Exclude lane loading? 4

[adamewood]

I am a recent grad, entry level structural engineer. A problem recently came up at work, where a client wants to drive a 6-axle, 180k drill rig over a 75ft span single lane bridge (with ends fixed in concrete headwall).
 
Is it ok to analyze this bridge assuming a single pass of the drill rig with no other applied loadings (i.e. do i have to apply the 640plf lane load specified in aashto lrfd)? I was under the impression that this lane load is for cases involving a train of hs-20 trucks, one after another. However a single vehicle making a single pass would only require one wheel line traveling on top of one girder (multiplied by a load distribution reduction factor). Is this correct?
 
I have found the girder to be over capacity with the lane load applied, and under capacity without the lane load, which is why i ask.
 
Thanks for all your help!

 

[FlashSet]

I do not know the answer, but wouldn't this vehicle have a train of support vehicles? At least a vehicle in the front and one behind with flashing strobe lights and "oversized load" signs?

 

[adamewood]

We can request that the client send only one vehicle across at a time.

We have done this before, but I have never had to analyze a bridge for such a heavy vehicle.

The bridge has no posted weight limit, and finding a detailed inspection report is proving difficult. I think the bridge should be fine, but obviously I would like to prove this.

Thnx for your response.

 

[bridgebuster]

You're correct, lane load only applies to HL -93 loading (LRFD). You can analyze your bridge with just the rig, make sure you specify that no other vehicles should be on the bridge (other than a passenger car or two).

 

[crossframe]

A six-axle, 180k drill rig on a 75-ft single span bridge should be analyzed as a permit load. You can certainly specify no other vehicles on the bridge if that's what it takes to avoid overstressing the girders. A six-axle vehicle will probably occupy a decent amount of the span length by itself. If you want to allow HL-93 (or any other) loading in front of or behind the drill rig, you can do that too. Since it's not a routine occurance, you can also take Operating Rating / Service Load II type allowances, which will reduce the load factor for live load.

 

[gwynn]

As others have noted, it is correct to analyse the bridge with only the drill rig on it, so long as it is specified that only the drill rig will be on the bridge. I'm not familiar with AASHTO design and overload procedures, but some codes will also allow a reduction in the dynamic load allowance if speeds are limited to certain amounts. Though this generally requires that the crossing be supervised by a professional engineer.

 

[adamewood]

Thank you all for your help. I saw that the aashto Manual for Bridge Evaluation includes a live load factor of only 1.15 for one-use heavy vehicles. I combined this load factor with other factors for Sevice Load Type II as crossframe suggested (in which just about all load factors are 1.0).

Thanks again for all your help. Feel free to offer any more suggestions!

 

[a7x1984]

Why is this "super load" not being submitted through the state's hauling permits section and analyzed by the appropriate bridge eng. section? Or is it? I ask, because I am in that section.

Before you doing anything creative, make sure the code likes the artwork you are trying to wow the world with.

 

[gwynn]

Though I am not the OP and do not know the answer to your question in this instance, at first glance it reads like a bridge on a mine access road. Here, those are not submitted to the authority as they are privately owned and do not allow public access. We also see a number of cases where overload evaluations are farmed out or the client gets approval to have an outside engineer do the evaluation due to time constraints.

 

[a7x1984]

gwynn - That makes more sense if it is a privately owned bridge. As far as public roads: I do see some overweight trucks that are evaluated by others, but I have never seen something that only weighs 180kips be outsourced for analysis.

In Russia building design you!

 

[gwynn]

After doing the weight conversion, I agree that seeing something that light be outsourced or done by outside sources due to the carrier's preference would be odd. I usually only see that for things like transformers.

 

[a7x1984]

Adam: for future reference - Try using WSR instead of LRFR with DF of S/7.0 (single lane) and impact = 1.10. That is how we do it in Maryland; it is quick and conservative in most cases.

Also: "but some codes will also allow a reduction in the dynamic load allowance if speeds are limited to certain amounts."
In MD that limiting value is 5 mph.

There is a reason Maryland his an 8% structural deficiency rating...

In Russia building design you!