Can structural health of bridges be determined solely by acceleration measurement? (no strain gages)

[Rob Stanovski]

Well, this article claims that the structural health of a bridge can be evaluated based only on acceleration (vibration) measurement.

This would definitely solve a lot of cost issues about monitoring a large number of existing ageing bridges. But the key question here is if it is actually possible to determine the health based solely on acceleration measurement with a low-noise triaxial accelerometer. Sure, operational modal analysis can be done. Modal shapes obtained, natural frequency determined. Velocity and displacement of oscillation can be integrated and observes. But to get stiffness, you would have to know what is the actual load that gives you certain displacement and the load (excitation) is difficult to measure (varying traffic, wind etc.).

I surely think it is good to gather the data, but can engineers with more experience in the field elaborate on the feasibility of the idea? If we could get rid of strain gauges, fbgs etc, it would be a dream come true for structural monitoring engineers.

What do you think?

 

[3DDave]

That information should be forwarded to the NTSB to be added as an appendix to their report.

 

[charliealphabravo]

I'm looking at the report again and realize I misunderstood what was being said. The failure initiated at the south end of the bridge at node U10. My quotes regarding Node U10(') refer to the north end of the bridge which was secondary.

Conclusions
Findings
1. The initiating event in the collapse of the I-35W bridge was a lateral shifting
instability of the upper end of the L9/U10W diagonal member and the
subsequent failure of the U10 node gusset plates on the center portion of the
deck truss.

I'm a bit surprised that the report contains no photographs of the recovered materials or the failed components at U10.

 

[BridgeSmith]

I don't know why there has not been an addendum to the NTSB's report issued. Possibly, since the later findings did not change the recommendations in the "final report", they decided not to issue any changes to the report. Plus, I can imagine there could be some hesitation to make changes to the document with the word "final" in the title. I have no documentation of the supplemental information I have provided. I have, as a reporter would say, 'a source close to the investigation', one of a handful of researchers at the National Highway Institute that reviews design failures such as this for the NHI.

 

[3DDave]

That's too bad. It means that a critical part of the story of the path to failure has been left out, carried only by hear-say. It means the loss of the piece that says, in effect, "we didn't do the analysis on the new piece because we didn't do the analysis on the old piece, because some rule-of-thumb said we could skip it to begin with."

Since the "Final" report would not be replaced, but amended, I doubt there's any way to reason that it should not be updated; I believe it said that gusset documentation had not been found. Now, apparently, it has, closing a gaping hole in the timeline.

It would also be beneficial to understand how the documents came to be found and why they weren't found at the exact time the investigation was conducted. That might help understand the failure in record retention and retrieval that interfered in gaining an understanding of the failure.

 

[BridgeSmith]

As I heard it, the documents were apparently located amongst a large volume of personal papers of the original design engineer, in a place where no one expected to find design calculations. They were stumbled upon several years after the collapse.