IEBC Allowable 5% Gravity Increase on Total Load or Load Type? 2

[waytsh]

My understanding based on previous editions of the IEBC is that the 5% increase was based on the total gravity load. The wording in the more current versions, however, could be read as 5% of the load type, i.e. dead, live, or snow.

“Any existing gravity load-carrying structural element for which an alteration causes an increase in design dead, live or snow load, including snow drift effects, of more than 5 percent shall be replaced or altered…”

How do you guys read this?

 

[Nick6781]

Note that it's the effect of the load. Think in axial, shear, moment, and overturning, etc.

 

[lexpatrie]

I've always viewed the "origin" as being intended for a lightweight roof overlay. That would lend to the "solo" 5% increase that would net, in some load combinations, 15%. I would NOT read it that way without signoff from the building official and disclosure on the drawings/letter that that's what is being done. (a la Florida 'violating' the code or standards).

Were it my project, logically, the 5% overstress would apply to a load case or a load combination under the appropriate code, and I suspect there've been a few code change proposals (that I also presume failed) that would have clarified that.

Further, the 'overlay' in my mind, would typically be shingles, not all that typical with the "current" vogue for flat roofs. This would technically be a dead load, but unless there's some kind of offsetting effect, a change of occupancy (i.e. live load increase), adding weight (like solar), and changing insulation (higher snow load), these all "eat" from the same trough of allowable 5% increase.

I'd also add something about roof slope. We've seen enough roof collapses in the media, better to try to avoid them on the design/repair side, if possible.

Mike Mike - code change proposals are "free". No need to go the drastic step of joining the organization... I've gotten one approved. That or just call that Reid Middleton guy who writes a hundred code change proposals a week, he can squeeze yours in, I'm sure.

EZ building - I've referenced that provision in several damage reports calling for engineering to satisfy those requirements. One was a residential, the other was a poured gypsum roof from 1970. It shows up in Hurricane damage projects with structural and roof damage, there's a dollar limit in the FBC/FEBC for residential and a precedence lists, and I often see public adjuster estimates that include renailing of the diaphragm (for uplift), as a precautionary measure. T.L. Smith discussed that some years ago. And What's SID, anyway?

 

[RWW0002]

I've always viewed the "origin" as being intended for a lightweight roof overlay. That would lend to the "solo" 5% increase that would net, in some load combinations, 15%.

I am not following the math here. Can you give me an example of how 5% increases in loads can lead to a load combination of anything over 5%??

My opinion is that the letter of the "new" wording is more restrictive and more conservative than previously. It seems that you are implying that it is less conservative than previous codes from your statement.

I agree with you though that the 5% has always, in my mind, applied to the total gravity load effect in the past.

 

[lexpatrie]

5% increase in dead load (hypothetically insulation and a 1 psf EPDM, say)
5% increase in snow load (due to R value crossing from Ct = 1.0 to Ct = 1.1, say)

D+S load case 10% increase?

Or a snow or rain load increase due to the revision in the definition of susceptible bay?

You're perhaps thinking of the D + 0.75 (W+S or W+Lr) cases? Or did the load combinations change while I wasn't paying attention again.

 

[Mike Mike]

Lex, in your case D+S would increase 5%, not 10%
1.05D + 1.05S = 1.05(D + S)

Thanks for the tips, I am way too busy with work to get into the committees right now, but I'm writing all this down on my to do list for the next housing bubble / virus leak. I assume all Reid Middleton's stuff goes thru a committee of experienced structural engineers, right? do you know if those committee positions are paid? do the majority of changes come from outside ICC staff?

 

[lexpatrie]

Uh, Reid Middleton is a company but the guy who submits a ton of code change proposals (or did) was like one guy "representing self" as I recall, and I've forgotten the name, it got a write error and has Phil Brazil in that spot now in my storage.

Ok, maybe I got that partially correct, except the proposals (of late) are for WABO.

Redirect Notice

I meant a 5% increase of dead load, and a 5% increase of snow load, 1.05D/D + 1.05S/S = 1.05 + 1.05 = 1.10.

One code change proposal isn't too bad, there's a lot of overhead in setting up an account and formatting things to submit them, and it's not intended for "gee this provision is unclear please clarify this" kind of commentary. What you do in that case is provide a language change and then all the more involved people can step in and public comment on a "better" version and then they can all fight it out. You don't have to be all that involved and the proposals come in three year spikes anyway. They aren't doing the 18 month revisions anymore. If you want to delay it, I'd write down some notes before sticking it at the bottom of the "to do" pile so it isn't so "what the heck was I thinking" when you pick it up in two or three years.

Influencing I-Codes.... Lorenz, Interface Magazine, June 2021.

As far as the "latest" anything on the I-codes, it's surprisingly hard to find, but the votes on the 2026 CAH from say, March, is here. (Almost useless without the proposals themselves as they just list D for denied, AS for As submitted, etc. There will be another hearing in October....

This might be more useful, but still lacks the proposal text. But it is almost transparent about what was done and why. Feels pretty disorienting)

2025 Group B ROCAH to CAH #1 Now Available​

I think this is the actual proposals (and I also think it's for the right year...).

https://www.iccsafe.org/wp-content/uploads/2025-Group-B-IBC-Structural.pdf

S172 is my personal favorite this time around.

S157 gets the dumbo award for being the most dangerously stupid idea. Went down, HAPPILY, in FLAMES 14-0 disapproving.

 

[waytsh]

Thanks for all the input, and the references to the commentary, which I don't currently have. It seems like the commentary is still written using the language from older versions of the code. Did they simply forget to update it?

Another thing I find frustrating is the inconsistency within Section 806.2, where the first half of the paragraph discusses loading, and the second half shifts to capacity. Like others have mentioned, why not just reference capacity throughout? For example, if I’m reviewing a building that was originally at 80% capacity and the new load takes it to 90%, the actual increase in capacity utilization is 10%. But if the added load represents a greater than 5% increase in the load itself, we’re forced to comply with the new code? That feels unnecessarily rigid in situations where we know the capacity.


Mike Mike, I fully support your involvement in the code process. Your input could bring some much-needed clarity.

 

[phamENG]

we’re forced to comply with the new code?

If you know the capacity, and the new demand is under that capacity, then you're good.

The 5% (whatever that 5% actually refers to - clearly it's, well....unclear) is a threshold for doing anything to it. If you don't exceed it, then you're good. If you do exceed it, then you need to go deeper. First stop is determining the capacity of the system as is and comparing it to the new load. If it works, you don't have to do anything. If it doesn't, then you reinforce, replace, etc.

So if you increase by 4% and it's now 103%, you're good - a part of that built in factor of safety is whether or not it survives construction, and it clearly did that, so we give it a pass. If you increase by 8% and now it's at 94%, you're still good since your demand is less than capacity. If it increases by 7% and now you're at 106%, then you need to reinforce it because we can't reliably depend on it to support design loads.