Sizes in Intermediate Moment Frame Bigger? 3

[wilberz]

Have you designed same kind of building both in Seismic Design Category C and D,E,F? Did you notice the member sizes of the columns are bigger in Intermediate Moment Frame than in Special Moment Frame? This is because as Force Reduction Factor gets lower, you need to increase the member sizes for elastic strength because they are not ductile (like in special moment frame).

in

http://www.researchgate.net/publica...CTILE_DETAILED_REINFORCED_CONCRETE_STRUCTURES

It is suggested that:

"Design provisions for ductile detailing need to be modified as it has been observed that with increased R values, the member size decreases and lead to structures having more damage compared to normal detailed structures thus R need to be defined more clearly as in other seismic codes."

Can you confirm that as Force Reduction Factor gets lower, the member sizes increase?

They say Special Moment Frames are more expensive than Intermediate Moment Frames.. but if the latter has bigger sizes.. how can it be least expensive? Can anyone clarify?

 

[KootK]

There are too many parameters at play to say with any certainty wilberz. I suspect that beams tend to dominate shear mode drift simply because your average beam is probably 2-3X as long as your average column is high.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wilberz]

but the contribution to plastic rotations is due to both column and beams at the joint.. so even if the beams deflect more and drift is more.. it is still the contribution of both column and beam to plastic rotations.. you said it is seismic story shear force that causes inelastic beam rotations.. are you saying when the beams deflect, it can stretch the joint and initiate elastic or inelastic beam rotations (instead of the column that drifts) that cauase it?

 

[KootK]

I'm not saying anything of the sort Wilberz. Frame drift in the shear mode is due to both beam and column flexural deformation. Just not in equal measure necessarily.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wilberz]

We mostly are given the above image.. where the columns swaying is what rotates the joins elastically or inelastically.. can you give references or image where the beam deflections alone can contribute to plastic rotations? I just want to picture it in my mind. But when beams deflect in the positive moments, there is corresponding rotations in the negative moments in the elastic limit (before reaching probable moment strength). So when you state "frame drift in the shear mode", are you referring merely to DRIFT without necessarily talking about plastic rotations caused by beam deflection or are you referring automatically/correspondingly the formation of plastic rotations caused by beam deflections?

 

[wilberz]

Or (don't miss above message) let's take an obvious example of a structured that is completely braced frame or the column is infinitely rigid and column can't deflect (just for sake of discussion). When the beams deflect from load.. this can cause drift, right? (or no?).. And in seismic activity, when shear force is introduced, it can make the beam deflect more even beyond elastic limit (pushing laterally) and this can cause column-beam rotations inelastically beyond the nominal moment cracking the joint (plastic rotations) even if the column is infinitely rigid and didn't deflect... right?

(or say the column slightly driftable being pushed by the beams and the beams deflections from shear force mode causing mainly the plastic rotations.. a picture is worth a thousand words.. hope I can see one picture of what you mean)

 

[KootK]

y.. can you give references or image where the beam deflections alone can contribute to plastic rotations?

Once again, I never said anything about beams alone. It's always part beams and part columns. You can make your own reference. Draw a moment frame in your favourite software package, apply a lateral load, and all will be revealed.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wilberz]

You mean like the non-reversing hinge on the right? So the beams must be strong flexurably to avoid more drift and lateral shear force bending it more. It's from

http://www.nehrp.gov/pdf/nistgcr8-917-1.pdf

 

[wilberz]

Once again, I never said anything about beams alone. It's always part beams and part columns. You can make your own reference. Draw a moment frame in your favourite software package, apply a lateral load, and all will be revealed.

The following is the moment frame you said to draw in Etabs.. I applied USB97 lateral load..

Fixed based:

Pinned based:

What all will be revealed. If you replaced the curved beam with straight beam. The frame would still drift. So how can beam cause drift.. Unless the following scenario:

In ultimate seismic shear force...
column deflection causes drift then plastic rotation...
beam deflection causes plastic rotation then drift

Would you agree to these order of events between columns and beams? If not.. how can beam deflection causes overall drift? what would happen if the beams are completely straight.. I can't selectively remove the dead load of the beam in etabs.

 

[KootK]

What all will be revealed

What has been revealed is that you now have what you asked me to create for you: a picture of what lateral frame deformation looks like when beam flexural deformation contributes to that lateral frame deformation. It's your second sketch. It would be more illustrative with a multi story frame but that's okay.

If you replaced the curved beam with straight beam. The frame would still drift. So how can beam cause drift..

Both beam and column flexural deformation contribute to frame lateral deformation. If your columns are rigid but your beams are not, there will be lateral drift. If your beams are rigid but your column are not, there will be lateral drift. Simply conduct these experiments with the frame model that you've created and you can easily answer these questions for yourself.

Would you agree to these order of events between columns and beams? I

I wouldn't. In all phases of the load history, bending flexibility in both beams and columns contribute to drift. As loads are increased and plastic hinges form, model joints that were originally fixed simply become pinned. That's all.

I'd recommend downloading a free program called Mastan2. It allows one to model frames with plastic hinges and track behaviour all the way through hinge formation and collapse. I'm afraid that the answer to most of your "what if" questions is "structural analysis". Doing some for yourself will give you a much better feel for things.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wilberz]

Many many thanks for all the insights Kootk!

 

[jayrod12]

KootK you deserve a star for your patience and determination on this one.. nice work.

 

[KootK]

Thanks Jayrod. That's very kind of you to say. And yummy stars never hurt of course...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.