Galvanized Bridges

[Bitmapped]

It seems fairly common for newer pony truss bridges to be made of galvanized steel, but other than that, I don't see galvanized bridge beams very often. With galvanization being common for other steel highway components like light posts, guardrail, and sign structures, is there a reason it's not used more often for bridges?

[Dirt Roads]

It seems fairly common for newer pony truss bridges to be made of galvanized steel, but other than that, I don't see galvanized bridge beams very often. With galvanization being common for other steel highway components like light posts, guardrail, and sign structures, is there a reason it's not used more often for bridges?

From what I can gather, galvanized steel is preferred for bridge construction.  However, it's not so much an issue for bridges to be constructed from galvanized steel, but rather an issue on how to galvanize extremely large bridge elements such as stringers and long girders. 

The primary (and most cost-efficient) method of galvanization is still the hot-dip method.  For a maximum-length 120-foot stringer, you need a monstrous hot bathtub full of molten zinc heated to 450-degree Fahrenheit over the entire tub and maintained there for about an hour.  That is certainly an expensive endeavor; smaller steel sections are easier to handle and take less time to fully galvanize.  Needless to say, there are similar issues related to clean, pickle and purify (adding zinc flux) to the stringer. 

Other bridge elements such as steel box beams and steel box girders have voids that are intrinsically difficult to galvanize.  I highly suspect that the temperature of the molten zinc inside the voids needs to be maintained at a similar temperature as that outside of the steel beam/girder.

[kalvado]

One thing may be the lifetime of the bridge. It's not like sign posts last 100 years.
There is also som effect of steel embrittlement, mostly for very hard steel though. Acid etch before coating can leave some hydrogen in metal, trapped after plating.
Then there are some interesting slow-rusting steels (remotely similar to stainless, but cheaper) where slow loss of metal on thick beam can be less critical %% wise than smaller structures...

[Dirt Roads]

One thing may be the lifetime of the bridge. It's not like sign posts last 100 years.

I totally agree with you, but the bridge building industry is convinced that 75-year galvanized steel elements is an improvement to bridge longevity.  The industry is closely watching the galvanized stringers on the Stearns Bayou Bridge in Ottawa County, Michigan (near Grand Haven).  Constructed in 1966, it was the first bridge to use hot-dipped galvanized steel stringers and is approaching 60 years old.  From all accounts, the stringers have not required any maintenance whatsoever and the zinc coating is still pretty thick.  Strength is not really getting tested here, as the bridge only has two 60-foot spans (and six 50-foot spans).  However, highway researchers did note that a pedestrian walkway was added in 2008 (but it is supported by new galvanized stringers, and not attached to the original ones).

There is also som effect of steel embrittlement, mostly for very hard steel though. Acid etch before coating can leave some hydrogen in metal, trapped after plating.
Then there are some interesting slow-rusting steels (remotely similar to stainless, but cheaper) where slow loss of metal on thick beam can be less critical %% wise than smaller structures...

For much of my early career, I got stuck with the installation of early versions of galvanized signal gantries and cantilevers.  Partly because of embrittlement concerns, the entire railroad industry switched over to aluminum structures.  Nowadays, those galvanized stand as shining testaments whereas their aluminum counterparts look dull and dingy.  The bridge industry is totally convinced that their hot-dipped technique produces high-quality thick zinc coating that will not damage the internal steel structure. 

On the other hand, bridge designers don't seem to be as convinced with the galvanization of diaphragms, connecting pins and bearing supports.  On the Stearns Bayou Bridge, the zinc coating thickness of the miscellaneous elements after 25 years was similar to that of the bridge railings above (averaging say 60% of the zinc thickness on the stringers themselves).  After 51 years, the stringers and railings are still going strong (but I noted that they didn't give inspection results for the other components).

Anywhoosit, the industry seems to currently be focused on painted galvanized steel to increase the lifespan to 100 years.  The paint will give an additional layer of corrosion protection, but require some level of repainting.  I might be wrong, but it seems like the goal is to get through the entire 100-year lifespan with some maintenance but no overhaul (ergo, replacement of bearings, pins, diaphragms, etc.)  One study indicates that researchers believe that they can make the 100-year lifespan goal by simply allowing the paint to disintegrate over time and allow the zinc coating to ride out the rest of time.  That might look goofy, but for some bridges aesthetics are not an issue.