Retrofit of Concrete Structure

[McGill10]

Adding braces to retrofit 5- story RCC structure in SDC-D.Planning to add braces in between existing concrete columns. Now adding only digonal steel member as braces and using exist concrete columns as vertical members and exist slab as beam members. Can this system be considered as STeel Special Concetrically braced frame (SCBF) per ASCE 7 and use the corresponding R and omega factors? I am doubting as it does not have steel column/beams as shown per AISC 341.
How you tackle this situation.
Thanks a lot in advance or any ideas and sharing your experiece.

 

[HTURKAK]

The proposed system should be considered as DUAL SYSTEM but not sure for ...(SCBF). If the existing system is RC Special Moment frame (SMF), the system should have dropped beams and you should not need to use exist slab as beam members.

I will suggest you to look ASCE Standart 41 (Seismic Evaluation and Retrofit of Existing Buildings ).

Pls post the structural plans with proposed vertical brace locations..

 

[KootK]

1) I think that you could certainly make the rational case for SCBF IF you can make the concrete columns, beams, and connections satisfy all of the same requirements that we'd normally be asking of steel columns and connections. This is a pretty big IF and I suspect that you'd run into trouble somewhere along the way.

2) I agree with the recommendation to investigate the approaches outlined in ASCE 41.

3) If braces are okay, would a concrete shear wall panel also be okay? That might simplify things given that:

a) This is something that's been done a lot in the past so there is precedent.

b) It obviates the need to try to apply single material requirements to a mixed material system. That always seems to wind up a bit clumsy.

 

[Deker]

I believe that the building can be classified as a braced frame building with some judgement. The behavior of your fuse element will be critical, and it is imperative that you are able to predict its behavior accurately. On that front, I would do the following:

1. Rather than SCBF, use BRBF for its higher response factor and better capacity predictability.
2. Use true pin connections at the ends of the BRBF to avoid unintentional moments being transferred into the concrete frame.
3. Design the brace connections, concrete slab, beams, and columns to remain elastic for capacity-based design loads.
4. Design the beams and columns for deformation compatibility demands at max drift. These loads are additive to the loads from #3. Again, the concrete members should remain elastic.

There is precedent for this system in some high seismic areas, albeit for new construction. If you have thorough documentation of your existing building you may be able to make it work.

John Wayne Airport, Santa Ana, CA

PCI Article

 

[McGill10]

That was a good recommendation to follow ASCE 41, thank you to both of you. Looking into ASCE 41, it seems as the right document for seimic retrofiting of an exsisting building and question about R values is not applicable (or say R =1 system).
As R =1, now foundation sizes will be larger compared to high ductile syemtem with R>3, however, it allows to use soil bearing pressure = 3*qallow.
In this system, we are planning to use Chevron braces ( for bay 10ft ht x 18 ft lengh). Based on my limited understanding of ASCE 41, I assumpe detailing rules of SCBF don't apply beign design without R factor. Any thoughts about this?
Thanks a lot.