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.
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
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
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
RE: Type 2 (Wind Only) Moment Frame
RE: Type 2 (Wind Only) Moment Frame
RE: Type 2 (Wind Only) Moment Frame
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
RE: Type 2 (Wind Only) Moment Frame
Mike McCann
MMC Engineering
RE: Type 2 (Wind Only) Moment Frame
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
RE: Type 2 (Wind Only) Moment Frame
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
EIT
RE: Type 2 (Wind Only) Moment Frame
RE: Type 2 (Wind Only) Moment Frame
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
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
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
RE: Type 2 (Wind Only) Moment Frame
RE: Type 2 (Wind Only) Moment Frame
RE: Type 2 (Wind Only) Moment Frame
RE: Type 2 (Wind Only) Moment Frame
RE: Type 2 (Wind Only) Moment Frame
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
RE: Type 2 (Wind Only) Moment Frame
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
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
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
RE: Type 2 (Wind Only) Moment Frame
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
RE: Type 2 (Wind Only) Moment Frame
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
Thanks.
EIT
RE: Type 2 (Wind Only) Moment Frame
RE: Type 2 (Wind Only) Moment Frame
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
RE: Type 2 (Wind Only) Moment Frame
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:
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I appreciate the efforts.
EIT
RE: Type 2 (Wind Only) Moment Frame
RE: Type 2 (Wind Only) Moment Frame
ht
RE: Type 2 (Wind Only) Moment Frame
Thanks again.
EIT
RE: Type 2 (Wind Only) Moment Frame
RE: Type 2 (Wind Only) Moment Frame
EIT
RE: Type 2 (Wind Only) Moment Frame
http:
RE: Type 2 (Wind Only) Moment Frame
"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
RE: Type 2 (Wind Only) Moment Frame
Would that it were so! We do our best, but the structure is frequently smarter than we are.