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Re: Shear Wall Rho

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Nels,
 
 
The rho factor currently in UBC and IBC is for new building design. For existing building retrofit, you may not need to use the rho factor at all. Existing building retrofit or strengthening is not quite the same as designing a new building. For new building design, one has no choice but to comply with the Code either UBC or IBC because it is a new building design that one can always make it complying with the code. But for existing building strengthening or retrofit, it may not be so easy to comply with the new building design code entirely up to 100%. Especially for shear wall buildings, there are lots of talks about the rho factor. For certain cases, the retrofit of existing buildings may not need to comply with current code for new buildings at all. For instance, in GSREB Chapter 1 for URM retrofit, you only need to use 75% of the current code shear force to design your retrofit and I believe that rho factor is not applicable in the URM retrofit. Existing building retrofit is not to make an old building brand new  to completely meet the code set for new building design. New building design is for the standard of so called life safety level of design force and existing strengthening is for a limited life safety which is very close to a collapse prevention. Existing building retrofit or strengthening is to make it safe enough so that hopefully when EQ comes the building will not collapse to really kill people;  meaning for retrofitted building in comparison with new building design today, there will be more damage to the retrofitted building than to the new building. We saw that after Northridge EQ. It is acceptable because at least we saved lots of life even those retrofitted URM buildings were still sustained cracks or local failure but they didn't collapse to kill people. I believe this is the purpose of retrofit. No one can bet that this kind of retrofit is good enough for next big EQ; but at least this is what the professional engineering community's understanding of the acceptable risk that existing building retrofit supposed to achieve.  It is important to have as much building retrofitted as possible to save life than to ask for perfection and end up with that the retrofit becomes too difficult to be possible at all. For your case, I am not sure up to what degree of safety you need to strengthen your building to. I also knew there are certain building owners out there want to strengthen their building up to continuous operation level that will be covered in the performance code. For basic existing building retrofit in the IEBC and GSREB, I am not that worried about the rho factor because the configuration and the layout of existing building may not be possible to have the ideal world of the rho factor as required in the new building design. It is not clearly specified in the code. I believe this is the way it should be. I hope the building official in the area where your project is located can consider this notion here that existing building retrofit is not the same as new building design. Again there is no guaranty here for both new building design and retrofit as what will exactly happen when next EQ comes. It is all based on certain probability study of occurring in terms of next EQ's location, size and the soil of the project site. Best regards to you and to your project.
 
/02 08:12AM >>>
I am designing strengthening of an old multistory wood-framed building with several unreinforced masonry chimneys.  My system for seismic bracing of the building is to install new masonry walls aligned with each chimney in each direction so that the bracing system has enough rigidity to protect the chimneys against seismic loads.  The old building has flexible diaphragms so that each wall acts to brace its tributary area of building; no two walls are aligned.  The lengths of the masonry walls are necessarily as short as practical.  I've determined rho for each wall per equation 30-3, UBC Section 1630.1. 
 
In determining E [per equation 30-1] should a single value of rho, the highest, to be used for all of the walls in a direction, or should each wall be assigned its own rho value?  Since each wall acts independently of the others in a flexible-diaphragm system, it seems that each wall should be designed on the basis of its own value of rho, but the Code seems to imply use of a single value: the maximum.
 
Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net