Plastic Hinges 5

[GalileoG]

I am having a hard time trying to understand exactly what or how a plastic hinge is formed during the seismic response of a structure. What exactly is a plastic hinge, why is it formed during dynamic (seismic activity) and where do they form? The texts that I possess explains it but I still have not completely grasped it. Thanks.

 

[Stillerz]

frv-
Given even just a fairly complicated structure in general; let's say a tower for arguments sake, isn't fair to say that there is no guarantee that the third plastic hinge would form in the fixed-fixed beam because force redistribution?
Once the plastic hinge forms at the ends of the beam and it becomes theorectically simply supported the forces it will "draw" from the surrounding structure will become significantly different and presumably significantly less since it is no longer and member capable of resisting lateral loads as, say, part of a moment frame. Meanwhile the surrounding structure with be absorbing the loads it once counted on the fixed-fixed beam to carry and thus redistributing loads "away" from the fixed-fixed beam and also worsening the P-Delta effects of the structure.
So where is the force creating this third hinge coming from?

 

[mexicali8]

FRV, I'm in this forum to learn, so what I say or ask next is within that purpose, not to prove a point.

On my practice:
1.- Plastic design is permited, but I will not design a plastic hinge for code or anticipated loads. If I do so I will have a deformed structure very soon.
2.- Unexpected loading for me is loads beyond that 1.6 load factor. For example, I will design a classroom building for code loads (for me that is anticipated load), but not for 250 students craming one room. In the event of a prank in wich those 250 try to fill one classroom in the second floor I will prefer for it to fail in a soft manner, ie, deform, deform so it will make the students desist on their intent. Designing the whole school to withstand the load imposed for that kind of improbable (but happens) event will make my structures so expensive that I will go out of bussines soon.

Rafael Guerra.

 

[Stillerz]

To be sure, my post was also a long-winded question...I am not well versed in desiging structures with plastic hinges.

 

[frv]

Stillerz-

The fixed-fixed example both I and mh819 gave dealt only with gravity loads.

You certainly would not use a "plastic-hinged" moment frame. However, I can't see any problem with a plastic-hinged braced frame.

I'm not sure if your comment was directed more toward what I mentioned about seismic detailing. If so, I will pass on something I recently learned which I thought was great; For dynamic loading, your structure must possess EITHER the strength OR the ductility to withstand that load (seismic), NOT both. In essence, the more ductile your structure, the less strength it needs to withstand the event.

 

[mexicali8]

And a fact is I have little experience in the design of structures.

Rafael Guerra.

 

[frv]

mexicali-

I think you overestimate the rotation required to form a plastic hinge. It almost certainly would not be perceptible to the naked eye.

As far as 250 students in a class, fine. But have you stopped to think how much 100 PSF is? Multiply by 1.6 and I think you could fit 250 Jerry-Springer-guest-sized people in that room and be well within safety margins.

 

[Stillerz]

frv-
Sorry if I went down a slippery slope there....
Working largely in the power industry where braced frames and virtually a given for most structures I guess I had simliar structures in mind during my post and assumed you were referring to a beam being used as part of a moment frame. For the most part, I use moment connections as part of momnet frames for resisting lateral loads as part of a moment frame.
Having said that, the original post was about plastic hinge design with regard to a seismic event, which is certainly a lateral load. With this in mind, maybe my last post makes more sense; maybe not.
In braced frames, more often than not, beams that are part of a braced frame are really beam-cloumns as can often times carry very large axial loads.
I guess what I am gettting at is, when or in what type of structure is this plastic hinge design approach used?

 

[mexicali8]

frv,

O.K. One more thing I know now.

Rafael Guerra.

 

[youngstructural]

I'll just throw in my two cents (an amount of money that coins can no longer do here in New Zealand... Best Id be able to do would be a 10 cent piece).

IF you are doing seismic design, you may "redistribute" up to 30% of any given moment thanks to plastic hinge formation in the beam. This is because although for a GRAVITY loads your system consists of simply supported beam section between your plastic hinges, in a SEISMIC frame (under a load combination like Gravity + 0.6 Live Load + EQ) your plastic hinges actually PERMIT the redistribution of the governing SEISMIC load. This is because of the rotation created by the plastic hinge and the fact that the seismic loads will travel through the beams using them as compression members.

If you want an excellent book on plastic hinging, moment redistribution and the (very useful and still very much state of the art) capacity design in concrete look no further than Park & Paulay's "Reinforced Concrete Structures".

Oh, and just a quick note: Moment redistribution is an absolutely fantastic technique. However, that said, you really need to be clear about when it does and does not work. In a gravity frame with beam governed in all cases by gravity load cases, and further reliant upon redistribution to work you had best be certain you will NOT have plastic hinge formation. In a seismic frame that you want to have survive an earthquake without collapsing it's actually much, much better to make you that you CAN experience plastic hinging, whether or not it is expected.

Hope that helps,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

One more thing:

Anyone trying to learn more about this issue really needs to study, read and think until they can understand what frv said when posting:

"For dynamic loading, your structure must possess EITHER the strength OR the ductility to withstand that load (seismic), NOT both. In essence, the more ductile your structure, the less strength it needs to withstand the event."

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

Okay, okay, last post:

Before anyone asks, YES, Capacity design (aka a structure reliant upon strong colum weak beam and plastic hinge energy dissipation) will almost certainly result in a completely useless structure post-catastrophic earthquake. HOWEVER, as long as we've done our jobs right, the building will not collapse and the occupants will survive.

Again, before anyone points it out, YES, base isolation of an elastic structure designed to a mu of 1.25 is better as it results in no flying contents injuries. But it's also very expensive and relies upon components that we do not KNOW will last the often 80+ year design life of a structure.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[frv]

youngstructural hit the nail on the head...

designing a structure for seismic is akin to designing a vehicle for a head-on collision.. not meant to be pretty- just keep the occupants alive.

 

[rtmote]

frv and youngstruct that was well-played.
Love the quote:

"For dynamic loading, your structure must possess EITHER the strength OR the ductility to withstand that load (seismic), NOT both. In essence, the more ductile your structure, the less strength it needs to withstand the event."

It is a common misconception.

Like trying to deal with project engineers who think it is possible to overdesign.....



Robert Mote

http://www.motagg.com

 

[structuralnerd]

Do any of you have any good reading material suggestions about this subject?

 

[youngstructural]

Yes, the same as I have already mentioned, namely
Park & Paulay's "Reinforced Concrete Structures".

Also:

1. Your structural code AND commentary (not trying to be smart!!!)
2. AS/NZS 1170 & NZS 3101 (with commentary in both cases)... I came to New Zealand to learn more about seismic engineering because they are the world's experts; Hands down. The codes reflect a wealth of local knowledge and a voratious apetite for anyone else's discoveries.

For the basics, or someone more interested in steel:

Plastic Methods of Structural Analysis: SI Version
B. G. Neal
ISBN: 0470990171
Format: Hardcover, 205pp
Pub. Date: December 1977
Publisher: Wiley, John & Sons, Incorporated
Edition Number: 99

NOTE: For this excellent introductory text you should try to find the oldest copy possible; The later editions stripped out alot of the "justification for use of plastic design", which in my opinion is a big mistake as it is a great additional value beyond the intended design purpose for the book.

I'd be very keen to hear anyone else's recommendations...

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[frv]

Really? New Zealand?

I'm surprised.

I would have assumed it would be Japan. Or (not to be ethnocentric) the US; they do a tremendous amount of research in this area at several universities: Berkley, Illinois, Texas, etc.

 

[youngstructural]

frv: You are suprised that I say New Zealanders are the world's experts in Seismic Engineering. And someone must agree with you pretty strongly as you've already gotten a star. Well I will say it again; Yes, New Zealand. And to be very clear I stand by my OPINION on that point. The States and Japan both do a great deal of Seismic Engineering research (so do others in Europe and around the world), as does NZ, however NZ goes further and put some of the most advanced methodologies available it into common practice...

When I say "learn about seismic engineering", I mean above all else Capacity Design. If you don't know it and are reading this thread, you should probably take some time and read about it.

Capacity Design was invented in New Zealand and is widely applied in New Zealand consulting practice. It is not commonly applied elsewhere (including USA & Japan as far as I am aware). If I wanted to learn about theoretical seismic engineering I may very well look to at studying at UC Berkley or another US school, however I wanted to actually practice the application of that research. In english. Without getting too detailed, I found quite quickly that this meant working in New Zealand.

I would be very happy to hear differently/be corrected. If you know of a US firm or juristiction applying Capacity Design, or advanced plastic hinge energy dissipation (in concrete, not just steel), I would be very keen to hear about it!

I'm not sure, but I am very confident in saying that New Zealand is still the best place to learn about Seismic Engineering. It is the practical actual application of Seismic Engineering in every day practice that makes NZ unique, in my honest opinion. I am a Canadian and could have gone to Vancouver to practice (and would be making MUCH more money), however Canada, like most countries, abounds in UNAPPLIED Seismic Eng. theory embedded into portions of codes that no one uses....

Just out of curiosity I typed in "seismic engineering expertise" for a Google search. Half of the hits talk about New Zealand research and engineering practice. Most of those sites are non-NZ engineers praising Professional practice in this country... Just like I do.

Good examples:

http://peer.berkeley.edu/course_modules/eqrd/index.htm?c227top.htm&227cont.htm&DesAppr/desappr4.htm

http://www.earthquakeengineering.com/news_events/articles/News_Istambul_Project_18Jan05.pdf

Finally, if you're curious about the application of Capacity Design, here is an excellent run-down on the theory:

http://www.wiratman.co.id/ximages/capdesign.pdf

Please, before anyone starts getting their patriotic feathers in a tussle, please remember that this is my professional opinion, and only an opinion, and when it comes down to it NO ONE is "BEST". I appreciate and respect the excellent research being done in many places around the world, but found an abnormally highly motivated consulting industry to learn from in New Zealand.

I look forward to your follow up posts.

Regards,

Maurice Quinn, P.Eng (Ontario)


B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[frv]

Maurice-

I was not disagreeing with you.

I was just stating that I was surprised.

I'd venture to say that if you ask even those in the structural engineering profession to name the country at the forefront of seismic design, you'd be hard pressed to get more than a small percentage saying "New Zealand".

I repeat, I'm not arguing with you; I'll take your word for it. I just wouldn't have thought of it.

And the reason I mentioned Japan and the US is because both those countries have high risk seismic areas (the entire country in the case of Japan) AND the financial wherewithal to pursue research in the area.

Europe certainly has the financial wherewithal; except for the countries that are actually at any significant risk of suffering seismic events (Turkey comes to mind).

Anyway- I'll take your word for it. Go Kiwis!

 

[youngstructural]

Fair enough; Point taken. I'm probably a bit sensitive about it all as I've made alot of sacrifices to move down here. It has cost me an untold amount of personal stress and professional challenges. It's also very hard to pay off Canadian school debt with a much weaker dollar and a significantly lower pay scale for engineers. Besides which your typical NZ Engineering graduate lives and breaths seismic engineering and practicing next to them with what I had learnt before comming here was not easy...

It is unfortunate, but I admit I never really gave New Zealand much thought either, until I looked into it. I was sure I'd wind up working in California, but even the first bit of research I did into where to go pointed very quickly to the other side of the world.

I've enjoyed and respected many of your posts here on Eng-tips. Hopefully if anyone's interested in Capacity Design I haven't put them off with my fanatical enthousiasm for it and my adopted home...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

Youngstructural is 100% correct. New Zealand is the leader in seismic design. And for good reason. It is also on the Pacific rim of fire, same as Japan and California. And I am not biased, because I am in Australia, in the middle of a plate so not too concerned about earthquakes.