Snow load on existing buildings
Snow load on existing buildings
(OP)
Section 3403.3.1 of IBC 2009 states that if the live load is not changed then the presumed live load at the time the building was constructed may be used in lieu of the new live load per IBC. Does anyone know if this applies to snow loads on roofs.
I have a condition whereby I am adding solar modules on a roof. The roof is adequate is I use the smaller snow load that existed when the building was constructed, however the roof is overstressed if I use the new snow load per IBC 2009.
I have a condition whereby I am adding solar modules on a roof. The roof is adequate is I use the smaller snow load that existed when the building was constructed, however the roof is overstressed if I use the new snow load per IBC 2009.






RE: Snow load on existing buildings
RE: Snow load on existing buildings
DO NOT try to be Mr. Nice Guy here and save the client money at the expense of safety. Life is too short and there are way too many humgry lawyers. I have repaired too many older buildings where the roof collapsed due to snow overload.
Mike McCann
MMC Engineering
RE: Snow load on existing buildings
RE: Snow load on existing buildings
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Snow load on existing buildings
RE: Snow load on existing buildings
RE: Snow load on existing buildings
But I have noticed people seem to call it snow only and seem to have gotten away from calling it a "live" load??
Me - old school!!?? Maybe I should go back??
RE: Snow load on existing buildings
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Snow load on existing buildings
Skiis, regarding drifting, I have given this much thought and run through many different scenarios with snow. The calculated drift height exceeds the height of the modules in almost all cases, so it typically doesn't present any issues. The solar array "disappears" under the snow at its max...at least up here in Ohio with our 20-25 pg.
RE: Snow load on existing buildings
Usually the increase is in the 1 - 2 psf range - not really significant. BUT check it out and CHARGE the client accordingly!!
RE: Snow load on existing buildings
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Snow load on existing buildings
RE: Snow load on existing buildings
Really folks. They are two completely different things.
Yes snow has a short duration but that does not make it a live load.
In the NDS for wood - snow has a different duration factor than live load.
Snow is specifically defined in codes as SNOW and live load is LIVE. They are treated differently in load combinations.
Per the IBC at least (not sure of the British or other codes) you cannot lump snow loads into provisions intended for live loads (which was the question of the original post above).
I agree with Garth above - live loads are loads on floors or roofs that are due to occupational loading (people, furniture, moveable equipment, books, etc.) Snow load is from that nasty white stuff.
RE: Snow load on existing buildings
As far as the 5%, I agree that the load needs to be looked at locally, not saying that a 100 sf array dead load can be looked at as uniform on a 20,000 sf roof (though I have seen this done pretty commonly).
RE: Snow load on existing buildings
Yes timber treated SL and LL differently, but when doing a steel beam or joist framed roof, we (upper midwest - MI, MN, & WI) could logically decide that if SL > Roof LL, it governed. And yes I am so old that drifting wasn't even considered when I first started.
Where we had Mech and Elec utilities hanging from roof steel, we used DL + LL + SL for those elements. Which was more conservative than how it would be combined now days.
And now we have so many combinations DL + LL, DL + 0.75(LLr + SL), DL + SL, not to mention all the wind direction/combinations, plus diagonal wind loads with torsional components (gag me) that one can't logically tell what will eventually govern.
Is it any wonder that we have developed so many black-box engineers?
gjc
RE: Snow load on existing buildings
Maybe engineers (in their heads) equated the two - but they aren't the same thing and have extremely different effects on the structure.
RE: Snow load on existing buildings
I have the older codes too, and there was never a "live load" reduction applied to snow since it is basically uniform in nature, outside of drifting, unlike a true "live load" that will be spotty in nature. So, it ofalls under different rules than a true live load.
Mike McCann
MMC Engineering
RE: Snow load on existing buildings
From a 1970 textbook -
"The snow load is an essential element of the live load on roofs in many geographical areas. 20 psf is frequently taken as a minimum, with higher allowances being necessary in localities. A map showng snow load allowances . . ."
From a 1969 textbook; 2nd paragraph in the Live Load section -
"In the colder states, snow and ice loads are often of importance. For roof designs, snow loads of from 10 to 40 psf are used; the magnitude depends promarily on the slope of the roof and to a lesser degree on the character of the roof surface."
So at the least let's agree it is a "subset" of Live Loads.
I never said anything about reducing SL like occupancy LL's. That's why I don't agree with the DL + 0.75(LL + SL): it's a reduction of snow load just because of other live loads in the building. Maybe statistically they shouldn't all be maximum at the same instance - but I've seen roofs with lots of snow on them, for very long times. The roof structure doesn't much care what's happening in the 2nd floor auditorium two levels below it. The roof itself is covered with DL + SL. But this is a reduction of the column loads when in combination with other Live Loads. We are getting as statistically challenged as bridge designers.
My entire life has been in snow country and I witnessed a roof collapse while still in college. When the Timber Design professor was discussing a case he was an expert witness on, I asked if he meant the beer distribution warehouse in my home town. He asked me to stay after class - as he was interested in how the collapse played out. He was never told there were witnesses - even though my brother was the one who broke the news to the building's owner, minutes after the collapse. This was a 1960's era pole barn put into a town of 700 that probably never required plan submittals. He shared that it was a case of a southern WI pole barn company that was totally oblivious to lake effect snow in upper MI.
In the words of Rodney King - "can't we all just get along?"
gjc
RE: Snow load on existing buildings
But in the view of the original post, snow load should not be considered a live load with regards to code adjustments or handling of live loads in my view. Your textbooks may lump them together in terminology, but building codes really don't.
There are load combinations where full snow is considered so reducing snow for the final design of roof elements shouldn't occur.
Yes collapses occur - but usually due to higher snow loads than mandated by the code, drifting occurring on roof where drifting wasn't considered, or clogged drains creating ponding under the snow on the roof.
RE: Snow load on existing buildings
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Snow load on existing buildings
RE: Snow load on existing buildings
RE: Snow load on existing buildings
"Live Load"...
Those loads produced by the use and occupancy of the building...and do not include...environmental loads such as wind load, snow load, rain load...
"Live Loads (Roof)"
Those loads produced (1) during maintenance...and (2) by moveable objects such as planters and by people. **commentary** "...This definition clarifies that roof loads, such as snow loads, are not live loads"
RE: Snow load on existing buildings
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com