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RE: ASD Load Combinations

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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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