Hello all,
Lately I have been working on a project involving retrofitting an existing structure where more weight would need to be added to the original system. Our retrofit puts us in the category of an Alteration 2 within the IEBC code. After investigation of this older structure it has been found that the structure was never designed for seismic loads, rather it was designed for wind loads. IEBC has a 10% trigger rule for lateral loads which states, "any existing lateral load-carrying structural element whose demand-capacity ratio with the alteration considered is no more than 10 percent greater than its demand-capacity ratio with the alteration ignored shall be permitted to remain unaltered." With that being stated, using my common sense engineering judgement I thought that this rule is considering that the structure/member was designed correctly originally. Meaning lets say that a member was designed originally so that the demand-capacity ratio is exactly 1.00 under original loads. Well then I feel comfortable taking the design to 1.1 interaction and eating ever so slightly into the safety factor for lateral loads. I feel comfortable doing this knowing that code updates have led us to more stringent requirements and it wouldn't be fair/economically viable to update/upgrade all buildings every time a new building code was adopted with more stringent guidelines...this was my understanding of why the IEBC was created in the first place.
Now here is the question/issue that has arisen from this project. After I analyzed this structure for current seismic loads, I have had interaction ratio's of 5.00 and higher at some braces due to the fact that this structure has a high mass at the top of the building and wind load obviously acts differently here. So my common sense engineering judgement/interpretation of what the code intends here is that these members need to be upgraded since we are 'touching' the structure and altering it. But here is where I have been apparently been proven wrong...since the new interaction ratio with the alteration considered is only lets say 5.05, then this structure is fine! We do not need to upgrade and bring it up to code! I have even received recent correspondence with the ICC on this and they too stated that you only need to compare the demand-capacity ratio's to each other before and after alteration...so apparently it doesn't matter what the interaction ratio is under original loads (could be 10000.000+) as long as the difference in the interaction ratios with the alteration considered is <10% for lateral loads!
Has anyone else here had experience with this portion of the code? Have you had other interpretations with this portions of the code that differ from what I have heard or would you go about it the way I originally interpreted the code? What do you guys think? It is just hard for me to swallow that the code says it is legal for something to be overstressed 1000% over as long as originally it was only overstressed 999%...this seemed to be a no brainer to me until I had other engineers arguing for this interpretation (though I bet these same engineers would not agree with using the 5% gravity trigger in the same way as this load is a 'real' load and seismic not so much...although show me where in IBC or ASCE that this is seperated in such a way
). Maybe my original interpretation was flawed and bias simply because my natural engineering judgement tells me to design members to not fail...crazy concept I know.
Lately I have been working on a project involving retrofitting an existing structure where more weight would need to be added to the original system. Our retrofit puts us in the category of an Alteration 2 within the IEBC code. After investigation of this older structure it has been found that the structure was never designed for seismic loads, rather it was designed for wind loads. IEBC has a 10% trigger rule for lateral loads which states, "any existing lateral load-carrying structural element whose demand-capacity ratio with the alteration considered is no more than 10 percent greater than its demand-capacity ratio with the alteration ignored shall be permitted to remain unaltered." With that being stated, using my common sense engineering judgement I thought that this rule is considering that the structure/member was designed correctly originally. Meaning lets say that a member was designed originally so that the demand-capacity ratio is exactly 1.00 under original loads. Well then I feel comfortable taking the design to 1.1 interaction and eating ever so slightly into the safety factor for lateral loads. I feel comfortable doing this knowing that code updates have led us to more stringent requirements and it wouldn't be fair/economically viable to update/upgrade all buildings every time a new building code was adopted with more stringent guidelines...this was my understanding of why the IEBC was created in the first place.
Now here is the question/issue that has arisen from this project. After I analyzed this structure for current seismic loads, I have had interaction ratio's of 5.00 and higher at some braces due to the fact that this structure has a high mass at the top of the building and wind load obviously acts differently here. So my common sense engineering judgement/interpretation of what the code intends here is that these members need to be upgraded since we are 'touching' the structure and altering it. But here is where I have been apparently been proven wrong...since the new interaction ratio with the alteration considered is only lets say 5.05, then this structure is fine! We do not need to upgrade and bring it up to code! I have even received recent correspondence with the ICC on this and they too stated that you only need to compare the demand-capacity ratio's to each other before and after alteration...so apparently it doesn't matter what the interaction ratio is under original loads (could be 10000.000+) as long as the difference in the interaction ratios with the alteration considered is <10% for lateral loads!
Has anyone else here had experience with this portion of the code? Have you had other interpretations with this portions of the code that differ from what I have heard or would you go about it the way I originally interpreted the code? What do you guys think? It is just hard for me to swallow that the code says it is legal for something to be overstressed 1000% over as long as originally it was only overstressed 999%...this seemed to be a no brainer to me until I had other engineers arguing for this interpretation (though I bet these same engineers would not agree with using the 5% gravity trigger in the same way as this load is a 'real' load and seismic not so much...although show me where in IBC or ASCE that this is seperated in such a way
). Maybe my original interpretation was flawed and bias simply because my natural engineering judgement tells me to design members to not fail...crazy concept I know.