Snow causes collapses in Maryland - pre-manufactured?
Snow causes collapses in Maryland - pre-manufactured?
(OP)
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
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






RE: Snow causes collapses in Maryland - pre-manufactured?
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.
RE: Snow causes collapses in Maryland - pre-manufactured?
RE: Snow causes collapses in Maryland - pre-manufactured?
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.
RE: Snow causes collapses in Maryland - pre-manufactured?
Another engineer I spoke with had measured some and got 23 pcf.
RE: Snow causes collapses in Maryland - pre-manufactured?
RE: Snow causes collapses in Maryland - pre-manufactured?
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.
RE: Snow causes collapses in Maryland - pre-manufactured?
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
RE: Snow causes collapses in Maryland - pre-manufactured?
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.
RE: Snow causes collapses in Maryland - pre-manufactured?
Dik
RE: Snow causes collapses in Maryland - pre-manufactured?
Dik
RE: Snow causes collapses in Maryland - pre-manufactured?
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
RE: Snow causes collapses in Maryland - pre-manufactured?
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.
RE: Snow causes collapses in Maryland - pre-manufactured?
Dik
RE: Snow causes collapses in Maryland - pre-manufactured?
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!