Shear Consideration In ductile building

[keewan]

I was attending municipality discussion about building design(to listen only),and through the discussion the municipality employee tell that since the building is ductile the calculation of shear done by etabs is not enough(gravity shear) ,and he open the details summary of a member, which include moment , shear and design,etc and said that you have to check another type of shear since the building is ductile ,you have to take positive moment right and negative moment left from summary page and divide by span to get shear, and he told us that etabs calculate this as well

I think that my boss didn't understand ,so I am here asking what exactly he was looking for?

Note: The building we design is ductile since there are no drop beams only hidden beams.

 

[KootK]

In a ductile moment frame, the beams are often designed to develop plastic flexural hinges at the ends. If you do a free body diagram of a beam, including the plastic hinge moments and the gravity loads, you'll likely see what the speaker was getting at.

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.

 

[keewan]

Thank you

Could you clarify what you said more, preferably with example.Since I need to get the full picture

One comment, As far as I know plastic hinges will be developed in order to redistribute the moment again, after reaching the capacity moment where plastic hinges occur(But I don't understand it completely)

 

[KootK]

You're most welcome.

Could you clarify what you said more, preferably with example.Since I need to get the full picture

Try the sketch below as an example of sorts. The logic goes something like this:

1) An ETABS elastic analysis will give you one value of the beam end moments (M_E).
2) Ductile moment frames develop plastic mechanisms under seismic load.
3) The beam moments associated with the plastic hinge mechanisms will be larger than those associated with #1. M_ph > M_E.
4) Beam shear due to lateral action will be 2 x M / L no matter which version of the beam end moments you're looking at. This is just statics as I mentioned in my first post.
5) V_ph = 2 x M_ph / L > 2 x M_E / L = V_E.

That's it.

As far as I know plastic hinges will be developed in order to redistribute the moment again, after reaching the capacity moment where plastic hinges occur(But I don't understand it completely)

Plastic hinges are developed as part of general moment redistribution. That's not what's going on here though. In ductile seismic design, plastic hinges are developed, and encouraged, as mechanisms for dissipating seismic energy.

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.

 

[keewan]

Dear Kootk

I do appreciate your help so much, to be honest it is not clear 100 %, thus I decide to read ACI code before I continue the discussion with you. Let me now summarize what I understand and then you can correct me or add what you see proper.

They are 3 types of system to be used ordinary sway, intermediate and special, theY all differ in ductility an reinforcement details, and I think values of moment will be less in special however more details are required so the value of saving money is debatable.

Now let’s return to our main point of discussion,

In ordinary system there is no plastic hinges to develop as far as I can see by code methodology, hence you can calculate the moment and consider the P, delta analysis to account for second order analysis, hence you can get the shear forces (You can calculate envelop shear,)

In intermediate it will be the same as ordinary regarding the shear

However in special energy dissipation occur, and plastic hinge is a must(as the code methodology), and to ensure no brittle failure will not occur due shear ( the critical earthquake combination result and gravity load combination are added to together),no envelope(Actually why he add I don't know, since both combination will occur togethor)

However regarding moment design in all system we only deal with envelop case only (no addition)

Based on what we said the municipality guy should consider this only and special sway only, not the intermediate we design on

If what I said so for is right(which may not be ), if I like to calculate the shear now from etab I get design value for moment right and left from earthquake combination and then get the shear by simple static then add to shear of gravity and design(so can etab calculate it by himself)


Please be patient by clarification

 

[KootK]

In ordinary system there is no plastic hinges to develop as far as I can see by code methodology, hence you can calculate the moment and consider the P, delta analysis to account for second order analysis, hence you can get the shear forces (You can calculate envelop shear,)

Yes, more or less. Some plastic hinging is to be expected but it is not anticipated to be severe enough to warrant capacity design protocols.

In intermediate it will be the same as ordinary regarding the shear

No. IMF beams are capacity designed for shear corresponding to the nominal moment capacity of the beams. This is less than the probable moment that you design SFM's for but is still capacity design.

Based on what we said the municipality guy should consider this only and special sway only, not the intermediate we design on

As discussed above, a level of capacity design does, in fact, apply to IMF's.

If what I said so for is right(which may not be ), if I like to calculate the shear now from etab I get design value for moment right and left from earthquake combination and then get the shear by simple static then add to shear of gravity and design(so can etab calculate it by himself)

Only for OMF. Not for IMF or SMF.

This would be an excellent, free, and short read on this subject: Link. In particular, see diagram 5-10 and the associated text.

If you can spare the time, the best place to get the theory is from the guys that more or less invented it: Link


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.

 

[keewan]

Now I fully understand it,

But this will involve manual calculations, however municipality employee said etab can calculate it, is this possible?(I think ETABS cant do it, we have to do it manually !!!right!!!)


Thank you again

 

[wilberz]

KootK. In the Northridge, Kobe, San Francisco, Nepal mega quakes.. can you show a picture of a beam that has reach probable moment strength (Mpr) and suffer shear failure? Did the whole beams just collapse to the floor or just hang in the ceiling.. I just want to see how an ACTUAL shear failed beam from Mpr look like.. or if there is not a single failure.. is it possible 99.9% of buildings collapse from the columns failure?

 

[KootK]

@Keewan: you're most welcome. I don't know if ETABS has a feature that specifically addresses capacity design. I've been handling it by hand.

@wilberz: I know of no such photos outside the lab. In the lab, beams just hang there but lose the ability to resist load. One of the goals of capacity designing for beam plastic hinge strength in this manner is to protect the columns from being overloaded precisely because it is the columns that fail most often in real events.



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.