Transfer column on a thickned slab 2

[slickdeals]

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?

 

[msquared48]

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

 

[slickdeals]

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".

 

[msquared48]

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

 

[msquared48]

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

 

[BAretired]

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

 

[msquared48]

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

Mike McCann
MMC Engineering

 

[BAretired]

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

 

[apsix]

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.

 

[steellion]

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.

 

[ron9876]

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.

 

[slickdeals]

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?

 

[rapt]

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.

 

[slickdeals]

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

 

[Tomfh]

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

 

[haynewp]

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.

 

[canstructural]

use stud rails, better than beams.

 

[ron9876]

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.

 

[BAretired]

Do I have the picture right?

BA

 

[csd72]

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.