How would you model this frame?
How would you model this frame?
(OP)
I have a concrete frame (columns are aprox. 3 m high and 3 m apart).
Columns have circular cross-section with diameter 250 mm.
Beams are 300/400 mm.
Columns are on single foundations.
I know there is no such thing as 100% fixed / pinned in real world, but if you have to model this would you rather choose pinned or fixed support as a colums supports?
Also, at the corner joints (column - beam) would you design this as pinned connection (no moment transfer)?
Columns have circular cross-section with diameter 250 mm.
Beams are 300/400 mm.
Columns are on single foundations.
I know there is no such thing as 100% fixed / pinned in real world, but if you have to model this would you rather choose pinned or fixed support as a colums supports?
Also, at the corner joints (column - beam) would you design this as pinned connection (no moment transfer)?






RE: How would you model this frame?
I am not that experienced but i am guessing that everyone will suggest to create 2 models
1) model it as fully fixed in order to capture the behaviour of the frame without considering the foundations, even though they are not fully fixed because in reality you will have some sort of soil structure interaction.
2) Since we know that there will be some soil structure interaction you should create another model that includes the foundations in you model and try to model the soil as well. Then you can use both models to dcide on the design of the structure.
RE: How would you model this frame?
For the column to beam connection it will be fixed as well. The interior columns would take very little moment since the spans are equal on both sides, but the end columns would get some, usually around a 16th moment.
Your beams could be designed using the coefficients give in the ACI code Chapter 8 (or Chapter 6 in ACI 2014)
RE: How would you model this frame?
I notice that in both models the columns are fixed to the beams.
another day in paradise, or is paradise one day closer ?
RE: How would you model this frame?
RE: How would you model this frame?
Dik
RE: How would you model this frame?
For the column/beam joints I would treat those as fixed unless you specifically detail it so that there is no moment transfer between the two. This isn't easy to do with concrete. If you do fix it, you can determine the moment transfer based on the deflection of the joint which takes into account the relative rigidities of the columns and beams. The easiest way to do this is to model it using RISA or SAP2000 unless you want to brush up on your structural matrix analysis.
RE: How would you model this frame?
Dik
RE: How would you model this frame?
http://www.eng-tips.com/viewthread.cfm?qid=430956
RE: How would you model this frame?
In analysis, Fixed has definite meaning. No Rotation.
In most cases you actually mean 100% moment connection. So the moment in the column is equal to the moment in the beam at edge connections and rotation occurs at the joint. That is NOT Fixed.
For footings, normally you would assume pinned unless you can guarantee that there is no rotation of the footing. Unless you have a rigid pile cap with tension/compression piles or socket into solid rock, there will always be some rotation of the footing, so you will never get full fixity.
RE: How would you model this frame?
RE: How would you model this frame?
It's a fairly slender structure so the footing/pile needed to fix the bases would probably be out of proportion to the superstructure. This suggests pinned bases, at least for strength design. For serviceability (deflection), you might consider a small fixity. 10~20% of the column stiffness (4EI/L) appears in some design guides for steel structures and these concrete columns are of similar size/stiffness.
Column-beam connection: If your bases are pinned, you need the tops fixed for stability. If the bases are fixed, you have a choice. I'd go fixed column-beam for insitu construction because that's how the real structure will tend to act. Zero moment in the real structure will need significant cracking. For precast, pinned connections might be significantly easier.
Rapt, I think your showing personal preference by not using 'fixed' at the beam-column connection. Fixed in this context refers to the connection between analysis element (member) and the node: there is no relative rotation; the angle between elements (members) is fixed.
RE: How would you model this frame?
It has nothing to do with a preference of mine.
Historically, "fixed" has a meaning for what happens at the end of a member.
If the people who develop analysis software want to use a term for the connection between a beam and a column, it should not be "fixed" as that has a different meaning.
RE: How would you model this frame?
Decide upon consequences.
* Column - Beam: Continuous - - - Means that the column have to take moment (more complex column design/detailing)
* Column - Beam: Pinned - - - Means that the beam is simple supported (more rebar steel to place in the middle span of the beam of height increased)
* Column - Footing: Continuous - - - Means that the Footing have to take moment from the column (the dimensions could be greater if stability issues found)
* Column - Footing: Pinned - - - Means that the Footing transfers mostly axial force (the dimensions could be lesser no stability issues found)
If I had to decide:
* If I had small loads on the beams (say lightweight roof) + Good soil: Column - Beam: Continuous + Column - Footing: Continuous
* If I had great loads on the beams + poor soil: Column - Beam: Pinned + Column - Footing: Pinned
Best Regards,
MSc. Eng. Serguei Joa
Structural Engineer
Bouygues Batiment International, Cuba.
RE: How would you model this frame?
How can you have both the column-beam and the column-footing pinned? you need at least one of those connections to resist moment for stability.
There are two options that most of us see, pinned base and moment resisting frame, or moment resisting base and pinned frame connections.
RE: How would you model this frame?
BA
RE: How would you model this frame?
BA
RE: How would you model this frame?
1) Pinned Base
2) Fixed Base
Or you could design the base as a spring to account for the estimated rotation of the footing.
Or you could model the column with a horizontal support at the foundation level and extend the column down a certain distance to a pinned base to provide the same rotational resistance at the foundation level. I believe this method and how to calculate the distance is described in the PCA Handbook (if I remember correctly).
RE: How would you model this frame?