Bridge check when demolishing by excavators 1

Hire a bridge engineer?

You double posted here and in the restoration forum. Ideally just pick one place to post your question.

Also, I'm unsure as to what you are requesting. What are you trying to analyze exactly? Weight of the equipment on existing bridge, vibratory effects on existing bridge, load capacity of bridge as excavators progress in demo?

We need more information to provide useful commentary.

Thanks, Enable. I will post at one topic section moving forward.

The client would like to make sure the old bridge deck is safe when both excavators are doing demolition work. Because the excavators will start demolition in the centre of the bridge toward both abutments. In other words, the deck can support both excavators when the deck itself is under demolishing.

Hope this help you understand the issue. Let me know if you need more information.

Hire a bridge engineer.

If you had to ask the question that you asked, in the place that you asked it, you don't have the necessary qualifications. Sorry to be blunt.

Yes, you want a staged analysis considering the effects of the excavators operating on the bridge (including dynamics, track load distributions, and which attachment/method is being used) as well as the progressive effects of the demolition on bridge capacity.

There are specialist companies like Foothills Bridge, Genesis, Priestly Demolition, UrbanTech, etc that are highly experienced in this work.

Equipment and crews working on the deck of a bridge being demolished means a medium or high risk operation (depending on the bridge). It's worth getting a qualified engineer.

----
just call me Lo.

What is the construction? I've not seen excavators used for selective demolition.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Thanks Lomarandil. It is a topic I am assigned to solve. Not for a real client.

I also have a question for excavator load distribution, how should I distribute the weight excavator on the girder? Since the bridge is long and there will be only one excavator on half of the deck, should the load distribute to correspondent girders along the entire bridge span? or just one section?

Thx

Thanks for reply, dik. I did see excavators on bridge to demolish the deck. There are girders supporting the deck so it is safe to demolish the deck after the structural stability check.

as long as you don't 'paint yourself into a corner'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

This isn't the place to make simplifying assumptions that overestimate strength or underestimate loading conditions. The way you assign load distribution is "the way that ensures that the mistake that we know we are making, is on the safe side". Up to you to figure out what that means.

Traditional girder load distribution assumptions often do not apply to tracked excavators. It is better to work from first principles or conservative assumptions based on the actual track pressures.

Note that depending on the action of the excavator (hammering in front, hammering over the side, prying up sawcut slabs), that excavator track pressure is almost never uniformly distributed.

Dik-- that's a good comment- lots of real projects are forced to start at one or both abutments, leaving the excavators on an 'island'. On a few projects, the contractor has been faced with the decision of hiring a big crane to pull an excavator off the last span of deck, or cutting the machine up for scrap. Sometimes the economics work either way. But always better to make that decision before work starts, not at the end of the job.

----
just call me Lo.

Thanks, BrianPetersen. I think I get what you mean.

Hi Lomarandil, thanks very much for your suggestions. since the actual track size of one excavator is much smaller than half of the deck, is it reasonable to consider the excavator as point loading (factored of course)?

Also, regarding the bridge capacity as the demolition goes, what are the typical cases I have to analyze? I believe the critical case will be the excavation approaches close to the abutment. What is the minimum capacity case I should assume? Is it the case that the excavator has to sit at the abutment end and the longest arm length it can reach (deck section from the far end of excavator arm to the abutment)?

It's conservative to treat an excavator or track load as a point load. Just make sure that doesn't mean you're bypassing any checks on local effects.

There's no one list of typical cases. That will depend on the bridge configuration and removal sequence. You'll have to apply engineering judgement -- and when in doubt, check more cases.

----
just call me Lo.

Magicming:
This is the std. age old Structural Engineering problem, with a few reverse twists. You first have to determine what the loads are, what the load combinations are, what the load paths are, etc. Then, it shouldn’t fall down under any loads it will see. The bridge owner should be able to provide plans, specs., design calcs., etc. for the bridge in question, which allows you to study the supporting structure at various stages of undress, with some confidence. This part of the problem becomes much more complex (difficult) if you have to go out and measure all this stuff, guess at hidden details and rebar, and make material properties assumptions too. Your total loads may not be greater than the max. total bridge loads during its operation (or they could be), but they certainly will be concentrated and distributed in ways not contemplated in the normal bridge design and operation, and you have to account for this. You have to know the dimensions, weights, imposed loads for the equipment the contractor will be using, and the various ways he will be using them, as this relates to loads on the tracks. Given the track width and length and c.c. spacing, how does this relate to the girder spacings? Can he reach both sides from a centered position, or does he have to move back and forth across the bridge deck to reach the bridge edges and railings. The track loads are not point loads in this situation, they are 2’ wide and 16’ long, or some such, and quite sizable over that area.

When you start in the middle of the bridge, maybe you only want one excavator working from third pt. of that girder span to the other third point, so two excavators aren’t loading the same girder from the moment standpoint. What is the worst location for the excavator track on the girders for max. moment and girder shear, and are they o.k.? Is stability of the girders, with some deck or no deck, a problem? What excavator location cause max. loads, moments, and shears in the cross-bridge framing members? What is the worst location on the conc. deck span, btwn. girders, for the excavator track to fall, and is it a problem under max. track loading? Where are dump trucks located during their loading, for debris removal, and their contribution to all of the questions above? Are there any debris piles on the deck before being removed from the bridge.?

You are writing the general scenario for this deck removal operation. What are the steps required to do this, and what are the potential problems with each step? Make lists of all of these things you can think of, and keep adding to it as you go along. What can the contractor do, and what should you warn him not to do? Are you just removing the conc. deck for replacement; what special precautions are req’d. to minimize harm to remaining bridge members? You do need the help and oversight of an experienced bridge engineer who has seen many of these things which you might not normally think of, during your first look.

I recall a double arch brick/stone bridge over a pair of railroad tracks. The railroad did not want to close both lines so they put padding to protect the rails under the one arch and quickly reduced it to rubble. Seconds later, the now unbalanced second arch also collapsed. Since the railroad did not want that line closed there was no padding. I guess that after the thrust resistance was gone they expected the bridge to just hold that pose a while.

I wonder who signed off on that demolition.

As I said in the now deleted thread in the Bridge Engineering forum, it comes down to capacity > demand at each stage in the demolition process. Place the excavators where they will be, and calculate the force effects on the components of the superstructure. Then, calculate the capacity of the superstructure with whatever portion of the superstructure is still intact at that point of the demolition.

As I also said in the other thread, these forums are for practicing engineers with real-world problems, not for "a topic...assigned" that's "Not for a real client."

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

So where are the responses from the deleted Bridge Engineering thread? How can a thread with responses just be deleted?

So where are the responses from the deleted Bridge Engineering thread? How can a thread with responses just be deleted?

Click to expand...

I assumed the mods deleted it because it was deemed to be 'student work' or the equivalent. Similar wording - 'assigned topic', 'not a real project' was used in the other thread.

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