Concrete Material Connection Rigid Or Pinned?

[JackEngTip]

Hello everyone, I guess this question has been discussed here but I still want some thoughts from you guys.

1. For concrete continuous beams analysis & design. I tend to model the frame that includes the concrete columns. However, I have seen many people model continuous beams with external & internal pin supports only, ignoring the moment attracted by RC columns, which is fine for beams (just conservative generally (although might be unsafe for top reo at end supports)). But I wonder why would people design it this way. I meant, moment attracted by RC columns cannot be ignored in column design. So why not design them as frame so they don't need to have another model for column design?

2. When it comes to footing design, I have seen many computation that takes axial force into consideration only (i.e. pinned connection to pad assumption) especially when it is a core wall structure. I understand that most lateral load will be carried by core wall but there will still be some lateral load carried by column and the vertical load will introduce moment as well. So why overturning is ignored in pad footing design then? Is it more like a experienced thing? (thinking compression in columns is normally large so stability is not a issue?)


3. I am currently designing a building that has a PT transfer slab. The PT slab is outsourced to another company but I still need to evaluate the loading on transfer slab. Apart from the axial force, do I need to give the bending moment at the bottom of the transferred column? I haven't run the calculation but I think the bending moment in column will be pretty large under dead load and live load. I don't have much experience in PT slab so I am wondering normally if the moment of the column will affect the PT slab design much?



Thank you.

 

[JAE]

From my old time (and I mean old time) mentors (1927 to 1967) they would assume pinned connections because:
1. Hand analysis used back then was easier with pins.
2. They added top rebar with the knowledge that there was indeed some fixity.
3. It was conservative to do so for positive moments.
4. They overdesigned the columns anyway.



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[jayrod12]

Agree with JAE on the older days.

My typical design is beam bottom reinforcement as pinned, beam top reinforcement at supports for wl^2/10, shear enveloped.

 

[gte447f]

JAE and Jayrod, I agree with your commments, but the OP seems more concerned with the columns that the beams. Is there significant accumulation of unbalanced moments in the columns as a result of dead and live loads that needs to be accounted for in design, or can they be designed for gravity only?

 

[JAE]

Well I think the "Old Guys" would first do a pin analysis/design of the beams to get a base design started.

Then, the column/beam frames were also analyzed by hand (moment area, portal, etc.) to get "wind" moments which then were used in columns.

But then again - I know for moderate height buildings sometimes they just designed columns based on gravity loads and were extremely conservative just to make sure.

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[BridgeSmith]

For multi-column bridge piers (we call them 'bents', for reasons I still don't know), we usually model it both ways (pinned and fully rigid connections) and use the worst cases. Occasionally, we will employ the "15% rule" from old days, where if the columns contribute less than 15% of the total stiffness of the members framing into the nodes, the column connections are considered pinned. We compared that approach to a frame analysis for a bent where the column was 23% of the total and found the moments were all within 5%.

 

[JackEngTip]

Thanks everyone . Looks like it is more of a "old style" way to comp the RC beams.
What about the pad footing (item 2) and PT transfer slab issue (item 3)? Can anyone please comment? Thank you.

 

[jayrod12]

For item 2, I would say generally speaking footings themselves will allow enough rotation that a pinned assumption is reasonable in my eyes.

I don't have experience with P.T. but I would say you should give them all the applied loads and moments acting on the slab to get a properly designed slab the first time.

 

[JackEngTip]

Thanks jayrod12.
I still don't understand why overturning is not an issue in pad footing design. Even if you design it as pin connection you will still have at least min. reinforcement in the columns and starter bars from the footing, which will give you a rigid connection and attract moment that these reinforcement can take in the footing. So footing stability might fail because of this overturning moment. Same as the transfer slab. I was told that we can make the transferred column to transfer slab to be pin connection but I doubt that because of the same reason.

 

[jayrod12]

If you model you columns as pinned and check the level of rotation at the base it's likely minor. Well within the range of tolerable for a fitting before overturning is a true concern.

I liken it to a standard steel shear connection. Can it resist some moment, sure, but the supporting beam and the connection generally allow for just enough rotation that the pinned assumption is still safe.