Composite Roadway Stringer Beam Web Replacement - Fatigue Check to Eurocode 2

[nwr9]

Hello,

We are assisting the council with the rehabilitation of a ~80-year-old steel bridge. They have identified corrosion to one of the roadway stringer beams. The corrosion has resulted in substantial but localised steel thickness loss to the lower web and flange. The stringer beams are simply supported by the cross girders and composite with the roadway slab. The corrosion is located at the end of the beam where the web is coped. It is proposed to replace the affected section of web and flange as shown in the diagram below. The lower flange would be pre-welded to the web section in the shop leaving only the upper horizontal web weld, vertical web weld and flange weld to be performed onsite. All welds to be full penetration butt welds performed from both sides.

Below are my questions:
[ul]
[li]Would anyone be able to sugguest a suitable Fatigue Detail Category for the vertical leg weld (blue in above diagram)? Given the proximity to the support, the only tangible action at this location is vertical shear stress.[/li]
[li]Is Fatigue Detail Category 80 (as per below) the most suitable for lower horizontal leg (purple in above diagram)?[/li]
[li]Are my assumptions for the remaining welds in the above diagram reasonable?[/li]
[/ul]

Thanks!

 

[SlideRuleEra]

We are assisting the council with the rehabilitation of a ~80-year-old steel bridge.

Verify that the existing steel can be welded. It probably can be welded, but best to find out now.

See "Field Welding to Existing Steel Structures" by David Ricker

 

[nwr9]

Thanks SlideRuleEra, that's a great reference.

 

[nwr9]

Is anyone able to shed some light on a suitable Detail Category for the vertical weld in the web?

Thanks!

 

[bugbus]

I don't have my code (I assume you're going off AS 5100.6 or AS 4100) in front of me, so I can't remember what the detail categories are specifically.

But don't forget to check fatigue at the corner of the cope, where you will tend to get a crack shown here in pink.

 

[nwr9]

Thank you bugbus, that is a good point that I had not considered. Is it correct to say that the stresses here are likely to be a higher fatigue risk due to stress concentration? For which the correction factor can be obtained from PD 6695-1-9, example excerpt below:

I assume the Detail Category 8 (as per my initial post) is still appropriate for this location? As long as the stress concentration has been accounted for.

Yes, AS 5100.6 is the governing standard for this repair.

Thanks!

 

[bugbus]

I'm not familiar with PD 6695-1-9 but it's probably a good place to start, since I don't think AS 5100.6 really covers the coped detail at all.

Yes, there is a stress concentration at the cope due to the radius, and also the fact that there is a force perpendicular to the flange at that point when you resolve the forces. I think a diagonal plate can sometimes be added to help resolve this component and reduce fatigue at the flange-web interface.

 

[nwr9]

Rather than using stress concentration factors, I've produced an FEA model which nicely shows the concentration of stress that forms at the cope as predicted by bugbus.

Notes on the model:
- Complete stringer is modelled. Only the end web portion is shown in the below model outputs
- 4 stress plots are provided: 1) Principal Stress 22, 2) Principal Stress 11, 3) Radial stress (RR) about the cope, 4) Tangential Stress (TT) about the cope
- Tension stress is +ve, compression stress is -ve
- The peak compressive stress shown is due to the knife-edge reaction (representative of cross-girder web below)
- Web stiffeners have not been modelled
- The load modelled is commensurate with the AS 5100 vehicle fatigue load model (70% M1600 including dynamic effects).
- I believe the stress values are very conservative as the composite slab has not been modelled. This is due to AS 5100.6 not permitting the composite slab to be used for shear capacity.

I have two follow-up questions:
1. Would I be correct in adopting the 75 MPa stress from the Principal Stress 11 plot as the design stress, and comparing this to Detail Category 80 (see my initial post)?
2. While the stress at the proposed location of the vertical web weld (blue line in my original post) is not as high as that at the cope, it is not insignificant and I would therefore like to check it against a suitable Detail Category. Is anyone able to assist with recommended a suitable Detail Category for this weld.

Thanks!!!