ASCE 7-22 Windward Drift

[SteelPE]

I have a project that I am currently working on that is to fall under FM Global Requirements. We are in snow country and it appears as if FM is requiring the roof to be in compliance with ASCE7-22 which uses ultimate snow load vs allowable snow load (which I know and love). Regardless, I am trying to calculate the leeward and windward snow drift (the old upper roof and lower roof snow drift respectively). I noticed that they changed the hd calculation considerable (at least it appears that way).... however, when it come to the calculation for the width of the drift W, there appears to be absolutely no limits on the legth of the windward drift..... they just say to use 8*.75*hd = 6hd.

Is this correct, we just W = 6*hd for windward drifts and there are no longer any limits when hd<hc (like there are still in place for leeward drift)?

 

[ANE91]

hd cannot exceed hc. Refer to Figure 7.7-2. O’Rourke’s textbook corroborates this notion.

 

[SteelPE]

The question is in reference to the drift width W not the height of the drift.

Maybe I didn't make that clear.... I will try to edit my question.

 

[DanKile]

Reading the commentary, yes the windward width is based on the calculated windward drift height, and is not limited by the clear height. The design drift height would stop at the clear height.

 

[Once20036]

I don't want to side track the conversation, but which FM document is referring to ASCE 7-22?

The FM wind document (FM1-54) largely plagiarizes ASCE 7-05. Interesting that their snow document would be SO up to date.

 

[SteelPE]

FM Global document 1-54 requires the use of ASCE 7-22 on page 6. This document was updated in October of 2024. This issue was picked up during the code review by the FM Engineer.

Prior to the start of the project, in June of 2024 we had a meeting with a FM Engineer to go over the design requirements of the project. Due to delays with the owner (not sure on what they exactly wanted) it took 6 months to award the project and get it going. In that time, FM revised their 1-54 document to update the requirements to ASCE7-22.

This whole ultimate snow loads was a complete shock to me...... as we were told Pg went from 65 psf to 98 psf. Took over a day for me to figure out that snow loads were not ultimate and not allowable.

 

[DanKile]

The wind document is 1-28. Looks like FM is doing the same in both, mixing multiple ASCE 7 years together as they please.