From: "Juan José Treff De la Mora" <jjtreff(--nospam--at)hotmail.com>
Date: Tue, 05 Jun 2001 14:28:17 -0500
I am sorry for my English. It is an elevated tank (not a high tank). The
tank is 179 in tall, 132 in diameter and holds 10565 gal of water. The wall
and the bottom are 1/4" steel plate and the top is 3/16" steel plate. It
should be placed at 72 ft above the ground. It is located on a high risk
seismic zone and in an open field with 70 mph winds. And yes, I am very new
at this. I have a bachelor degree in civil engineering and a masters degree
in structural engineering. I just finished the second one. I am 27 years old
and this is the 6th job I am doing.
From: Lutz, James [mailto:JLUTZ(--nospam--at)earthtech.com]
Sent: Martes 5 de Junio de 2001 12:52
Subject: RE: HIGH TANK
Is this an elevated tank or a standpipe, and what are the dimensions? Design
for hydrostatic load is a no-brainer, however I would encourage you to use
high strength steel, since the allowable stress increases are usually well
worth the premium. It's the lateral load design that gets interesting.
Hydrodynamic loads due to seismic motions are usually estimated separately
as impulsive and convective (sloshing) loads. If the tank is a standpipe and
very tall relative to the diameter, you can probably crudely estimate the
seismic base shear by treating all of the load as impulsive, and using the
mass of the water and tank, combined with the beam stiffness of the shell,
to find the period of the structure and the associated acceleration from a
response spectrum. The base shear can then be applied as an equivalent
uniform pressure surcharge to estimate shell force increases.
I would compare this to the answer you get using AWWA and use the biggest
number. I'm not sure that the AWWA methodology, which is still based on
rigid tank assumptions from George Housner's early work, necessarily
accounts for amplification effects in more flexible standpipes, which is
what you might be dealing with here. The other thing with tall standpipes is
that you will probably get some rocking of the foundation in a seismic
event, and soil-structure interaction will tend to increase the natural
In the recent Nisqually earthquake near Seattle, I heard reports of 3 foot
drift at the tops of a couple of welded steel ground-storage reservoirs.
These had height to diameter ratios of about 3 to 1. At those sorts of
drifts, you might also want to add in P-delta effects.
You will probably need to anchor your tank. You should get your hands on
AISI's "Design of Plate Structures" for anchor chair design methodology,
since AWWA's tank design standards don't get into the real design details.
AWWA also does not tell you how to design shell manholes. You will need API
350, which also provides a little help for roof and top angle design.
Things can get pretty complicated if you think about it long enough. Sounds
like you are new at this. Have fun!
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