Arguably, this is a better location for it... but there's a parallel thread here: http://www.eng-tips.com/viewthread.cfm?qid=420024

Here's the fracture:

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JAE:

I see what appears to be two horizontal pins in the end of the broken face. each at the third points and centered.

Seems strange...

Mike McCann, PE, SE (WA)

Those may be the plug welds referred to in the other thread.

I may be all wet but it looks to like the inside face of the crack is rusted. That would indicate that the crack had been present for some time.

Shear failure in a compression member resulting in brittle fracture from cyclical loading at plug weld locations, welding that crystalized and strain hardened the local mild steel matrix.

Interesting...

Mike McCann, PE, SE (WA)

That thing broke clean through. Different member, different time and a bunch of people could of been killed.
I consider this a fortunate occurrence in a way.

Freshly painted, so I think the crack is new. And it looks like a tension member, continuous top chord near support. The member is, I think, a wide flange, and we see the outside flange. Maybe all the way through, but not conclusive from the photo.

Saw this from two days ago in a news report:

Officials say the 14-inch truss below the westbound lanes cracked suddenly, causing the bridge to sink slightly. It’s not clear when the steel fractured. Scientists with Lehigh University are working to determine the cause and extent of damage.

Karl Frank, professor emeritus of engineering at the University of Texas at Austin, told the Inquirer that, decades ago, workers may have accidentally drilled holes into the bridge’s steel beams and filled them with plug welds, which could have led to the fracture. It’s a quick fix typical of repair work in the 1950’s (the bridge opened in 1956), he said, but something that engineers would avoid doing today."

Here's another overall photo of the fractured wide flange (W14 I think)

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And some repair work underway:

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Quote (JAE)

workers may have accidentally drilled holes...and filled them with plug welds...It’s a quick fix...but something that engineers would avoid doing today

How would a hole in a steel beam be fixed today?

I would think fillet welds around a plate plus a plug weld. Is he saying that they only did the plug weld?

appot - don't know the answer to that. I think perhaps leaving the hole would be just as bad as the weld infill but maybe not in terms of fatigue.

AISC's Table A-3.1, section 1.4 kicks you into Stress Category C for holes while section 8.3 and 8.4 (plug welds) imply a Stress Category E or F, respectively.

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I believe the recommended fix is just to fill the hole with a bolt and nut.

Mike McCann, PE, SE (WA)

They might want to check the sulphur content of the steel... to see if potential for hot short brittle fracture.

Dik

Mike is right. In the bridge world we put a bolt in the hole and tension it.

A brief report from FHWA.

• http://files.engineering.com/getfile.aspx?folder=54c31606-24c9-4f3f-827c-28

Here's a couple of photos from bridgebuster's linked report and it included the statement:

Based on the length of time from when the failure is presumed to have occurred to when it was discovered, and the bridge’s continued performance under full live load during that period, it is unlikely that the bridge was in immediate jeopardy of loss of stability. It is clear that the fractured member was carrying no load, and the structure had redistributed the load to stable load paths. It is unlikely that the load paths would suddenly change such that the fractured member would again be relied upon for any load carrying capacity, without restoring the original truss geometry. However, the immediate splice plate repair was necessary to provide the owners and consultants with a level of comfort with the safety of workers accessing the bridge.

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http://www.burlingtoncountytimes.com/news/local/re...

That is a brittle fracture--no shear lips present at the fracture periphery. I bet it made a pretty good bang when it went.

Quote (bridgebuster's linked)

It is clear that the fractured member was carrying no load

Gotta throw the BS flag on that statement.
How to explain the 2 inch plus displacement when the ability to carry the non-existent load was lost.

Bill
--------------------
"Why not the best?"
Jimmy Carter

waross,
If you read closer, it is clear that the writer is talking about the condition after the truss chord parted. They were analyzing the bridge without that member.

It is clear that the fractured member was is carrying no load
Silly me. I stupidly assumed that the writer understood tense matching.

Bill
--------------------
"Why not the best?"
Jimmy Carter

swall... if no one heard it, then, did it make a noise?

Dik

The answer to your question may be contained in the FHWA report linked by bridgebuster in his 22 Feb post.

Quote (FHWA Report)

Incident: On Friday, January 20th at approximately noon, the resident engineer on the active painting job noticed that the upper chord on one of the PA deck truss spans was fractured. Second hand, we have heard that, when asked, the painting contractor reported hearing a huge noise and feeling the structure and surrounding ground shake back prior to Christmas.

Some one heard it.
It made a noise.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Looks to me that the hole is just for drainage. Fracture appears to be not related to it.

Amazing how straight the break is!

From Bridgebuster link report from FHWA.

really surprised at the lack of fatigue striations and the brittle fracture

Sawbux,
The holes referred to are the weld filled holes in the flange of the chord member, not the one that is open in the web.

The original was riveted. I've just been involved with a riveted bridge crane that material coupons indicated a high sulphur content... made it hot short, so welding was not to be considered for repair... could be the material is hot short and not weldable.

Dik

Hey dik; what's "hot short" mean?

Keith Cress
kcress - http://www.flaminsystems.com

it smoked,
see the attachment, hot shortening is explained right after cold shortening.
B.E.

You are judged not by what you know, but by what you can do.

• http://www.tf.uni-kiel.de/matwis/amat/iss/kap_9/illustr/s9_1_1.html

Or, each particular portion of that particular member comprising an uncontrolled and undocumented plug weld and associated HAZ comprise an alloy puddle of uncontrolled and uneven composition, some of which may be cold short, i.e., brittle at the temperatures encountered over the river on the day the big bang was heard.

... Okay, I'm not completely happy with that above, so I'll try again:

Each of the plug welds was not documented, and likely not controlled wrt to electrode selection other than using what might have been available in the shop on very short notice without leaving a paper trail. The remainder of the bridge clearly has a DBT low enough to have survived a fair number of NJ/PA winters, but the plug welds in the failed member presumably have a DBT a bit higher than would be suitable.

If a brittle fracture starts in a localized area like that, will it be stopped when it propagates into a less brittle portion of the same member? Would the member have failed if there were only one mis-drilled hole? Should there be some sort of transition visible in the fracture surface as a fracture that started in brittle material reaches slightly less brittle material?

I hope they'll preserve the fractured ends for study.

It sounds like the sort of real-world problem that, e.g., AISC, should suggest as a research topic for someone. Those are probably not the only undocumented plug welds in standing riveted bridges.

Mike Halloran
Pembroke Pines, FL, USA

My paper had this story

Link

Two quotes in the story by the Chief Engineer for the Pennsylvania Turnpike bother me:

Quote ("It pretty much demonstrated a simple fact — that it's 1956 steel," )

He should have qualified what he meant because there are people who will read it and think every 1950's era bridge is in danger.

Another statement that should have been qualified (bold text):

Quote:

"it also is possible that an unpermitted load put too much weight on the structure, and temperature may have played a role"

Someone will think they shouldn't cross a bridge on a cold day. Granted, a lay person knows nothing about brittle fracture but as engineers we have to keep in mind how we say things.

itsmoked... as noted in the information included above, hot short basically means that base steel in the heat affected zones of the weld become 'brittle' when the welded material cools. This can be caused by sulphur being in the original steel 'mix'. This brittleness can lead to a sudden failure of the steel because it has a greatly reduced ductility. Once the failure is initiated, due to the reduced steel section, failure continues and can proceed very rapidly/catastrophically.

Dik

dik; Thanks much, I learned a bunch on that sword article and see what you're saying.
Glad I took a material science course doing Charpy and yield tests and being taught what 'body centered cubic' was.

Keith Cress
kcress - http://www.flaminsystems.com

Thanks for that link, berkshire.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Berkshire... Great link and a star,

You are welcome guys,
I learned about hot shortening and cold shortening in Metallurgy class in Technical College. It actually took me a while to find that article that described what I had been taught.
B.E.

You are judged not by what you know, but by what you can do.

Yes, thanks for a good link - My Spatial Reasoning isn't all that good and as a result, I have always found a lot of Material Science quite hard work. This guy comes at a few things from different angles that work quite well for me.

A.