Seismic and Additions to Existing Buildings

[connect2]

So if you are connecting an addition to an existing building what is the conscensus on the use of the Response Modification Factor and the system Overstrength Factor? By that I mean if the existing, older building, has different factors than the new addition what Factors would you use for the building as a whole, existing and addition as one building? It is the intent to not seismically separate the two.
Thanks

 

[SteelPE]

Every time I do an addition I do my best to make them two separate structures. My view is that it is easier to design the structure as two separate structures and not spend the time investigating the existing structures.

The answers to your questions can be found in either IBC chapter 34 or the IEBC. This part of the code places limits on how much you can increase the "demand/capacity ratio" of members before you need to update the member/structure to comply with the existing building code.

 

[jvvse]

Which code are you using?

ASCE 7-10 section 12.2.3.3 says you have to use the least value of R in any given direction. There is an exception for small structures with flexible diaphragms that you can design each line of resistance with its own R factor, but the diaphragm always must use the lower R.

As far as the Overstrength, use whichever goes with the R factor you are using. Since the diaphragm is always using the lower R, us the Ω that goes with it.

 

[TDIengineer]

I think your question is a little off base. You need to first look at your administrative code and determine what they will allow in terms of added mass to your existing structural system without triggering a code upgrade of the entire structure. Often around 5% to 10%. No more than 5% of total change in original mass.. and not a single element increased by 10% i believe is what the California Code says (CBC).

Once that is triggered, your question about R value goes away as it would now need to meet current code and the R value listed, for the whole thing.

This is why often many additions are added with a separation, especially if the existing structure has a rigid diaphragm, since adding elements could change the entire lateral distribution.. not just at your area of work.