Bridges and rust

I noticed recently, driving on Rt 95 N of Boston, how the overpass bridges are rusted only on the side that faces oncoming traffic. That is, the side that you see as you drive under it. The side you see in your rear-view mirror is clean. Same applies N or S bound.

After mulling this over, the only idea I have is that salt particles kicked up by traffic hits the bridge mostly on the side facing traffic, so that side rusts faster.

Any other ideas?

There are exceptions, of course. Bridges newly painted or built, or ones that are badly rusted on both sides.

Here is a Google earth photo

Seems logical. I never noticed that before. Interesting observation.

I’d suggest adding sand and dust and anything on the road along with that salt. That debris blasts the paint off so rust occurs.

Your theory of the reason is as good as any I can think of.

Only in the past 10 years have I’ve seen bridges in New England start using Weathering Steel. I’ve seen in being used in NY for over 30 years. But New England is just now catching.

That picture almost looks like a frequency distribution plot. The left lane is less traveled than the middle and right lanes, and all 4 are more traveled than the breakdown lane. So it stands to reason grit particles will impact the bridge more where more cars are, with some outliers from windy days. That would lend support to your theory.


Wonder if bridge designers know about this? Difficult to believe they don’t. But I see no attempt to add extra protection in the areas that are most vulnerable…

I’m not sure it needs extra protection, just fresh primer and paint after a good sanding.

Where I work the seawalls in the ship channel are thick steel, and I can see where rust has bubbled up under the paint. I asked what will happen when the seawalls rust all the way through. Nobody had an answer.

You can always stick a fresh sheet of metal on the seawall.

But as we learned in MN with I-35, rust in the wrong spot on a “critical” design bridge (i.e, one part of the bridge fails, and there’s no back up so the whole bridge fails) can be very deadly.

May that distribution skew is the semi-truck effect?

Many semis travel in the middle lane to avoid clueless drivers on the on-ramps and many states restrict left lane travel by trucks so middle and right lanes are mostly where semi-trucks travel. Trucks kick up a heavier wake of air than cars and should have a greater effect on the sand blasting.

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The kicked up sand/stone dust etc does the same thing to your windshield around here. I travel that route every day.

They just completed the renovation to those bridges this last summer. Media blasted all of them (with the tarping and trucks to capture the media and paint) and then painted them with the strongest smelling epoxy paint you can imagine. All part of the work leading to the new Whittier bridges over the Merrimac.

Yep, it’s always easy to tell the hacks from the consummate professionals. The pros follow lane protocol, knowing the cars driven by clueless merging drivers come standard with brakes.

Just drove by that very bridge and I was wrong. They haven’t gotten quite that far south. The renovated bridges start a couple miles up the road. That particular bridge looks even worse now. :open_mouth:

What does the other side of the bridge look like? That might confirm your hypothesis.



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It looks similar, rust patches in the end facing oncoming traffic, fairly clean other end.

Amazing! It’s as if you flipped the picture!


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Interesting observation.
Perhaps the snowplows and salt/sand trucks have something to do with this phenomenon.

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Yes, the plows that operate after the road is salted throw a tremendous amount of salty slush.

I was going North on I-81 between Syracuse and Watertown in 1972 approaching an overpass where Tinker Tavern road goes over and one of the huge plows was plowing Tinker Tavern Rd. , it threw a wall of mostly Ice off onto I-81 that hit the window and momentarily knocked me out. I awoke in pain thinking I had hit the underpass and was dying, I quickly realized I was going 70 down the road with no windshields and it was way, way below zero. Nothing to do but continue, there were no cell phones or CBs back then and no services short of Watertown. Got to Lomgways truck stop and went to bed while waiting for someone to come and put windshields in. Only minor cuts and bruises and mild frostbite. Had a sheepskin jacket and a wool hat and always carried a wool blanket with me back then. If you broke down, help could be a long time coming.

Now that’s funny.