Plastic hinge region of RC building

[Dylan0022]

Without running non linear analysis, is there anyway to evaluate where the plastic hinge is going to be under EQ? I have a run elastic analysis and can see that not only the core wall cracks but also some other walls crack as well. Should I consider these cracked walls as plastic hinge as well as a simplified method? Or it depends how much it cracked since cracking doesn’t necessary lead to plastic and if so how do I evaluate?

Anyone please advice?

 

[STrctPono]

You can have a good idea where the initial and any secondary hinging might occur from your elastic analysis but you won't know for sure unless you run a pushover. The standard method would be to run your elastic dynamic analysis and then reduce your response spectrum by a seismic response modification factor based on the expected ductility of the system. You then need to detail accordingly.

 

[HTURKAK]

The plastic hinge locations could be estimated , more over a requirement of the seismic codes ( sway mechanism requirement).
How did you see the core wall cracks and other walls crack ? will you please be more specific?

If you want to see the sequence of hinging, you shall run PUSH OVER analysis.

 

[Dylan0022]

Thank you StrctPono and HTURKAK
can you please elaborate on how to evaluate after this?

I run a non ductile analysis first and check the the stress under EQ combo. Understand that pushover can be performed but like I said, is there anyway to estimate which walls are in plastic hinge region without running non linear analysis based on the level of cracking? I believe just because this wall cracks under non ductile EQ doesnt mean it will go plastic. Am I wrong?

 

[STrctPono]

can you please elaborate on how to evaluate after this?

I recommend you talk to a senior level Engineer in your office to help guide you on running a response spectrum analysis.

Either way, the idea is to reduce your lateral demand on your structure based on the understanding that you will have an inelastic response from your structure's lateral force resisting system. You may have a reduced force demand but if you take advantage of a high R value, you will then need to detail accordingly to ensure ductile hinging and not a brittle failure. The inelastic straining of the individual elements releases energy from the system and you can account for this by scaling your response spectrum curve accordingly.

I believe just because this wall cracks under non ductile EQ doesn't mean it will go plastic. Am I wrong?

Correct. Cracking does not equate to plastic hinging. Inelastic straining of the reinforcing steel is required in order to hinge. You would need to run a moment curvature analysis using non-linear constitutive material models for the element to understand the load deformation behavior. Your columns and beams may crack. You are also supposed to reduce the effective stiffness of these elements based on their expected level of cracking. When you run a seismic analysis, your columns and beams should not be modeled with gross EI. A cracked (effective EI) should be used.

 

[Dylan0022]

Hi STrctPono Thanks for your reply.
I understand the concept of response and I actually had cracked the walls and run a ductile analysis. What I was not sure was how to evaluate the plastic hinge region after this.
As you mention about moment curvature, is the following process correct?
Step 1. Run non ductile analysis and check the cracking conditions.
Step 2. Crack all walls that have exceeded their tensile strength and reduce the seismic demand, re-run analysis based on ductile structure.
Step 3. Design walls to step 2, compare Mu to design Moment M* from step 1 (instead of step 2), if Mu is less than M* than it is assumed plastic hinge.

Step 4. Iterating the above to find the second and third yield of the structure

Please correct me if I am wrong. Thanks

 

[HTURKAK]

Dear Mr DYLAN,

Your approach is not reasonable and the outcomes may mislead . The elastic analysis could be acceptable for small earthquakes which the structure should remain elastic.

The reasoning behind the RSA and the use of behavior factor or Response modification factor (R) is that, during large EQ, the deformations of elastic and elasto plastic systems are similar.

The locations of plastic hinge regions ( for the structural wall systems , the plastic hinge length in general , is assumed to be half the length of the wall ) can be easily detected.

If this is a real project, you may post more details ( structural plan , seismicity etc) to get more useful responds.



I will suggest you to look the following doc and the link .

https://www.nehrp.gov/pdf/GCR 14-917-25_RecommendationsforRCWallBuildings_std.pdf

 

[Dylan0022]

Thanks for the doc. I will have a look.
Just a quick question before I go through these doc, without running non linear analysis like push over, how do you evaluate the plastic hinge region since my above process is wrong? Can you give ur steps if possible? It is a general question and I was testing not a real project in particular. Thank you.