North Carolina school roof collapse - Micro-burst 1

[JAE]

Supposedly a micro-burst knocked out an end wall and part of the end bay of roof while kids were playing basketball inside.

A news report of the event:

https://www.youtube.com/watch?v=Tw6MplxjSqs

A video of post-storm damage:

https://www.youtube.com/watch?v=EEc87_nHpgU

From the videos:
1. Wood 2x roof joists with probably some kind of plywood or wood sheathing
2. Steel angle trusses with an end-bay horizontal diaphragm
3. End wall was block and brick - with no reinforcement in the block.
4. The top portion of the end wall is gone along with the end bay over the stage area.
5. This results in no significant lateral shear wall system left for the roof diaphragm, yet the principal and others were standing inside the gym afterwards.

Probably either the roof edge peeled up, leaving the end wall unstable, or a portion of the end wall got blown in (no rebar) and the end bay roof fell in or was pulled upward.

 

[IRstuff]

Is it normal for block walls to not have reinforcement?

It looks to me from the second video that the exposed truss is sitting on another block wall, so that might be the actual shear wall. At 0:22, the black-painted block wall is visible.

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[jayrod12]

There is still empirical rules in the Canadian Masonry Code that allow for unreinforced masonry walls. I would never do it for a shear wall or loadbearing wall. But apparently that was really a thing back in the day.

 

[HorsieJuice]

The masonry work in the unpainted block wall visible at 00:13 in the second video looks incredibly sloppy. Is that damage, shoddy workmanship, or both?

 

[IRstuff]

I vaguely recall San Francisco didn't do mandatory reinforcement of brick walls until Loma Prieta.

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[r13]

The area looks like is under construction at the time of incident.

 

[bnickeson]

Unreinforced block walls were pretty much the norm prior to about the 90's. It's borderline frightening to see how some of them were designed/built, as many don't work for 50 mph winds let alone 90+.

 

[Alistair_Heaton]

it won't be only the wind that's done it.

Wet Micro burst has tons of water per second travelling at 70knts plus hitting the structure.

 

[XR250]

This does not surprise me.
I practice in Central NC. From my field experience I can tell you that it is common for masonry end walls to have no attachment to the roof diaphragm - other than the occasional girder bearing.
Many times I find edge angles drilled for bolts that were never installed.

 

[thebard3]

In Ohio most basement walls were unreinforced block construction until maybe the 70s or 80s. I assume that's the norm for anywhere in the US where basements are common. If the backfill isn't correct or proper drainage maintained, then the walls start to move inward years or decades later, with few good options to fix it properly. It's the first thing one should look at when buying an older home.

Brad Waybright

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