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Load Rating - Existing Bridge to be Demolished
4

Load Rating - Existing Bridge to be Demolished

Load Rating - Existing Bridge to be Demolished

(OP)
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.

RE: Load Rating - Existing Bridge to be Demolished

I can offer no help on the actual load rating of the bridge - sorry - but I can point you to this recent thread in way of warning. It's about a bridge being demolished with equipment moving across. Don't sharpen your pencil too much...

RE: Load Rating - Existing Bridge to be Demolished

So you're trying to max out a NINETY YEAR OLD bridge???

What is is made of? Timber or concrete?

Doesn't matter - Anything that old has a high risk of collapse from heavy point load machinery.

When they send something over, be sure to video it from teh other side so you can post it on YT if / when anything goes wrong.

At least get them to drive the excavator off the truck this time....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Load Rating - Existing Bridge to be Demolished

2
I vote for the Experimental Method:

I recommend using a truck and load that you are just about to drive to the junkyard anyway.
You still have the risk of driver injury and the cost of fishing all that crap out of the river.
I don't suppose it helps a whole lot to note that you (hopefully) won't have any snow on the day you do this.
You could do isolated repair, add cribbing or mats, etc., where needed, but whether that would be cost effective, I don't know.

RE: Load Rating - Existing Bridge to be Demolished

(OP)
Anecdotes are fun, but I'm left curious about the analysis. Always like videos of collapse, thanks.

We are wrecking over BNSF rail yard. They don't like things falling into R.O.W. !

RE: Load Rating - Existing Bridge to be Demolished

Quote (TempStructures)

We are wrecking over BNSF rail yard. They don't like things falling into R.O.W. !

Then an excavator is probably not the best choice of equipment for the job. Depending on the bridge's original design, it likely should be dismantled in discrete "chunks", using small equipment to cut the superstructure apart and a crawler crane to lift and remove the "chunks".

If a maximum sized excavator will be used anyway, once demolition begins what use is the load rating?
Load capacity of structure that remains will decrease as demolition proceeds.

www.SlideRuleEra.net idea

RE: Load Rating - Existing Bridge to be Demolished

(OP)
Excavator will be used to hoist sawcut pieces off deck off the stringers. It's behaving like a crane.

The excavator will only be operating on portions of the bridge that are intact, so a load rating comparison still valid.

RE: Load Rating - Existing Bridge to be Demolished

Wouldn't be temporary support eliminates the guess work? Demo can be more riskier than new construction.

RE: Load Rating - Existing Bridge to be Demolished

(OP)
Shutting down the trains, covering the tracks, and wrecking conventional letting it all fall would be easiest / safest. However, RR closures come at significant cost.

At the end of the day, an excavator is going to walk on this bridge. Any help in confirming correct use of Rating Factor would be appreciated.

RE: Load Rating - Existing Bridge to be Demolished

I would never count of the load rating of an intact 1930's bridge to equal the load rating of a partially demolished 1930's bridge.
Very simple designs at that time.

A crane has the reach to be positioned well back from the work area; an excavator cannot do that.

www.SlideRuleEra.net idea

RE: Load Rating - Existing Bridge to be Demolished

(OP)
Only the overhangs will be demolished with excavator up top. Everything else is sawcut crane show from grade.

The load rating was performed 2016. It is a fracture critical structure still in use by public at this point, so it has been inspected every 2 years for a while.

RE: Load Rating - Existing Bridge to be Demolished

If it's a fracture critical structure from the 1930's, I'm picturing some sort of truss bridge like the one in the linked video. Can you confirm?

I think the issue here is the analysis only gets you so far. Even for a new and properly designed structure, the risk of collapse is not 0% - there is some acceptable probability of failure that is built into design codes. Failure either due to overloading or faulty material/workmanship.

If the excavator is acting as a crane, then the tread pressure is going to be a function of the load and lift radius - which adds more uncertainty and/or room for human error in your load condition compared to its typical daily use. Given the age of the structure and I'm assuming poor condition, there is much more uncertainty in the materials as well - especially at connections where you're likely to have more corrosion and less accessibility for inspections.

Given the uncertainty in both loading and strength, I would be very uncomfortable comparing this condition to the posted maximum rating as a means of approving this method. It would be a good start to see if it's feasible, but I think you would want to run a full analysis of each stage of demolition...and then spend a lot of time on-site looking at the structure, especially connections.

Without knowing anything else, this doesn't pass my initial gut check. There is no guarantee that the bridge has seen its full design load recently or ever, so it may not have ever been tested at its rated limit in its current condition and now the intention is to push the bridge to its limits one last time while its being demolished? Not saying it can't be done, but regardless of what the analysis says, I'd be a bag of nerves for the actual demo.

RE: Load Rating - Existing Bridge to be Demolished

No, if they're sawcutting bridge overhangs, you're talking about a concrete deck bridge, either on A7 steel girders or concrete girders. Some of those steel girder bridges had "quasi-composite" details thrown in near the deck. I was never sure how effective they were, but you'll want to be careful about that if you are getting too close to the girder lines with your sawcuts. And if you have concrete girders, make sure that your sawcutting plan doesn't allow the pieces to deflect and bind against each other during the cut, or you'll definitely overrun your closure window!

Yes, working from a recent bridge rating to build a capacity envelope is a valid approach for capacity. I have analyzed this exact situation for several bridge demo jobs. Of course, you want to allow a healthy safety factor if the inspection reports aren't recent (I'd be fine with 2-3 years) or leave any ambiguity about the condition of the bridge, areas that are hard to inspect, etc. But it's a good first pass, and for some jobs it's enough to save a lot of more tedious modeling and analysis from scratch.

Figuring out the excavator demand (picking) is a different animal, as CANPRO alludes. Picking over the side you can easily get to 80/20 or 90/10 splits between the tracks if the picks are large enough relative to the excavator (or you're at max reach). If you pick/swing over a corner, you can get to triangular track loading with 30-50% of the track lifted off. So make sure you're understanding (or restricting and enforcing) how the contractor wants to use the excavator.

----
just call me Lo.

RE: Load Rating - Existing Bridge to be Demolished

Any photos or drawings of this bridge??

When you post next time please remember we can't see what you can so from the OP we didn't know it was a railway bridge or that it has been recently inspected so difficult to make sensible comments without a bit of back ground.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Load Rating - Existing Bridge to be Demolished

Quote (TempStructures)

Only the overhangs will be demolished with excavator up top.
It is a fracture critical structure...

"Fracture critical" implies that the superstructure has simply supported steel stringers instead of concrete deck girders.

If the remainder of the superstructure (everything except the overhang) will be worked with a crane from below, seems prudent to use the crane for the overhang, also. Avoid the maxed-out excavator problem completely.

www.SlideRuleEra.net idea

RE: Load Rating - Existing Bridge to be Demolished

(OP)
Some pics.

I'm looking for some pretty narrow feedback on correct use of load rating factor. I recognize we don't know each other, so there is no context for this request for help.

Yes understood on operating restrictions and track pressures. I typically take 100% of excavator over single track. I've observed lots of bridge demolition and the excavators are definitely dancing around....most operators are cowboys. You have to be to operate heavy equipment on 100 yr. old structures. Most operators that have been around have a story or two.

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

RE: Load Rating - Existing Bridge to be Demolished

Ah, yes, good catch SRE... I initially thought steel girder, hedged my bets, and then forgot about it being fracture critical!

Cranes are less desirable for sequencing, not to mention that sawcutting and cranes make for very slow work. Sometimes you need to do it that way, but being able to cut/break most of the way through and then pull on a chunk of deck at your feet with the excavator is much more efficient.

Seeing those asbuilts -- I'll definitely vouch for your method. That's a great fit for a bridge like this.

----
just call me Lo.

RE: Load Rating - Existing Bridge to be Demolished

Quote (Lomarandil)

Seeing those asbuilts -- I'll definitely vouch for your method. That's a great fit for a bridge like this.

This is obviously contrary to my opinion posted earlier. I'm not saying this method is wrong, and please don't take the following questions as anything more than me trying to get a better understanding of this process.

This is my understanding of the proposed method:

  • Take the stated capacity of the bridge, with consideration of the load rating factor, and analyze the existing structure based on this loading
  • Apply the staged demolition loads to the existing structure and compare the bending/shear in each element to the values of the first analysis with the stated capacity x load rating factor
  • Ensure the bending/shear in each member due to the demolition loads don't exceed the baseline bending/shear produced by the reference analysis
My biggest question is: do you review the calculations of the engineer that determined the stated capacity of the bridge as part of this process? If not, who is responsible for any failures during the demolition as a result of errors in the original analysis?

Hypothetical scenario - the engineer who determined the capacity/load rating factor of the bridge made an error in their calculations and overestimates the capacity of the bridge by say 10%. The engineer responsible for the demo process relies on the capacity/load rating factor to determine the demo loads are acceptable, but pushing the limits of the structure. A structural failure occurs during demolition. Assuming the 2nd engineer made no mistakes in their analysis, who is responsible for the failure?

RE: Load Rating - Existing Bridge to be Demolished

Ultimately, the demolition engineer usually owns that scenario. Same as if there were an error in the demo engineer's own calcs, or if both sets of calcs were perfect, but something about the bridge condition was missed during inspection.

The repercussions will vary depending on the demo engineer's contractual relationship to the contractor, and the contractor's contract with the bridge owner. Many bridge owners will CYA by providing the inspection reports for information only, in which case the contractor needs to price in risk (and do their own inspection). Some inspection reports can be held to a more factual standard of evidence as the benchmark for existing condition. Whether the contractor or engineer carries that risk varies.

That said, not a legal expert, but I believe basing design on a state-approved, recent bridge inspection report (that was not provided "for information only") at least meets the "standard practice of care".

There is risk. I've been bitten by that specific risk, on a much more complex bridge demo. But not that much more than any job with existing structures.

----
just call me Lo.

RE: Load Rating - Existing Bridge to be Demolished

Quote (TempStructures)

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.



www.SlideRuleEra.net idea

RE: Load Rating - Existing Bridge to be Demolished

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.

RE: Load Rating - Existing Bridge to be Demolished

(OP)
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.

RE: Load Rating - Existing Bridge to be Demolished

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.

----
just call me Lo.

RE: Load Rating - Existing Bridge to be Demolished

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

RE: Load Rating - Existing Bridge to be Demolished

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.

RE: Load Rating - Existing Bridge to be Demolished

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

RE: Load Rating - Existing Bridge to be Demolished

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.

RE: Load Rating - Existing Bridge to be Demolished

(OP)
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!

RE: Load Rating - Existing Bridge to be Demolished

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.

RE: Load Rating - Existing Bridge to be Demolished

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.

RE: Load Rating - Existing Bridge to be Demolished

Lo,

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

RE: Load Rating - Existing Bridge to be Demolished

Quote:

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

RE: Load Rating - Existing Bridge to be Demolished

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.

RE: Load Rating - Existing Bridge to be Demolished

Quote (Lo)

...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

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