By Adam Ludwig

Ludwig Structural Consulting, LLC

 The Cost Benefit Analysis (CBA) on replacement verses repair options for the West Seattle Bridge is now publicly available. The analysis tells us to repair the bridge. Because the repair option is so low cost, can be completed so quickly the choice to repair is a no-brainer and the CBA bears this out. So it is mesmerizing to witness SDOT’s public information campaign designed to convince us that bridge replacement is the right answer. SDOT is motivated to replace the span. They will have more business (read budget) to conduct with a new bridge to procure, less maintenance to be hassled with than if the bridge is repaired and a long outage may help get a few hundred more of us on bicycles. But while a win for SDOT, replacement of this good, repairable bridge represents a sad losing proposition for City of Seattle taxpayers and our communities affected by the bridge outage.

We will be harmed by having to pay and wait for an expensive new bridge. I expect there would be much less debate about this choice on repair/replace if we could have some straight talk about the bridge and the repair that has been designed to extend its life forty years or more.

First, let’s get to the bottom line result on what the CBA says about the total “performance” of the repair verses replacement over the life of the bridge. The CBA is looking at a period of time in the study that extends to the year 2100 and includes the cost of a bridge replacement at some point of time over that duration extending 80 years hence. WSP, the firm responsible for preparation of the CBA, reports that the repair will perform 1.5 times better than replacement over that period of time. That includes the cost of replacing the bridge at some point before the year 2100. This is a long period of time that everyone at SDOT is coached to refer to with sad faces as “some point in the future when we’ll have to replace the bridge.” WSP’s reported “LCCA Value Index” – a number that represents bridge performance divided by costs – gives a number of 0.93 for the repair verses 0.66 for replace. For those looking for cliff-notes to understand the CBA document, all you need to understand here is the higher the index number, the better. 0.93 is much better than 0.66. So, skipping to the end, the CBA provides a strong recommendation to repair rather than replacing the bridge.

Now let’s shed some light on why repair is so clearly the right choice:

• The engineers have determined that the problem with the bridge cracks is related to issues associated with where and how the post-tensioning (PT) strands at the bottom of the bridge box girders were terminated. The original bridge stopped them too far from the piers. All of the observed cracking that caused the bridge to be closed for service is related to this flaw in the original detailing of the PT. But beyond the crack zone, the engineers have determined that the bridge is not deficient. The “as built” bridge structure satisfies the original bridge design criteria.

• The bridge’s seismic design was ahead of its time. WSP has noted several times in the CBA that the repaired bridge is expected to perform on par with an enhanced level of seismic performance that is above the minimum standard that SDOT maintains for existing bridge infrastructure and moreover nearly on par with their standard for new construction.

• The work being done in Phase 1 repairs – a phase that SDOT misleadingly refers to as “shoring and stabilization” – is nearly complete and will correct the known deficiency associated with the bottom deck PT. Once this work is done, the bridge could be opened back up to traffic. Let’s repeat that - SDOT could choose to have traffic back on this bridge by Christmas of 2020. However, they appear to be inclined to keep it out of service until either they complete the phase 2 strengthening or until we have a shiny new bridge in 3 to 6 years.

• The CBA assumes a schedule for repair being completed by mid-2022. However, that is not at all representative of how soon the phase 2 repairs could be complete. The CBA assumes that the work will be contracted anew, rather than adding to the scope of the current contracting repair team. If they chose to go as fast as possible, the Phase 2 strengthening could be done less than a year from now. This strengthening is intended to bring the level of service up to and above how it was being used prior to closure, including four lanes in the east-bound direction that exceeded the original bridge design criteria. The strengthening will also allow the bridge to accommodate the live loads with less strain. This will improve longevity. This is critical to WSP’s design to allow the repair to last at least as long as the remaining life of the original – an AASHTO rehabilitation design requirement.

• Up-front repair costs, including voluntary upgrades to the foundations that aren’t scheduled to be performed for at least 10 years are reported to be $47M. I have learned from sources inside of SDOT that the work to repair and strengthen the superstructure alone, sans foundation work, is likely less than half of this amount. Thus, near term repair expenses of less than $25M are required to strengthen the superstructure in a way that will ensure a long remaining service life.

SDOT has had a heavy hand in the presentation of results in the CBA. Readers need to understand that the “Base Line” performance assumptions that are reflected in the tables are derived from direction given to WSP by SDOT. However, WSP has rendered an engineering opinion on what values to “report” and these are listed in the executive summary adjacent to the base line figures. For the base-line assumption, SDOT has chosen to penalize the existing bridge in every conceivable way, assuming the repair will only last 15 years, assigning it a normal, or below-expected seismic performance, and choosing a repair procurement method that will take until 2022 to complete. However, the CBA captures the expectation that the repair will last longer, will perform better seismically, and can be done faster. The upsides of any combination of these events are clearly exhibited in the range of potential “LCCA” index values that the repair can achieve. Take a look at Figure D. Do you see that tall bar noted Alt 2? That’s the repair. Note the potential range of outcomes that vastly exceed any replacement option. Repairing the bridge is a classic low-risk, high-reward proposition. We can spend very little right now and it could go like the energizer bunny until 2075 incurring only average maintenance costs throughout that period. But even when repair is given the worst benefit of the doubt, as expressed in the base line figures, it still compares well with the other options.

Do not be dazzled by fancy renderings of a new bridge. That is a new bridge we do not need. So what if federal funding for a new bridge is easier to come by? What percentage of the up-front costs will local taxpayers have to foot, 75%? The ROM costs for the new bridge are around $400million, so in this case we’ll be paying $300M of that. Taxes are taxes. Any city politician who is considering we apply $300M of tax dollars to a bridge we don’t need instead of using those dollars to address other vital problems facing our city has some explaining to do. Read the CBA. It doesn’t tell us to replace. It tells us to repair. That’s some sad news for SDOT and great news for the City of Seattle.