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RE: ASD Load Combinations

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A couple items to keep in mind...

First, if I recall correctly, the load combination in question actually
was D+L+(Lr or S or R)+(W or E).  I will agree that the likelihood of a
seismic event hitting with a full floor live load and a full roof live
load (i.e. reroofing, etc.) is very remote (a slight understatment
<grin>).  However, note that the load comibination also indicates S (snow)
and R (rain/ponding).

Now, in most of the high seismic zones in the US, snow is not much of a
problem.  However, I do believe that there are areas in the mountains in
California and certainly all or most of the high seismic regions in Alaska
that could have snow piled on the roof for a large part of EVERY year.
Therefore, there is a VERY good chance that seismic event could occur with
a full compliment of a roof snow load.

A similar discussion can occur about rain load.

Second, the inclusion of the roof live load shouldn't effect the design
too much.  It will obviously effect roof members that are part of a moment
frame and could effect the overall lateral system by the way of
P-delta/leaning column effects.  But that load combination does not mean
that the roof live load is included as part of the seismic mass used to
determine the seismic load.  Now, I will admit that I might be missing
something along the way that might make it a more conservative design
since seismic design in not nearly as big of an issue out this way.

Scott Maxwell, PE, SE


On Mon, 21 Aug 2000, Dave Meney wrote:

> I haven't been following this thread - but I thought I'd add an Australian
> perspective to this last response by Syed.
> 
> Our loads and load combinations code does not require roof load on
> non-trafficable roofs to be combined with wind or earthquake loads.  I
> believe as Bill Allen does it would be ridiculous to combine this load,
> which is usually only seen during construction (or re-roofing), with a
> long-term effect such as earthquake or design wind speed.
> 
> Additionally, I believe different factors are required for different floor
> live loads.  Much of my work involves the design of mining structures.  The
> floor loads on grated areas is often 5 kPa (you do the conversion!), which
> accounts for the abuse these structures get from time to time.  But in any
> instant in time, there is usually NO live load on the grated areas.  Our
> codes (building codes) do not cover industrial and mining aspects and
> non-building structures well, as don't many foreign codes in my experience.
> The code uses a factor of 0.4 x the floor live load for combinations
> involving earthquake.  In an office building, this would be 0.4 x 3.0 kPa =
> 1.2 kPa.  In a mining structure, it is in my opinion too conservative to
> design for 0.4 x 5.0 kPa = 2.0 kPa and I would usually consider a reduced
> value which results in about 1.0 kPa.  After all, equipment is separately
> considered, and in an office floor it is not, taken to be part of the
> uniformly distributed load.
> 
> These days with computers, Syed, I would think (hope) nobody has a problem
> with multiple factors for various live loads.  The paying customer would not
> be happy knowing we've deliberately gone conservative to save some
> computational effort, especially when he already sees us collectively as a
> conservative bunch!
> 
> Dave Meney,
> Registered structural engineer
> Yenem Engineering Services
> Perth, Western Australia
> 
> > -----Original Message-----
> > From: Syed A Masroor [mailto:smasroor(--nospam--at)gem.net.pk]
> > Sent: Monday, 21 August 2000 1:57 PM
> > To: seaint(--nospam--at)seaint.org
> > Subject: Re: ASD Load Combinations
> >
> >
> > The reason we need L and E both in this case is for ordinary floor
> > loads, which may be there during an earth-quake. (May be a factor of
> > 0.75L or something would be acceptable for this). It would be
> > difficult
> > for all of us if the code differentiated between different live loads
> > and put a proper factor for each type under various combinations.  I
> > vote for keeping all live loads together, even if it means
> > designing for
> > improbable events in some structures.
> >
> > Bill Allen wrote:
> > Even if you can visualize an earthquake occurring during a re-roofing
> > job, I
> > have to ask:
> >
> > What is the probability of THAT happening?
> >
> > Isn't the design level earthquake something like 2% exceedance in 50
> > years?
> >
> > A re-roof occurs every 10-20 years?
> >
> > Are we talking about a 500-1,000 year event for a structure that won't
> > last
> > 50 years anyway?
> >
> > Yes, it CAN happen. But, so can an engine falling off an airplane
> > hitting a
> > roof. Are we going to design for THAT?
> >
> > O.K., so my statistical analysis isn't accurate, but you get the idea.
> >
> > IMO, combining wind OR earthquake load with roof live load (even 50%
> > roof
> > live load) is pretty ridiculous.
> >
> > Regards,
> >
> > Bill Allen, S.E. (CA #2607)
> > ALLEN DESIGNS
> > Consulting Structural Engineers
> > Laguna Niguel, CA
> > http://www.AllenDesigns.com
> > V (949) 365-5696
> > F (949) 249-2297
> >
> >
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