IBC 2006 Force increase to existing building clarification

[jeffhed]

Our firm is looking at a small addition to an existing tilt up building. The original building is 12,676 sq. ft. with a reentrant corner. The addition will fill in the reentrant corner and is 1,224 sq. ft. No exsiting walls will be altered in any way so the only concern is the increase of load to the existing building. My question is this 3403.2 states that no force imposed on the existing structure may be more than a 5% increase. Then 3403.2.3 states that smic load increase must be less than the 10% allowed by 3403.2.3. Both of these items trigger a reanalysis of the structure elements that are experiencing additional loads. 3403.2.3 aslo has 3 exceptions for addition not stand alone that will not trigger a reanalysis. Seismic loads will not be increased by more than 10% but the gravity load for one wall will increase more than 5%. Do I still need to reanalyze the wall even though it falls under the exceptions for seismic? The reason I am asking is because the structure is old with no construction docs to be found. We are trying to find a way besides designing the addition as completely stand alone to avoid reanalyzing the walls due to absence of documents on the project. Any ideas?

 

[MiketheEngineer]

I would re-check for gravity loads....

I think this is an "AND" situation. Re-check gravity loads if over 5% AND re-check seismic if over 10%.

That's the way I see it... sorry

 

[civilperson]

Re-analysis is required for both criteria. Straight forward and probably satisfactory if the original design was satisfactory.

 

[jeffhed]

I agree that the reanalysis won't be a problem. The real problem is to get the steel layout we will have to pay a local geotech firm to scan the walls and see where the bars are. Also, the tilt up building is 15+ years old so I am wondering if the walls would stand up to current codes like reinforcement ratios etc. Does the wall have to conform to all current codes, or just have adequate capacity to resist the current code lateral and gravity loads?