Snow causes collapses in Maryland - pre-manufactured? 1

[JAE]

Here in the midwest USA we had some significant snowfall in December and quite a few calls to come inspect damaged roofs - one we looked at was 22,000 s.f. - with a progressive collapse mechanism going on that was prevented by some shoring and struts. It happened to be a pre-manufactured building.

This article describes several buildings in Maryland that collapsed - some don't sound like pre-manufactured structures but I wonder what the percentage of troubled buildings happen to be these types of "tin" structures.

I know the ones around us were more often than not pre-manufactured. Seems to me that it shows that if you build right at the minimum standard code you pose the risk of trouble should statistically higher loads get applied.

http://www.wtop.com/?nid=25&sid=1882549

 

[JAE]

Here's a snow cover map of the eastern US. Looks like up to 30" plus in some areas...maybe more. The scale is in the lower left hand corner.

We calculated that a 30 psf live load capacity of a roof could take about 4.3 feet (52") of snow at the theoretical failure point. 30 psf was the traditional snow load used in the midwest through the 1960's to the 1980's before ASCE 7 started lightening them up a bit.

 

[JAE]

sorry - here's the link

 

[concretemasonry]

What was the actual moisture content of the wet snow that fell.

The loads could have been out of the bounds of the projected frequency, which is a minimal design (code).

If it can be proven, the EOR and employer could be facing some problems.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[JAE]

The snow we measured here was 15 pcf. The IBC suggests 17.
Another engineer I spoke with had measured some and got 23 pcf.

 

[hokie66]

Structures in that whole region should have been designed for at least 30 PSF snow load. But there are a lot of minor structures which are not designed, just built. The hangar at Dulles may be one of the skinny prefabs.

 

[Ron]

JAE...we see the construction of PEMB's to be notoriously bad...I will be testifying at trial this month on a small aircraft hangar that has numerous construction deficiencies (missing bolts, loose bolts, bracing missing, sliding doors misaligned....). In addition, the building was designed as an enclosed building under Exposure B (ASCE 7). The exposure category is a design error, but the construction of the doors was so poor that the gap between the doors and the frame exceeds 4 sq. ft., thus putting it into a partially enclosed category.

In my ever-so-humble opinion, these buildings are so "optimized" in design, that the construction has to be almost perfect...and we know that's not likely.

 

[msquared48]

I heard that the snow was, ie is, "very wet", so I believe the 23 pcf figure.

I agree with the PEMB comments here, and that's why I never go to the line.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[paddingtongreen]

I had to modify one PEMB in the Baltimore area, It had suffered a progressive collapse in the interior bays, the purlins went first, and in failing, no longer provided lateral stability for the frame. The cause had been analyzed by a local engineer and the building fixed. I was changing the use.

The analysis was interesting, we had two heavy snow storms in quick succession, both finishing with freezing rain and then another ice storm. The first ice trapped the snow as it gripped the avalanche preventers (I'm not sure what they're called). The same happened with the second storm, it laying on top of the first, and the next ice storm finished the job. The actual snow load was way over code, and the analysis showed that the collapse would not have happened under considerably more than the code load even though the actual design did not meet code.

These PEMBs were bad enough before the computer programs arrived, but now they can take advantage of secondary and tertiary load paths, leaving no redundancy at all.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dik]

National Building Code of Canada (and yes, we know something about the product) suggests 19 pcf (Imperial conversion).

Dik

 

[dik]

Great topic JAE...

Dik

 

[BAretired]

The density of snow varies depending on the texture and age of the snow. As it gets older, it melts. If it re-freezes, it turns to ice and the density of ice is close to 62 pcf. Ground snow load values in Canada are provided in the NBC or the various provincial codes. Roof design snow load is a function of ground snow load.

In my experience, wet new fallen snow can weigh as much as 20 pounds per cubic foot. If it remains on a surface, it settles or consolidates and becomes denser.

BA

 

[TXStructural]

My understanding is that AISC is/has considered language to address non-prismatic, shop built members more thoroughly.

Pre-manufactured buildings seem to be designed for flexural demand at any section, and shear demand at any section. And the design considers minimum weight and minimum fabrication costs as optimizers.

In my forensic work, I never met one that exceeded code by my calculations. The ones that hadn't failed probably had not experienced code-level loads.

The inherent "extra" capacity of prismatic, hot-rolled members seems to make them more robust.

 

[dik]

Attached is part of NBCC... with snow loadings. The 3 kN/m^3 is approximately 19 pcf.

Dik

 

[jike]

In the winter of '75-76, in Wisconsin, 18 buildings collapsed, all PEMB! One manufacturer experienced the majority of collapses, which lead some to speculate that there was a problem with adequate bracing of that system compared to other manufacturers.

We measured the snow on one of the buildings that collapsed and it measured out at exactly the design LL times the factor of safety plus the excess capacity in the safety factor of the DL!