Effect of sharp indentation on steel beam capacity 2

[bugbus]

I am trying to assess the capacity of a steel I girder (on a bridge) that has been struck by an over-height vehicle, causing the bottom flange to be quite deformed locally with a sharp indentation in its edge - see photo below.

If it were the case of a bolt hole in the bottom flange, there would be no issue here. As the beam approaches its plastic section capacity, there would be localised yielding and strain hardening around the hole with associated load redistribution, the end effect being that the full plastic section capacity of the beam could be realised. The Australian codes allow this, provided that the combined area of the holes does not exceed a certain fraction of the flange area, this fraction depending on the yield and tensile strength of the material.

However, where there is a sharp notch in the side of a ductile element (sharp being the important word here), my understanding is that there can be a significant reduction in ductility.

Could this be enough to cause the beam not to reach its plastic section capacity (i.e. fail in some other way, perhaps more along the lines of a tearing type failure?)

 

[dik]

It doesn't look too bad, to me... a matter of adding a 1/2" or so plate to the bottom, unless there is more damage than what appears... Is it bent in plan?

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

-Dik

 

[bugbus]

Dik, the beam is not bent in plan. The damage is all localised at the edge of the flange circled in the photo.

 

[Settingsun]

Gut feel is it's OK, but I'll keep watch in this topic for learning opportunities.

The bearings don't look too far away. Is it in danger of getting to yield stress at the damaged location?

 

[dik]

Any sharp edges can be ground off and a metallurgist should be contacted to see if there are any real 'notch' problems... be a bigger problem where it's cold. Check with the metallurgist if the area can be heated to reduce/eliminate any residual stresses. I think that adding a plate of similar thickness as the flange and fully securing it... would allow you to cut out the damage, but I've leave the damage where it is... maybe fix it cosmetically/metallurgically. Maybe even weld up the notch. I assume the material allows you to do this. Then just a matter of having the patch sealed and coated to eliminate rust issues.

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

-Dik

 

[HTURKAK]

Apparently , the local deformation around support. Are beams continuous above the pier ?

You may perform a qualification calculation to see the reduction and effect on total capacity of the deck.

If you provide more pictures, sketches showing the spans , no. of girders etc. you may get better responds.

 

[kingnero]

I agree with dik. This is at the boundary between leave it as is (and grinding away the sharp edges + PT/MT + repainting/apply cold zinc) or repair by means of welding.
I assume this is a low carbon mild steel, so welding is a possibility without introducing any significant problems (if executed correctly!).
You should have a qualified person or company take a proper look at it (in person) and assess both possibilities.
Having proper dimensions, load ratings, material test reports, environmental factors like extreme temperatures, ... ready, will certainly be helpful.

 

[dik]

KN: Repair by welding is good... I didn't consider it (overlooked it)... My first approach was to patch, just adding extra material. Welding may also alleviate any notch stress issues.

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

-Dik

 

[kingnero]

Dik,

As a welding engineer, so generally in favour of welding, in unknown circumstances I would try to avoid welding, when I see threads like this...
So I fully agree that grinding would be the first option to consider...
Patching requires both more welding work (needs a serious all-around fillet in order to avoid corrosion issues), and transverse welding on the flanges, which might initiate fatigue cracks on the long run. So, if repair by welding is chosen, I'd say stringer beads in the longitudinal direction of the beam, with the start/stop areas staggered ( = trapezoid shaped welding patch).

 

[dik]

KN: Thanks... bad thread choice, I think.

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

-Dik

 

[MIStructE_IRE]

Its so local i’d probably live with it and do nothing..

 

[EdStainless]

As a materials guy two things come to mind.
- Is there any local cracking (penetrant inspection) or local hardening (hardness testing). If no crack and no hardness then there is no metallurgical damage.
- Does the distortion of the flange alter load path/eccentricity/stiffness?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[dik]

I think it is likely that there could be local hardening.

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

-Dik

 

[JedClampett]

What happened to the over-height vehicle?
We had a epidemic of these about 15 years ago. Except they were prestressed concrete beams and the vehicles (I think they were hoists) ripped right through the bottom flange of the beam(s). Traffic was restricted for a while until the bridges were completely replaced.
So score one for steel beams.

 

[bugbus]

Thanks everyone for the replies so far

I'll try to answer as many of these as I can

The bearings don't look too far away. Is it in danger of getting to yield stress at the damaged location?

It is near mid-span, the span is not long (only ~10 m). Photo makes it look closer to the bearings but it is just the angle it's been taken at.
In terms of capacity, there is not much reserve for the loads we have on it.

Apparently , the local deformation around support. Are beams continuous above the pier ? If you provide more pictures, sketches showing the spans , no. of girders etc. you may get better responds.

Beams are simply supported. I'm hesitant to provide too much detail for the client's sake.

- Does the distortion of the flange alter load path/eccentricity/stiffness?

Short answer is no, it's a very simple structure, all simply supported

What happened to the over-height vehicle?

No one knows. It could have happened any time in the last ~60 years for all I know.

 

[Tomfh]

If it ain't broke don't fit it.

 

[MIStructE_IRE]

I really wouldn’t be concerned. The service guys will drill holes in the bottom flange that will cause a greater loss of cross sectional area and you won’t bat an eyelid. You still have to my eye 90% of the bottom flange remaining in tact at that point.

 

[bones206]

For comparison, this happened in 2019 and bridge is still in service. Not sure for how much longer...

 

[BridgeSmith]

At most, we'd reduce the capacity of the girder by discounting the damaged area (5-10% of the flange?), and check the demand/capacity ratio for the girder compared to the other girders. Unless the cantilever is very large relative to the interior girder spacing, (not typical for a shallow girder), the exterior girders usually have a demand well below that of the interior girders. AASHTO, for a long time, has required exterior girders to have the same load-carrying capacity as the interior girders, for future widening, so even with some minor damage, like what's shown, it'll still have a higher load rating than the interior girders. If so, there's no reason to do anything (unless the trucker's insurance is footing the bill - then do the 'cadillac' fix)

We've got dozens of bridges in our state system with damage like that, and we haven't done anything to them, nor do we plan to.

Btw, due to the location (midspan of a simple span, the highest tensile stress location) I would not recommend welding, due to the reduction in fatigue resistance that would result if it's not done really well, which would be rare for a field weld.

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

 

[Ron]

Insignificant.