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RE: Life span of painted steel

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Alex,

It depends on the exposure.  I have seen office, retail and some industrial
buildings constructed in California, Missouri, Kansas, Rhode Island, etc. as
far back as 1925 and 1917.  These structures had the original primer paint
or were uncoated for structural steel and bar joists with only light spotty
surface corrosion.  I have also observed no rust on uncoated steel in
shopping centers constructed in 1960.

I have also seen very badly corroded bar joists in a school above a locker
room and shower.  The schools age was less than 25 years old.  The primer
paint was long since gone.  The rest of the school was fine.

When in doubt, apply a good finish paint.  The primer paints used then and
today are too variable to predict longevity for any degree of aggressive
exposure.  A good finish paint can produce a somewhat predictable life
expectancy for a given exposure.

In your particular application, I would not see the necessity of a finish
protective coating for the concert hall or the church.  Be more careful in
the gym locker room areas.  The main gym roof could go unprotected.  The
other variables are insulation (condensation on the steel) and exposure in
high humidity areas like Florida.

Also if the gym steel is exposed, I would suggest a finish paint just
because the spotty corrosion would be unsightly.

In some aggressive applications a corrosion allowance (1/16 inch is common)
is used along with protective coatings.

Knowing the exposure, you can predict annual corrosion in mils per year.
Chemical engineers commonly calculate this.

Regards,
Harold Sprague
The Neenan Company
harold.sprague(--nospam--at)neenan.com


-----Original Message-----
From: Alex C. Nacionales [mailto:anacio(--nospam--at)skyinet.net]
Sent: Monday, January 11, 1999 1:49 PM
To: seaint(--nospam--at)seaint.org
Subject: Life span of painted steel



Does anybody have an idea, experience or data on the life span of structural
steel like trusses in an enclosed building like a gym, concert hall or a
church using only paint as rust protection. Do you know of any steel truss
or other structural steel member still existing after 50 years.

Thanks in advance