Forum Discussion
Horizon170 wrote:
Non related question.
When a sign says 10 ton limit, does that mean no vehicle over ten tons or the combined weight of all vehicles on bridge may not exceed ten tons?
I think the latter.
It is a good question. I've been involved in the design of more than 25 bridges in my professional career.
The answer is, . . . . it depends. It depends on the mode of failure which can change as the bridge ages. For suspension bridges there are multiple modes of failure. You can have a failure of the main span where the whole main span collapses, or, you can have smaller but equally problematic failures like punching through the bridge deck because of an overloaded axle/wheel. You can also have a failure of the approach structure. There are others too.
Suffice to say that you can cause a failure because of excessive combined weight or because of excessive point (axle/wheel) loads. Best bet, if the sign says 10 tons, the combined weight of your vehicle should not exceed the limit sign. As you can see from the video, going significantly over the limit doesn't always cause a failure, but it doesn't help. Abuse over time adds up.
Chum lee- I found it interesting to see the dynamics of a suspension bridge in action but I agree with everyone about the driver being a knobhead.
- In the County I live in, the power company sent a 50 ton crane with pole trailer over a bridge that was posted at 10 tons. This is a one lane wooden bridge built in the 1930's. The front tires of the crane fell through the deck, his front axle caught some stringers before completely falling through. So yeah, those postings mean something. The electric company got a rather large bill for the repairs.
DutchmenSport wrote:
Remember the footage of the Golden Gate bridge in San Francisco when gale force winds caused it to flop around like a tissue paper.
That was the Tacoma Narrows Bridge in Washington state, and it collapsed due to design problems that were exacerbated by the wind whipping through the Narrows. It was later redesigned and rebuilt.Horizon170 wrote:
Non related question.
When a sign says 10 ton limit, does that mean no vehicle over ten tons or the combined weight of all vehicles on bridge may not exceed ten tons?
I think the latter.
My understanding is it's the vehicle weight, not the total bridge weight, unless indicated otherwise. Many posted bridges are small enough and narrow enough that it's a moot point; you can't fit much more than one large vehicle on it at a time anyhow. (There are others, of course, including the one in the video.)
Bridges (and other safety-critical rigging or supports) are generally designed with a safety factor of several times the expected load, maybe around five times for a typical bridge if such a thing exists. A ten ton bridge would be designed to support something like 50 tons without failing catastrophically.- That bus was more likely 35,000 pounds not 35 Ton. but.. Still the driver had no business taking that bridge... THankfully the bridge held.
- The bridge can take it, (for a while) deflection is normal. (just not that much) Just like overloaded trucks on a roadway, it causes greatly accelerated wear which leads to early failures. Commercial drivers should know better than that. That was no accident. Load restricted bridges are clearly marked on their route maps. IMO, they deserve any fines they get.
Chum lee - Non related question.
When a sign says 10 ton limit, does that mean no vehicle over ten tons or the combined weight of all vehicles on bridge may not exceed ten tons?
I think the latter. noteven wrote:
The writer of the article described a “35 ton bus”.
That all you can eat buffet musta bin a good one.
Yeah 70,000 lbs. is kind of tough to swallow. Even half that would be a bit hard to believe.- Both drivers, there was a second bus (probably from the same company) that crossed afterwards that wasn't in the video.
