IEBC 5% - Increased Pg

[rharting]

We have a project where we are adding new RTU's and replacing the existing roof with increased insulation. During our analysis of the roof for the new RTU's, it was determined that the ground snow load increased from 40 PSF to 50PSF in the mid 2000's (we are in a site specific location). The original building was built in the early 1970's and based on the sizing of the roof members, the flat roof snow load was lower than 28 psf.

Per the IEBC, analysis is only required where the "alterations cause an increase in design dead, live or snow load, including snow drift effects, of more than 5% shall be replaced or altered as needed to carry the gravity loads required by the International Building Code for new structures." If the increase in insulation adds less than 5% more dead load, can it be justified that the structure not impacted by the RTU does not need to be reinforced to resist the increased snow load due to the changing code? In other words, can the increase in snow load be ignored since the increased loading is not caused by the alterations, but by a change in the code.

 

[driftLimiter]

Probably would be up to the building official. Sensibly I would agree with

In other words, can the increase in snow load be ignored since the increased loading is not caused by the alterations, but by a change in the code.

But sensibility is not always the basis of these decisions "D

 

[straub46]

I agree with your assessment, as long as the roof works for the original 40 psf ground snow. Also, any new members or those with an increased demand/capacity ratio of more than 5%, would need designed for the 50 psf ground snow. But I also agree with driftLimiter's comment as well about it being up to the building official. There's a reason the site specific snow load went up; could be due to climate data or because there were some snow related roof collapses.

Go Bucks!

 

[phamENG]

I disagree with drift and straub. The code specifically says "increase in design dead, live or snow load, including snow drift effects...required by the International Building Code for new structures. So you have to look at it with current loading, do the analysis, and see if you have an increase of more than 5%. It's not a question of analysis, it's a question of reinforcement. If the stress in the supporting members is not increased by more than 5%, then you don't need to reinforce it. So if snow load resistance requirements have increased, you have to consider the new ones in your 5% check.

 

[driftLimiter]

The provision says ....for which an alteration causes an increase in design dead, live or snow load, including drift, of more than 5 percent shall be replaced or altered as needed to carry the gravity loads required by the IBC for new structures

And alteration is defined as Any Construction or renovation to an existing structure other than a repair or addition.

So by this logic, if the building official changes the snow load requirements, and then a building owner does ANY CONSTRUCTION to an existing building then the entire roof system would need to be repaired/replaced?

I do not believe this is the intent of the code. Yes you need to reinforce members that are impacted by the RTU alteration, and yes I would design them with the new snow loads. But OP specifically asked

can it be justified that the structure not impacted by the RTU does not need to be reinforced to resist the increased snow load due to the changing code?

My take is, Yes it can, adding an RTU unit in 1 location does not justify reinforcing the entire roof. Only the members effected by the RTU unit itself, or snowdrifts that might accumulate because of the RTU.

Now please don't mistake my argument here. If there is good reason to believe that the roof is unsafe then it is our duty to report this to the owner and building official, and recommend a sound solution. I am simply reasoning with the language of the code.

I do agree with PhamENG that the 5% increase should be evaluated using the current snow load for both the condition before and after alteration. This way it isn't a question of whether the element is meeting the current code, but rather an analysis of how the alteration changes the loads in a given element. However if you do it this way, it is irrelevant whether you use 40 or 50 psf.

 

[straub46]

The way driftLimiter just explained it is what I believe the code intends and how I have always treated these situations as well. If the existing member has load effect from the RTU/drift around it, use current loadings (wind included), and see if you need to reinforce. If it doesn't have any load effect from the RTU, then those members don't need reviewed. New members are designed with current code loadings.

Go Bucks!

 

[rharting]

Thank you all for your responses.

The AHJ agreed that we do not need to evaluate the entire roof for the new 50 PSF ground snow load.

In all honesty, i believe we may be removing a nominal load from the roof as we believe it was designed to be ballasted (we cannot really confirm that). I had a really hard time recommending to the client to reinforce the joists in the roof since we are likely reducing the dead load on the roof from 10 years ago.

Trust me... we will be notifying the client and being up front and honest about what we are seeing. They will be informed of the issue and be aware of the risks.

 

[MotorCity]

The key phrase is "alteration causes". An alteration did not increase the snow load from 40 psf to 50 psf.
So, according to the code, as long as the added insulation (or any other construction, does not increase the load by more than 5%, you do not need to consider the additional load due to the alterations, including the larger snow load. If that was not the case, every building within the vicinity of the increased snow load would be required to upgrade even if it was not currently under construction.

Be care though, because if you are adding RTU's, you are required to consider snow drift around the RTU unless it is less than some length (15' I think). So in that case, your alterations ARE increasing the snow load...and if its more than 5%...you know the drill.

That said, if it were me, I'd try like heck to find a way to reinforce the roof for the larger snow load. Thats a 25% increase in snow load, not a small change. And, presumably the increased snow load was based on records indicating such a snowfall is possible in that location.