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Posted by Matt Whiting on November 4, 2006, 1:52 pm
Kats wrote:
> First time on this forum and I hope that you might be patient with me. We
> have hired a licensed contractor to build our 4600sq. ft. home. I am not a
> contractor nor am I experienced in any terms used in construction so much of
> what is happening goes straight over my head. But I am hoping someone on
> this forum might explain what our options are to the following....
>
> Apparently our trusses company is working on another home at our elevation
> about 1/2 mile away. They came and asked our general contractor (GC) why we
> didn't need snowload trusses for our house since the one they're working on
> now had to have them. So GC called code enforcement and this is what he found
> out... it looks like our licensed architect (LA) didn't know that at 4300
> elevation we are in a snowload zone. I know this zone code covers from 3300
> to 5400 elevation but do not know the load amount it should carry. Anyway,
> the LA didn't design snowload trusses into our plans. Engineering didn't
> catch the mistake and neither did our County Permit Department. But now...
> it looks like we might need to remove our roof and if I understand correctly
> put on different trusses! GC has pulled his crew off the house until this
> can be resolved so all work has stopped. At this point other companies have
> done their work, the roof is felted. HVAC, central vac, plumbing, electric
> wiring all in. Much of this runs through the existing trusses.
> A meeting was held by the GC, LA, the engineer, and County to see what
> measure might be taken. GC has come up with the idea of possibly
> retrofitting our roof based on the current snowload code using calculations
> at 200 feet elevation increments instead of the entire 5400 elevation. If
> County approves this plan on Monday, it will go to engineering to figure out
> exactly how to do this and then back to County for final approval. Can
> someone enlighten me if this is a safe solution for this problem or at least
> explain exactly what I need to concern myself with to make sure this is
> handled properly? I sure don't want the roof to collapse onto of my g-kids!
>
If I understand this correctly, and I'm skeptical that I do from your
description alone, it sounds like the code is applying the worst case
snow load that would exist in the 3300' to 5400' elevation range, which
would likely be that at 5400', to the entire range of elevations. It
sounds like your GC is asking if they can instead interpolate the snow
loads between the value at 3300' and that at 5400' to get a value for
the actual elevation of your house which is 4300'. This seems
reasonable to me, but is certainly less conservative an approach. So,
for example, let's say the snow load at 3300' is 20 psf and at 5400' it
is 40 psf (I'm just making up these numbers for illustration!). Since
4300' is roughly halfway between 3300 and 5400, a linear interpolation
would suggest that the appropriate snow load for your house would be
about halfway between the upper and lower snow load values and thus
would be 30 psf. Since your trusses don't meet the 40 psf required by
code for this entire 3300-5400 range, but might meet the interpolated
value of 30 psf, this approach might let you get by.
Is this safe? Probably, but it all depends on how close to linear the
actual snow fall averages are between 3300 and 5400 feet. There may be
other options available that a structural engineer could devise that
wouldn't require replacing the roof. It may be possible to sister
members to the existing trusses. Trusses typically assume no
load-bearing walls in the span. However, if your floor plan happens to
have a wall that runs perpendicular to the trusses and is near the
center of the truss span, it might be possible to install a king post in
the trusses and transfer load to the wall and effectively halve the
length of the trusses. This would limit future remodeling, however, as
you can't easily move a load bearing wall later.
It is pretty sad that your LA and engineer aren't providing solutions
and it is resting entirely on the GC. However, it sounds to me like
your GC is pretty sharp as his idea seems to be reasonable to me.
However, I'm a structural engineer in training (halfway through my
masters degree), so don't take my comments too seriously. :-)
Maybe Bob Morrison will weigh in...
Matt
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