A Concrete OMRF in a High Seismic Risk Area? 1

[aiden]

A colleague of mine designed a concrete OMRF in a High Seismic Risk area. As ACI code requirement, it should have been a SMRF (this requirement was overlooked, his previous designs were all in very low seismic areas).

The designed OMRF structure obviously resulted to a massive structure since the seismic load is more than two times if it were a SMRF,... the story drifts are also very small. However, as an OMRF structure, the reinforcing details does not conform to the detailing and minimum requirements set forth in ACI Chapter 21.

Since it can be demonstrated in the analysis that joint rotations and stress reversals during earthquake are not high enough due to the stiffness of the structure, what would be the consequences with such a structure should an earthquake occur?

Can anyone share their opinion on this?

 

[youngstructural]

Detailing for plastic hinges is only required in the New Zealand code if you are reducing your stresses as a result of soil factors, or the structural system factor (Sp) or, of course, if you are designing a ductile frame.

There is no reason that an elastically designed frame would behave badly in the design earthquake. The issue is that we are very bad at predicting the effects of ACTUAL earthquakes, as well as the magnitude, frequency, etc, etc, of the real design period earthquake.

The consequence is that it will perform better than an SMRF until the loads on the frame exceed the frame capacity. At that point plastic hinges will form in regions which cannot sustain plastic hinges due to insufficient reinforcement and catastrophic failure would become a possibility. I would also not vouch for the safety of your beam column joints in anything but a moderate earthquake. That said, I do not know ACI 318 well, but do have a good portion of seismic experience in my five years of experience.

Hope that helps,
Looking forward to other peoples' answers,
Cheers,

YS

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

 

[youngstructural]

*bump*

I know it's rude, but I am curious to see if anyone else has anything to say on this one...

Cheers,

YS

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

 

[ARLORD]

ACI does require ductile frame design where the hinge forms in the beams and not at the column, for hign seismic areas. In ACI, SMF design depends on the ductility of the structure, meaning deformation and redistribution.

I agree with young structural, that the OMF should be fine elastically up to the point where the actual loads equal the design loads. After that hopefully, redistrubution, up to the point where hinges form, will help to resist the earthquake loads. The loads are short lived.

However, I'm no seismic expert. I have just reviewed ACI Chapter 21 in prep for the STR II Exam.

 

[aiden]

hi guys,

got busy the last few months. well, my colleague asked me for my opinion and I advised him to do a quick revision of reinforcement details to conform with ACI 318 chap 21 code and project requirements. he dont have enough time to perform a quick redesign of the structure since it was the start of construction stage.

here is my personal evaluation of the problem and the resulting structure.

1. the structure should behave inelastically at approximately twice the code required earthquake force
2. code recommended strength modification factor for SMRF is deemed not appropriate for the type of structure (industrial) since excessive joint rotations and translations would disrupt plant activities from damaged machineries and equipment interconnection. therefore redesign using SMRF seismic factors is not recommended.
3. for comparison, latest IBC code strength modification factor for OMRF requires a massive ten times the earthquake force required for SMRF.
4. ACI 21 requires that joints for structures in high seismic risk areas (IBC SDC=D or UBC Zone 4) should be detailed with seismic detailing requirements.

 

[prsconsultant]

OMRF is not an acceptable system in SDC "D" or higher. The actual forces on a structure during a real seismic event are much higher than the code specified values. The code forces are minimum required to maintain life safety. ACI requires special detailing to enable significant energy absorption during a seismic event.

 

[msquared48]

I believe, in either ASCE7 or the IBC that there is an exception to the OMRF restriction if the structure is only one story. I will have to look that up, but I ran across that last summer and used it.

Mike McCann
MMC Engineering