Type 2 (Wind Only) Moment Frame 8

[RFreund]

I have a couple of questions regarding wind only moment frames. Meaning when analyzing gravity loads the beams in the frame are pin-pin. However when analyzing lateral loads the connection is fixed.

1. Is my description correct in describing the design process.
2. Are these typical?
3. Can you provide me a reference that discuses the design of these frames.

I just have a hard "stomaching" this design philosophy but it seems like many frames in the past have been designed this way.

Any other insight is appreciated.

EIT

 

[JoeBaseplate]

Useful thread. Thanks for the input Lion06. I have designed a few Type-2 moment frames in the past as well. I am finding the comments here very informative.

 

[Lion06]

willis-
I appreciate the conversation. The only real downside that I can see to using the PR connections is that the connections will be designed for greater moments than the FMC, and that the analysis gets a little more complicated. That being said, it seems like the DAM would be applied like any other structure, which is a definite plus, in my book.

Also, correct me if I'm wrong, but since you'll be designing for gravity + lateral moments, the only difference between the FR and PR connections is the increased deformations and, consequently, second order effects on the structure, correct?

 

[WillisV]

Lion - you won't necessarily be designing for greater moments, the FMC method does represent what is happening in reality to some extent if the connections are sufficiently ductile throughout their moment rotation curve in that the connections do yield under gravity loads and then load and unload unload under lateral loads. A true PR analysis will also capture this behavior by not only considering the moment-rotation curve but also the loading history on the connection (i.e. you apply gravity loads to your model, then apply lateral loads in a staged analysis) so the effect will be similar to FMC but with the benefits of knowing the actual deflection of the structure and being able to track the true moments through the connection behavior to make sure there are no non-ductile failures. As far as how do you know what loading history the structure might see, there are good recommendations in "Rex and Goverdhan (2002) - Connections in Steel Structures IV". I admit that designing a PR building is a lot of work, but there is a certain class of building that the FMC method, which ignores all of this, can be unconservative and a more rigorous analysis is warranted.

 

[RFreund]

Thanks again guys.

However getting back to simple structures such as the structure Lion described above. Say you modeled only the frame, couldn't you have a frame with a 2 fixed bases and one fixed end connection and the other pin. Then use Dam to analyse this frame? I guess I'm asking if you not trying to model the entire structure can you set up a frame with one pin one fixed connection.

I've attached a simple example Illustrating my question.

EIT

 

[Lion06]

I suppose you could do that, but I would have a couple questions/comments.
1) How are you determining the actual lateral shear getting into the frame?2) The moment at the base is not quite what you have estimated. The far end pin throws the portal frame analysis off a bit.
3) This is extremely simplified. You won't have many single frames like this unless it's a small building with only two columns in the line.

 

[RFreund]

Lion

1.) I'm not sure I understand the question - The lateral shear gets into the moment frame via the roof diaphragm.

2.) Yeah True but I didn't feel like setting up this frame in Risa.

3.) Couldn't you apply this even if you have any number of columns? You would just model the frame as pin one side and fixed on the other for the interior columns? I'm not talking about setting up a 3-D model of the building in Risa (or any other program), just a 2-D frame and run the analysis with the Lateral load in the one direction only.

Thanks again


EIT

 

[Lion06]

1) I know how the shear gets into the frame, but without a 3-D model, how are you determinig the magnitude of the lateral force getting into the frame? With all of the different wind cases, I would not tackle that by hand. I would do a sanity check by hand to make sure I'm in the right ballpark.

 

[Lion06]

Willis-
Do you know of any software that allows a rotational spring stiffness input at a beam/column joint?

 

[WillisV]

Sap2000 or Perform3d will allow you to input a full moment rotation curve as needed for PR design. Other programs allow basic rotational spring stiffnesses (even the new version of RAM, which I definitely would not recommend for this kind of analysis).

 

[paddingtongreen]

It used to be known as "Semi Rigid Design". I used it a lot, both before and after computers.

It's years since I used STAAD but it allowed changes in members, joints and restraints for different cases but would still combined them.




Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[RFreund]

Lion06 can repost that last attachment with the 1.5 to 2.0 increase in drift reference?

Thanks.

EIT

 

[Lion06]

It's on page 7 of the attached pdf.

 

[RFreund]

Lion - I've tried several browsers and it still does not work for me. Did the attachement not upload? Are you able to open it? basically it opens to this

http://files.engineering.com/downlo...2f095d&file=Wind_Clip_Drift_-_L_Geschwindner_

A blank screen.

Sorry for being a pest. I apprecieate the help though. I could give you me email but I know that is fround upon so I will avoid that for now.

Thanks again.

EIT

 

[Lion06]

It's not opening for me through the link. I don't know why. Do you know of any sites I can post it to and then post the link to that site?

 

[RFreund]

It is a pdf right?
I'm not sure why it would not work same as the others.

I'm sure there are a few different ways we could do this but the only one I'm familiar with is this - if you have a gmail/google documents account you could upload it to there and then open it. This will open the document in a different tab or window and then you can copy that URL. That is what I have done with the other document below:

https://docs.google.com/viewer?a=v&...NTM0Zi00MmM3LWI1MTAtMTg4NGNhNzFiMTEx&hl=en_US

I appreciate the efforts.

EIT

 

[Lion06]

Alright, I'm still at the office. I'll look into this when I get home later.

 

[Lion06]

Let me know if this works.

https://docs.google.com/viewer?a=v&...YWNlMS00MTAwLTliZDEtYmFmZTI3MjEzNzNk&hl=en_US

 

[RFreund]

Lion - Sorry I forgot one important step, go to the upper right hand corner and click share then under who has access then public (or anyone with link should work) then paste link.

Thanks again.

EIT

 

[Lion06]

Done. Let me know if you can get it now.

 

[RFreund]

Chyeah! Thanks!

EIT