Load Rating - Existing Bridge to be Demolished 4

[TempStructures]

I am attempting to compare excavator loading on a bridge with rated loads. My typical approach is to compare load effects of design load with proposed equipment. In this case, contractor is trying to maximize equipment size and I'm looking to bridge rating to sharpen my pencil. The bridge is being demolished, it will never be opened to public use again.

The design live load is H-15, built in 1930's. Load Factor method is noted. See attached photo for rating report excerpt.

Is the following the correct way to compare moments using the Rating Factor:
H-15 Moment x 1.32 Rating Factor >= Excavator Moment

I am using Lever Rule to determine distribution factors to girders and all comparisons are on a girder basis.

Thanks for any feedback.

 

[SlideRuleEra]

Crane cannot be used for overhang for sequencing reasons. There are limited RR closure windows to work in.

Understood. See in a previous thread that you are the contractor's employee (I can identify with that). Why the push to maximize size of excavator? (Expect the quick answer to be "speed").

From my experience, overhang removal is one of the easiest parts of bridge demo. Overhang is going to have to be removed is small chunks to minimize chance of damage below. From the drawing, the overhang does not appear to be particularly robust.

A smaller machine should be more nimble, capable of the job in a timely manner, and able to maneuver without each movement being a "nail biter" from concern about overload.

http://www.SlideRuleEra.net

 

[JLNJ]

The wheel load on a big excavator might be considerably more than the wheel load expected from an H-15 truck, even if the total load of the vehicle is less. Don’t forget about local effects.

 

[TempStructures]

All good points. We have an outside 2nd check engineer on board; wanting to avoid errors.

Can anyone address the question of correct use of Rating Factor? This is my main question:

Is the following the correct way to compare moments using the Rating Factor?
H-15 Moment x 1.32 Rating Factor >= Excavator Moment

My main concern is that I'm not missing some load factors in multiplying H-15 moment (service level) x 1.32 Rating Factor (Load Factor Method) to compare with Excavator Moment (service level).

Please advise.

 

[Lomarandil]

Once you are comparing the service level load rating moment and shear envelope* with your excavator's service shear and moment envelope, the factors have been cancelled out. *E.g. (HS-15 envelope * RF) / (operating load factor for vehicle)

As long as excavator travel speed is limited (say 10mph), deck condition is good, and you've accounted for track pressures separately, low to zero impact factors are appropriate. Just like an overload permit analysis.

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just call me Lo.

 

[TempStructures]

Thank you Lomarandil, et al. Look forward to contributing more to this forum. I've mainly been a lurker.

 

[Lomarandil]

Glad to have you around.. I always think that construction and demolition engineering problems give us some of the most room to be flexible and creative. It's a good niche.

----
just call me Lo.

 

[BridgeSmith]

I've never used rating factors in this type of analysis. It's always been a straight application of the loads, calculation of the resulting moments and shears, and comparing to the allowable stresses on the components. In your case, since yielding is not a concern, but only avoiding collapse, I'd suggest comparing the applied stresses to the buckling capacity of the girders, paying particular attention to the changing bracing conditions as the demolition progresses.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

It is what I though should be. The risks/applied forces are differ for an old bridge in continued service, and one subjects to destructive force from demolition.

 

[TempStructures]

Straight application of loads needs to take into account any deterioration on these old structures. That is why reliance on Load Rating is key. It is the same process DOT's rely on to allow overload permits, etc.

Yes, paying attention to stability as deck is removed is key. Make sure those old riveted diaphragms are up to the task!

 

[Lomarandil]

Retired13, maybe you are confused between the initial bridge rating/design loads and the load rating/rating factor?

Because using a load rating does account for continued service and aging.. which is why we use it to check that bridges are still adequate for everyday traffic!

----
just call me Lo.

 

[Settingsun]

Are you working out the HS-15 moments and shears etc, or were they given in the report? Were the capacities not given? I'm wondering why you can't go direct to assessed capacity and need to use the rating factor at all.

Your method sounds ok in principle but it's something of a concern that you don't seem to have enough of the rating methodology and assumptions from the report to know with certainty how to make the comparison. How do you know your analysis is consistent with theirs?

On responsibility as queried by others, the demolition engineers are choosing to rely on a report beyond what it was produced for and are modifying the structure. The original engineer might wear a little cost but I can't imagine much.

 

[r13]

Lo,

Read again. I think you were confused by my bad writing.

 

[BridgeSmith]

That is why reliance on Load Rating is key. It is the same process DOT's rely on to allow overload permits, etc.

DOTs use load rating as a way to assess the adequacy of a bridge for a number of representative vehicles that approximate the massive variety of different vehicles using the bridge. It allows you to remove the dead load and just evaluate the capacity available for various live load types. In evaluating a single bridge for a specific loading, doing a load rating is just extra work, unless you're trying to use it as a way to avoid doing a capacity analysis of the structure, which I wouldn't recommend.

An excavator has a much different axle gage and loading distribution than the H-15 truck, or any truck, for that matter. With there being deterioration and changing bracing and loading conditions as the work progresses, I don't see that you can get around manually calculating the loads and the capacity and doing a straight up check of DL + LL < capacity.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Lomarandil]

You cannot get around manually calculating the loads and demand from the excavator. Agreed.

However, if the capacity envelopes for a design vehicle acting on the bridge, as scaled by the rating factor, totally envelope those excavator demands for the operation -- why spend the time to re-build the bridge capacity element by element? Especially for older bridges designed for material efficiency (multiple cover plate cross sections, etc.), that can be quite an undertaking.

(Speaking here to a operation like overhang removal which will not reduce the capacity or remove bracing of the main bridge elements. If you're taking the bridge deck and bracing down with equipment working from the bridge, that's another question, and will require the detailed and staged analysis.)

----
just call me Lo.

 

[BridgeSmith]

...if the capacity envelopes for a design vehicle acting on the bridge, as scaled by the rating factor, totally envelope those excavator demands for the operation -- why spend the time to re-build the bridge capacity element by element?

While it's possible back the capacity out of the load rating, it's more work and more prone to errors than just calculating the capacity of the components. If the capacity needs to be reduced for damage, section loss, or changing bracing conditions, the person who's checking the sufficiency of the bridge needs to understand how the capacity of the bridge was calculated for the rating. A person that understands how the rated capacity was determined, can and should calculate the capacity for themselves. Trying to scale something from a truck load to an excavator load, by just doing some fancy math, but not understanding why the structure has the capacity it does, is a dangerous way to conduct the analysis. That's why I said I don't recommend it. Taking shortcuts in bridge analysis is a shortcut to potential disaster, IMO.

Rod Smith, P.E., The artist formerly known as HotRod10