Seismic IEBC - UBC to IBC Differences & Importance Factor

[s-anthem]

I have a building that was designed under the 1991 UBC as an Essential Facility (Hospital Parking Garage), so it has an Importance Factor of 1.25 (Note: they did not go up to 1.5 importance factor in UBC 1991). However, the garage is no longer servicing a hospital and they are making updates to it putting it now under Risk Category II Building with an Important Factor of 1.0 for these updates.

I wanted to compare the buildings original seismic story shear/forces and base shear to the new buildings story shear/forces and base shear. I was wondering if my calculations for the existing building should still have an importance factor of 1.25 or because the UBC is such a different code from the modern day IBC that it would be too conservative to get my existing loads with an additional 1.25 factor on them?

This will greatly affect my IEBC checks for the 10% lateral rules so I wanted to make sure that I am comparing apples to apples.

Thank you!

 

[andrews2]

The philosophy of me and others in my office when trying to make use of the 10% increase exception has been to use the governing code of today for both the before and after cases. This means there is no need to compute the 1991 seismic loads. You would run the lateral analysis of the building as it is using the current governing code with Ie = 1.0, then run it again with the additions they want to make. If the controlling demand/capacity ratio for every lateral member has not increased by 10%, no retrofit is required.

The point of the exception is to prevent you from having to satisfy newer codes because of very small additions, so your comparison needs to take into account the demands that are added due to the addition only and not the demand added due to the change in codes or occupancy.

As you say, the comparison needs to be apples to apples, so in my opinion you should have the same importance factor and calculate loads using the same code.