IBC 3402.2 Renovation / Alteration
IBC 3402.2 Renovation / Alteration
(OP)
We have a difference of opinion in our office, and I am hoping to get clarification from some other colleagues. IBC 3403.2 permits an increase in gravity load on the existing structure, provided the stresses in the member do not increase by more than 5%. The language varies slightly between IBC 2006, IBC 2009, and IEBC. We are checking existing joists for the weight of a new small mechanical unit. The joist falls within a drifting snow zone that apparently was not considered when the building was designed in 1962.
1. If I ignore the snow drift and check the joist for original loads plus the weight of the unit, the stresses do not increase by more than 5%. At this point, is a reanalysis required?
2. If I include the snow drift, the stresses increase by more than 5%. We exceed the allowable load by approximately 4%. Do we need to reinforce this joist?
3. Hypothetically, if the joist is stressed to 98% of capacity based on existing loads, and I add a mechanical unit that increases the stress to 103% of capacity, is it acceptable? We are not increasing stress by more than 5%, but we have exceeded the allowable load
I think IBC needs address the ambiguity of section 3403.2. Your thoughts?
1. If I ignore the snow drift and check the joist for original loads plus the weight of the unit, the stresses do not increase by more than 5%. At this point, is a reanalysis required?
2. If I include the snow drift, the stresses increase by more than 5%. We exceed the allowable load by approximately 4%. Do we need to reinforce this joist?
3. Hypothetically, if the joist is stressed to 98% of capacity based on existing loads, and I add a mechanical unit that increases the stress to 103% of capacity, is it acceptable? We are not increasing stress by more than 5%, but we have exceeded the allowable load
I think IBC needs address the ambiguity of section 3403.2. Your thoughts?





RE: IBC 3402.2 Renovation / Alteration
Your structure is over 40 years old. By most accounts, the building has exceeded its expected useful life (from an economic standpoint..physically it might be just fine); however, that put you in a "damned if I do, damned if I don't position. If you take the non-conservative, original design assertion, then you save your client money (consider that the building has been performing well for its life thus far). If you take the conservative approach and consider current code, then you'll do one of two things...accept the slight overstress or strengthen the joists. Risk to you or cost to your client.
Considering that the original design was incorrect (assuming drift was required by the code at that time), then don't compound that issue by summarily accepting it now. Unfortunately it isn't a "no harm, no foul" situation. It could just be that luck has allowed the building to perform.
Take the second approach, even if you decide to accept the overstress.
RE: IBC 3402.2 Renovation / Alteration
The mechanical unit that we are adding only weights 600 lbs spread out over 3 joists (joists are 38' long). Although I feel the same way you do about taking the conservative approach, it is hard to justify reinforcing large portions of the joist because we are adding a small 200 lb concentrated load. Our client is going to think that we are crazy. It definitely puts us in a bit of a predicament, and since IBC leaves the interpretation of section 3403.2 to "engineering judgement" there are probably just as many engineers willing to accept a 4% overstress before demanding costly rehabilitation.
Thanks for sharing your opinion.
RE: IBC 3402.2 Renovation / Alteration
RE: IBC 3402.2 Renovation / Alteration
Snow loading from that era, at least in my neck of the woods, was HIGHER than it is today. This generally gives us some extra capacity assuming the owner didn't overlay the original roofing with new layers and add dead load.
When checking an existing structural member, you should check it against the CURRENT code - not the code in effect at the time of its design/construcion. You can use the older codes to help you get a handle on what the original designer was doing, but for a check today, use today's code.
I also accept about 5% overstress on bar joists and beams when it is an existing condition and a new applied load.
RE: IBC 3402.2 Renovation / Alteration
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: IBC 3402.2 Renovation / Alteration
I agree with JAE that you need to check per the current code, and I will often accept 5% over on an existing structure.
RE: IBC 3402.2 Renovation / Alteration
@dcarr82775 - The unit is located away from the drift. I always include the base snow load in addition to the unti because i suppose snow will still accummulate on top in most cases.
My question is more general. I know that several options exist that could solve my specific problem.
RE: IBC 3402.2 Renovation / Alteration
What msquared was referring to was the wood duration of load factor (1.15) where allowable wood stresses can be increased 15% for load combinations with snow.