Renovation to existing structure, structural evaluation needed per IEBC?

[SHOHOLA_GENE]

As a structural engineer, I have always taken it that an existing structure is permitted to continue to remain in service with the same loadings as long as there are no glaring issues with the structure (i.e. the thing is so undersized it is going to collapse tomorrow).

Now, I know that if I am increasing the load in any member by more than 5%, i need to re-evaluate the member to be safe with the new loadings (IEBC 807.4), that is not the question.

Let's say I have recently been pressed on a job for which a building is undergoing extensive floor plan changes, which qualifies as a "Level 2 Alteration"" within the IEBC (I am using 2015 version but it is very similar in previous versions as well, just the chapters are off by one number). I am repairing some very rotted portions of the floor structure, and my repairs are sized per the modern code, with LVL beams. But directly adjacent to these repairs are perfectly good (non-rotten) pine floor beams, which have stood for 40+ years, and I know are undersized not only per the current code, but also if you put any realistic floor loading on the beams. It was my plan to simply leave these in place, not ask any questions. BUUUUUT.....

IEBC 807.3 states (with commentary):
"the minimum design loads on existing elements of a structure that do not support additional loads as a result of an alteration shall be the applicable loads at the time the building was built, COMMENTARY Unless an alteration adds load to an existing structural member, there is no need to re-evaluate it against current code-loading criteria. this is in contrast to the treatment of existing members that have increased loading in accordance with section 807.4. THE APPLICABLE LOADS OF THE CODE AT THE TIME THE BUILDING WAS BUILT APPLY."

So, my question is: when opening up an existing structure to renovate, does this section of the code require you to evaluate the structure against the "code at the time of construction"? Many times that Code is unknown, and in this case I know that these beams will not meet ANY code, i have looked at the bea, on paper and it does not even hold a 10 PSF dead load, before i even put any live load on it. and they've been standing for 40+ years. the jpys of being a structural engineer.

Thanks! Gene

 

[kipfoot]

Regardless of the code interpretation, why leave grossly inadequate floor members unreinforced? Presumably, the owner has the intention of using this space and with the renovation underway this would be the appropriate time to make the floor system adequate.

For me, it becomes a more difficult call starting at 110% or 115% stress ratio for wood, but it sounds like your case is a simple life safety issue.

 

[JAE]

I'd agree with kipfoot. You have a case here where you KNOW that an existing floor member cannot support the intended loads. Who gives a rip if the engineer, or architect, 40 years ago designed a very weak member.

You need to protect the safety and welfare of the public...also you should look out for the interests of your client as if they were your own interests.

It may be that the wood values used 40 years ago were significantly higher than what you are using now...but it's difficult to pin that down sometimes.

And the code is a minimum. You can always be more conservative over and above what it mandates.

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