Bridge Load Ratings

Posts that don't fit in the other train categories. Off Subject Chit Chat I tell you. :)
Raildudes dad
Roadmaster
Posts: 4753
Joined: Fri Dec 23, 2005 9:12 am
Location: Grand Rapids, MI

Bridge Load Ratings

Unread post by Raildudes dad »

Ok, here comes Load Rating 101.

I cannot copy the posting sign(s) for Michigan. You will have to look up the Michigan DOT posting guidance document.

http://www.michigan.gov/documents/mdot/ ... 7901_7.pdf

This where this sign is a joke. The 3 types of trucks shown cover 28 different truck / trailer/ axle configurations. So if I’m driving a conventional 5 axle tractor trailer like you see all over the country I might be allowed to cross without any restriction but if I’m driving a tandem tractor, 4 axle dump trailer I might be restricted to the tonnage on the sign. How is a driver supposed to know?

Another “joke” for this analysis is “we” (me) have to analyze 28 different combinations. You can take a peek in Part 1 – Section 2 for the axle configurations. Fortunately the computer and software helps do the calculations. The ridiculous part of this is some of these axle configurations are not manufactured. “We” have bridges where the Truck 2 controls. I’d like to have someone find me a truck with 9 feet from front axle to the first tandem. Back in the olden days “we” manually analyzed 5 combinations.

http://www.michigan.gov/documents/mdot/ ... 8697_7.pdf
http://www.michigan.gov/documents/mdot/ ... 8698_7.pdf

Ok, moving on:

To address Alex’s comments.
Load limits signs are erected if needed regardless of length of the bridge. Any legal axle load combination that rates at 1.0 or lower gets posted. It does matter where on the bridge the load is when load rating a bridge. When a road authority posts a single load limit tonnage, they can be playing games. They should post the 3 truck types. If one truck type is restricted, the other 2 should be legal load tonnage. But that goes back to why the sign is a joke.

Here are the load restrictions for the Big Mac
Truck Regulations

Travel in right lane – no passing.
500-foot spacing – no tailgating.
Use four-way flashers.
Maximum speed is 20 mph.
Rules Enforced from toll plaza to Jamet Street exit.
Trucks with a gross load of over 40 tons must request escort prior to crossing.
Maximum allowable vehicle weight is 72 tons.


Axle loadings are significant as well. Most states limit gross loads to 80,000 lbs. 16,000 steer axle and 2- 32,000 tandems. (Length from kingpin to the trailer tandem gets in the equation as well but that’s Load Rating 201 :wink: ). So why does Michigan allow 154,000 gross? When one does the calculations, a 16,000 steer axle with 13,000 axle loading with 3'-6" spacing to 9 foot spacing doesn’t create any more stress than the 32,000 lb tandem. All those axles on the 11 and 13 axle combo’s are 80% of the “Interstate” tandems.

Amy questions class? :)

GreatLakesRailfan
Railroadfan...fan
Posts: 4828
Joined: Wed Aug 24, 2005 12:28 am
Location: Marysville, Michigan

Re: Bridge Load Ratings

Unread post by GreatLakesRailfan »

The signs aren't the only joke with the load rating system. I'm aware of at least one county whose weighmaster was (is?) a joke as well. What purpose is there in putting a weighmaster on the road commissions payroll, if he doesn't have the ability to ticket overweight trucks?
~ Charles W.

Raildudes dad
Roadmaster
Posts: 4753
Joined: Fri Dec 23, 2005 9:12 am
Location: Grand Rapids, MI

Re: Bridge Load Ratings

Unread post by Raildudes dad »

Our weigh-masters can write over weight tickets.

hoborich
Railroadfan...fan
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Joined: Thu Nov 13, 2008 1:05 am
Location: Northern Michigan

Re: Bridge Load Ratings

Unread post by hoborich »

So why does Michigan allow 154,000 gross?
Cause the trucking lobby is more powerful than the taxpayers. Next question?
"Ask your doctor if medical advice from a TV commercial is right for you".

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