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Type 2 (Wind Only) Moment Frame
7

Type 2 (Wind Only) Moment Frame

Type 2 (Wind Only) Moment Frame

(OP)
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

RE: Type 2 (Wind Only) Moment Frame

NO! NO! NO!  Use your modeling as a best effort to predict how the structure will react under ALL applicable loading conditions. Don't pick and choose.  The worst case will control.

If the structural need is lateral support for wind, then the gravity loads will probably be fine....but don't change the analysis to just accommodate the one condition.

With available software, you can model the entire structure (or at least a significant portion thereof), so changing the loading conditions is a simple process....leaving the structural model conditions the same...

RE: Type 2 (Wind Only) Moment Frame

4
Here is my two cents.  I've designed a number of "wind clip" buildings, and I have, in fact, designed them exactly as you say, but I'll add a few comments.

The idea behind designing for pinned connections under gravity load is that the connections are designed for specific, wind-only moments.  It may be true that gravity end moments exceed your lateral end moments, but that doesn't matter here.  If your gravity end moment exceeds your lateral end moments then the connection simply yields under gravity loads.  It's important that the plate or angle be designed to yield BEFORE the weld or bolts fail - this ensures the rotational ductility of the connection and that it can actually undergo the deformations necessary to allow it to act as a pin for gravity.  

It's common to design the beams for gravity only as pins, then check those beams with fixed ends for lateral only.  Do not combine gravity and lateral for this type of system when checking the beams.  When you check the beams for lateral moments, be sure to use the entire length as unbraced for the negative bending.

Also, please note that this lateral system is not a candidate for the DAM.  Well, technically speaking, it is, but it would be so burdensome to do that, that it's not worth it.  You'll need to use the effective length method, but there are a couple modifiers you'll need to apply when calculating k.  The nature of the connection is such that both end connections of a beam yield under gravity load.  When a lateral load is applied, the windward connection tries to unload (which means it has moment capacity), but the leeward connection tries to load further (which means it's acting as a pin).  So, the frame is really only fixed at the leeward ends of the beam for wind.  See the commentary with the alignment charts for modifications to L and G for the girders.

Also, I typically use the actual drift for a 50 year wind for the drift that will be seen during a 10-year event.  The reason is that a 10 year event is allowed to be multiplied by 0.7 for the 50-year event, and the flexibility of the connections typically results in a 50% increase in drift.  So 0.7*1.5 = 1.05 * 50-year drift (I just use 1.0 * 50-year drift).

Lou Geschwindner has a couple papers on the topic.  I'll post them.

RE: Type 2 (Wind Only) Moment Frame

The reason it's ok to design the beams as pinned for gravity (and this is conservative), is because the beam will never see more negative moment than the connection can handle.  You will design the connection for very specific moment capacities, so that this negative moment is never exceeded in the beam.  Once you check the beam for that negative moment with the full unbraced length, you're good, as far as the beam is concerned.

Now when it comes to columns - Because you're either designing a lateral system with full fixity or a gravity system with zero fixity, you need to play some tricks with the program to capture the actual column moments.  I typically will specify 50 % of a fixed end moment to get cranked into the columns from the beam.  This is often more than will actually occur when you do the analysis, but it makes it so you don't have to think to hard about it.  It's not changing your beam or connection design, and I don't mind being conservative on columns.

RE: Type 2 (Wind Only) Moment Frame

As a side note, the leeward connection being pinned increases k pretty significantly for the columns.  This is really only a viable lateral system for 7 stories and under.  You'll need to grab almost every column and beam framing into the column as lateral elements in your wind-moment frame.  Depending on the height of the building, you'll probably need to fix the base connections, too, but you can design that for 50% of the fixed moment, since that's what you're limiting the moment getting cranked into the column to.  I think you can justify using less, but that's sort of the approach I've adopted.

RE: Type 2 (Wind Only) Moment Frame

Why would you ever model anything other than reality?

Mike McCann
MMC Engineering

 

RE: Type 2 (Wind Only) Moment Frame

Because it gets you out of having to do testing for the moment-rotation curvature of what could be called semi-rigid moment connections.  Also, if you model it as fixed for combined gravity and lateral, you will get very large moments on the leeward connections (gravity FEM + lateral moments, because they are additive at that location), and that goes against the very principle of the system.

The whole point of the system is that the connections yield under gravity load, but still provide moment resistance for lateral loads.  If you design for gravity + lateral moments then you're losing the economy of the system.

RE: Type 2 (Wind Only) Moment Frame

Thanks for your eloquent discussion, Lion.  Sometimes we have to be reminded of how "rigid" analysis differs from tried and true design methods.

RE: Type 2 (Wind Only) Moment Frame

(OP)
Wow - Lion that was more than I could have hoped for - Your the man.

I'll have a look through and see if I have any questions.  One question is why do you say it would be very burdensome to apply damn?

EIT

RE: Type 2 (Wind Only) Moment Frame

(OP)
Lion any chance you can re-post the last paper, for some reason it does not open for me.

EIT

RE: Type 2 (Wind Only) Moment Frame

The reason it would be almost impossible to apply the DAM, as far as I'm concerned, is this.  Let's look at notional loads in one direction (say in the positive X direction (which would be to the right on the page).  Say you have 10 frames with 3 columns and two beams in each frame.  The left end of each beam would need to be modeled as fixed (since this is the only connection that has LATERAL moment capacity, because the direction of lateral moment is opposite the direction of gravity moment so it's trying to unload) and the right end would need to be pinned (because this yields under gravity load and is trying to load further under lateral load, but has already yielded).  Now, for notional loads in the negative X direction you'll need to flip those pinned and fixed ends because the direction of the lateral load has changed.  You'll need to do the same thing for lateral loads in both Y directions.  Don't even get me started on the torsional wind cases with lateral loads in both directions simultaneously.  I think it's too much book keeping as to which connections need to be fixed and which need to be pinned - too many models to check.   

RE: Type 2 (Wind Only) Moment Frame

3
The FMC (Flexible Moment Connection)/Type 2 connection as put forth by Disque and later carried on by Geschwindner is a simplification of true PR analysis that has its place for simple buildings meeting certain limits (more on that below), but in my view for more complicated structures its use would be as a preliminary design tool and would be followed with a true analysis of the structure taking into account the actual moment rotation curves and loading history of the connections.  

Eurocodes actually impose strict limits on the use of the FMC method to low rise simplistic type buildings (regular geometry, 8 floors max, 4 spans max, 16ft floor-floor max, 40ft span max with spans not more than twice or less than half of adjacent spans).  Though these limits are not present in U.S. codes (the method is not as widely used in the U.S. anyway), it is my opinion that it would be prudent to use them as a guide.  

The FMC method has several shortcomings as one would expect from an approximate method.  First of all, connection deformations are ignored for the determination of lateral forces, thus the drift is unknown (you are relying on far reaching "fudge" factors to amplify your drift based on recommendations derived from a small class of 2-d frames).  Also, since the force-deformation response characteristics of the connection are ignored, the distribution of forces is only correct if the connection can travel through its actual moment rotation curve in the shake down process without a non-ductile failure such as bolt shear.

White and Goverdhan lay out a method showing how to use the Direct Analysis method with the actual moment rotation curves to perform a true PR design, using the same example problem as Geschwindner and Dique that I would highly recommend reviewing as a more rigorous alternative to the FMC.  (you can get this paper in the proceedings for Steel Connections VI conference for free here http://www.aisc.org/WorkArea/showcontent.aspx?id=26572)

Note that the FMC shortcomings are (sort of) recognized in the 14th Ed. steel manual which states on p11-2 that "When used, the analytical model of the PR connection MUST include the force-deformation characteristics of the specific connection. As an alternative, flexible moment connections (FMC) may be used as a simplified approach to PR moment connection design, particularly for preliminary design."

I am not against the FMC method, just wanted to make sure everyone who uses it considers its possible shortcomings and considers a true PR analysis if warranted.  

  

RE: Type 2 (Wind Only) Moment Frame

Willis-
If you use the DAM with moment rotation curves, you would need to design for gravity + lateral moments, correct?  This is not quite the same as the flexible wind moment connection.  I agree there are a lot of limitations with the flexible wind moment connections, and that it's important to understand them before using this lateral system, but it has been used successfully on a pretty large number of projects.

AISC 360-05, section 11 gives a nice blurb on the uses of the FMC.

RE: Type 2 (Wind Only) Moment Frame

Lion - correct - no it's not the FMC, it's better (and what FMC is approximating).  The AISC manual blurb from the 13th ed. manual has been updated to what I quoted above in the 14th ed. manual to be more in line with current research and to more clearly denote that the FMC is an approximate method best suited to preliminary analysis.   

RE: Type 2 (Wind Only) Moment Frame

Note that the Section title in the 14th Ed. Manual has even been changed from Design of Flexible Moment Connections to Design of Partially Restrained Moment Connections to move the emphasis away from solely the use of FMC.   

RE: Type 2 (Wind Only) Moment Frame

PR connections, from what I've heard are not usually a cost-effective option, because they require testing to determine the moment rotation curve.  Is that a true statement?

RE: Type 2 (Wind Only) Moment Frame

Yes and No.  If you are using one never used before, yes, just like in seismic if you were not using a pre-qualified connection you would need to get a P-R curve.  However, for the generally used top and bottom angle connections (same ones you probably used in your FMC analysis) there already have readily available published moment rotation curves that can be used, with references provided to them in the 14 Ed. manual, so there is no additional cost other than design time.  Speaking of which, as you pointed out it is often actually harder to try to figure out how to use direct analysis with FMC to meet all the simplifying assumptions than it is to just use it with real PR analysis.   

RE: Type 2 (Wind Only) Moment Frame

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.

RE: Type 2 (Wind Only) Moment Frame

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?

RE: Type 2 (Wind Only) Moment Frame

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.   

RE: Type 2 (Wind Only) Moment Frame

(OP)
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

RE: Type 2 (Wind Only) Moment Frame

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.

RE: Type 2 (Wind Only) Moment Frame

(OP)
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

RE: Type 2 (Wind Only) Moment Frame

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.

RE: Type 2 (Wind Only) Moment Frame

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

RE: Type 2 (Wind Only) Moment Frame

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).   

RE: Type 2 (Wind Only) Moment Frame

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.

RE: Type 2 (Wind Only) Moment Frame

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

Thanks.

EIT

RE: Type 2 (Wind Only) Moment Frame

(OP)
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/download.aspx?folder=b1bfc49e-f67b-4da0-ae4e-ef0fa72f095d&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

RE: Type 2 (Wind Only) Moment Frame

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?
 

RE: Type 2 (Wind Only) Moment Frame

(OP)
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&pid=explorer&chrome=true&srcid=0Bz-ld2aFpGVZZDU2ZmM3NjMtNTM0Zi00MmM3LWI1MTAtMTg4NGNhNzFiMTEx&hl=en_US

I appreciate the efforts.  

EIT

RE: Type 2 (Wind Only) Moment Frame

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

RE: Type 2 (Wind Only) Moment Frame

(OP)
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

RE: Type 2 (Wind Only) Moment Frame

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

RE: Type 2 (Wind Only) Moment Frame

(OP)
Chyeah! Thanks!

EIT

RE: Type 2 (Wind Only) Moment Frame

Lion, you quoted the 14th Edition with this

"As an alternative, flexible moment connections (FMC) may be used as a simplified approach to PR moment connection design, particularly for preliminary design."

My 13th Edition says the exact same thing except it does not contain the last four words "particularly for preliminary design."  I'm curious why AISC would add that phrase.

I think the concern about joint rotation is taken care of by the prescriptive geometry and welding locations shown in AISC's FMC sketches.  As long as you follow their "weld here" and "don't weld here" guidance, then you'll be ok.

RE: Type 2 (Wind Only) Moment Frame

Remember, the structure will behave as you have designed it.  Designing structures is you shaping how it will act, not you being subjected to it's actions.

RE: Type 2 (Wind Only) Moment Frame

peal724,

Would that it were so!  We do our best, but the structure is frequently smarter than we are.

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