Solution to repair bridge cross girder/floor beam?

[Redacted]

Hi there,

I am working on the temporary repair of a bridge that is badly corroded on the cross girders. The cross girders are continuous beams that span over 5 main girder supports. The cross girders support the bridge deck.

To come up with a solution, I analysed the max applied moment and shear loads for each cross girder section of the continuous beam and determined if the existing corroded beam section can sustain the applied loads. If the specific section can sustain the loads I left that section, however, if it cannot sustain the loads I proposed to slide another cross girder right along side (~12” away) the affected section to share the live loads. For clarity I have provided a sketch. Is it fine to only add beams to sections that would potentially fail (i.e sections that needed the repair), or would it be necessary to slide in new cross girders that are the entire length of the existing cross girder (even when the other sections of the existing cross girder are fine to sustain the loads)?

In the case of this explanation, the sections are the respective span lengths between the supports.

See attached sketch.

 

[bridgebuster]

You're probably OK. In your analysis did you check the cross beams assuming one section is missing?

 

[SlideRuleEra]

Agree with bridgebuster, you should be ok.

If/when the continuous existing floor beam fails, the replacement floor beam (with simple supports) will pickup the load. At the same time, the existing floor beam goes from four-span-continuous to something less. These two simultaneous events will change how the five girders share the load from that floor beam (Sketch shown below). Should not be a big deal, but something to check.

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[Redacted]

Thanks for the responses so far. I did the analysis today as bridge buster suggested, assuming multiple affected sections were missing. This did in fact increase the shear and bending moment in a few scenarios.

Is it fair to assume that when the replacement floor beam is placed next to the existing cross girder that it takes half of the live load for the respective section?

 

[bridgebuster]

Designing the replacement beam for the entire load mwould be conservative but may not be necessary depending on when permanent repairs will take place. If the beams are as bad as described a 50-50 split becomes a judgement call.

 

[Redacted]

@bridgebuster

The max moment that I get in the continuous beam is only 46kNm so the specced 8" new beams resist this quite easily. The repairs are meant to last no more than 5 years, until the new bridge is built. I am more worried about potentially missing an unknown yet necessary repair for the existing cross girders. The existing cross girders are centred at 4 feet. So wondering what the worst case scenario would be if one of the sections fail and there isn't a new support beam right next to it? Would the failed section transfer 50 percent of its load both ways to the other existing floor beam sections that are in line with it? For example the floor beams that are 4' to the north of it and 4' to the south? Would this cause the steel deck to sag/sink down, considering the 4' c/c?

The conditions of the beams were pretty bad in some spots however, I was quite conservative with their measurements. I spot checked the thicknesses of the web and flanges over the supports and at mid spans. For each section I took the lowest spot check thickness measurement and applied it to the entire section that the measurement was in or near. Eg. if I measured a flange thickness of 3mm in one spot of the section I would apply that to the entire section even if the other spot check flange measurements were above 5mm.

Using the worst case measurements I calculated the individual section properties to determine the moment and shear capacities for each floor beam.

 

[SlideRuleEra]

RStars - IMHO, you are making a big mistake to try to share load between the second span of an existing continuous beam (I'll bet it's the W10x21 from your other post) with a new simple span W8. For starters, under load the deflection characteristics of the two beams will be very different. As live load varies, so does the load distribution between the two beams.

As a former bridge contractor, I would never add any member to an existing structure assuming that it relied of load sharing with a existing member to work. There are other complications too. We can get into those if you want to... or not.

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