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."

 

[ldeem]

There is a similar style foot bridge design in a park near me and I have always wondered about out of plane buckling on the top chord. I thought about measuring it one of these days just to see what the kl/r ratio looks like.

On the repair/replace question from the owners perspective they have to spend the money on a repair design and then find out if it is more economical than replacement. Where the cost to replace is known right from the beginning.

On a semi-less serious note in my world I would figure the dead load stress and added load from a repair crew given the existing damage and if that works out get some input from iron workers about heating and bending it back in shape or replacement. I would guess the original design included heavy live load, snow load and wind load, so the existing dead load only stress is probably quite low so probably can support a repair activities. But I am in private sector and for Public Works they probably need to send an RFP for design, get stamped bid set then go out to bid and take the lowest bid.

 

[bridgebuster]

They could try heat straightening BUT straightening a compression member is more trickier than a tension member. If done incorrectly it could make the situation worse. The damage portion of the top chord could be replaced without out shoring from below. Temporary struts could be installed to do the repair.

 

[racookpe1978]

Simple fix. Two days.

Brace the bent upper truss member at the 3/4 height point, cut the bent section of the tube steel.
Reweld tube steel section.
Bend and straighten the two broken sections, clamp them back in place, reweld.
Cut out the temporary brace at the 3/4 height point.
Paint.

If anybody is concerned about loads, brace from below with a vertical 4x4 column on a jack and jack stand base. Take the load off the truss by observing when that side of the bridge begins to move, then go 1/4 inch higher. (Go further than 1/4 inch displacement, and you start putting a tension load back in the upper truss.

 

[NorthCivil]

that damage shouldnt be too hard to fix, but there may be other issues with the bridge that warrant bringing it down.

Just like when I sent my 92 ford explorer to the Autowreckers. It was running until the fuel pump went, and I could have replaced the fuel pump and had it running again. But the cost of the repair wasn't worth it.

Same concept applies to public infrastructure.

 

[Guest090822]

It was the right call to close the bridge. Trusses are non redundant, if that continues to deform who knows what will happen.

I’m experienced in heat straightening, never heard of it being used on tubular sections. Not to say someone hasn’t tried it, but I’ve never seen it.

 

[rhodesrt]

Thanks for all the great feedback from everyone. I understand there may be a lot of other things at play with respect to the call to replace (i.e. politics, age of the structure, costs associated with repairs). This damage happened over 2 years ago and we have been told that it is going to be fixed "soon" ever since.

Based on this feedback, I am going to reach out to the public works department and see what the cost of replacement is and if they have a report explaining what options were explored. I am also going to reach out to my city representative and talk to her about the options that were mentioned by some of the replies. I think we deserve to understand how our money is being spent. It wouldn't surprise me if a new bridge ends up costing $400k-$500k which might be able to be avoided or at least explained to the citizens paying for it.

 

[bridgebuster]

I got a little ahead of myself. I was thinking more on the order of a laced box, which can and have been straightened. I think this could be straightened somewhat and a little Splash-zone could level out the top when it's done. However, I would go with replacing the damaged section.


We did a truss chord repair, laced box, that got hit several times. The last hit really mangled it. We had to amputate about 3/4 of it and splice it together. It is ugly; then again, so is the whole bridge.

 

[BridgeSmith]

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.

All of those could be factors, but my money's on #2 as the primary driver of replacement vs. repair. I'm fairly sure it's similar to highway bridges, where a low capacity bridge is 'grandfathered' as long as it's in service without rehabilitation, but if it requires major rehabilitation or structural repair, it must meet the current minimum load carrying capacity when the work is finished.

I don't think the replacement would be as expensive as some of the numbers that have been thrown out above. As long as they don't go crazy with 'aesthetic' features, BigR could probably supply a prefab, 1-piece replacement for about $150,000. Add $50,000 to get it in place and anchored to the abutments, and it's done.

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

 

[STrctPono]

As long as they don't go crazy with 'aesthetic' features, BigR could probably supply a prefab, 1-piece replacement for about $150,000. Add $50,000 to get it in place and anchored to the abutments, and it's done

I would tend to agree with that number if it were a wood deck. With a concrete deck, I don't think you're getting it cheaper than $250,000.

 

[BridgeSmith]

I would tend to agree with that number if it were a wood deck. With a concrete deck, I don't think you're getting it cheaper than $250,000.

I would expect a timber deck to be the more expensive option. Granted, the concrete deck adds 15 to 20 kips to the total weight, which could be an issue, depending on crane access to the site. The cost for the 7 CY or so of concrete for the deck shouldn't be more than $10,000.

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