By Adam Ludwig PE, SE

Dear Mayor Durkan,

Please direct SDOT to repair the West Seattle bridge. Why? Because the repair will be safe, it can be completed quickly, and it will be a cost-effective means to shorten the duration of an emergency bridge outage. Repairing the bridge also comes with the great promise of extending the life of the bridge out 40 years.

Last Wednesday’s Community Task Force (CTF) meeting included a presentation by Greg Banks of WSP, the international design firm that is designing the repair. We can be very confident in WSP’s expertise and that their repair design will be safe for the load rating that they assign the bridge. According to SDOT, the planned repair will allow this bridge to be back in service by 2022 and I would challenge them to shorten that to what should be possible: traffic on the bridge within a year from now. But even if it does take until 2022, it is imperative that this bridge be repaired, and traffic restored as quickly as possible.

The bridge repair will restore a safe and vital transportation service to our community. This is my professional opinion. I am a practicing structural engineer with over 29 years of experience in the design and construction of complex renovations to existing structures to give them new purpose and extended life. When the bridge was closed, I consumed all information that SDOT made available, including structural engineering memorandums written in 2013 and 2014 that, while inconclusive about the cause of the cracks, gave me relief that the issue was not related to shear strength. I’ll explain why this was important to me from an engineering standpoint. But first, understand that the information in these documents suggested instead that the cracks were primarily flexural in nature, a much less concerning condition and one that the memos explained did not adversely impact the bridge’s ability to safely support the loads it was originally designed to support.

However, those findings were incorrect. The bridge cracks were pointing to a serious shear deficiency affecting bridge safety. In fact, if the city had authorized a comprehensive structural analysis using an independent assessment of the bridge loading rather than one that relied upon the old modeling results, they would have discovered the problem then and would have been compelled to fix it then, using the same technology that is being employed now: external steel post-tensioning.

When the bridge was closed, after reading all the available information, I was incredulous that the closure was even necessary. Able to perform my own due-diligence evaluation, I tracked down a copy of the original bridge drawings from one of the original bridge engineers and performed my own structural analysis. As a peer reviewer might do, I focused on shear loading (i.e. shear demands) and shear strength (i.e. shear capacity) at the point in the girder where the cracks occur. The 2014 memo includedan Appendix containing a GTSTRUDL (a structural analysis and design software tool) computer model input/output file and also a summary of the box girder loading at a particular location on the bridge near the cracks I will refer to as joint 38 (See Figure 1). Here I had the design model’s assessment of the shear loads that I could compare to my own independent analysis. In the engineering world, we are trained to verify model results by “checking reactions.” This is what I was doing – taking an aspect of the loading that I could verify through relatively simple analysis means and then comparing that with the model results. My checks did not result in worst case demands because they assumed balanced loading conditions used to simplify the analysis.

But if this simple analysis, done properly, shows loading that significantly exceeds the model results, that is a major red flag that there is a problem with the model. Drilling down further, it is a straightforward structural engineering exercise to determine the shear strength of the cracked bridge beam by using the information on bridge geometry provided in the drawings, and assuming that the reinforcing was, at a minimum, equivalent or better to what had been specified on the drawings (see Figures 2 and 3 in illustrations at the end of this article).

INFOGRAPHIC

I discovered, to my dismay, that the bridge girders, if reinforced as shown on the design drawings, are not strong enough in shear to an alarming degree. I also found that my assessment of dead load and live load shear demands at the crack location significantly exceeded what was shown in the modeling summary included in the report. The model data was flat wrong and wrong in the worst way – showing shear loads far less than actual. Erroneous modeling results, from the circa 1980 computer model, led the engineers to under-reinforce the bridge to varying degrees. However, as happens more frequently in construction than engineers would like to admit, mistakes made by the original design team were compensated for by the contractor who made improvements. The contractor is responsible for means

and methods of construction. A responsible contractor will validate and propose augmentations to the design as needed to suit their requirements which include considerations for temporary construction loads that are very different from loading the bridge structure undergoes in the final service condition.

The contractor made post tensioning substitutions across the board that improved the strength over what was specified. Unfortunately, the shop drawing records of the as-placed condition were not preserved. These were critical to understanding the severity and extent of the strength deficiencies.

Based on recollections from the original bridge engineer with whom I corresponded about the nature of the shear steel substitutions, I was optimistic that the problem was solvable. However, the answer on repair feasibility could only be determined through rigorous modeling, analysis, and testing that WSP would perform and oversee.

Through the spring and early summer, while this work went on, we waited to learn from SDOT if their engineers would be able to fix the problem. The announcement that a safe repair was feasible came as a relief but not so much as a surprise to me. The surprise came after, when SDOT announced that, despite the feasibility of a repair, they weren’t certain if it should be repaired. I was surprised, given my understanding of the construction techniques and costs associated with the proposed repair versus those associated with a span replacement. These costs are not close, nor is the time needed to complete construction of a repair versus span replacement. Repair is cheaper, it is faster, and I know it will be safe. Safe, because we can be confident that WSP’s very excellent engineers hold public safety as their first and primary objective. When WSP says a fix is feasible, they don’t mean feasible but unsafe. They mean the repair is feasible with safety being the number one concern among many other design concerns.

During last Wednesday’s CTF meeting, Mr. Banks of WSP noted they will have the ability to test many of their important design assumptions by observing how the bridge will respond and displace to loading applied as they tension the phase 1 bottom deck repair tendons and via test loads that will be applied to the temporary, movable work platforms. They will also be able to use these same platforms to do load testing to confirm the performance of the final repair when construction is complete.

The question that remains to be answered is what load rating the repair can achieve to ensure long term structural performance. WSP’s final determination of the load rating will be informed by this load testing.

We have learned that WSP is optimistic that this process will substantiate a load rating for the repaired bridge up to what had been intended by the original construction. However, they have determined, at a minimum, that bridge service can be safely restored to some level, with a load rating that will in any case support passenger vehicles. It will safely support, even in a worst case scenario, all of the cars.

Imagine what that bare minimum scenario will achieve – it will essentially eliminate the destructive effects of a transportation crisis that is coming on the heels of the economic emergency created by the Coronavirus pandemic. If not repaired, the economic impacts imparted by the Coronavirus will becompounded by the effects of the ensuing congestion to come as we “open back up,” a ruinous situation that will make the terrible conditions already impacting our community appear trivial. SDOT’s path toward a repair/replace decision made logical sense in May when there was a question about whether the bridge could be repaired. Now that a repair is known to be feasible, given the availability and expense, there is no justification for anything other than a bridge repair now – one that can last up to 40 years and certainly long enough to plan for myriad other options should the final bridge rating be determined to be anything less than the level of service the bridge was providing before it was closed.

If this bridge outage goes on for years it will destroy neighborhoods and lives. But while we are not empowered to avoid the damage imparted by COVID, the congestion issues related to the bridge problem can be solved safely and quickly with a repair.

SDOT’s team is in the process of a very time consuming cost benefit analysis (CBA) that is loaded with subjective engineering assumptions that must be made about unknown structural, environmental, and human behaviors in a calculation that will be used to attempt to “monetize risk”. The CBA also requires WSP to receive direction from SDOT on certain criteria that will have a substantial outcome on the results. One of the most influential inputs relates to the seismic design criteria. SDOT insists that the repair costs include a voluntary seismic upgrade to the bridge foundations that has no bearing on the superstructure concerns for which the bridge was closed. The foundation work includes strengthening the soils around the piles using injection grouting – work that can be done while the bridge is in service.

This foundation work is said to be a very expensive component of the proposed scope of work included in the CBA for the repair option.

During the CTF meeting last week, SDOT would not permit Mr. Banks to answer questions about his current opinion on the construction cost for the superstructure repair work – that needed to fix the cracks – versus the voluntary foundation seismic upgrades. Nor was he permitted to offer his current opinion on the cost of the least expensive bridge span replacement option that is being considered.

SDOT refuses to allow this information to be disclosed for fear of how the public will perceive the vast difference between construction costs and construction time needed for repair versus replacement.

Instead, they insist that we wait for the CBA process to be complete, so that we can be shown how it may make sense to throw away a bridge that can be repaired for pennies on the dollar for what a replacement will cost and be completed by 2022 at the latest versus a replacement scheme that could drag on for years and years.

I have high confidence that, even hamstrung with the costs of the voluntary foundation work, the CBA, in all of its wonkish glory, will still reveal that repair is the right decision. But, consider for a moment, that modified by all the subjective multipliers used to weight the outcome, the CBA gives a result showing the replacement option to be even moderately competitive with repair. What then?

Repair the bridge. This is not the time to shut off West Seattle from this critical transportation lifeline.

Temporary reductions in traffic demands that are a short-term side effect of COVID do not afford us the luxury of time, just the opposite. They provide a precious, once-a-century opportunity to complete these repairs faster and with the least disruption than ever would have been possible. The economic circumstances compel us to call the ball and get this repair done.

Please, Mayor Durkan, direct SDOT to fix the bridge.

Don’t add insult to the injury caused by the failed stewardship of this vital asset. We need this bridge now and this city cannot afford to wait for a replacement. We can thank our lucky stars for the reinforcing substitutions made by the original contractors that saved our bacon. With the pandemic, and the protests, and the economic doom, and with the entire west coast ablaze, we needed that. Don’t toss away the bridge like it’s a cheap piece of trash. It’s an in-place, multi-billion dollar asset. Fix it!

Adam Ludwig, PE, SE

Adam Ludwig is a Structural Engineer with 29 years of experience whose firm Ludwig Structural Consulting, established in 2012 has completed projects from Nashville to Honolulu, to Denver.

ENGINEERING ILLUSTRATIONS

Referenced in the text 

These illustrations can also be downloaded here as Page One, Page Two, and Page Three