Is this a moment frame?

[penpal97]

I am designing an ordinary moment frame as defined by AISC 341-05. All beams and braces are pin connections. Please see the attached.

Frame A - just an example of a perfectly acceptable moment frame using knee braces.

Frame B - Is this allowed to be qualified as an ordinary moment frame? I would guess "no" because it precludes yielding by bending of the beams. But does the yielding mechanism of the have to be in beams, or can it be in columns for OMF as well? AISC 341 seems to allow it for OMF, but not SMF.

Frame C - Does this now become a special truss frame? or can I still consider this an ordinary moment frame?

 

[EChristensen]

PenPal97,

I just joined Eng-tips and came across your question and I think I can help.

A moment frame generates it's lateral stability by using partially or fully restrained moment connections to transfer bending moments through the joint, usually by bolted end plates, flange bolted or flange welded connections.

Since, as you stated, all the beam and brace connections are pinned, by definition none of the frames you propose are moment frames as there is no moment transfer through the joint.

Frame A would be classified as an Eccentrically Braced Frame. Frame B looks like a Truss Frame, Frame C should be treated as an Eccentrically Braced Frame similar to Frame A.

Regards,


Eric C. Christensen, P.E.
EagleSpan Steel Structures

 

[COEngineeer]

I disagree with eric. I think all of them are moment frames.

It would be eccentrically braced frame if the braces goes to the base of the beam. What do you guys think?

Never, but never question engineer's judgement

 

[penpal97]

Thank you all for responding.

AISC 341-05 Commentary C11.1
OMF Knee-Brace Systems - ... knee-brace moment frames may be design as ordinary moment frames.

ASCE 7-05 does not list Truss Moment Frames anymore; I would guess that Truss Moment Frames are no longer allowed. Does this mean that Frame B is no longer an acceptable moment frame?

 

[penpal97]

does anyone else have an opinion on the matter?

 

[nicam]

I agree with Eric, the fact that your connections are all pinned, no moments would transfer throught the connections.

Why is frame 'A' a moment frame when no moments will transfer to the columns or beam?

 

[dgkhan]

I will agree with nicam and EChristensen . COEngineer, you have been very reasonable before. So I am not sure if you are not very correct this time.Can you show me how moment from column will go into beam or vice versa if connectons are shear conn only?
Secondly
penpal97
it is not a typical frame... looks like two columns with lateral brace, braceis kind of built up member…

 

[Lion06]

I would be very interested to hear some other opinion's on this. It is clearly not a conventional "moment frame" as we typically think of them, but I would just like to offer this opinion.
If the braces were closer to the bottom of the columns, I would agree that they would qualify as an EBF. The whole idea of a "braced frame" assumes no relative lateral translation between the top and bottom of the column under lateral loads. I don't know if I believe that to be true for frame "A". It is really at what might be considered the demising line - braces being any lower on the column would likely qualify as EBF, any higher would likely not (just IMO).
The point I'm trying to make is that while it may not be the traditional moment frame, it would be more likely to behave as a moment frame than a braced frame (at least if the brace connection to the column were moved up).
I think most would agree that as the knee brace were made shorter (i.e. the brace was at the ends of the beam and column) that it would certainly behave more like a moment frame even though it might be classified as such. That would certainly allow for a sidesway buckling case - which, almost by definition, precludes it from being a "braced frame".
Again, I would be interested to hear from others.

 

[WillisV]

Agreed with your main thoughts StructuralEIT - the classification is dependent on the behavior, as braces move from the base of the column up the length of the column, the columns are mobilized as bending members and the frame becomes more and more "moment frame" like. I do not think I would use a knee brace system for R>3 applications.

Also see Charlie Carter's thoughts:

http://www.seaintarchive.org/group/seaoc/mailarchive/2002d/msg01654.html

 

[penpal97]

I mentioned this a bit earlier up the thread, but here it is expanded...

AISC 341-05 Commentary C11.1
OMF Knee-Brace Systems - ... knee-brace moment frames may be design as ordinary moment frames. The system can be considered as analgous to a moment frame with haunch type connections. The brace represents the sloping bottom flange and the beam represents the web and top flange of the haunch. The knee brace carries axial loads only, while the beam-column connection carries both axial load and shear.
...
This system is not anticipated to be as susceptible to column failures as a K-type braced frame since the column is designed for the moments resulting from forces from the knee brace(s).

 

[penpal97]

Any thouhgts on Frames B & C?

-------------------------
Frame B - Is this allowed to be qualified as an ordinary moment frame? I would guess "no" because it precludes yielding by bending of the beams. But does the yielding mechanism of the have to be in beams, or can it be in columns for OMF as well? AISC 341 seems to allow it for OMF, but not SMF.

Frame C - Does this now become a special truss frame? or can I still consider this an ordinary moment frame?

 

[structuresguy]

Wow, I can't believe some of you don't think that Frame A is a moment frame. We use knee braced frames all the time, where the beam-column connection is simple shear connection, and they work very well to resist lateral loads. And moment is generated at the supports through the lever arm distance from the brace to the beam-column connection and the axial load created by the brace.

 

[COEngineeer]

I am not sure why some of you think those are trusses. Just forget about the trusses, imagine they were big beams. The columns will resist bending like a moment frame. If the braces goes to the base like inverted V or X, then it would be a truss.

Never, but never question engineer's judgement

 

[civilperson]

The pair of beam and brace or top chord and bottom chord will impart moments into the columns in each case. Thus moment frames.

 

[EChristensen]

I hadn't read that about the knee brace frames in the commentary. Thanks for pointing that out.

On a macro scale I can see how a knee brace frame will behave similar to an OMF. A few years ago when I did design a knee brace frame in a seismic area I used the provisions for EBF, because I felt the EBF most closely resembled the actual conditions. I felt the provisions for designing the beam link in the EMF were most appropriate for how the knee brace system works. Purely a judgment call on my part.

Be careful with the connections.

Since there is discussion on which frames are truss and which are not, this is the way I see it. Frame B is definitely a moment truss frame. Frame C is not, it looks like a truss at first glance but the vertical members at the third points are clearly zero force members (unless there are loads applied at those points) therefore Frame C will really behave the same as Frame A, it just has more stuff.

Regards,

Eric C. Christensen, P.E.
EagleSpan Steel Structures

 

[tngv752]

My idea,

I think it's depend on the global structure, you can decide it 's a moment frame or pinned connection.

Frame A: you can design as pinned connection because the beam at mid height of the column (column can be long) so you dont want to tranfer the moment to the column. Otherwise, it look like a portal frame with moment connection between beam and column and pinned connection for knee bracing.

Frame B & C is look like a truss to me. The beams are top and bottom chord.

 

[msquared48]

Frame "A" is just that, a knee-braced frame, with the knee brace triangle serving as a moment arm to develop a moment through the pinned connection of the beam to the column. Without the knee brace there is no moment or moment frame here. It's the same structural arrangement as a bent three-hinged arch where the haunch is deeper due to the moment capacity, the same as the knee brace.

Diagrams "b" and "C" look to me like a transfer truss and a modified transfer, Vierendeel (sp) truss to allow Mechanical ducting to pass through. Technically, there would not have to be moment connections here to develop a moment at the columnn if both the top and bottom flanges of the truss are pinned to the column. As previously mentioned though, I would have to research the code to see if this is allowed. I would be concerned with a lack of redundancy here.

Mike McCann
MMC Engineering