CEBC / IEBC Relocated Structures Seismic Requirements (Ch14)

[ByInspection]

Hello -

We are working with a client who wants to move a canopy structure about 100 yds from its current location. There will be zero changes to the structure and no change in seismic or wind loading conditions (SDC D). There is some discussion right now with the AHJ about evaluating/retrofitting the structure to current standards. We understand that the foundation and anchorage need to be designed to the current code, but this discussion is about the structure itself and whether it needs to be evaluated against the current code.

I've read CEBC 1402.3 & 1402.4 as well as the commentary in IEBC for these sections. We would be taking advantage of the 10% exception rule, "Structural elements whose stress is not increased by more than 10 percent". This language is not clear on whether this 10% is the difference between the old site and new site design parameters, OR if the 10% difference is between as-built design parameters and current design parameters. Based on the language in section 503 of the CEBC, we feel this 10% has to do with any change to the structure that would result in a change in D/C ratio of 10% (like cutting a hole in a shear wall or something).

Lastly, the commentary in the IEBC: "Exception 2 allows additional seismic loads that do not increase stresses in affected structural elements by more than 10 percent. Allowing overstresses of up to 10 percent in existing structural members has been a long-standing rule of thumb used by structural engineers."

What does "additional seismic loads" mean? Additional loads at the new site, or additional loads in the current code (compared with the old code).

Thanks for your help on this!

 

[steveray]

Welcome to the forum!...I don't do a lot of seismic so you may want to wait for some Cali people....But there does not seem to be any stress increases by moving it 100 yards, so I say game on! Stresses are based on location to location, not old vs. new....

 

[classicT]

I have interpreted the 10% increase allowance to be taken from existing structural design to proposed structural design. Basically, if exposure or some other variable changes such that loads are increased, the maximum allowable increase without fully complying with current code is 10%.

Additional seismic loads would likely be from installation of new systems or equipment, particularly those of substantial weight at greater installed heights that can increase the seismic stresses acting on the building.

 

[ByInspection]

Thanks for the welcome!

Our interpretation was what you stated classicT. Unfortunately, we just got an email from the AHJ that they want us to compare seismic and wind loads against the original design and check for 10% increase. I may reach out to ICC to get a ruling, but that requires a membership.

 

[steveray]

I would suggest purchasing ICC Commentaries:

Exception 2 allows additional seismic loads that do not increase stresses in affected structural elements by more than 10 percent. Allowing overstresses of up to 10 percent in existing structural members has been a long-standing rule of thumb used by structural engineers.

 

[ByInspection]

Yes, I saw this explanation in the IEBC commentary, but it still isn't clear whether the 'additional seismic load' is from the existing location to the new location, or as-built design code to the current design code, or from changes to the structure. This is where we are having a disagreement with the AHJ.

 

[steveray]

I am not an engineer, but I would say the "increase stresses" are "installed" stresses...How old is the structure? It has been "field tested" for that many years....

 

[ByInspection]

It was built in 1991. And I agree... based on the other language in the code, 'increase stresses' typically refers to changes or alterations to the structure that would cause an increase in load or a reduction in capacity... e.g. increased stress.