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RE: Design of Environmental Structures

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Although I was in favor of direct sum of these seismic components in the
past, I am starting to come around to accepting the SRSS approach.  It
is true that a direct comparison of the calculated impulsive period and
convective period will often show multiple cycles occurring
simultaneously near the peaks.  However, the actual responses will not
act so uniformly during a real earthquake - peak values will occur more
randomly.  I have recently read about time history analyses of tanks
that have compared the actual peak values and timeline occurrences of
the impulsive and convective components - it was found that the peak
values of these components generally occur at noticeably different
times.  Of course there can be no guarantee that the peaks will not
occur simultaneously during a future earthquake, but the probability is
considered to be low. 

The part that still bothers me a little is that when a relatively low
convective force is combined with a much larger impulsive force, the
effect of the convective force is mostly negated (i.e. when a small
value is combined by SRSS with a much larger value, the resultant
remains relatively close to the larger value).  But of course, if the
convective force is a small percentage of the impulsive force, it
wouldn't have as much effect on the design anyway.  

Thus, unless a good argument can be made that using SRSS is too
unconservative, we will likely continue to use that methodology.  (Note
that other reference standards, such as by AWWA, also appear to be
adopting the SRSS methodology.) 

William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

> -----Original Message-----
> From: Lutz, James [mailto:James.Lutz(--nospam--at)] 
> Sent: Tuesday, July 05, 2005 2:48 PM
> To: 'seaint(--nospam--at)'
> Subject: RE: Design of Environmental Structures
> Bill, I see the convective and impulsive lateral hydrodynamic 
> loads are still being combined using the square root of the 
> sum of the squares method, even though the differences in 
> period make it likely that numerous cycles of the impulsive 
> mode will occur within a single cycle of the convective mode.
> Any discussion within the 350.3 committee about revising this 
> to simple addition of the horizontal modes? You and I had a 
> brief exchange about this at the Seattle ACI seminar you 
> presented on ACI 350.3, and I sensed that maybe there was 
> less than unanimity within the committee on this subject.
> Jim Lutz, P.E., S.E.
> Earth Tech, Inc.
> 10800 NE 8th St - 7th Floor
> Bellevue, WA 98004
> 425-455-9494

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