bridge loading

[AD10]

I'm trying to determine the appropraite desing loading of a bridge to accomodate off-highway type articulated dump trucks. Any thoughts would be appreciated. Thanks in advance.

 

[JAE]

I'd design it to the AASHTO specification. They have truck loads that are generally accepted for most uses. If you have specific trucks that are heavier, off-road types, then I'd get specific data on those trucks.

 

[unclesyd]

Is this for the bridge in Virginia that was in the news today? The new said it was case of a 6 ton truck on a 3 ton bridge and the boom hit the lateral brace.

 

[DRC1]

Typical truck is 40 ton, of which 20 ton is payload. Alternate vehical slightly larger. Off Roads can be significantly larger. Best check with who ever will be using trucks for guidence. Otherwise I would talk to Catipiller. Plants have to size crossings etc. for these trucks and they may be able to give you some design standards.

 

[oldrunner]

You need to know the weight and axle spacing of the particular trucks that you will be designing for. We have designed small bridges for huge axle loads for trucks carrying rocks from a quarry to the crushing plant. We also considered trying to limit the speed of the trucks to reduce the impact forces, but the drivers didn't pay any attention to their speed. Are these trucks wider than one lane?

 

[AD10]

thanks for all the responses.

the trucks we're looking at are/similar to a CAT 3-axle articulated truck (model 730). they have a GMW of 110,000 pounds and an operating width of 9'6"

 

[jimaitken]

You can use the AASHTO Standard Specs for Highway Bridges 17th ed. chapter 3 for determining the impact and distribution of the wheel loads. I would agree with oldrunner that you need the axle spacing, but what you specifically want is the weight per axle. Then use the AASHTO distribution factors to get the axle loads on a single beam. There are different rules for moment distribution vs. shear distribution as well as the girder type and deck type combination. Chapter 3 outlines this.

Is this a temporary or permanant bridge? It sounds temporary. You may want to look at the temporary works specification that AASHTO publishes to see if there are any requirements that can be overlooked for the temporary structure. Obviously fatigue will not be an issue for a construction brigde constucted from new steel.

Hope this is helpful.

 

[JStephen]

Consider whether the customer might buy heavier trucks at some point, too.

 

[Dinosaur]

Before skipping fatigue checks, consider the possibility that the bridge will be used in a highway application in the future. I would ask the client about his future plans at the site and when the design is over issue a clear statement regarding the maximum loading and its HS rating in case the bridge is someday released for public use.

 

[jimaitken]

The point I was trying to make in my last paragraph was not to find ways to skip critical design steps and checks, but as everyone is saying in their posts, to determine the usage of the structure and design using the appropriate code or guide specs to determine the loads and complete the design. If this is a temporary structure then why design it for a 75 year service life and over 2,000,000 cycles when a less stringent code can be applied. The design should be suitible to the intended use.

I recommend looking at AASHTO Standard Specifications for Highway Bridges 17th edition (2002) and AASHTO Guide Design Specifications for Bridge Temporary Works (1995 or latest edition). Some combination of the information in these two publications may be applied as engineering judement dictates.

 

[AD10]

thanks again for all the posts.

the bridge will be permanent. the vehicle I noted previously with a GMW of 110,000 lbs has an axle distribution of 30%, 34.9%, and 35.1% between the front/steering axle and the two axles under the dump body.

from research I've done, the design rating would probably be HS-25, the HS-20 loading is max'd at 80,000 lbs.

 

[MichSt]

Keep in mind that if the axle spacing on your vehicle is less than an HS truck the loading on your bridge could be more severe than HS-25 even though the overall vehicle weights are the similar.