ETABS selected SMRF

[pkjr618]

Good day all,

I am in the midst of modelling a R.C. 4-storey residential bldg and initial analysis suggests that the bldg. is very stiff when all of the frames are used as lateral force resisting elements (SMRF class, Zone 4). Moreover, the detailing requirement that the ACI code imposes on the special moment resisting frames messes up the sizes of the frames. Specifically, requiring a larger section due to the required beam-column capacity ratio and joint shear stress.

Since the story drift of the structure is way below the value allowed by ACI (from initial analysis), I would like to assign the special moment frame only to some and not all the frames. How do I do this in ETABS?

Thanks and Regards,

 

[dadomago75]

Hi pkjr618,
first of all it is better to understand your situation to avoid giving you wrong advice.
Because I don't use the ACI 318 code for analysis, I found this document about what you called SMRF Link, because I never heard about "special" moment resisting frames in Europe, but I understood it refers to seismic provisions for MRF structures.
If your structure is just columns and beams and is located in a high seismic area, then it is right to call it SMRF, whereas if you have any kind of core system (lift shaft or stair core), then it is not a MRF, but it is a mix.
If this is the case, than you can decide to consider the resisting system as a core walls system or consider both walls and frame.
Depending on which one you choose, the ductility factor for your RSA is different.
Sorry if I am tedious, I don't want to lecture anyone but I don't know your experience, so I prefer to repeat even the basics to avoid incomprehension
In my opinion, because your structure is just 4 storeys height, you can just consider only the core system, in which case I would suggest to model all the columns as pinned both ends.
But if your structure is just a frame structure, then you are right, you might have to comply with the requirement of the code.
I said "might" becase as I said, I don't know much about the ACI for seismic design, but in the Eurocode is specified that if you are in the lowest seismic area (which was the old "non seismic" region) and if your structure is not taller than...., you don't have to comply with the geometric/mechanic ratios for seismic design; but you better check it out.
By the way, if yours is a mixed system, at page 2 (bottom left corner) of the pdf file I posted the link, it is specified that not more than 25% can be addressed to the frame system.
Anyway, if you want to use only few elements for your SMRF, you just have to select all the other columns and release the moments at the ends in both y,z direction (not x, the longitudinal axis!), but I don't think it is correct, because all the columns (if of the same section) have the same stiffness, and the actions will be shared based on the stiffness of the elements (all columns have the same stiffness so the same design actions) and also based on their distance from the stiffness centre (if the mass centre does not coincide with the stiffness centre you will have a torsional moment).
I hope I have been clear. If you have any other query, go for it.
Cheers mate

 

[pkjr618]

Hi dadomago75,

I've taken your moment release suggestion into consideration. The lateral system is all frames by the way. I will have a separate model considering only the selected frames taking the seismic load and other columns with moment releases. The beams not considered in the lateral system will serve as collector/transfer elements or just detail the concrete slab to serve as a rigid diaphragm.

Thanks.