IBC 3402.2 Renovation / Alteration

[McCalf]

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?

 

[Ron]

McCalf...unfortunately the code is full of ambiguities. To resolve them requires one of two things...a code interpretation, which you can get from ICC or professional risk, something in which your engineering judgment has to guide you.

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.

 

[McCalf]

Thanks for the response Ron. I probably need to do more research into the codes applicable in the early 1960's, but my general understanding is that drfiting snow was not a code consideration at the time. Therefore, the original joist design was sufficient to meet code at the time of construction.

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.

 

[nhstruct]

Typical when I look at an existing building I will accept the building as being fine if I'm under 5% overstress. I am willing to accept this for several reasons, the theory of design does not account for load distribution through the floor/roof diaphram, the there is a safety factor in determining the load and member strength. Also assuming your 600lb load does make the 3 members overstressed, you can reinforce those and leave the rest since you are not touching those joists. The other thing to remember is that the client does not know what the joists need if you tell them there fine they will believe you, if you tell them it needs minor reinforcing, they will believe you. If I inform a client that something needs to be done and they don't like it, I try and give them other options , from suggested they look at lighter units, or different locations.

 

[JAE]

I would agree that the older codes (1960's era) didn't have snow drifting in them.

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.

 

[msquared48]

Are you talking a 5% overstress IN ADDITION TO the 15% time duration factor allowed for snow, or 5% not considering this factor?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dcarr82775]

Are you including the weight of drifting snow on top of the mechanical unit? If you are you might be able to remove the snow from on top of the unit and get your forces down a bit.

I agree with JAE that you need to check per the current code, and I will often accept 5% over on an existing structure.

 

[McCalf]

@msquared48 - The overstress does not include any time-dependent factors. I was not aware that IBC allows a 15% increase on snow loads. Can you reference this section?

@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.

 

[JAE]

McCalf - is this a wood structure (wood joists) or steel?

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.