Look at the tire pressure in each tire. This will be the psi on the road. check to make sure that measure the square inches each tire has in contact with the ground. also look at the AASHTO tables. for loads on streets from different types of vehicles. My guess would be in the nieghborhood of 65-80 psi.
Some tire pressures are as high as 100 psi. I've used different weight distributions (rear/front) in the past: 80/20, 70/30, 60/40. When I have no information on the actual vehicle and load, I use the most severe case for design. For tandem axles, I assume that the axles share the load equally. That usually means an 80/20 split with each tandem axle carrying 40 percent of the total weight.
To get a handle on the importance of load distribution, run the calcs on the three cases I listed above.
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
Hold it folks, one of my business functions is the operation of a very large tandem axle unit, and therefore I wish to share some of my knowledge.
First of all, completely disregard the 80/20 split, or any other ratio because it is meaningless. For design purposes, use an axle rating that would be average, or worse case. A typical axle for gravel trucks would be 46,000 lbs or 48,000 lbs. In most cases however, it is not the axle rating that will limitl loads, but tire size. The most common tire for the rears is 5,500 lbs per tire, therefore the max loading will be 44,000 lbs....on the rears only. Since you are trying to enable a design, the splitting of the weight 80/20 is not advised because it is not a realistic value.
Let's take my truck for instance. My gross vehicle weight is 58,000 lbs. My tare is 30,000 lbs (empty) and my front and rear tires limit my loading capacity. I load to about 43,000 lbs on the rears, or about 28,000 lbs of gravel/sand from the pit.
My point is that for an actual surface loading, each rear tire is loading the road contact surface to 5,500 lbs. THe psi at the tire contact is about 60 to 75 psi (depending on the tire width and contact area).
Also, tire pressure has nothing to do with it's load rating. The recommended tire pressure on my front tire is 125 psi, and it's recommended load limit is 8,500 lbs. My rear tire pressures are 100 psi and they are rated for 5,500 lbs.
For your design, use the very common rear axle split of 46,000 lbs (2 x 23,000 single axles), even though the tires will likely limit it to 44,000 lbs or so. The equals 5,750 lbs. per tire.
Let's see - 43,000 pounds on the tandem axles divided by a gross vehicle weight of 58,000 pounds. That's about 74 percent of the weight on the rear, and 26 percent on the front. Fits right there between 80/20 and 70/30.
Using an 80/20 split and a GVW of 58 kips, I'd assign 0.4*58,000 = 23,200 pounds per axle - or 5,800 pounds per tire. Compare this to [blue]KRSServices[/blue]'s 5,750 pounds per tire. I don't mind being off by 50 pounds in 5,750...and on the conservative side, to boot.
Don't get me wrong - it's always better to have vehicle-specific loads and vehicle counts to use when designing roads. But it's also important to know what range of values represents reasonable results, too. And I have seldom had a complete traffic analysis to use in design; with no crystal ball to know what will appear on the roads in 10 or 20 years, either.
Remember - "It's better to be approximate and correct than precise and wrong."
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
LOL, don't get me wrong, the entire purpose for my rambling was to demonstrate that your rule, is not incorrect, but should be premised on realistic fugures. In a highway design, for example, I would utilize the loading based on the 46,000 lb axle rating and then compare the results to a 50,000 lb axle rating. Since 46,000 lb ratings are common today, that is a good start, however, who knows what will be allowed in the future (tandems vs. tri-axle and super-B). Anyway you get the picture. The one point that I wanted to strive for was that commonly, the axle rating should be a good start for your design premise, not gvwr's. Of course, these must then be converted to frequencies and assumed ADSL's to enable the design of the structure.
What do you do in the case where the gvw on the unit (trailers) is 80,000 or 100,000 lbs? The 80/20 does not quite work well, yet the axle ratings remain the same, there is only more of them. The loading limit on the road surface per tire remains constant as 5,750 lbs. Applying a split in this case would lead to a different design criteria. The bottom line is to utilize whichever methodolgy the designer is comfortable with. I really wanted to clarify the tire air pressure to loading comments.
If the pavement has to be designed to AASHTO standards, you could have axle loads of 32,000# (HS20-44) or 40,000# (HS25). Sometimes, highway projects are to be designed for even heavier truck loading.
I wanted to tell all of you who have responded what a great experience this has been. I am certainly excited that I was able to sign up for Eng-tips. Thanks for the comments.
You're right, of course - the ratios don't apply to everything, and an 80/20 split will get you in the wrong arena with school buses, for instance. But they can help in evaluating some loadings, when you don't have anything else to go on -
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
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