Let's say your rig weighs 5 tons.
You come upon a small bridge with a sign that says;
WEIGHT LIMIT 4 TONS.

At first, I thought this would be bad.
But, isn't there also a sign on the other side that says the same thing for someone coming the other way?

So, isn't the bridge actually rated for 8 tons?

And if the bridge were long enough for say, four vehicles in each direction, and they were all legal (like 3 tons each) making a total 24 tons, what then?

What I'm trying to ask is what does:
WEIGHT LIMIT 4 TONS, mean?

 

Simple.

Dates back to the early days of motoring when bridges were narrow -- and attempting to pass two vehicles in opposite directions involved a sort of 'playing chicken'.

The term "Chicken" came into the vernacular of the time to describe the risky act of passing two vehicles in opposite directions on the narrow bridge.

As part of a government-sponsored project, the U.S. Department of Weights and Measures observed that when two trucks, each hauling chickens in opposite directions and about to cross a bridge, hit that inevitable Bump which immediately precedes the crossing of any bridge – caused the chickens in each of the two trucks start to fly -- thereby causing the trucks to lose approximately half of their weight – and ultimately allowing the bridge to carry the weight of both of the trucks simultaneously.

The bridges from then on had the same load restriction posted on each end to account for this phenomenon.




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Dang, Raoul. You may have some competition from jgorka. That was a great story about chicken trucks!

While I don't know the answer to the question, I can tell you that I've driven my 3.5 ton truck over a "2 Ton" bridge. I think you'll have to analyze the bridges on a case-by-case basis to determine whether they are "4" or "8" ton capable. There are some old WPA-built bridges around here that I wouldn't walk across!

Perhaps the DOT could answer this?

 

When a weight limit is placed on a bridge, the general rule is that it will support roughly 3 times that amount. Somewhere in the architects and civil engineers diary it is written that loads shall be measured to withstand 3 times the static load listed. Which means if a bridge is rated at 5 tons, its really rated for 15 tons, and it's actually rated for even more than that! The static load (not moving) is less than the dynamic load (definately moving) because a moving vehicle provides up and down and side to side additional forces that need to be accounted for!! So just drive your truck over the bridge and don't worry. If you don't make it, don't blame me... I'm just a Phone Guy!!

 

Think about something like the mackinac bridge, Ambassador Bridge, Tacoma Narrows, etc... if any of those longer bridges were only rated for say, 5 tons, what if there were 4 casr on it? Shouldn't break it. What about when traffic backs up on it? DEFINATELY shouldn't break! They generally figure out the loads with an overdesign of roughly 3, but not for static loading... the bigger problem they face is fatigue loading. (The cycling of a load being repeatedly applied and released) This happens due to axle loads on the vehicles, expansion joints bouncing the vehicle, as well as wind applying forces on the bridge. Fatigue loading is what causes it to fail in the long term. Those weight limits are set in such a way that in theory, if you never exceed that load, the bridge will last to the designed fatigue limit. When you start exceeding those loads, though the bridge will probably not fail at that exact moment, it shortens the fatigue life of the bridge.
I'll ask some CEs at school for some more detailed bridge design info....

(kind of off the subject, but the latest experiments being conducted involve bridges that are reinforced by carbon fiber cables instead of steel re-rod... they're going to be the next major improvement in bridge design... They'll never rust!)

 

Isn't it usually accompanied by a per axle rating too? All poultry aside...

jgorka, you been smokin that wild weed again, ain't cha?

 

So thats where the term playing chicken came from?lol So if I am hauling a donkey in my truck what does that mean? I think that is the only time that a person could honestly say he was hauling a$$.

 

Actually, the term haulin' a$$ is but another mis-nomer and actually changed to that over time.

It seems that the early moonshine runners got to drinking their cargo on those long runs, and it seems that the home-brew mix caused them to, er, pass wind, if you know what I mean.

Since they often got caught in this situation away from their home/farm 'privvy', and since the, er, 'wind' and soon-to-accompany lower-intestinal distress resulted in a "Howling A$$" as they drove like crazy for the nearest (safe from the 'Law') out-house.

You see, it simply was the fact that they drove like wild-men with their Howlin' A$$ a-Tootin and a-wheezin eventually mutated into the term Hullin' A$$.