Why we haven't had a forensic engineering study on why the West Seattle Bridge failed
In July of 2020, Westside Seattle published story regarding the lack of a full explanation for the failure of the West Seattle Bridge. We called for what is known as a Forensic Engineering study of the failure, by an outside engineering firm. Such studies examine everything from the original plans, to the modification of those plans during construction, to the reports about the construction and obviously the history of the bridge and incidents that led to the failure. Forensic Engineering is not inexpensive with fees ranging in the hundreds of thousands of dollars or more. But as we wrote in 2020:
'The author of Why Buildings Fail, Ken Carper Professor Emeritus, Washington State University, Past National Chair of ASCE Forensic Engineering Division and Chief Editor Emeritus, ASCE Journal of Performance of Constructed Facilities said that understanding why structures fail is as important as understanding why they succeed.
“The investigation of constructed facilities that have not met performance expectations is vital to successful design, construction, and maintenance of future constructed works. Forensic engineering can bring about improvements in engineering design in much the same way as medical pathology has contributed to advances in the practice of medicine. But in order for the lessons to be learned, the investigations must be accurate and thorough, and the results must be disseminated openly to the relevant professionals. Most failures, particularly in facilities that have been in use for many years, result from multiple causes involving errors in design, construction, material deficiencies, and/or maintenance. A thorough investigation may provide lessons for all parties. The opportunities for learning these lessons should not be overlooked. The study of failed projects nearly always provides more useful lessons than does the study of successful ones.” '
Now on the heels of an audit of Seattle's history of bridge maintenance that notes maintenance on the city's 77 bridges has been sorely lacking Councilmember Lisa Herbold has responded to our request for more information. We asserted in our request that no central comprehensive set of reasons for the failure, no full report had been assembled and published.
Here is Councilmember Herbold's reply:
SDOT published a number of documents in April 2020. A technical assessment memo in 2014 raised a number of potential causes for the cracks that appeared in 2013. I don’t think it is the case as you suggest that “no comprehensive set of reasons for their failure has been produced.” The comprehensive set of reasons considered in the analysis included:
- Dead Load
- PT Losses
- Live Load
- Seismic Displacement
- Differential Temperature
- Local Effects
- Termination of Reinforcing at Intermediate Anchorages
- Location of Cantilever Moment Post-tensioning
And then the analysis concludes:
“The analytical studies did not reveal a definitive cause for the cracking although the longitudinal earthquake (2001 Nisqually) could have produce tensile stresses sufficient to cause cracking in the region of the observed cracks. The cracks were not reported in the post-earthquake inspection. The resultant stresses for all load combinations deemed likely were tensile at the location of the observed cracking but less than the probable tensile strength of the concrete. Therefore it must be concluded that the principal cause of the cracking is due to the combination of global loads covered by the analytical studies and local effects not quantified here or the result of the 2001 Nisqually earthquake.”
The Council hired a structural engineering firm to provide independent technical expertise to the Council; they noted some of the same issues, including load, creep, temperature and concrete tension.
Accountability can often come in the form of legal liability. The City Attorney’s Office examined the possibility of taking legal action, but concluded that it wouldn’t be viable to pursue.
At the request of Councilmember Pedersen, chair of the committee with oversight of SDOT, the City Auditor published an audit report on bridge maintenance. In March the Transportation and Seattle Public Utilities Committee heard an update on SDOT’s implementation of the Auditor’s recommendations. Most of the recommendations are pending; the next update will be later this year. Significantly increasing funding for maintenance is a key recommendation.
The Council reserved $12 million in funding for bridge-related transit improvements in the 2023 budget. The Council previously approved an ordinance allowing for $100 million in bond funding , but it has not been implemented.