Transfer column on a thickned slab

use stud rails, better than beams.

RE: Transfer column on a thickned slab

slaickdeals I don't have any experience with steel shapes embedded in concrete. However if the concrete stress is greater than the max then solve for the required area of concrete and make some type of change. Are you using max concrete strength that the code allows? Can the column above or below widen so they overlap and eliminate the shear? What is the architects objection to a deeper section. Is it legit. If it simply means they have to route some MEP lines around the area that likely would be the easiest solution. My experience is that when you present the facts to an architect that something simply doesn't work they are more receptive. For instance the stress in the concrete is whatever and the max allowable is whatever. 800 kips on a 20 inch slab doesn't sound like it would work. By the way don't forget punching shear. A column real close to a support is the toughest to make work. But don't play with it, you must be certain  that what you do works because the consequences are servere.

RE: Transfer column on a thickned slab

If buildings could all be designed using a 'recipe book' approach then Structural Engineers would not be a necessary profession. It is precisely problems like this that are the reason for our profession!

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

(OP)
@BA,
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

(OP)
*you have the picture

RE: Transfer column on a thickned slab

It looks like to me the band beam would just work, with a lot of shear stirrups and 6000 psi concrete.  But I would still like to deepen the section under the column, say the first 60" from the outside face, to 24".  Not too much to ask.  

RE: Transfer column on a thickned slab

Actually, that answer is only for the column load.  We really need the total band beam reaction to know if it works or not in beam shear.   

RE: Transfer column on a thickned slab

This design is a little outside of my comfort zone.  Considering one way shear of the 60" x 20" beam, the round column is almost totally within the distance 'd' from the edge of the rectangular column, so most of its reaction does not have to be included in beam shear by code.  

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 do not think you could justify using all of the 60" wide band beam to transfer an 18" diameter column over this length (3" gap) to a 48" wide column below. I would find it hard to justify much more than the width of the column above, possibly 24" maximum.

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 disagree with you, rapt, as to the effective width of the band beam.  The reaction is 48" wide, so I wouldn't have a problem with distibuting this over a width of 60" in beam shear.  60" bands are commonly supported on smaller columns than that.  Punching shear has to be checked for both the circular column above and the rectangular edge column below, and those calculations are complicated by the proximity of the load to the reaction.

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

Looking at BA's sketch it would appear that the only way failure could occur is by crushing of the strut between upper column base and lower column top, or bending in the beam due to the offset. The strut is inclined therefore there must be horizontal forces to resist.

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

Strut failure will not be an issue, as the strut is confined within the 60" wide band.  Anchorage of the bottom tie bars is the most important consideration.

RE: Transfer column on a thickned slab

hokie66,

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

You can also use shear friction (ACI318 section 11.6) to greatly increase the shear capacity.  With shear friction you can increase shear capacity to 0.2f`c*Ac or 800Ac.  That is 6 times more allowable shear capacity Vc.  Shear capacity is so low because of the unpredictable nature of shearing cracks and the catastrophic consequences of a brittle failure.  Shear friction provides ductility to any potential shear failure so you can go deeper into the capacity.  The precast industry uses shear friction all the time and has done extensive testing in both lab and real world conditions.

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

Shear friction is for direct shear not diagonal tension. In my opinion doesn't apply here.

RE: Transfer column on a thickned slab

All this talk of shear ties and shear heads.

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

csd72,

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

(OP)
I agree that a compression strut will form between the two columns and all the shear will flow through that strut. But what reinforcing needs to be provided across that strut. The code asks for minimum steel crossing the plane of the strut.

RE: Transfer column on a thickned slab

Personally I feel the strut is working too hard, i.e. the concrete is approaching ultimate shear in this zone.

RE: Transfer column on a thickned slab

BeRetired,

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

rapt,

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

I have changed the sketch a bit.  The top bars have been extended to develop in bond.  The bottom bars have been removed from the sketch as they do not play a role in the strut and tie model

With the assumed strut dimensions, it seems to me that the structure is quite doable.

BA

RE: Transfer column on a thickned slab

Hokie66,

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

Are the adjacent columns concentric?

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

paddingtongreen,

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

csd72,

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

hokie,

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

Quote:

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.

Well said.

Quote:

The strut under the 18" column is confined by a 60" wide beam.  It's not going to fail by splitting or crushing.

I think you should still check strut stresses and check they are below code requirements.

RE: Transfer column on a thickned slab

I'm sorry but I still can't see how this joint can be separated from the adjacent columns and joints, i.e. from the frame as a whole. An eccentric moment is introduced, even if the slab beam set up is strong enough it will not be rigid, it will rotate and the upper column will try to sway from the vertical, but the upper beams, supported by the other columns, will push back, relieving some of the moment at the joint.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Transfer column on a thickned slab

paddingtongreen,

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

BAretired,

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

I agree, except that the stress field can spread as it goes down to the wider column.  Therefore, the width of the bottom horizontal tie need not be restricted to the width of the upper column.  This is similar to strut-tie design of a pile cap, where rarely is strut confinement reinforcement necessary, and where anchorage of the bottom bars is the most important consideration.

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

In my last sketch, I considered the struts to fan out to 8" x 48" which resulted in a maximum factored compressive stress of 2.58 ksi for a factored load of 700k.  

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

I might design this with the beam support both fixed and pinned, as it doesn't hurt to be conservative with a situation like this.

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