Your videos are extremely exceptional sir. Please keep on posting. It’s hard to find such a good ref these days. Thanks once again.

There is a design usable life of the structure, maybe 70 years or so. There should slso be inspection and maintenance requirements on the building, just like cars. The roof has been functional for 50 years. I would say the design was proper.

ASCE 7-22 updated the ground snow load maps to be reliability-based (which I believe means ultimate) while also dropping the LRFD load factor for snow from 1.6 to 1.0. Does that mean that the flat roof snow load is an ultimate value in 7-22 and service level in 7-16 (and earlier)?

@StructuresProfH

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Yes. The new ASCE 7-22 snow load is meant to be an ultimate load with a load factor of 1.0. Previous versions of ASCE 7 prescribed the service level snow load and then applied a load factor of 1.6 to get the ultimate load. Applying the 1.6 factor to a service level snow load doesn't always make sense from an uncertainty point of view, hence the change. Some places, mostly up north, will have service and ultimate snow loads that are not that different and other places may have very little service level snow but could have a rare, extreme snow event that is much more than 1.6 times the service load. The new method aims for a consistent probability of failure given the prescribed ultimate snow load rather than apply the same 1.6 factor everywhere. Of course, the Miller Hill Mall was built before LRFD was even a thing, so that's a whole other can of worms. Interestingly, in Duluth, using older editions of ASCE 7 may give you a bigger flat roof snow load. For example, in the 2005 edition, service-level ground snow for Duluth is 60 psf. Exposure factor Ce is the same, but often the thermal factor Ct = 1.0 for any unventilated but heated building. Importance factor (which is now incorporated directly into the Hazard Tool) I = 1.1 for Risk Category III. This gives pf = 0.7*1*1.1*1*60 = 46.2 psf service load. Apply the 1.6 factor, and that gives about 74 psf for ultimate, a bit higher than the 68.5 psf we found using the 2022 edition!

Folks, if you are engineers, and design this way, pay for the E&O insurance and have a good attorney on speed-dial! Codes are a MINIMUM; your JUDGEMENT is what keeps you sane!

@StructuresProfH

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Yep. It's right in the title of ASCE 7: MINIMUM Design Loads and Associated Criteria for Buildings and Other Structures.

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Sounds like the mall roof was substantially under strength for that region.

@StructuresProfH

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That may very well be. I don't know the design capacity of that roof, but it was designed over 50 years ago - things have changed quite a bit since then.

@LTVoyager

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@@StructuresProfH It will be interesting to see the forensic analysis. It sure didn’t seem to me as though it had all that much snow on it for that region of the country.

@StructuresProfH

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I would definitely like to see the forensics. Duluth is very close to record snow levels this year, so I think the load may be larger than it appears. Snow density and loading is a tough thing to estimate visually, but I'm guessing this was very dense snow. There were also some significant drifts on the east side of the collapse against where the roof was higher. It's possible that there was an overload and consequent failure under that drift, and the rest of the roof was pulled down with it. Tough to say for now, but I do agree, it doesn't inspire much confidence in the capacity of that roof.