Scrapers and off road haul trucks on new bridge 2

[OSUCivlEng]

I got a request from a contractor who wants to drive scrapers and off road haul trucks on a new PC beam bridge that I designed. My first response was to send me the model information and wheelbases for the equipment. I haven't received that information yet.

However, thinking about it some more, I seriously doubt there is enough capacity in the bridge. I looked up one of the scrapers I know is on the job, a CAT 651E. The loaded front axle weight is 126 kips and the loaded back axle weight is 112 kips. The axle spacing is 32.5 feet.

Anyone ever done this before? How did you distribute the wheel loads to the girder, because AASHTO distribution factors don't apply to something like this. Also I suspect the impact load is much different, though you might be able to control that with a speed limit on the bridge.

 

[BridgeSmith]

You'll have to calculate the distribution factor (DF) by hand if the wheelbase is significantly more than the 6ft the DF in the specs assume. The most common way would be using the lever rule (assuming the deck as simple spans between girders), but depending on the superstructure type, that may be fairly conservative.

Figuring out how slow they'd have to go so that there's no impact could be tough. Per AASHTO, it's 5mph, but that's for highway trucks with typically 80psi in the tires. What it would be for a scraper with much larger tires at a much lower pressure is beyond my knowledge. I think one could confidently assume slowing them to 5mph would be adequate to get impact to a negligible level.

I wouldn't assume that the scraper would overload the bridge. After all, the HL93 truck load, including load factors, is 126 kips, and the design loading includes a lane load on top of that. If the load is distributed over more girders that a standard truck load, you may find that it's adequate. Much depends on the girder spacing relative to the wheel gauge distance (axle length) and the bridge span lengths.

Rod Smith, P.E.

 

[BridgeEI]

Several of the OL-1 trucks are near 200kip so I bet it checks closer than you think. Is it a 1 time thing or using it as the main movement from one side to the other? If it was unloaded and a one time deal I think it would check. But I don’t know particulars on the span etc so hard to say.

I would use the lever rule to distribute like Hotrod mentioned. I kinda think your deck may control more so than the beams since you would have maybe 2x the normal wheel load.

 

[IDS]

Why not do a computer analysis and check the deck for the resulting actions?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeSmith]

IDS, short of a finite element analysis, checking the capacity of a concrete deck carries a large amount of uncertainty. However, the deck is not typically the controlling component for the loading capacity, anyway. Usually, the capacity of the girders will be more critical, even for a load of this magnitude, due to the larger contact area of the wheels.

Full-scale testing has shown that there is considerably more horizontal arching action within a concrete deck than traditional analysis techniques account for, provided that a few criteria are met (See the empirical deck design provisions in contrast to the traditional design in the AASHTO LRFD Art. 9.7). The results of the testing showed that concrete bridge decks essentially do not fail in flexure; when they fail (at around 5 times the traditionally-calculated capacity), it's actually a punching shear failure mode.

Computer analysis is the typical way to analyze the loads and capacity for bridge girders, but it requires the proper distribution factors to be input, so the correct loading to each girder is applied in the analysis. For a vehicle that has a non-standard gauge distance between the wheels at an axle, the distribution of load can be determined conservatively by hand with minimal effort (using the lever rule). If the girders are adequate for the loads using the simplified, conservative approach, there is no reason to expend the effort for a more rigorous analysis.

Rod Smith, P.E.

 

[IDS]

Computer analysis is the typical way to analyze the loads and capacity for bridge girders, but it requires the proper distribution factors to be input, so the correct loading to each girder is applied in the analysis. For a vehicle that has a non-standard gauge distance between the wheels at an axle, the distribution of load can be determined conservatively by hand with minimal effort (using the lever rule). If the girders are adequate for the loads using the simplified, conservative approach, there is no reason to expend the effort for a more rigorous analysis.

Use of a computer grillage or plate and beam analysis for bridge design is standard practice outside the USA and doesn't require the use of pre-determined distribution factors. For analysis of the distribution of non-standard loading with very high wheel loads it seems to me to be the easiest and most reliable way to go.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Lomarandil]

Yes, I've typically found that limiting equipment to walking speed reduces impact to negligible levels. If you end up requiring a defined travel path, the slow speed won't typically be a lot of burden on the contractor anyway.

All of the advice above seems sound in my experience. I've previously supplemented permanent bridge decks with timber mats (especially if you fabricate up timber mats with filch plates) to improve load distribution in a few cases. It's a lot of work with limited payoff, but sometimes is just enough to make something work.

----
just call me Lo.

 

[BridgeSmith]

From what I understand, those are often good options, IDS, depending on the type of superstructure. However, as you say, they're not widely used in the US, and I'm not familiar enough with them to give advice on how to use them, so I confined my comments to what I know, which is the method in the AASHTO spec.

Rod Smith, P.E.

 

[PEinc]

BridgeEI makes good points. We don't know if this is a temporary or permanent bridge, The attached PDF shows how an Acrow modular truss bridge (80' span) was used for BIG, heavy (324K GVW), off-road trucks to cross a creek during highway construction. If the existing bridge is not strong enough, perhaps a similar Acrow bridge would be a solution.

www.PeirceEngineering.com