Roof Collapses and last week's East Coast USA Snow
Roof Collapses and last week's East Coast USA Snow
(OP)
Doesn't anyone design for snow loads any more?
A school? A Toy Store? A large retail outlet? A smoker's pavillion?
http://www.nbc4.com/news/1997936/detail.html
http://www.nbc4.com/news/1997887/detail.html
http://www.theiowachannel.com/news/1997950/detail.html
http://www.cbsnews.com/stories/2003/02/18/winter/main540920.shtml
A school? A Toy Store? A large retail outlet? A smoker's pavillion?
http://www.nbc4.com/news/1997936/detail.html
http://www.nbc4.com/news/1997887/detail.html
http://www.theiowachannel.com/news/1997950/detail.html
http://www.cbsnews.com/stories/2003/02/18/winter/main540920.shtml
RE: Roof Collapses and last week's East Coast USA Snow
RE: Roof Collapses and last week's East Coast USA Snow
Just ask the members of the El Paso, TX surf club. They are waiting for the western half of the U.S. to drop into the Pacific from the "big one".
RE: Roof Collapses and last week's East Coast USA Snow
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.
RE: Roof Collapses and last week's East Coast USA Snow
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!
RE: Roof Collapses and last week's East Coast USA Snow
RE: Roof Collapses and last week's East Coast USA Snow
RE: Roof Collapses and last week's East Coast USA Snow
Backsmith
RE: Roof Collapses and last week's East Coast USA Snow
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.
RE: Roof Collapses and last week's East Coast USA Snow
(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...
RE: Roof Collapses and last week's East Coast USA Snow
RE: Roof Collapses and last week's East Coast USA Snow
Old timers around my city typically used a roof snow load of 30 psf. Some years later, the UBC changed to a ground snow of 30 which gave us a roof snow of 21 psf. We'd add 5 psf for rain on snow and get 26 psf for our design. Now, the IBC requires 25 psf for ground, making the roof snow 17 psf. We on the committee said no way and set the minimum roof live load at 25 psf.
Beyond loads, the codes and standards out there are constantly researching the "exact" capacity of any system or material. So we get more and more complex codes and design methodologies that take lots more time. Just look at what you really have to do to properly design a concrete column these days.
Most of the failures due to snow that I saw back in 1984 in San Antonio, Texas (13" of snow in Texas! Do you believe it!) were due to stability issues and not capacity issues. Many times there were beams running over the tops of columns and no web stiffener plates were provided. The beams, with the significant snow load, simply rotated off the top of the column and dropped the roof....with the columns still standing vertical.
RE: Roof Collapses and last week's East Coast USA Snow
RE: Roof Collapses and last week's East Coast USA Snow
A few points to ponder....
The code requirements are statistically based minimums. These are based on PROBABILITIES not POSSIBILITIES. These recent failures could easily have fallen into the "possibilities" not probabilities. This simply means that given the appropriate information at the time and designing, constructing, and maintaining the structure in a reasonable or "non-negligent" manner might not have prevented the failures.
There is a need for local review and application as JAE pointed out....it just doesn't usually get done in a formalized fashion as a code supplement. Our local engineering societies should be driving committees such as the one JAE described. Remember....one size fits all usually doesn't fit anyone well!
Our Standard of Care does not require perfection. It requires a lack of negligence. There's a big difference between those two points on the practice map and let's hope they don't get too close together.
RE: Roof Collapses and last week's East Coast USA Snow
If you use a generally good detailing practice, i.e. web stiffeners on overhanging beams at the column, proper lateral bracing, etc, even if the actual code forces showed that they weren't required, etc., then you can probably tolerate 30 vs 26 psf of snow. Most catastrophic failures are the ones that were SUDDEN, (Hiatt Regency mezzanine)and not ductile.
Designing to Code is not enough, you must focus on proper detailing, and CHECK THE SHOP DRAWINGS...
GA
RE: Roof Collapses and last week's East Coast USA Snow
Details, Details, Details..."Hyatt" not "Hiatt".
Sorry, I've been checking shop drawings all day in Kansas City, Mo. I couldn't resist the jab.
Your point is a valid one!
Thanks for the post.
RE: Roof Collapses and last week's East Coast USA Snow
RE: Roof Collapses and last week's East Coast USA Snow
RE: Roof Collapses and last week's East Coast USA Snow
RE: Roof Collapses and last week's East Coast USA Snow
I had the fortune of visting the VMI campus last July. Really fine engineering university, I must say.
Anyway, USACE is leaning heavily towards IBC 2000 as "the" code. However, the Uniform Facilities Criteria folks in TI 809-02 "Structural Design Criteria for Buildings" dated 01 September 1999 (http://www.hnd.usace.army.mil/techinfo/ti.htm) references EVERYTHING, including ASCE-7, which it should, as pertinent to structural design. For snow and wind, MIL-HNDBK 1002/2A "Loads" has all sorts of geographical info, though my opinion is that every project is case-by-case and I use these loads as a starting point.
I've heard the "well, if it's really windy, all the snow will blow off the roof, so wind governs" argument. OK, but, gosh, my next-door neighbor's got a lot of FROZEN SNOW stuck to his roof and it hasn't blown off in these gail-force winds. Strange...
I go with whichever code or reg is the most conservative; if the local code says 30 psf and the USACE/NAVFAC/AFM says 15 psf, I would use 30 psf and make a note in the calcs why I chose 30 psf. Federal LAW may trump state and local law, but engineering judgement answers to a higher power, no?
I am somwhat bugged by the recent collapses because it makes structural engineers look bad - even though it *might* not be the fault of the engineer.
RE: Roof Collapses and last week's East Coast USA Snow
RE: Roof Collapses and last week's East Coast USA Snow
The collapse of the Borders Book Store in Newark, Delaware, during a severe winter storm in January of 1996.
http://www.sbcmag.info/past/2001/01nov/borders.htm
The caused - The open web bar joists involved in the collapse had been poorly manufactured and the defective welds involved simply failed to sustain the loads transmitted thereon.
RE: Roof Collapses and last week's East Coast USA Snow
My point is that before we are too hasty in blaming the building design, let's wait for the investigation. While it could be a bad design, there are a several other reasons like bad construction, materials (see post above)and owner misuse which might of caused or contributed to the failure.
RE: Roof Collapses and last week's East Coast USA Snow
RE: Roof Collapses and last week's East Coast USA Snow
I think my rafter tables were based on span/something max deflection with max stress in parentheses. The few buildings I have seen collapse were either damaged from rot or not built to code.
RE: Roof Collapses and last week's East Coast USA Snow
You're absolutely wright! My mistake...