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RE: Subject: Re: Seismic/Non-Seismic design

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Stewart,

I chair the Nonbuilding Structures Technical Subcommittee of the BSSC.
What references do you have regarding serviceability requirements for
process plants and piers and wharfs?  You will see a marked expansion of
this section in the NEHRP and the IBC in the years to come.

Your input would be greatly appreciated.

Regards,
Harold Sprague
BSSC TS13 Chair
Black & Veatch
 ----------
From: Stewart, Warren
To: 'Seaoc - Send to mail list'
Subject: Subject: Re: Seismic/Non-Seismic design
Date: Monday, September 08, 1997 12:24PM

>From: James_F_Fulton(--nospam--at)rohmhaas.com (James F Fulton)
>To: seaoc(--nospam--at)seaoc.org
>Subject: Re: Seismic/Non-Seismic design

>     I am interested in this topic also for one and two story industrial
>     buildings and chemical process structures that could extend as much as
>     5 or 6 stories high.  We typically design these as x-braced frames
>     with simply suppported girder-to-column connections. Most of our
>     designs are in non-seismic areas (Texas), but what about this type of
>     structure for other seismic zones ? What are the drawbacks ? This type
>     of steel construction seems to be much more economical to design and
>     construct than moment frames or even eccentrically braced simple
>     frames.

Refer to 1632.3 of the UBC for non-building seismic loads.  Braced
frames typically result in "rigid" structures as defined therein.  The
coefficient 0.5 in formula 32-1 is equivalent to a prescribed C/Rw in
equation 28-1.  If the period is greater than 0.06, equation 28-1 can be
used but C/Rw can not be less than 0.4.  The bottom line is that the UBC
does't cut you much slack for non-building structures.  Fortunately, the
UBC recognizes that it really isn't an authority on non-building
structures, and allows other nationally recognized standards to govern.
Perhaps API has better take on the subject.  So much for the
legal/theoretical side.  As for practicality, the value of the more
ductile framing systems is that it allows the engineer to reduce the
initial cost of the structure with the understanding that the
non-structural elements as well as the structural elements may need
extensive repairs after a major event, generally assumed as having a 10%
chance in 50 years.  Other non-building designers (e.g., wharves and
piers) have adopted a two level seismic approach that  looks at an
additional smaller seismic event (50% chance in 50 years), with the
intention of resisting these forces without significant damage or
interuption of activites.  This may be appropriate in your case.
Remember that rigid structures will have a higher design force than
flexible structues, but less displacement.  In the design of chemical
process structures, the displacement may be a more critical factor.
Also important is keeping the value of the chemical process in mind.
The cost for shutting the process down to make major repairs maybe
significantly more than providing the extra cost in the framing system,
even discounting the repairs to present value.