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Re: Snow Load + Seismic

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> From: Scott Maxwell <smaxwell(--nospam--at)>

> The point is that to me section 1630.1.1 means that by the 1997 UBC code,
> you must use 100% of the design snow load as part of the seismic dead load
> if that seismic design load is greater than 30 pdf UNLESS the builing code
> offical gives you the approval to use something less that 100% of that
> load, but even the building official cannot let you use less than 25%.
> That deals with the seismic dead load that is used to determine the
> seismic loading (i.e. V).
> Then if you choose to use a load combination from section 1612.3.2 (i.e.
> an alternative ASD load combination) that has snow load in combination
> with seismic (i.e. equation 12-16), then the exception allows you to use
> zero (0) psf for the S part of the combination if the design snow load is
> 30 psf or less.  If the design snow load is greater than 30 psf, then you
> must use 100% of the snow design load for the S part of the load
> combination, UNLESS the building official give you the approval to use
> something less than 100%, but again the builing official cannot let you
> less that 25% of the snow design load.

What is the rationale for including a seismic mass if it doesn't also
exist similarly as a gravity load (e.g. applying different ratios of
snow load effect in the same load combination)?

Is it reasonable to assume the maximum snow gravity effect (1 in 50 yr),
as either gravity mass or horizontal inertial mass, to occur at the same
time as the maximum seismic effect (1 in 500 yr)? Doesn't this blow the
statistical basis into microscopically small probability ranges?

I always figured that
a) the snow mass reduction in E was a statistical twiddle for seismic.
Also, snow does not necessarily act as a rigid body during a dynamic
event, and,
b) applying the full snow gravity load in combinations with E was,
generally, a conservative simplification and I could avoid calculating
different S for use in different combinations.

f(r*S) <= E <= f(1.0*S) when S => 30 psf
E = f(0*S) when S < 30 psf

Now I interpret your remarks to be a reverse of this:
0 <= r2*(S < 30 psf), E => f(r*S)
1.0*(S => 30 psf), E => f(r*S)
r is the reduced factor for the snow mass applied to E
r2 is the reduced factor for snow gravity load effects applied in
combination with E.
(dependent on reductions permitted in each load effect)  

Canadian code was conservative if applied prima facie but latest edition
decided to explicitly use a consistent mass effect for both E and S in
the same load combination
e.g. Load effects = E(0.25*S) & 0.25*S. 

R. Paul Ransom, P. Eng.
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)> <>

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