Galvanizing Insights, Volume 12, Issue 2
American Galvanizers Association
www.galvanizeit.org
Galvanizing Innovations
Churchill River Bridge

Steel Bridge Design for 100 Years Plus

From the 1500 to 1800’s, Europeans designed bridges with stone because of its availability and its characteristics to support foot, horse, and wagon traffic without much maintenance for literally hundreds of years. The desire for longer span bridges with an architectural flair initiated the age of steel and/or concrete bridge design in the late 1800’s and 1900’s, without much consideration of longevity. Currently, in this second decade of the 21st century, there is a renewed focus on the durability well known by the English, French, Spanish, Germans, etc. of yore. And so, due to budget constraints, heavy traffic loads, complex environmental erection sites, and inevitable population shifts, bridge designers today have their sights on a 100 year, virtually maintenance-free bridge - modest compared to those of old.

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Ask Dr. Galv
Woodwards Building

How does surface preparation of hot-dip galvanized steel differ when coating it with paint or powder coating?

Though many specifiers are familiar with one type of corrosion protection or another, few realize the intrinsic value of utilizing two corrosion protection systems together, known as a duplex system. A duplex system is formed by painting or powder coating over hot-dip galvanized steel. When used together, the two systems work synergistically to provide superior protection rather than when used independently.

The paint or powder coating provides barrier protection to the galvanized coating. Then, when the paint or powder coating has served its useful life, the galvanized coating offers barrier and cathodic protection to the base steel. In turn, the hot-dip galvanized coating serves as an incredibly durable ‘primer’ that prevents the underfilm iron-oxide corrosion which causes paints and powder coatings to fail. The combined protection of the two protective coatings (hot-dip galvanizing covered by paint or powder coating) provides corrosion protection to the steel 1.5 to 2.3 times longer than the expected life of each protective coating alone. In addition to long lasting, superior protection, duplex systems provide architects, engineers, and other specifiers design freedom. Utilizing a combination of paint/powder coating with galvanizing means you don’t have to sacrifice corrosoin protection for aesthetics, or vice versa. This advantage has led to more specification of duplex systems each year.

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Featured Project
DIA Parking Garage

DIA Parking Garage Stairs

Denver, CO

Denver International Airport (DIA) found out the hard way painted stairs on the parking garage only last 15 years. After learning this lesson, the stair towers were hot-dip galvanized (HDG) to provide durability, and realize sustainable, cost effectiveness on that portion of the $7.5 million renovation for worn out components. Safety has always been a major concern so HDG's projected 50 years or more of life lessened the concern. The HDG coating is harder than the steel itself making it perfect for the abrasive wear and tear of the 600 million passengers passing through the airport. The airport environment is very corrosive due to engine exhaust and exteme temperatures of hot and cold weather. Painted stairs in the airport environment have a proven track record of premature failure, and other forms of corrosion protection such as weathering steel would have required tremendous design modifications. Hot-dip galvanizing with it's metelurgical bond was the wise choice.

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© 2012 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.