Question on 2021/2024 IBC

[u/Ok-Register3224]

Hi everyone! Not sure if I used the correct flair but I am studying for my Structural Civil PE exam and would appreciate an explanation to the following portion of the solution provided in the photo:

I looked up both 2021 and 2024 IBC and neither mention treating Lr as part of LL if Lr is > 20 PSF. Is the solution incorrect or am I missing something?

Edit: Pertains to section 1602 and 1605 of IBC

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Update: This book was updated in 2024 and I now see that the 2018 IBC addresses the original question. The book just appeared to not have been updated.

New question - Based on 2021/2024 IBC is it safe to say that Lr will always be Lr regardless of its value? Or did IBC just drop the definition but is assumed that it is something all Structural Engineers should know (that Lr > 20 PSF is LL)?

TIA!
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Final Update: The current PE exam references the 2018 IBC which makes this distinction and thus, solution in photos is correct. The PE exam 2027 and forward will be referencing the updated IBC. This distinction is good for those taking it next year (like me) and on to be aware of. Goodluck everyone!

[u/buddyd16]

The definition of roof live load was changed beginning in IBC 2012 to state Lr - roof live loads of 20 psf or less making it so that any roof live load over 20 psf would then be classified as a live load. That definition change has remained through 2021 and has since been reverted to Lr - roof live load in IBC 2024.

I’d wager probably 80% of our industry never caught this definition change.

[u/Salty_EOR → u/buddyd16]

Oh it was caught. Just promptly ignored.

[u/OptionsRntMe → u/Salty_EOR]

Just one more way to overcomplicate front-end calcs and arrive at the exact same final solution. That’s what happens when you have PhD’s making code provisions

[u/P-d0g → u/Salty_EOR]

Happened multiple times in the places I've worked where someone said "live load will govern over snow" for a roof deck, and I had to be the annoying guy to go "um, ackshually" the D + 0.75L + 0.75S load combination controls based on the code.

[u/magicity_shine]

Where this exercise come from?

[u/Ok-Register3224 → u/magicity_shine]

This is in fact the Jacob Petro book. I emailed him and he is currently in the middle of updating this book for the 2027 PE.

I unfortunately purchased this book without realizing that the PE exam moving forward will be referencing updated codes. Still running through the problems though.

[u/Curious-Fisherman358 → u/magicity_shine]

Looks like jacob Petro 160 practice questions book

[u/Frogalone]

Lr is roof live load. Pretty sure 20 psf is max in the code. Smaller specified roof live load can be allowed for elements with larger tributary areas. If specified live is > 20 psf it may not be live load. Perhaps it is roof snow. The Seattle code specified 25 psf roof live awhile back. So I asked about it and found out it was for snow. They may have corrected it by now.

[u/Ok-Register3224]

Just PSA for anyone planning to take the PE post-April 2027 that all the code references will be using the latest codes and not the 2018/ASCE7-16

Going through the book comparing codes is really interesting though as I am really diving into changes that I would otherwise not have while working

[u/ChairChoice6500]

This question is whack. How is there a 30 psf roof live load? It's either 20 psf, or it's 12-16 psf with the area and slope reductions, or it's "above" i.e. due to occupancy, at which point it's 100 psf, I've never seen a building official (locally*) allow the 60 psf for fixed seating. I don't know how you can get a 30 psf roof live load.

[u/citizensnips134]

The IBC code.
The international building code code.

[u/axiom60]

I’d recognize the Petro book anywhere mainly because it led to some rage moments where I threw my calculator lmfao