Sydney Hail Storm - Steel Framed Factory Collapse

[Trenno]

Sydney, Australia copped it pretty bad this week in terms of storms and flooding.

This afternoon was particularly bad with large amounts of hail. Several steel framed buildings collapsed due to accumulation of hail on the roof.

My question is how did the hail accumulate? Surely the pitch of the roof would prevent this? Pictures below.

 

[cloa]

It looks like a flat roof- they don't get hail much and rarely of that scale.

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[Ron]

All look like "pre-engineered metal buildings" (PEMB's).....those structures are severely "optimized" (structurally) and were likely not designed for such accumulations. They typically have a 1:12 or 2:12 slope, so they are relatively flat and the localized deflection of the panels and purlins exacerbates the issue.

 

[msquared48]

Agree with above two posts. Totally understandable.

Mike McCann, PE, SE (WA)

 

[JAE]

Sometimes they go to 1/2" per foot as well.

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[hokie66]

Those roofs are relatively flat, with steel roofing. Only pitched enough for water to run off, not hail. We do get big hailstorms in many areas of Australia, and they invariably lead to failures, but not usually of this magnitude. The most common type failure is filling of box gutters, then when melting starts, water runs back into the building.

Ron, I doubt those buildings are PEMB as you think of them, because we don't have much of that in Australia. But we do have all too many substandard old warehouse and factory structures which are even worse than your PEMB structures. As snow load is not an issue in most of Australia, hail is often forgotten as well. And many buildings are "designed" for only .25 kPa live load (about 5 psf).

Hopefully, we can get some more information about how those buildings were structured.

 

[Trenno]

Must have been a huge dump of hail in a very short time. I made the mistake of thinking rain runoff would be the same as ice.

I've honestly never considered hail accumulation. Increase the minimum live load? Have a minimum pitch for roofs?

 

[hokie66]

Yes, I think it should be a load case. There have been a lot of buildings damaged by hailstorms. Hail usually occurs with rain, and when the ice blocks internal drains and box gutters, the water as well as the ice has nowhere to go.

 

[msquared48]

That's why most roofs here have a minimum live load of 20 psf - construction related. Snow still controls though overall. It would well cover the hail load.


Mike McCann, PE, SE (WA)

 

[Trenno]

I would have thought this would just cause the gutter system to fail and thus relieve the accumulation.

What do you think a ballpark figure for hail loads should be? Reoccurrence period? Load combo = 1.2G + 1.2SDL + 1.5Hail? Have you ever actually designed for this? Did it kill your frame?

Interested to hear your opinion/experience.

 

[hokie66]

I don't really have a formed opinion as to the magnitude of loading which should be codified.

Most of my experience in Australia has been in high wind areas, so the wind loading generally controls.

The somewhat separate issue of roof drainage, especially involving any kind of internal drains, also needs a fresh look. This is sometimes left to architects, but in recent times hydraulic consultants are frequently involved. The roofs which collapsed in Sydney had internal gutters.

 

[JAE]

hokie66 - I think your point about drainage is very applicable. I had a large snowstorm occur in San Antonio, TX back in the 1980's with quite a few roof collapses - most occuring due to either poor construction or clogged roof drains and the resulting ponding (melting happened almost within 24 hours of the storm).

Ice and snow can clog drains quickly - especially ice/hail I would bet.


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[Ingenuity]

I doubt those buildings are PEMB as you think of them, because we don't have much of that in Australia.

I agree. It is interesting that Butler Manufacturing™ - one of largest PEMB in the US - are owned (and have been for more than a decade) by BlusScope in Australia, but the concept of PEMB never took off in Australia, except for possibly rural/ag/farm buildings/sheds.