Plastic Hinges
Plastic Hinges
(OP)
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.






RE: Plastic Hinges
RE: Plastic Hinges
RE: Plastic Hinges
RE: Plastic Hinges
Check out the recommended steel moment connection details coming out of the Northridge earthquake. There is plenty of literature on line. You will see how they necked down the beam flanges coming into the column to limit moments on the connection and column. In that case the plastic hinge was chosen to be in the beam.
RE: Plastic Hinges
I'm sure you're more experienced than me, but I've heard it's ALWAYS strong columns, weak beams for seismic.
I must disclose that working in Texas, I haven't had the opportunity to do any seismic detailing, nor have I taken seismic design in the masters program yet. What I'm saying is based entirely on a seminar I attended dealing with seismic design.
In essence, as far as I understood, you would always design your beams to dissipate the energy, while the columns remained (mostly) elastic.
Please correct me if I'm mistaken.
RE: Plastic Hinges
My experience is in bridges. In the earthquake prone bridges I have worked on, the plastic hinge was purposely located in the columns, not the superstructure or foundation for the reason stated above.
RE: Plastic Hinges
How exactly do you "locate" / choose where the plastic hinge occurs? Thanks.
RE: Plastic Hinges
For example, suppose you had a fixed-fixed beam, to which you apply a distributed load. The moments at the fixed supports are highest, therefore, as you increase the load, hinges will develop at the supports. At this point, you can idealize the structure as a simply supported span and the moments will redistribute throughout the member until the final hinge forms (collapse mechanism) at the center.
RE: Plastic Hinges
RE: Plastic Hinges
if the beam is undersized, the steel will begin to yield at the supports since that is the region of highest stress… once the entire cross section has yielded it is said to be “plastic”…. with plastic deformation comes excessive rotation… similar to what you could would get if you had a simply supported (ie hinged) beam instead of fixed-fixed. Once these hinges form, forces redistribute and the beam behaves like a determinate simply supported beam… the high stress area is now at the middle... but once a hinge starts to form (the cross section begin yielding) there the beam’s forces can’t redistribute (since it’s now essentially determinate) and the beam fails
RE: Plastic Hinges
In fact, it is a requirement for the section to be compact in order to do plastic design.
RE: Plastic Hinges
You design your structure for some maximum loads according to applicable code or anticipated loads. But then there is always the possibility of some unexpected event putting bigger loads on the structure than design ones: your structure is going to fail. But if you anticipate this you have three options:
1.- Design for greater loads than code or anticipated, wich eventualy will become antieconomical.
2.- Let the structure fail catastrophicaly.
3.- Locate the points on your structure that will initialy fail on such event and design them to fail in a controled manner, that is, make some "plastic hinges" that will deform rather than break. During this deformation, if more load is imposed on the structure the hinge will not take more stress wich will be taken by another member wich you will design as a "plastic hinge", and so on while it is practical.
Rafael Guerra.
RE: Plastic Hinges
Download a copy of ANSI/AISC 358-05.....Section 2.4.2. These are the prequalified connections to use in moment frames, the location of the plastic hinge is dependent upon the prequalification of the connection, and the geometry for each type of prequalified connection is spelled out for you. If I recall correctly, FEMA 350 is the old version.
In my interpretation, if a connection hasn't been tested (i.e. "prequalified") then you are on you own. So creativity is more or less ruled out.
RE: Plastic Hinges
I hate to contradict another engineer, but plastic design is permitted generally; not specifically for "catastrophic loading".
Furthermore, your "design load" already includes unexpected loading (hence the 1.6 load factor on live load).
But I'll agree that it's generally used for "extreme events" (such as earthquakes)- but these are hardly unexpected.
It is worth mentioning that (at least in the AISC code) you can only carry out plastic design in LRFD, not ASD.
RE: Plastic Hinges
RE: Plastic Hinges
If so, where does it end; that is to say, is there a point at which the last theorectical plastic hinge is formed and the structure has absorbed enough energy through defromation that we can consider the event to be over before catastrophic collpase?
RE: Plastic Hinges
Think of the fixed-fixed beam example above. Once the two hinges form at the supports, you can consider the beam as simply supported. As soon as the third hinge forms, the structure is no longer statically determinate (like a simply-supported beam with a hinge in the middle)
In fact, thats half the challenge of plastic design, figuring out what collapse mechanism forms first. Some structures may have several different collapse mechanism.
RE: Plastic Hinges
RE: Plastic Hinges
Seismic detailing is intended to allow for the formation of plastic hinges (in order to dissipate energy), but its main purpose is to allow for ductility. You are not actually doing plastic design.
RE: Plastic Hinges
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?
RE: Plastic Hinges
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.
RE: Plastic Hinges
RE: Plastic Hinges
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.
RE: Plastic Hinges
And a fact is I have little experience in the design of structures.
Rafael Guerra.
RE: Plastic Hinges
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.
RE: Plastic Hinges
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?
RE: Plastic Hinges
frv,
O.K. One more thing I know now.
Rafael Guerra.
RE: Plastic Hinges
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...
RE: Plastic Hinges
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...
RE: Plastic Hinges
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...
RE: Plastic Hinges
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.
RE: Plastic Hinges
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
www.motagg.com
RE: Plastic Hinges
RE: Plastic Hinges
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...
RE: Plastic Hinges
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.
RE: Plastic Hinges
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://
h
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...
RE: Plastic Hinges
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!
RE: Plastic Hinges
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...
RE: Plastic Hinges
RE: Plastic Hinges
You just simply never hear about devastating earthquakes in New Zealand... but that may have more to do with how sparsely populated it is..
You always hear about earthquakes (significant ones, anyway) in Japan, Western US, Chile, Turkey, Mexico, Central America, Indonesia..
Of those countries I just mentioned, only Japan and the US have the research institutions and the capital to do significant research.. it's good to know about New Zealand, though..
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The city of Napier on the east coast of New Zealand has the largest concentrated collection of Art Deco buildings in the world because an earthquake in 1931 destroyed the city, and the whole business district was rebuilt in that style.
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I'm not sure if you've lumped me into that category. I assure you my interests extend beyond my immediate surroundings. Never having heard of a major earthquake in New Zealand is NOT the same as never having heard of New Zealand, their people or their culture.
This is not a matter of arrogance, ethnocentric or otherwise, nor is it a matter of "criminal" ignorance..
I'll bet you the majority of people on this forum, many of whom are very bright (at least judging form some very lucid and insightful posts), and many of whom practice in other English-speaking parts of the world, would find themselves surprised, as I was, the New Zealand is the place to go to practice seismic design... (anybody care to chime in?)
Again.. and for the last time.. I'm not disputing it.. I simply found myself thinking .. "hmm.. I did not know that.." when I read the post..
RE: Plastic Hinges
frv: Those lucid and insightful posts as often as not come from you; So I certainly wasn't thinking you were "a [typical] arrogant American". You might be surprised to find that I actually don't think the typical American is arrogant. I think the typical American comes from such a large, diverse and busy country that they simply don't see the forest for the trees that immediately surround them. Again, as you've pointed out regarding your knowledge of New Zealand, there is a difference.
Secondly, let's not let this degrade any further. I want to take responsibility for the degrading that has occurred. Meaculpa. However, please help me move forward and get back to the technical point au courant: Plastic hinging under Seismic loading.
I do have to say that it'll be quite a treat to point out to my own little Kiwi at home tonight that an Aussie came to the defence of "our" countries technical prowess.... DESPITE the fact that such defence was not necessary in the first place as it was all a big misunderstanding mostly of my making.
Anybody want to discuss their views of the finer points of bar development in a plastic hinge zone (such as never, never, never do it, ever?), or hooking into a column in a ductile seismic frame?
Respectfully to all the worthy commentators,
And in the hopes that Structuralnerd hasn't been scared off and might accept some of my literature recommendations,
YS
P.S. Since it's Friday for Hokie66 and I, have a great week-end all. And if you're ever in Christchurch there Hokie66, drop by the Connell Wagner offices and say hello. Beer's on me.
P.P.S. I wonder if I have violated the site policies enough in one day to get my hand slapped or post removed. Hopefully not, as there is a value to seeing deteriorating threads come right! *hint hint*
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Plastic Hinges
I agree. Truce! But just to clarify, I have some standing in being able to bag Americans (not you, frv) for ignorance of the wide world, because I am one. I just happen to have married an Aussie girl and have lived here for 26 years. Now, like most Aussies, I tend to complain about all manner of things that occur in the rest of the world, including how many Kiwis are coming to Oz.
That's a decent company you work for, ys. Small world indeed. At the risk of further offending the site police, I retired from CW a few years ago, but still get involved from time to time when the office is busy.
Since you are shouting, I might join you in a bottle of that plonk NZ is becoming known for. As well as earthquakes, that is.
RE: Plastic Hinges
The wine certainly is good; Moved here for the Engineering, stayed for the woman (singular) and wine. Plus the job's not half bad. We could actually use a hand at the moment. Saturday morning and I'm at my desk with a new job on the go. Can't complain though as I really do love the work.
Do drop in if you're in town. And wine it is.
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Plastic Hinges
RE: Plastic Hinges
I do feel confident that I can say that if you believe in Capacity Design, then New Zealand is number one. They invented it, they have pursued research focussing on it and the issues involved for around 40 years and they have implimented it widely throught the country.
If you're a base isolation fan, I think you've got to look at Europe or the states. For active anti-seismic measures (such as active damping or energy dissipation) I'm pretty sure Japan is your ticket.
I prefer and strongly believe in Capacity Design because of how straight forward it is, how robust a system it is, and how it's essentially just our regular Structural Engineering knowledge applied with a view to ensuring certain areas experience the yielding that is inevitable anyways. Capacity Design does not fail if an Earthquake's energy output (or input to the structure) is higher than anticipated; The structure will simply drift more, but still not collapse. It's far more robust than most other design measures for Earthquakes.
dgkhan: You say you're very keen on seismic issues, and I am not doubting that. I am very curious: Where do you practice and what theory is commonly applied there? What theory do you apply?
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Plastic Hinges
Those are all examples of New Zealanders dedicated to the understanding of and teaching earthquake engineering.
Regards,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Plastic Hinges
Would be keen to hear if you're sure he's a Kiwi Qshake!
In any case, I think there are at least a few people around who share my belief that NZ is a great place to learn about what to do when the earth shakes...
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...