Steel girder cracks 1

[skewl]

Hi All,

Found a crack on the bottom flange of a steel girder while doing fatigue/NDT inspection. Not entirely sure of the cause of this crack. Any ideas?

Additional Info:
- Bridge was built in 1960s. Girder is a 36WF160. Potentially fabrication defect? The fact there are two cracks almost symmetrical about the girder centerline seems too big of a coincidence.
- The cracks occur at the termination of the cover plate near the ends of the girder. Stress concentration area due to abrupt change in section properties. Given that the longitudinal crack is near the ends of a simply-supported span, I am not too worried as it is not a high moment area (and the crack is parallel to the load path). What was surprising to me was that I was expecting a transverse crack rather than a longitudinal one (

https://www.researchgate.net/figure...ge-I-girder-with-cover-plate_fig5_248879216).

- Cracks goes all the way from the cover plate weld (weld does not appear to be cracked) to the bearing.
- Original quality images here:

https://1drv.ms/f/s!As8lKkwt3K1Vgvs3SZITPSLQhz3Acg

 

[bridgebuster]

That's a weird one. Are you able to scrape away the paint and probe the "crack" with a dental pick? Maybe it was scratched during rolling? Another thought, maybe the fabricator reworked the WF prior to installing the cover plate and introduced cracks. Is this the first time the cp's are being NDT'd? If there pictures from previous inspections you might be able to spot something.

 

[skewl]

We actually had to grind off the coating to spot these. Haven't done any probing yet but we plan on doing additional grinding to see if this is just a surface defect and if not, order some ultrasonic testing. The sentiment around the office right now is that these are manufacturing defects.

I am not sure if previous NDT were done. The bridge was rehab'd back in 1990s (fresh coatings) and the coating seems untouched until we tore it up last night. We did spot coating breakdown there which was why I prescribed NDT there. We did NDT on other cover plate areas but this was the first time we saw something.

 

[bridgebuster]

Thanks for the update. I'd be interested in knowing the final result of the inspection.

 

[dik]

To get this straight, a cover plate was welded to a W section. The crack occurred on the bottom side of the cover plate in the region of the outer edge of the bottom flange, parallel to the length. Have you checked the metalurgy of the cover plate to see if it is weldable? What were the circumstances of the plate being added, and has a section of the weld been examined microscopically? Need more information.

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

-Dik

 

[skewl]

@bridgebuster

We found this type of cracking on another completely independent structure (although we only found this at 2/20+ similar locations we looked at). We shaved off about 5mm of steel and the crack seems to not go in any deeper.

The senior engineer and myself were on-site. We agreed that it was likely a fabrication defect that developed into a crack. In fact, when they coated this back in 1990s, the crack was probably already there (we can still see coating after shaving off 1-2 mm of steel).

 

[skewl]

@dik

The crack occurred on the bottom flange of the rolled section, not the cover plate. I assume the cover plate is weldable since it is part of the original design. The weld has not been examined microscopically, and I doubt we will have a chance to do that.

 

[desertfox]

Hi kewli

If the cracks were due to tensile loading they would run at ninety degrees to the cracks shown so I wouldn't of thought they were structural but I'm no bridge expert.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[dik]

no explanation, then... desert fox, you're correct.

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

-Dik

 

[desertfox]

Hi dik

No not found anything definite about the cracks, ahowever while digging it appears that the cracks might be defects in beam due to poor manufacture.



“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[desertfox]

Hi

I found a thread with a similar discussion on cracks in the I beam bottom flange and possible manufacturing flaws which might have caused them :-
thread507-432949

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[skewl]

Thanks desertfox, that is a great find.
I been looking at some old threads myself and found this:

https://www.eng-tips.com/viewthread.cfm?qid=447854

 

[desertfox]

Hi Kewli

You’re very welcome, iF I find anything else I’ll let you Know.
Have you any plans to examine those cracks any further?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[bridgebuster]

Thanks for the update Kewli.

AISC didn't have much info. I found a reference to ASTM A6, which has some general info on non-injurious and injurious defects.

 

[skewl]

@desertfox
Unfortunately we have concluded our field work and won't be going back to do any more investigation. As suggested this is not a structural crack and very likely a manufacturing defect. This definitely would have been rejected at the plant nowadays but since it was fabricated in 1960s, anything could have happened.

 

[desertfox]

Hi kewli

Thanks for the update

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein