Pinned, continuous and partial fixity

[Ramaaaljh]

Hi everyone
For design a simple frame (2 columns and a beam) in Etabs,I really want to know about the difference between those 3 issues:

1- using a continuous beam in modelling.
2- using a pinned beam in modelling.
3- using a continuous beam with partial fixity at the start and end of beam ( with using a reducing factor n/(n-1) and multiply it with K ).

So what is the difference between those, and actually what is the cases I should use each one of them?

 

[klaus.]

Well the difference you should see on the results of your software....

 

[phamENG]

If the confusion is with the basic concepts of frame analysis, then you should review your structural analysis textbook and discuss the issues with your supervisor.

If the confusion is with the terminology used in the program, klaus is right. Run the analysis each of the three ways and review the results. The moment distributions, deflections, etc. should make the underlying structural behavior fairly simple to identify.

 

[Ramaaaljh]

Sure I know but why there are those difference between them, what is the type of the connection between beam and column in each one of them?

 

[Ramaaaljh]

PhamENG ok I understand what you mean and I can see the result but I want to know the difference

Like what I said in cases 2 and 3, I think we should get the same results but as I can see in case 2 I get a different value of moment in beam

 

[structSU10]

continuous beam would have a moment connection at each end. this costs more money.

pinned beam is a standard connection, theoretically having zero moment at beam support and is the cheapest.

partial fixity is a special detail where the connection has some moment transfer capability, reducing the beam midspan moment. it should be somewhere between the others in cost.

What you chose will depend on the build lateral system, and or which one is most economical to achieve the desired performance.

 

[Celt83]

looking at a simply case of 1-span with a full uniform load of w and the same fixity condition at both supports:

Total static moment in the beam is w*L^2/8

with pinned connections, ie hinges introduced in etabs at the beam ends, the is no moment resistance at the columns thus peak positive moment in the beam is w*L^2/8

with idealized fully fixed connections, ie the beam column joint cannot experience rotation, the moment resistance at the columns is -w*L^2/12 and the peak positive moment in the beam span is (-w*L^2/12)+(w*L^2/8) = w*L^2/24

with the actual rigid condition where the beam column joint can undergo rotation based on the stiffness of the joint the negative moment will be something less than 0 but greater than -w*L^2/12 call this m,end and the peak positive moment in the beam would be m,end + w*L^2/8

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[phamENG]

Are you familiar with the concept of end fixity and how it influences the moment in a beam? The three methods of modeling a joint are fixed (full moment transferred, beam and column remain at 90 degrees to one another), pinned (no moment is transferred, relative rotation is freely permitted), and partially fixed (moment is transferred, but some relative rotation occurs). If you aren't familiar with these, then I don't know how much help we'll be able to give you as that's a topic that should be covered in basic university courses.

Do some hand calculations using the inputs you used for your test runs of Etabs and the three end fixity models and compare them. That should give you a good idea of what they mean.

 

[Ramaaaljh]

Okay Thank you for answer


Celt83 but what if there is a shear wall instead of the column? Is it still the same connection?

 

[Ramaaaljh]

I noticed that if I used pinned beam or partial fixity in the end of beam which connects with shear wall I get a negative moment in that joint in beam but what if I wanted it to be 0 in that joint?

 

[Celt83]

...if I used pinned beam..

...I get a negative moment in that joint...

Then you have not used a pinned beam, I suggest you read the Etabs manual.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Ramaaaljh]

Celt83 thank you for answer but I really want to know more about this, how I didn't use pinned beam and why? When I drew the beam I choose pinned in release moment,so what is the problem?
I'm sorry but I can't understand this point I tried to read the Etabs manual but I couldn't find what I want

 

[Celt83]

the function of a pin/hinge in an analysis model is to provide a location of unrestrained rotation if the location can rotate then there is no resistance to twist. I am personally not familiar with Etabs but if it is like most other stiffness matrix based software packages you may need to introduce a hinge a the beam end. If you can;t figure it out from reading the manual then I would suggest emailing their technical support team for help, general consensus I have heard is their support team is wonderful to work with.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[phamENG]

Ramaaaljh - a pinned support is one with all moment released. It is idealized as an actually pin around which with beam can rotate. So if you use a pin support, it cannot have moment. Check your settings again. You either applied it to the wrong end, or you hit "cancel" rather than "ok" or "apply" (I haven't used Etabs in a few months so I don't remember all of the user interface details).

 

[Ramaaaljh]

Okay I will try to search more about it
Thanks alot..

 

[Ramaaaljh]

PhamENG I used it when I drew the beam, so when I can choose from properties of object- moment releases and in this choice I can use continuous or pinned and in that way I drew the pinned beam

 

[Ramaaaljh]

But I noticed that not in all cases I get 0 value of negative moment, as I said when the beam connects with a shear wall I still have a value for negative moment in connection joint