Return to index: [Subject] [Thread] [Date] [Author]

Elevated Water Tanks

[Subject Prev][Subject Next][Thread Prev][Thread Next]
You did not indicate the type or size of the tank.  I am assuming it is a
welded steel tank, probably a legged tank?  If another style, let me know
and I'll adjust the comments.

First-  Warning! -  Be extremely careful climbing old tanks!  Ladders and
other access devices are often corroded or do not meet current standards for
safe access.  Of particular concernn is the old rolling ladders that were
pivoted around the top of the tank.  Our inspection crews have specific
safety guidelines and rigging to reach the many areas of the tank that must
be inspected.  I recommend you look at safe access requirements first.

The basic design standard for welded steel elevated tanks for water storage
is AWWA D100-96.   This covers the fundamental loads and allowables,
materials, fabrication and construction details, etc.  It does require a
considerable knowledge of tanks and shell design to use it properly.  Much
is left to the user with only the minimum requirements and very little
information on analysis or design methods typically used by tank designers.
The Steel Plate Fabricators Association has a few publications that you
might find helpful.  See also Gaylord and Gaylord's Structural Engrg
Handbook which has a section on tanks written by a friend of mine- Bob
Wozniak.  A former teacher of mine at Cornell-- Bill McGuire Steel
Structures text book has some info that might be helpful.

We do a considerable number of elevated tank evaluations every year.  A good
evaluation includes such items as a detailed inspection and assessment of
the condition of the  structural elements (extent of corrosion, UT
measurements, documentation of the original members, comparison with
as-built drawings,etc), an assessment of the coating (condition, options for
top coat, re-coat, lead abatement issues, containment design, etc),
evaluation of safety related items (climbing protection,  handrails,
platforms, etc) and operating /health deficiencies.

If this is a legged tank, there are several ways to determine the wind
pressures on each of the elements.

1.  Use basic D100 approach, scaling your design wind speed relative to D100
value 100 mph and modify pressures by ratio of wind speed squared .  This is
the common approach used for many years by tank designers.  An interesting
note, for the 100 mph wind, no increase in allowable foundation pressures is
allowed.  tank is assumed to be empty, unless you have a process tank that
is always full, or operating mandates that say tank must be full during a
hurricane.
2.  Use design guides such as ASCE 7 to determine and equivalent pressure.
This is most often used today, and it is likely we will incorporate ASCE 7
into the next edition of D100.

Again assuming this is a leg tank, the lateral force system is most likely a
tension rod/compression strut design ( only a few tanks are tension /
compression brace design).  I recommend in high wind areas ( or any tank for
that matter) the seismic criteria be followed ( upset rods, anchor bolt
chairs that will yield the anchor bolts, compression struts that will yield
the rods, etc).  One particular areas to be concerned about is the
connection of the rod clevises to the wing plates.  The pins are often not
retained properly and can vibrate loose or fall out during an overload,
causing a failure of the lateral support system. Another area to watch, is
how the rods are "tuned".  To avoid excessive drift and torsion, it is
important that the rods be equally pre - tensioned (usually 8 to 12 ksi is
sufficient).

Elevated tanks weather high winds extremely well. There are numerous
examples of tanks taking direct hits from hurricanes and tornadoes with only
minor damage.  Most often if a legged tank fails it is caused by debris
impacting the horizontal compression struts causing it to buckle the rods
are impacted breaking a connection. Occasionally an impact on a leg is
sufficient to buckle the support.

If the tank is more than 4 legs or skip post bracing, be careful defining
the rod loads and the orientation of the wind alignment to get the worst
scenario.  If  it a post to post rod system, the in and out of plane force
distribution can be derived from a free body diagram easily.  If it is a
skip post system, the free body diagram is more complex.  Another area often
overlooked in the analysis is the momnet magnification in the uppermost
panel from the balcony to the rod line reactive force.  Determining the
proper forces in the tower is a bit tricky, be careful.

Also, try to find the drawings on the tank.  Few owners can locate the
drawings, but there may be a remote possibility that you can retrieve them
from the original contractor with authorization from the owner.

Good luck with this evaluation

Stephen W. Meier PE, SE
Tank Industry Consultants, Inc
    7740 West New York St                       684 West Boughton, Suite 101
    Indianapolis, IN 46214                          Bolingbrook, IL 60440
    317-271-3100                                       630-226-0745
    317-271-3300 fax                                  630-226-0802   Fax
    800-539-7102 pager
Meier(--nospam--at)TankIndustry.com