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?

 

[TRAK.Structural]

I would still interpret it as meaning a maximum of 5% increase in the total load effect.

 

[Respawn]

I do total load as well.

 

[EZBuilding]

I would consider total load as well.

With roof live loads decreasing to 20 psf from 30 psf, that is an easy location to find some additional SID capacity in renovations...

The existing building code is really poor in it's wording. I find designers have wildly varying interpretations and AHJ's often have a very inadequate enforcement. I think the IBC should include multiple different "example" projects with the correct interpretations.

 

[Flotsam7018]

Yes - you must become one with your inner lawyer when it comes to the IEBC. I'm going to go against the consensus so far and say that my interpretation of the 2021 IEBC is for each load case (not sure I completely agree with that, but that is how it reads). The 2015 IEBC was clear this exception was limited to less than 5% stress increases, not even mentioning individual cases. Why would they diverge from this very clear requirement to this wording unless there was a deliberate change of intent. If their intent was to essentially have the design checks be equivalent between 2015 and 2021, they did a very poor job.

Regardless, I would send an inquiry to ICC and will likely do so myself next time I run into this limitation.

 

[gte447f]

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?

Based only on the text of the quoted passage above, I agree with @Flotsam7018 that the 5% limit is for the magnitude of each load, not the total load or load effect. That is what the text says, but I do not know if that is what was meant. In practice, I would probably stick to 5% total load or load effect.

 

[HTURKAK]

“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…”

Sometimes it is necessary to write the total story to get the requirement:

[BS] 706.2 Addition or replacement of roofing or replacement of equipment. 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 as needed to carry the gravity loads required by the International Building Code for new structures.

My interpretation is ;

- 706.2 is for Addition or replacement of roofing or replacement of equipment,
- the 5% limit is for the magnitude of each load,
- If the alteration causes an increase in dead, live or snow load..of more than 5 percent , replacement or alteration is required if necessary. That is , if the addition or alteration causes an increase more than 5 percent , it is necessary to perform calculation and prove that replacement or alteration is not needed or shall be replaced or altered as needed to carry the gravity loads required by the IBC for new structures.

 

[waytsh]

Thank you all for the responses. Sorry for the delay in posting again. Sometimes life gets in the way.

- 706.2 is for Addition or replacement of roofing or replacement of equipment,

Correct, in my particular situation the client is adding a roof top unit but I think the Prescriptive and Alteration Level 1 chapters have the same wording.

I think I am landing with the majority opinion that it is 5% of the total load and they just wrote it poorly. I don't understand what the logic would be to have it delineated by load case unless it is a duration concern. A building doesn't know whether we are considering the load as a dead, collateral, live, or snow. It just knows that there is a load of a certain magnitude on it. Maybe I am missing something.

 

[Celt83]

My interpretation is they likely removed the stress language as most of the design methodologies now are based on LRFD so I believe the intent is a 5% increase in demand in whatever form that takes shear, moment, axial, torsion. To me this also doesn't mean directly allowing a demand/capacity utilization ratio of 1.05, my read on this would only allow for this if the existing element was already at a 1.0 utilization.

 

[Mike Mike]

I agree with the majority that the rule should be 5% of total load.
I disagree with the majority that the rule is 5% of total load.
ICC has clearly stated their requirements. From a legal standpoint, we are generally not permitted to flaunt any requirements, even where the committee has demonstrated they need to return to high school to learn how to write.

I should add the commentary contradicts the code, see screenshots below. I would guess this discrepancy exists because commentary has not been updated to match main code. I understand Waytsh is in chapter 7, but in my opinion the scope of this discussion should include chapters 8 and 9 too.

View attachment 10823
View attachment 10824

View attachment 10825

 

[Mike Mike]

 

[Flotsam7018]

My interpretation is they likely removed the stress language as most of the design methodologies now are based on LRFD

The removal of the 5% stress increase isn't the root of the confusion, but rather the language change from gravity loads to dead, live or snow.

ICC has clearly stated their requirements.

I disagree; it is clearly...not clear. See above. The goal isn't to sidestep any requirements but to reflect the code adopted level of risk and extend any impacts, positive or negative, onto clients.

 

[Mike Mike]

"... causes an increase in ... dead, live, or snow ... of more than 5% ..."
if Waysh's roof top unit increases the dead load by 6%, this clause returns a value of true, even if the total load only increases by 3%. it's just english. can anyone think of a way to interpret this clause as false?

 

[EZBuilding]

The Florida Existing Building Code also has the following section:

"[BS] 707.3.2 Roof diaphragms resisting wind loads in high-wind regions.
Where the structural roof deck is removed from more than 30 percent of the structural diaphragm of a building or section of a building located where the ultimate design wind speed, Vult, determined in accordance with Figure 1609.3(1) of the Florida Building Code, Building, is greater than 115 mph (51 m/s), as defined in Section 1609 (the High-Velocity Hurricane Zone shall comply with Section 1620) of the Florida Building Code, Building, roof diaphragms, connections of the roof diaphragm to roof framing members, and roof-towall connections shall be evaluated for the wind loads specified in the Florida Building Code, Building, including wind uplift. If the diaphragms and connections in their current condition are not capable of resisting at least 75 percent of those wind loads, they shall be replaced or strengthened in accordance with the loads specified in the Florida Building Code, Building.

Exception: This section does not apply to buildings permitted subject to the Florida Building Code"

I have never in my entire career seen this be applied in residential or commercial projects. I have never seen a building department enforce it. It is my understanding that re-roofing permits are often pulled with limited Structural Engineering support.

I agree with some of the comments that the wording is not clear and could leave an engineer open to being found non-compliant. Regardless - I think the approach of looking at it per load combination is appropriate.

 

[Flotsam7018]

"... causes an increase in ... dead, live, or snow ... of more than 5% ..."
if Waysh's roof top unit increases the dead load by 6%, this clause returns a value of true, even if the total load only increases by 3%. it's just english. can anyone think of a way to interpret this clause as false?

Our interpretation is the same - see my initial response, but clearly others have a different take. And what if you have a 4.9% increase in dead, live and snow on a member subject to all (3) load cases?

 

[Mike Mike]

Flotsam,

If dead, live, and snow all increase by 4.9%, the combined load would increase 4.9%. In this case the increase is not sufficient to mandate member reevaluation per either edition: 2018 IEBC and later or 2015 IEBC and earlier. Does that answer your question? Not sure I understand the intent. By the way my screenshots above were 2018 IEBC, sorry I forgot to mention that above.

Could you please breakdown your argument that "... causes an increase in ... dead, live, or snow ... of more than 5% ..." returns a value of false for a 6% increase in dead load, 3% increase in total load? In my opinion, if the 2018 IEBC language were an excel spreadsheet, it would say:
=if(or(dead ratio>1.05,live ratio>1.05,snow ratio>1.05),evalute member,no evaluation required)
I'm not seeing how else it could be interpreted.

If ICC decided to fire their staff and hire me instead, I would rewrite this section to say "... causes an increase in the vertical load of more than 5% as determined in accordance with the strength load combinations specified in ASCE 7 section 2.3 ...". Until that day I will follow what the code says, not what it should say.

 

[Flotsam7018]

Not sure I understand the intent.

Simply that an increase below the 5% threshold for all (3) load cases can result in a greater stress increase than 5% or more for a single load case.

I'm not seeing how else it could be interpreted.

I agree and have only expressed the same interpretation throughout this thread - no argument from me.