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]

With a high R value, you'll have lighter main members but much more costly connections (plastic hinge location detailing requirements ect). With a low R value, you'll have heavier members but more economical connections. The costly connections cost more than the lighter members save pretty much without fail. If special moment frames had lighter main members and cost neutral connections, why would we ever use anything else?

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.

 

[jdgengineer]

While I haven't down a direct comparison on member sizes between ordinary, intermediate, and special member sizes shouldn't be drastically different. Moment frames are typically controlled by drift and the Cd factor is somewhat invserly related to the R factor.

Ordinary -R 3.5, Cd 3 ratio = 0.86
Intermediate -R 4.5, Cd 4 ratio = 0.89
Special - R 8, Cd 5.5 ratio = 0.69

So special member sizes should be roughly 20% lighter than ordinary or intermediate

 

[wilberz]

With a high R value, you'll have lighter main members but much more costly connections (plastic hinge location detailing requirements ect). With a low R value, you'll have heavier members but more economical connections. The costly connections cost more than the lighter members save pretty much without fail. If special moment frames had lighter main members and cost neutral connections, why would we ever use anything else?

This may be true for steel frames where the connection detailing are more expensive. But in pure reinforced concrete.. it is just cheap 10mm rebars in the detailing in terms of closed stirrups and ties. This is cheaper than bigger size concrete. Don't you agree?

In the following paper in Thailand...

http://thescipub.com/html/10.3844/ajeassp.2011.17.36

It is stated that "ODF is the most expensive among ODF, IDF and SDF. Costs of SDF and IDF in Bangkok are quite similar."

ODF being Ordinary Moment Frame which is more expensive than Special Moment Frame.

I wonder before I wrote this thread why they don't use all Moment Frames instead?

 

[KootK]

But in pure reinforced concrete.. it is just cheap 10mm rebars in the detailing in terms of closed stirrups and ties. This is cheaper than bigger size concrete. Don't you agree?

I suppose that it is market dependent. In North America, most of the installed concrete cost is due to formwork and rebar placement. Larger concrete members don't add much to the cost but intense rebar congestion does. You'd also have some minor things like requirements for weldable reinforcing, mechanical splices, and intense inspection. I agree that the penalty in concrete is probably less than in steel.


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]

I suppose that it is market dependent. In North America, most of the installed concrete cost is due to formwork and rebar placement. Larger concrete members don't add much to the cost but intense rebar congestion does. You'd also have some minor things like requirements for weldable reinforcing, mechanical splices, and intense inspection. I agree that the penalty in concrete is probably less than in steel.

I notice in frame analysis that if you make the columns twice larger in special moment frames.. there is lesser moments at the beam ends.. this result in higher seismic activity before reaching probable moment strength Mpr. So if money is no object like important institution like hospitals.. doubling the column size and rebars after considering the standard special moment frames of selecting beam sizes from the Mpr, joint sizes from the beams and columns framing into it and the column sizes.. so after getting the required column sizes and just doubling them.. would it make the Reduction Factor R lower or does it stay the same at 8? If it gets lower. In hospitals and other places where the owners have no budget limit.. what would happen if the column sizes are large like in intermediate moment frames yet the beams ends were detailed for ductile moment strength.. what force reduction factor would this fall under, 5 or 8 and what would be the effect?

 

[KootK]

If a frame is provided with more elastic strength than required for the R value selected at the beginning of the design process, then the ratio of inelastic deformation to total deformation will be less than expected for that R value. Effectively, the structure will be endowed with more displacement ductility than it needs to satisfy code requirements. What the resulting effective R value would be is difficult to say with much certainty.

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.

 

[KootK]

If a frame is provided with more elastic strength than required for the R value selected at the beginning of the design process, then the ratio of inelastic deformation to total deformation will be less than expected for that R value. Effectively, the structure will be endowed with more displacement ductility than it needs to satisfy code requirements. That's about all that I'm willing to say with any confidence.

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]

If a frame is provided with more elastic strength than required for the R value selected at the beginning of the design process, then the ratio of inelastic deformation to total deformation will be less than expected for that R value. Effectively, the structure will be endowed with more displacement ductility than it needs to satisfy code requirements. That's about all that I'm willing to say with any confidence.

I'm asking this because some buildings are designed for say 5 storeys but only 3 storeys built in the meantime.. so the columns are bigger than for a 3 storey. So in cases like this. The moments of the beam ends would be less. For a given seismic activity say Magnitude 7. There is less rotations in the joint because of the stronger column with more moment resistance. So it will take higher seismic movement to reach the same rotation and it will take a larger seismic force to develop the beam plastic hinges.

Or in short. Building with columns designed for 5 storey and only 3 storey built would be stronger, won't it?

 

[KootK]

So it will take higher seismic movement to reach the same rotation

It will take higher seismic movement to reach the same plastic rotation. Higher seismic movement will always result in higher total joint rotations.

Or in short. Building with columns designed for 5 storey and only 3 storey built would be stronger, won't it?

Yesir.

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]

Remember my building has probable moment shear taken up by Vc + Vs intead of Vs (stirrup) only. It is designed for 5 storey but I only built 3. So not building it higher would maybe compensate a bit. I don't trust carbon fiber (these are installed).. but it the same carbon fiber used to retrofit shear capacity in airports.

Anyway. For 30 storey buildings with one meter size column with normal beam at each floor. Does this mean each floor beams are stronger than the average houses.. because the columns are so big.. could the deflection at higher floors cause the 1 meter column to bend more causing more rotations of the beams? If not.. then the failure mode of high rise buildings are columns failure more than beam failures because of lesser moments in the beams? (in other words, high rise buildings have smaller beams dimensions compare to houses)?

 

[KootK]

(in other words, high rise buildings have smaller beams dimensions compare to houses)?

In the unlikely event that the house and high rise were both moment frames, had similar spans and gravity loads, and somehow had the same story shears at the same floors then, yes, the larger columns of the high rise would attract lateral load moment away from the beams and permit them to be designed as smaller members. My experience of concrete homes in Asia, however, is that they are shear wall buildings with short spans and nowhere near the the story shear demands of 30 story buildings.

If not.. then the failure mode of high rise buildings are columns failure more than beam failures because of lesser moments in the beams?

Lesser moments in the beams would usually result in smaller beams so, no, I would not expect the columns to fail before the beams. In seismic areas, modern buildings are designed to ensure that failure generally occurs in the beams rather than the columns.

could the deflection at higher floors cause the 1 meter column to bend more causing more rotations of the beams?

No. The deflection at higher floors is primarily the result of global frame cantilever bending (axial column deformation). It is only the story shear mode of deflection that causes relative beam/column joint rotation.

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]

In the unlikely event that the house and high rise were both moment frames, had similar spans and gravity loads, and somehow had the same story shears at the same floors then, yes, the larger columns of the high rise would attract lateral load moment away from the beams and permit them to be designed as smaller members. My experience of concrete homes in Asia, however, is that they are shear wall buildings with short spans and nowhere near the the story shear demands of 30 story buildings.

I live in the Philippines. Here we rarely use shear wall buildings.. most just moment frames.. but here's the scary parts. We don't have course in structural engineering. So almost all so called "structural" engineers are just operators of cad and some civil engineers and even plumbers who memorized some rule of thumb. They don't know how to design special moment frames. The first "structural engineer" who designed my building did weak column-strong beams and the second one is just operator of Etabs (but fortunately working under experienced mentor) who don't even know the meaning of strain-stress curve for example. So in spite of our country located in seismic zone.. most buildings use moment frames only but not special moment frames (in steel frames.. city hall banned any special moment detailing (in the popular guide paper you sent for example) which they consider proprietary because they only understand the pre Northridge panel detailing).. nor do we use shear walls or braced frames.. 98% of "structural engineers" here are only willing to design building if the beam span won't be more than 4 meters.. they would deny any work of 6 meters beam span because they no longer have confidence.. so it is only very few competent engineers who design them but they most work with high-rise. So most of our homes are 4 meter beam span with hollow blocks wall (not rc shear walls) and they insert rebars mostly without calculations because 4 meter beam span needs say just a few rebars (they memorize it already). The government has prepared earthquake drills because they expected massive casualities when magnitude 7.0 arrive.

Anyway. I'm studying structural engineering just to check on the design of my 6 meter span building and learning to manually compute and understand the concept. I'm a physicist though.. and also a communications engineer so I'm still an engineer hence qualified to participate in eng-tips (lest I be banned.. lol)..

No. The deflection at higher floors is primarily the result of global frame cantilever bending (axial column deformation). It is only the story shear mode of deflection that causes relative beam/column joint rotation.

Thanks. I'm studying about the connection of drift and plastic rotations today. But see the following

https://courses.cit.cornell.edu/arch264/calculators/seismic-wind/index.html

Higher floors have higher story shear... so this what caused the story shear at higher floor and not the deflection.. thanks for this clarifications. So drift is same as deflection and won't cause story shear? I thought drift can cause beam-column joint rotations. Back to fixed column bases.. it lessens drift in ground floor and can lessen moment at beams framing in the ground floor ceiling.. unless you mean it is the story shear which affects the moments in the beams which affect the column (momentum redistribution because the column base is fixed)?

Let this be my last question as I just need basic understanding to find seasoned local structural engineer should I need real design. I know this forum is only for professional structural engineers. Thank you.

 

[wilberz]

forgot to ask this above

In the unlikely event that the house and high rise were both moment frames, had similar spans and gravity loads, and somehow had the same story shears at the same floors then, yes, the larger columns of the high rise would attract lateral load moment away from the beams and permit them to be designed as smaller members. My experience of concrete homes in Asia, however, is that they are shear wall buildings with short spans and nowhere near the the story shear demands of 30 story buildings.

If you will see in the following

https://courses.cit.cornell.edu/arch264/calculators/seismic-wind/index.html

the seismic story force of the second and third floor of a 3-storey is much more than the seismic story force of a 20-storey building. This means the beams of 3 storey building top floor must be stronger (flexual and shear wize) than the beams of any 20-storey building floor (as the following clearly shows)??

20-storey

3-storey

 

[KootK]

. So drift is same as deflection and won't cause story shear?

They are the same. The story shear contribution to drift will cause plastic joint rotation. The cantilever contribution will not cause plastic rotation although it will cause elastic rotation.

This means the beams of 3 storey building top floor must be stronger (flexual and shear wize) than the beams of any 20-storey building floor (as the following clearly shows)??

You are making an important error here. Seismic story force is not the same as story seismic shear force. Story seismic shear force at each level will be the SUM of the seismic story forces for ALL levels above the story under consideration. And it is the seismic story shear force that causes inelastic beam rotations.

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]

Thanks a lot Kootk! Many papers and references begin to make sense.

Do you know of a company outside the Philippines who can do seismic analysis by giving them the exact sizes of the members, rebars, live load.. soil condition etc.? In my country. No one even knows how to manually compute for special moment frames (I interview a lot).

Initially I could have used steel frames in the building. Unfortunately. As the following image shows.. our steel column-beam details where not only pre-Northridge.. it is even pre-pre Northridge, even presently.. with lack of stiffener at the middle and other modern detailing. So in seismic activity.. beam hinging won't occur but joint breakup. This is because our steel mostly came from china.

The following is technical paper of a civil engineer warning of the clear and present danger. When I visited him (prior to deciding whether to use RC of steel frame), he said I'm the only one who inquired him about this.

http://www.pgatech.com.ph/documents...ructural Steel - A Clear & Present Danger.pdf

We have complete lack of modern joint as depicted in the paper "Seismic Design of Steel Moment Frames" because in china, they don't do the detailing...

By the way... inside the paper "Seismic Design of Steel Moment Frame" there is a statement:
"The contributions to the shear mode of drift vary with configuration, but in general beam bending is the largest
contributor while column bending is the smallest. Panel zone shear deformations contribute on the order of 15 % to 30 % to
the total shear mode of drift. Estimates of the contributions to story shear drift can be obtained from the equations presented
in Section 4.2.1. ASCE 7, §12.7.3b requires that the contribution of panel zone deformation to story drift be included when
checking drift limits."

Is this also true for reinforced concrete where beam bending is the larger contributor to drift just like in steel frames or is column the major contributor in reinforced concrete beams (this details is not included in the paper "Seismic Design of Reinforced Concrete Moment Frames")?

Many thanks!

 

[KootK]

Do you know of a company outside the Philippines who can do seismic analysis by giving them the exact sizes of the members, rebars, live load.. soil condition etc.?

Certainly, most west coast US and Canadian firms would be up to the task. I hear New Zealand engineers are pretty good at seismic too. I've also had good experiences with engineers from Eastern Europe as well. The bigger challenge would probably be working out something economical with a group that you trust and communicate with well.

Is this also true for reinforced concrete where beam bending is the larger contributor to drift just like in steel frames or is column the major contributor in reinforced concrete beams (this details is not included in the paper "Seismic Design of Reinforced Concrete Moment Frames")?

I'd expect this to be even more true of concrete columns which derive extra flexural stiffness from axial loads in a way that steel columns do not.

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]

I'd expect this to be even more true of concrete columns which derive extra flexural stiffness from axial loads in a way that steel columns do not.

What? You don't mention about beam. You mean in reinforced concrete the major contributor to shear mode of drift is column... or

* in reinforced concrete, column bending is the largest contributor while beam bending is the smallest.. ?

opposite to that of steel where

* in steel, beam bending is the largest contributor while column bending is the smallest

or are you saying

* in reinforced concrete too, beam bending is the largest contributor while column bending is the smallest

please clarify, thanks..

 

[KootK]

I'm saying that, because axial load stiffens concrete columns, it is even more likely that beam deformations will account for the lion's share of the frame drift in the shear mode.

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]

I'm saying that, because axial load stiffens concrete columns, it is even more likely that beam deformations will account for the lion's share of the frame drift in the shear mode.

Oh.. I thought it's mostly RC column that controls the drift.. we talked about fixed bases and how making the column sizes bigger can control drift better.. how column is good for drift control.. so the beam is the hidden drift master after all...

Hmm... the consequence seems to be that in strong beam-weak columns.. there is less drift than weak beam-strong column then... so less drift means design equally strong columns and equally strong beams?