Repair of Plastic Hinges 1

[releky]

For concrete structures that form plastic hinges (in other words the flexural bars have yielded at the support), how are they commonly repaired?

Just curious as a construction management engineer.

 

[dik]

For a plastic hinge to have formed, the concrete must have encountered a fair degree of rotation and the concrete is likely badly cracked or even crushed. It would be a matter of removing any loose material and epoxy grouting and/or epoxy injection. First 'kick at the cat'...

Dik

 

[releky]

But the longitudinal bars have already yield in plastic hinges, so the bars have to be replaced... isn't the beam just demolished and place new one again?

Is there any actual picture of plastic hinges formed in seismic regions that has happened before?

 

[CELinOttawa]

The fact is that each structure damaged by earthquake must be individually evaluated and it is not often possible to repair plastic hinging zones which have been pushed to a great degree of rotation.

Dik is correct, of course, the cracked concre the needs to be repaired, however there is also the question of how much elongation the principal longitudinal steel has been subjected to... In areas of high seismicity where plastic hinge design is common, reinforcement is often made from specialty microalloyed steels. This is certainly the case for both New Zealand and Japan... Possibly California as well, but I am not familiar with their standards and construction practices. Bear in mind that to be a competent structure, plastic hinges must be able to competently undergo the NEXT EQ's rotational demands... Following and long and strong shake it is possible that the main steel in a hinge will no longer fit back into the hinge. Such structures would likely become borderline for repair, with an economic evaluation of repair versus demo and replace being difficult to do with any great accuracy, but essential.

I know of no one formal list of scales of intervention in such repairs, through both the NZSEE and FEMA (among others) have good resources. Most documents focus on immediate evaluation for safety, not repair.

Generally, in order of escalating interventions:

-full and detailed visual review. Epoxy resin injection of cracks.

-NDT, some chipping of concrete and epoxy resin injection of cracks.

-NDT, significant chipping (often requiring propping of sections of floors/beams) and repouring hinge section with specialised purpose batched concretes.

-Full gambit of testing. Replacement stirrups and potentially some new main steel (though thus is very difficult and cost prohibitive).

-Full gambit of testing. Significant and specialised propping. Hinges, potentially whole beams, removed and replaced. Costs are likely to run close to or above cost of a replacement structure, but such projects are undertaken in fear of the unknown time and cost for full demo and full design, bid, build of new structure.

-Not economically feasible to repair.

 

[SteelPE]

I'm sure you can find pictures of plastic hinges that form in a lab.

I am no where near the best engineer in the world and I don't work in a high seismic area however, this is a question that I have also pondered myself.

I remember attending a seminar once where the lecturer basically said that the plastic hinges form in the building to dissipate the energy of the seismic event. If successful they will prevent the building from collapsing. He went on to say that the intent of the code is life safety and not preserving the building for use after the event. While there is a possibility of repairing the joints, cost of the repairs may exceed the replacement value of the building and the entire building my need to be demolished and reconstructed.

This was from an AISC seminar but I imagine the ACI has a similar opinion.

 

[dik]

That's the neat thing about plastic hinges... once they have formed when the load is taken off, there is a residual stress... but, once the load is re-applied, the beam behaves elastically up to the load condition that caused the plastic hinge to form... it doesn't matter that the steel has already yielded unless the deflection is an issue.

Dik

 

[hokie66]

dik,
That might be true to an extent in steel structures, but not in reinforced concrete.

 

[releky]

You mean there is no actual plastic hinges that formed during real seismic activity anywhere in the world hence no pictures have been taken?

CELinOttawa, etc.. how does repair of plastic hinges compared to repair of diagonal shear failure?

Diagonal shear failure means the stirrups have already yielded and cracks width have opened up with loss of aggregate interlock. This is supposed to be a brittle failure but if the longitudinal bars and stirrups are adequate for live and dead load but diagonal shear failure formed during seismic activity.. could there be enough shear friction and containment to make the beam not fall?

Since you have to re place the entire beams when plastic hinges form. Likewise in diagonal shear failure (without plastic hinges forming), you also need to re place the entire beams. Are there actual pictures of diagonal shear failure where the crack widths have opened up reaching the compression zone and stirrups yield? Or are such pictures as nonexistent as plastic hinges? For knowledge, pictures of these must be shared.

 

[PicoStruc]

Example of seismic-cyclic plastic hinge :

http://www.fhwa.dot.gov/hfl/pubs/hif13037/images/image019.jpg

Of course, plastic hinge means yielding
But rebar yielding do not necessarily means plastic hinge !

To resist static load, rebar yield, This is a normal behiavor of the resistance mecanism in reinforced concrete and do not necessarily require repairs if within normal range of yielding or curvature demand. !

Plastic hinge formation in seismic «rare» event is used to dissipate energy and let the people get out of the building. In that kind of event, repair is required.

 

[dik]

Thanks Hokie, I agree to a large extent. With my first post, I was thinking of severe damage to the concrete.

In the event of minor damage (contrary to my first comment) the structure will maintain the residual stresses will behave 'elastically' if subjected to a similar loading.

A hinge, however, can form with minor damage to the concrete, in particular if the section is doubly reinforced (not uncommon) and the concrete section is basically undamaged. Based on my first reply, I can easily see the confusion caused.

Releky:
If a hinge has formed in an area of high shear (not uncommon at supports due to the high moment, but not necessary) and the damage is such to compromise shear capacity, then with minor damage, it is possible to repair the structure and not replace it.

Picostruc:
Rebar yielding generally occurs at the formation of a plastic hinge. The rotation of the beam continues (maybe briefly) with little change in moment capacity... my understanding of a plastic hinge condition.

Dik