RE: Trucks vs. Cars

[jheidt2543]

I am re-reading a paper by Alfred M. Freudenthal, “The Safety of Structures”, ASCE Transactions, Volume 112, 1947, Paper No. 2296. In it is the following quote pertaining to highway and highway bridge loadings:

“According to registration records, between one third and one fifth of all motor vehicles are trucks. … It is anticipated that future expansion in the number of motor vehicles will be in passenger cars rather than in trucks. A reduction in the ratio of trucks to the total number of vehicles registered, amounting to one sixth by 1960 has been predicted.”

In thinking about this, it seems that the prediction of a 17% drop may even have been low for the 1960 period. But, now my impression is that the ratio of tractor-trailer trucks to the total number of vehicles in the traffic stream is significantly larger (I’m considering SUV’s passenger vehicles not trucks for this discussion). Of course, this would impact the fatigue life of bridges.

My question is, what does current research have to say about the proportion of trucks to cars and is the trend on the rise? I am trying to get on the Federal Highway Administration's, but wow is it slow today!

 

[MotorCity]

From a structural standpoint I don't think it makes a difference. If one truck or 2 million trucks pass over a bridge, it should still be designed to carry such a load regardless of the car-truck ratio.

 

[dimjim]

I too have wondered about the surfaces where
they are now scoring the road noticeably
for extreme weather conditions and how much
effect the extra vibrations is deteriating the
sub structure and life.

 

[jheidt2543]

MotorCity:

I beg to differ, while the maximum design load MIGHT not change (it could based on the increase in tandom trailers etc.), the fatigue life of a structure would definitely be shortened by an increase in the ratio.

The main reason I asked the question though, is to see how accurate the 1947 prediction of the trend of the ratio to 1960 was and what the ratio now. Even though there are design codes that tell us what to use for a design load, the engineering judgement to select a more conservative design based on expected increases in loads - say for highway or railroad bridges as an example - is basically a guess. How good are those guesses?

 

[jike]

On the south side of Chicago on I-294, it sure seems that the ratio is 2/3 to 3/4 trucks and 1/3 to 1/4 cars, especially when you are sitting in traffic. I wonder if those bridge designs anticipated that much truck traffic?

 

[MichSt]

I did a quick google search on this topic. Here’s an article from the TRB you might want to take a look at.

http://pubsindex.trb.org/document/view/default.asp?lbid=469455

You should be able to read through the abstract and compared measured truck loads to the AASHTO design trucks.

Also, with regard to truck traffic in general, I believe more a more semi-truck freight is being carried via train due to increased fuel costs.

 

[MikeHalloran]

A lot of freight travels long distances by train, but it rides the train in truck trailers. Sea containers also reach their ultimate destination via the highway system.



Mike Halloran
Pembroke Pines, FL, USA

 

[LittleWheels]

The story of the Severn Bridge and the effects of changes in loading over time is reasonably well-known.

http://www.severnbridge.co.uk/history_strengthening.shtml

The popular story is that the designer was driving over the original Severn Bridge early one morning and noticed that he was virtually the only car amongst a fleet of lorries. When he got to work, he calculated that the bridge could fall down if there was a traffic jam of loaded lorries (rather than a mix of cars:lorries) and initiated the strengthening.

Apparently the bridge met the standards current at the time of design but, apart from the increase in lorry traffic generally, the traffic loadings didn't allow for variations in the ratio of cars:lorries. At times, the majority of vehicles on the road are lorries, although they are in the minority when averaged over a day.

 

[jheidt2543]

DaveMinter:

You hit the nail on the head! Most codes of the past were only updated every ten years or so. Now, most are on a 3-year update cycle. I don't like it because of the cost of renewing manuals and code books, but it does address the problem in my original question. Cynically, the sale of the code changes raises a lot of money for the code organizations and the seminars they give to explain new codes.

The paper by Alfred M. Freudenthal noted in my first post addresses this very topic and the author proposes a probability based approach to loading and factors of safety. This seems more resonable and puts the decisions on a rational basis and back into the hands of the design engineer.

 

[jen4950]

Fatigue and associated pavement and highway designs are based upon ESAL's- Equivalent Single Axle Loadings if I remember correctly.

Strength, Servicability and Fatigue are 3 entirely different beasts. If something is designed for it's limit state strength capacity- that doesn't neccessarily mean it will meet the required deflection criteria, and certainly doesn't mean it will survive a hundred million cycles of 99 percent of it's strength capacity.


(been a while since grad school- so forgive the misinformation if my memory has failed me..)

 

[TpaRAF]

The AASHTO LRFD design code uses a notional vehicle, which is a three axle truck, along with a uniform lane load. The design vehicles also include a tandem truck and the military loading. All these add extra axles to the loading.

So ... I think MotorCity's opinion of the design load is correct -- the bridge is designed for a maximum load. The passenger-car type load isn't even considered.

Regarding fatigue, this is generally a consideration for steel girders. The members and connections are generally designed for "infinite life" by limiting stresses to 50% of the fatigue threshold. For concrete, additional reinforcing is added to insure fatigue stresses do not control.

I do not think the vehicle mix is a consideration for the AASHTO design codes.

Regards, RAF