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Re: Snow load in combination with seismic.

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Randy,

The European Standards Eurocode 1 and Eurocode 2 give variable load combination factors for various load types to account for this. Basically, where there is more than one variable load, a load combination is set up for each variable load with that variable load at full ultimate factor (1.6 in your case) and all other variable loads with reduced factors, as you are suggesting. I do not have the Eurocode for earthquakes here so cannot confirm the factors used for the case you are considering. The Chinese code, which follows Euriocode closely, uses a factor of .5 for snow load when combining with earthquake.

Basically the same as you would do when combining Live Load and Wind Load where a reduced live load factor would be applied when combining with full wind load.

For those who do not like statistics governing design, unfortunately for you all loading data and design material factors are based on statics as are strength reduction factors etc. All design is based on the statistical probability of an event occurring such as return periods for floods, wind, tornados, hurricanes etc.and of a material being outside the design strength assigned to it such as Characteristic Strengths of concrete and combinations of loads being applied at the full values, such as wind and live load. These statistical methods are hidden from us in the factors.


At 08:44 PM 7/06/01 -0700, you wrote:
Many have responded to this topic.  All have good points and I have to agree
that NO one can predict the future.  That being said I think that it is
ridiculous to throw the maximum loads at every situation just to sleep at
night.  Our job as engineers is to economically apply the strength most
effective way.

My point is that there is NO scientific study that supports the use of more
than 1/4 of the SL as seismic DL.  Sure we can use 100% of the SL, but why
single out that load case?  Why not use a basic wind speed or 120mph,  after
all it COULD  happen.  Most of you don't have to deal with the reality of
using 50% (without rationality) of a 300psf SL as DL on a single family
residence.  Believe me, we are at the edge of timber construction.  Do we
want to limit construction of these mountain homes to steel and concrete?
Up here in the Sierras we sit on basically solid granite with a ground
frequency very dissimilar to the frequency of a typical wooden structure,
yet when using these extremely high dead loads the seismic base shears are
among the highest in the world!

I just hate spending money inefficiently, even if it is my clients' and not
mine.

Randy Vogelgesang S.E.

----- Original Message -----
From: <Rhkratzse(--nospam--at)aol.com>
To: <conrad(--nospam--at)karren.com>; <seaint(--nospam--at)seaint.org>
Sent: Wednesday, June 06, 2001 10:19 PM
Subject: Re: Snow load in combination with seismic.


> I'm no statistician, but I doubt that the probability of this happening is
> "none."
>
> About 30 years ago a floating bridge in the Seattle area was sunk by a
> combination of wind speed, wind direction (straight down the very long and
> narrow Hood Canal), state of the tide, etc.  If any of these factors had
not
> occurred in the manner they did the bridge would not have been flooded and
> sunk.  The probability of their combination in the required manner is
> "probably" very small, but it happened.
>
> I'd be wary of considering unlikely events impossible.
>
> Remember that kid who threw the basketball the length of the court and won
$1
> million?  What were the chances of that?
>
> Ralph Hueston Kratz, S.E.
> Richmond CA USA
>
> In a message dated 6/6/01 3:11:37 PM, conrad(--nospam--at)karren.com writes:
>
> << 100% of snow in combination with seismic?  Good Grief!!  The
probability
> of having full snow load at the same time as a design seismic event is
> somewhere between slim and none.   >>
>



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