roof collapses and last weeks east coast usa snow

huangyhg

roof collapses and last week's east coast usa snow
doesn't anyone design for snow loads any more?
a school? a toy store? a large retail outlet? a smoker's pavillion?
i suspect that the extra deep snow, coupled with drifting and the recent heavy rains, loaded the roofs much more than their design snow loads. it would be interesting to know what the actual loads and design loads were in each case. but, i'm sure we will never find out.
one hopes that the code "officials" out there will be updating their ground snow load maps.  but even so, every now and then the statistical probability of any loading map will be exceeded.
well, as far as the snow maps go... i suppose they're adequate for design.
but, in "my" part of the country (i'm in new york city, grew up in montgomery county, md) we'd get hit with "a big snow storm" once every couple of years. in washington, the washinton's birthday storm where we got 4 ft of snow in 1978 (i think that's when it hit) is still talked about. in nyc, every couple of years we'd get nailed with big snow in february, sometimes in december, too. "big snow" means in the 36" within a couple of days. it made the news this time because we've experienced a six-year lull in the big snow so people got all bent out of shape, etc.
daveviking & peinc are both right.  i am in the middle of a project where 2 wood beams (3 ply, spf center, with 1" plywood on each side) have failed, but didn't collapse.  luckily the restaurant manager had the presence of mind to shut the place down until it was safe.  
the beam was a 24' simple span and the failure location was 6' from the nearest support.  in addition to the recent snow, the beams also had hvac equipment on the roof.
when i inspected the structure i was shocked to see that each failure was the result of the center spf ply not being continuous throughout the span.  the connection was only a butt joint, which was only toe nailed.  the 2 outer plys were all that was keeping this beam up.  the thought that any engineer would design something like this, or a building inspector would let this go absolutely horrifies me!
i was wondering the same kinds of things myself when i heard about these failures.  my suspicion is that these collapses were probably due to a combination of extreme loading conditions especially in some localized spots, poor or incomplete structural detailing, maybe a construction error or two, and what i might call 'gnat's butt' designing, by which i mean when the designer, in a justifiable attempt to keep costs to an absolute provable minimum, meets just exactly what the building and design codes require and not one thing more, a practice very typical in our profession.  of the four, only the last can be planned for or controlled, as the others are unforeseeable.  from engineering school onwards (and though i've been away from there for a longer period of time than i might like to admit, i'd bet it's still basically the same), we as engineers always look to the most efficient solution or lightest structural section when doing a structural design.  while there's nothing wrong with this, and there's no doubt we are doing our jobs when we do this, there is also something to say for what one might call 'judicious overdesign' when it comes to a basic structure.  i'll give you an example.  i work for an institution which has a design manual for new construction.  it calls for using a 30 psf live load for roofs or the prevailing snow load whichever is higher.  i live in a region where the boca roof load would generally be around 25 psf or so when you factor in everything.  so is designing a roof for a live load of 30 psf 'overdesign'?  not in my view.  it allows for a margin of safety (i didn't have to worry too much about any roof collapses here) plus it adds a little safety against minor construction or detailing flaws.  it also affords flexibility, which often proves to be its most beneficial aspect.  we do a lot of renovations to existing buildings here, and when the mechanical guy says he wants to put a new hvac unit up on the roof, i can often accommodate this without expensive structural renovations which could often make the whole project infeasible.   the first cost for adding a little extra beef in a building's basic structure is pretty cheap when you compare it to many other first cost items. after all, the entire structural cost on a new building is probably around 10-15% of the final cost.  if you added 10% to the structural costs, which is probably more than necessary, you'd still wind up adding 1.5% to the overall building costs at most.  10 years down the road, or even just during construction, that may be the best 1.5% you could spend.  i know most engineers don't get to make these kinds of decisions, unless you happen to be in a facilities job like i have, but it's worth thinking about.  anyway, just my opinion on the subject.   
we need to re  
the combination of heavy snow, followed by rain, making it heavier wet snow, may have exceeded some of the minimums.  while some margin is advisible, maybe you can't design for everything.  drainage may have been stopped up with leaves or debris (lack of maintenance) and contributed to the loading.
backsmith
we will forever grapple with the economics of our designs, that is certain.
i've been amazed in my short career how varied the interpretations and opinions can be of highly experienced engineers on particular subjects.  just look at some of these threads.  it pushes me further along watermelon's line of reasoning, but then again, i've never owned a business and had to make payroll either.   
i think jcoffey nailed it!
(i would also have a little "fluff" in there for re-roofing as well.)
looking at the overall life of the building should be in the back of our minds. yes we look at what the current client wants but we usually get dragged into the fray when the lawyers come and play. regardless of who now owns the building now.
that's my 2 cents...
i also think the code acts as relative maximum as well as an absolute minimum.  taking jcoffey's example of using 30 psf when 25 is required, i would argue that even if the code was increased to 30, engineers would begin using 33 or 35 just to be safe.  heck, i'd design for 200 psf and never worry about it.  that is our nature.  if it becomes widespread that engineers are using 30 instead of 25, again only an example, then the code needs to be revisited.  although we are the ones who end up in the courtroom and it is our conscience that will haunt us with "why didn't i use 30 even though the code required 25" if there is a failure, we must have some faith in the integrity of the code and those who develop it.