City Condemning Pedestrian Truss Bridge 2

[rhodesrt]

Hi All, I am new to the forum and hoping to get some expert advice. I am a mech eng with no experience with bridge design since college. A tree fell on a pedestrian bridge in my neighborhood and the city closed the bridge. We reached out to our city council member and were told it is going to be replaced. I did a little research and it looks to me like this is something that could be repaired. I hate to see my tax dollars wasted if they don't have to be.

Can anyone take a look at the attached pictures and let me know if it looks like, on the face of it, like a total replacement of this pedestrian bridge is needed? I know this is very limited info so let me know if I can take some measurements or additional pictures to help in the assessment.

I also reached out to the city to see if they have a report, and this was the response:

"I am the Director of Infrastructure Project Management in Public Works. We inspect and maintain almost all of the bridges with in Denver. We inspected the bridge last spring and determined the damage is extensive enough to close it for safety reasons. At first glance, the damage can appear minimal. Our major concern is the bridge is a functioning truss type structure that gets it load carrying capability from the members that appear to be handrails. When the tree struck the bridge back in March, a sizable branch landed on the west top chord member, directly in the center. The tree bent the top member and failed the welds to the two supporting diagonal members. This damage drastically lowers the amount of live load (or people) that can stand on the bridge. It is also very difficult to repair this type damage."

 

[miningman]

IME, local authorities are extremely reluctant to take non solicitated advice from members of the general public. Would you be willing to place your PE stamp ond repair drawings?? If not , stop wasting your time right now,

 

[Trippelsewe]

Tree struck the top chord in pretty much the most stressed spot but imo it's repairable. Only think is that the repair I had in mind will stand out a bit...

 

[Settingsun]

They wouldn't take engineering advice but they did reply to the enquiry from a resident, as they should. I suggest asking the cost of the replacement bridge. These can be quite economical sometimes, then add in new asset vs old / whole-of-life cost and the replacement might stack up fairly well.

It would certainly be repairable if there were the will to do so.

 

[Lomarandil]

While they are correct that the damage is somewhat substantial, it would not in fact be incredibly difficult to repair.

A solution could be developed to reinforce the top chord instead of replacement, but it wouldnt be very aesthetically pleasing.

Presuming they want to cut out and replace the top chord (to have an invisible fix) the most expensive part would be the crane rental (as I doubt they want to touch the waterway for permitting reasons). You would likely pick the bridge and set it to the side, repair, then replace the same bridge. Expensive, but cheaper than a complete replacement (where you need the same crane rental).

Lots of good bridge engineers in Denver that could handle this project easily. If you find they are interested in referrals, I have plenty. But unfortunately, government spending doesn't tend to be managed logically.

The good news is that BigR is local, so that might bring the replacement bridge cost down.




----
just call me Lo.

 

[Tomfh]

Seems a bit excessive to condemn it based on that.

 

[STrctPono]

Sure looks like a Contech Connector Truss Bridge to me (They are same company as BigR). Damage is significant and repair would be costly but I would think it still beats out total replacement. Abutments are salvageable so for a new superstructure with labor for install, I would roughly estimate $500 per square foot for the type of bridge replacement. Looks to be about 8ft width by maybe about 70ft long? So say about $280,000.

They should be able to bring in a crane and install an overhead shoring gantry to provide continuous support for that side of the truss while they cut out and weld in the new repaired HSS members. Lomarandil is correct, they won't be able to shore the bridge from the underside because of the stream.

 

[kingnero]

If that impact really failed those welds (as in, the welds tore off), I am also in favour of replacing the bridge.
If the diagonals punched through the top member, that would seem more likely, depending on the detail that was used / size of the HDG drain holes.

 

[Settingsun]

USD 5400/m2 just for the span? I thought they were meant to be cheap.

 

[hokie66]

The top chord could be repaired, for much less than replacing the whole bridge. But I wonder if the city's answer is a silent acknowledgement that the bridge design is faulty. What braces that top chord?

 

[Tomfh]

What braces that top chord?

Presumably the web, which would be rigidly connected to the deck.

 

[Settingsun]

It's a through-truss. They're common and they work.

 

[LittleInch]

Could they not simply lift the bridge out in one lift to the shore, repair it then lift it back?

Or simply add a second beam over the damaged area for two or three bays, weld it to the existing beam then remove, repair the damaged bit and then remove the temporary beam?

full replacement looks like a whole heap more expense than a repair.

I think in the city response "very difficult to repair" is just code for we can't be bothered to even look into it...

The only course of action is probably getting your locally elected representative interested in why the city authority want to spend more of your dollars replacing something which could be repaired and closing off a public amenity and hence getting a proper engineering review of the options.

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

 

[bones206]

Maybe they have an insurance policy for the bridge.

 

[phamENG]

I'd be curious to see if there was any damage below the deck. Doesn't look like it from the pictures, but then we can't see anything down below so we can't be certain. If connections were damaged down there and the deck needs to be chipped off for a section of the bridge to make all of the steel repairs, I could see how the repair cost could approach the current asset value if not the replacement cost.

They may also have been able to select a less expensive design that, while may not be as pleasing to the eye, is functional and comparable in cost to repairing this one.

If the career engineering officials at the City won't answer your questions, talk to your elected rep. If you can get their attention and they put pressure on public works to divulge the info, it might happen.

 

[XR250]

From these pics, it seems like a fairly simple repair - other than how to shore it in the process. Perhaps there is more going on behind the scenes politically?

 

[OHIOMatt]

To repair the bridge, you would need to provide temporary shoring, which can be difficult in a stream bed. By the time you factor in everything that has to be done to make the repairs, the cost differential will likely be minimal. In my opinion, replacing the bridge is can be easily justified.

 

[kostast88]

This has to do more with policies rather than pure engineering.
Sure you can propose plenty of repairs that work on paper. I doubt anyone would ever approve them. That's not how bridges & infrastructure work. Worked 4 years in the field and I can tell you that decisions have more to do about eliminating risk, rather than thinking about costs. The paperwork alone for the repair could be just as expensive as a new design.

 

[CrabbyT]

I used to design pony trusses similar to the one depicted in your post. My belief is that the truss in your pic was inadequately designed.

When designing these trusses, you have to calculate a K-factor for the top chord. More often than not, the K-factor is greater than 1 and it reduces the top chord's compressive capacity. If the designer doesn't take the K-factor into account, the compressive capacity of the top chord will be over-estimated and it can buckle. Also, I've noticed that people who design pony trusses don't often take ice into account and that can cause trouble too.

Not sure why they wouldn't opt to shore up the truss beneath the panel points, cut off the top chords, and replace them with a properly sized tubes.

 

[JoshPlumSE]

There are many question that may come into play in the cost of repair vs replacement.

1) Who's paying? Does the city pay, or does their insurance company pay?
2) Does the bridge meet current code requirements? Depending on how long ago it was built, it might not meet current design requirements. So, it may require some form of strengthening or upgrade. If that's the case, then replacement could surpass the repair costs.
3) Impact of replacement vs repair on the community. By this, I merely mean to echo some of the comments others have mentioned. That shoring and such could be problematic for the community or the environment or such.
4) How old is the bridge and could it be experiencing some problems related to age / deterioration.