Snow load combination factor conflict between ASCE7-22 (IBC 2024) and ACI 318-19? 2

[YHW2012PE]

I found the snow load combination factor conflict between ASCE7-22 (IBC 2024) and ACI 318-19. The ASCE 7 (IBC 2024) LC factor is 1.0 but the ACI 31819 still uses 1.6. So please tell how about your current designs? How do you solve this conflict? Appreciate it very much if you can write your ideas.

Youhai Wang, P.E./P.Eng.
Principal and Chief Structural Engineer
Maple Creek Engineering Ltd.

http://www.maplecreekengineering.com

 

[STpipe]

Is ACI 318-19 the referenced standard in IBC 2024? If so, that's a bit of an oversight by the code committee.

Personally, I would focus on the intent of the code in terms of what the desired level of structural reliability. From my understanding, ASCE 7-22 transitioned snow loads to follow the same approach as wind loads and seismic loads by specifying the ultimate load rather than the service load.

So at the end of the day, there is no contradiction. The design under limit states design requires the evaluation of the structure by comparing the factored resistance of the members to the factored loads applied to those same members. Therefore, using the load combinations as specified in ASCE 7-22 would satisfy the requirements of ACI 318-19 because the load combinations that have snow loads as the primary load action with a load factor of 1.0 represent the ultimate limit state.

 

[canwesteng]

318-19 would have to be the reference code, since it is the most up to date. More of an oversight on part of ACI to include load cases at all

 

[YHW2012PE]

Thanks.

Youhai Wang, P.E./P.Eng.
Principal and Chief Structural Engineer
Maple Creek Engineering Ltd.

http://www.maplecreekengineering.com

 

[lexpatrie]

The issue stems from factored snow loads in 2022 and 2019 ACI likely uses ASCE 7-16, whixh is service loass, so either use service snow loads and ACI 7-16 load combos (ACI 318 2019) or the current 7-22 ones.

Sounds right to me.

 

[YHW2012PE]

Thanks. I think ACI should give us an answer about this question. They should do something like AISC does, which always points out latest ASCE7.

Youhai Wang, P.E./P.Eng.
Principal and Chief Structural Engineer
Maple Creek Engineering Ltd.

http://www.maplecreekengineering.com

 

[JAE]

Agree with lexpatrie.
Also keep in mind that both the ACI and ASCE 7 are referenced codes UNDER the governing code - in the US this is typically the IBC.

ACI makes use of language (Chapter 5) that suggests that the "nature" of loads in 318 should be consistent with that used by the "governing code" and if the nature of the loads is different then the provisions of 318 should be adjusted accordingly (i.e. what lexpatrie said).

 

[YHW2012PE]

Agree with you two. In addition, I found ACI 318-25 public review on ACI website. You can register to read the draft version. Thanks for all Reponses.

Youhai Wang, P.E./P.Eng.
Principal and Chief Structural Engineer
Maple Creek Engineering Ltd.

http://www.maplecreekengineering.com

 

[YHW2012PE]

Good news. The equations in Table 5.3.1 of ACI 318-25 for loads combinations have been changed to consistent with ASCE7-22. That is U=1.2D+(1.6Lr or 1.0S or 1.6R)+(1.0L or 0.5R. So the discussion is over. Thanks all.

Youhai Wang, P.E./P.Eng.
Principal and Chief Structural Engineer
Maple Creek Engineering Ltd.

http://www.maplecreekengineering.com

 

[YHW2012PE]

Youhai Wang, P.E./P.Eng.
Principal and Chief Structural Engineer
Maple Creek Engineering Ltd.

http://www.maplecreekengineering.com

 

[STpipe]

lexpatrie,

My understanding is that the change in the -22 edition wasn't just specifying the snow loads as ultimate loads rather than service loads. Some of the specified ground snow load values were increased such that the ultimate load could be up to 15% higher than if you performed the calculation using the specified snow loads in the -16 edition and factored them by 1.6.

So in that case, using the service loads and the ASCE 7-16 load combinations would potentially result in unconservative ultimate snow load.

 

[lexpatrie]

Fair comment, in that case you could rework it appropriately if needed.

I don't think the ground loads changed in my area (Well, no since we're on the 2020 code based on the 2018 IBC based on ASCE 7-2016), I don't think the ground snow changed in 2022 for my area.

Plus they added that winter wind parameter for any snow drifts. Might as well mention that, too.

Concrete has higher dead loads, so that snow load change would be less significant than, say, a metal building or a steel structure.

As a side note, anybody ever seen an R value for a concrete plank? With or without a bonded structural topping?