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.

 

[csd72]

Forget seismic, look up plastic design for an explanation of this. Hinge is a bit of a misdenomer as it does offer resistance to rotation as opposed to a real hinge that doesnt.

 

[graybeach]

For steel, the plastic moment is generallyequal to Z*Fy. This means that all of the material in the section is at yield. Theoretically, this is the maximum moment that can be transmitted to the connecting structural members. There can be more rotation, but no more moment. Plastic hinges can form from either static or dynamic loading. The significance for earthquake loading is that we can select where the plastic hinge will form by intentionally make certain members weaker. For example, one would want a plastic hinge to form, say, in a column rather than in the piles under the ground because the column would be easier to repair. Earthquakes can impose large displacements on structures. If you try to keep everything elastic, you get into an infinite loop because the forces just get higher. Plastic hinges act like fuses. By introducing them into a design, you limit the forces.

 

[structuralnerd]

I have not done a whole lot of seismic design, so please excuse my ignorant questions. Which beams/columns should you strategically make elastic and which should you make plastic?

 

[graybeach]

This has to be decided on a case by case basis. I worked on a bridge project where prestressed concrete box girders were framed into multicolumn reinforced concrete piers. We selected the columns because it would be easier to shore them up and get traffic moving again in the event of a strong earthquake. We calculated the plastic moment capacity of the columns, multiplied by a factor (1.1 if I remember correctly) and designed the girders for that moment.

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.

 

[frv]

graybeach-

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.

 

[graybeach]

That rule makes a lot of sense for building structures and is a better answer to the OP's question.

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.

 

[GalileoG]

Thanks all for the responses, but I'm more confused than ever!

How exactly do you "locate" / choose where the plastic hinge occurs? Thanks.

 

[frv]

It develops at the point of highest moment on the member in question;

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.

 

[csd72]

And you also have to make sure that no other section fails by buckling before this plastic moment is reached.

 

[NS4U]

plastic hinges occur in indeterminate structures- for example take the case of a steel fixed-fixed beam which is 2 degrees indeterminate… the maximum moment occurs at the supports

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

 

[frv]

Yes.

In fact, it is a requirement for the section to be compact in order to do plastic design.

 

[mexicali8]

It is something like:
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.

 

[blake989]

"how exactly do you locate the plastic hinge location"

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.

 

[frv]

mexicali-

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.

 

[frv]

way too many quotes in my post

 

[Stillerz]

Can structures utilizing the plastic hinge approach be modeled in applicaitons like STAAD and RISA?
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?

 

[frv]

Yes-

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.

 

[structuralnerd]

Also, when is it required to use plastic design? Which seismic design categories?

 

[frv]

I think people are getting plastic design and seismic detailing confused.

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.