Concrete gravity only columns

[WiscEIT]

Hi

Any ideas on how to achieve pin connection at the top of an interior column and rigid concrete beam? My intention is to have that interior column not to resist seismic lateral forces. I know you can never truly have actual pin connection in concrete members but for the practical purpose of design, how do you achieve pin connection at the top of the column construction wise? Would that include not bending the beam bars into the column and just let them pass through the core without bending them into the column?

Thanks

 

[JAE]

Are you talking about a rotationally pinned connection (no moment)? Or a translational pin (horizontal movement of the beam not transferring lateral shear into the top of the column)?

If only rotational - then perhaps do a precast concrete-like connection where the beam sits on an elastomeric bearing pad.


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[WiscEIT]

Thank you JAE for your reply. I am talking about both. No "rigid" joint for the purpose of lateral forces. The forces could be both shear and moment. Basically, I want to separate the column from the lateral resisting system. And achieve that in cast-in place concrete. Chapter 21 of ACI has provisions for gravity only members. But I am trying to understand how to bridge the gap between analysis(pin joint in a frame model) and achieving a pin connection in construction. It is so easy in a steel, wood or precast column. I wish there was more literature out there on it. What are your thoughts?

Thanks

 

[WiscEIT]

P.O.: Construct the connection such that only axial forces are carried by the column due to gravity loads. Acts like a leaning column in a steel frame.

Thanks again

 

[JAE]

I suppose you could design a slip connection like this:
1. Build the concrete column up to a point below the bottom of the beam - perhaps 3 inch gap.
2. Supply a vertical smooth dowel in the center of the column sticking upward some distance.
3. Install an elastomeric pad bonded with a teflon/slip material on top. Usually there is elastomer/stainless steel plate/teflon in the assembly. This is the bottom plate.
4. The pad/slip plate assembly would have a slotted hole in the center for the dowel.
5. Install over the bottom assembly a top teflon and steel plate assembly such that the teflon pads are in contact.
6. The upper assembly is usually larger than the lower assembly - the overhang keeps dust and debris from accumulating on the lower pad and possibly gouging the teflon over time with movements.
7. The upper assembly would have headed studs sticking up to bond into the concrete beam above.
8. Install an open PVC tube over the dowel - sized to allow the horizontal movement in the beam you expect. PVC tube to have a top cap of course.
9. Cast the upper concrete beam over the pad assembly and the PVC.

Even with this there is still some level of friction (teflon or other slip materials are not perfectly smooth). That means that your "gravity-only" column will still have some lateral movement due to the beam's horizontal displacement.

To account for this you'd have to perform a deformation compatibility analysis on the column....assuming some degree of lateral delta at the top.
Since you have to do this in any case, I would recommend ignoring steps 1-9 above and simply use an elastomeric pad to take out most of the rotational moment forces and design the gravity-only column to be laterally displaced at the top by your frame Delta displacement. Simply back-calculate the moment that would be produced in the column by this lateral displacement and design for the Pu and Mu forces you get. Chances are it may be that the columns minimum 1% steel reinforcement is adequate anyway.


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[WiscEIT]

JAE, I understand your procedure and the logic behind it. But isnt it same as a precast beam bearing on a precast column. The construction practices in India are not this sophisticated (do not even incoporate elastometric pad on small projects especially). The currect practice over there is not never design them as gravity columns. I should have probably mentioned where this project is located at. Hence, I wanted to achieve this without monolithic concrete construction without the use of bearing pads. I was thinking something like follow the seismic provisions for stirrup spacing to be ensure flexural yielding mode. Backcalculate the moment produced by the lateral forces like you mentioned. And size the column for the obtained forces. But not detail the flexural reinforcement at the column beam joint as detailed for special concrete moment frames. Even less stringent than ordinary concrete moment frames. Just continue the column and beam longitudinal reinforcement through the joint. Would that serve my purpose? What do you think?

Thanks a lot!

 

[JAE]

I think you are on the right track. I'd be a little concerned, if this was a high seismic, that the beam-column joint would still need proper confinement steel - and the column perhaps would need its essential amount of ties, despite the longitudinal reinforcement being based more on deformation compatibility than strength.

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[slickdeals]

Can you be a bit more specific on the reasons why you are trying to achieve what you are. I agree with you that the construction practices in India are not that sophisticated. Therefore, you shouldn't try to create a home-cooked detail that could create other problems.
1. Why don't you want the columns to resist those forces? Why are you creating a pin connection?
2. What type of building is it? A single story project?
3. As JAE was asking, is this in a higher seismic zone?

 

[slickdeals]

That said, there is some guidance in SP 34, Chapter 11 on hinged columns. One thing to consider is that by pinning the beam-column joint, you are perhaps creating a cantilevered column condition which is less desirable. What are you doing at the foundation?