By Tracy Record
West Seattle Blog editor
Just released by SDOT, here’s the Cost-Benefit Analysis (CBA) meant to help inform the city’s decision of what to do about the seven-months-closed West Seattle Bridge:
Though it looks at six options for repairing or replacing the bridge, the renderings and costs are NOT by any means the final set of options – they’re what was roughed out for comparison purposes. It should also be noted that the CBA does NOT address the newly emerged option that some have nicknamed “rapid replacement” – though that will be explained at tomorrow’s West Seattle Bridge Community Task Force meeting, along with the CBA. But if you just want to skip ahead to the bottom line, those estimates are on page 68 of the 89-page document, with basic up-front construction cost rough estimates from $47 million for repairs to $1.9 billion for an immersed-tube tunnel.
Here’s the CBA document’s purpose as explained in its Executive Summary:
This CBA addresses some of the questions we have all grappled with since March 23 – What long-term impacts will this closure have? Can the City afford to wait for a replacement? What is the life span of a rehabilitated structure? What benefits does rehabilitation offer? What benefits does replacement offer? While the purpose of this report is to objectively present the benefits and drawbacks of multiple alternatives, to help inform the decision to rehabilitate or replace the bridge, its findings will not, and should not, dictate that decision.
The CBA is, in fact, just one factor in this decision, focused on helping to support decisions being made around public safety and technical risk. The decision will also be informed by external exigencies and current events, by the recommendation of the asset owner, SDOT, as well as by the guidance provided by the Technical Advisory Panel (TAP) and Community Task Force (CTF). Ultimately, the imminent decision to rehabilitate or replace the structure will be a City decision based on a value judgment for the safety and well-being of the people of Seattle.
Now, the toplines of the document, prepared by WSP, a consulting firm that SDOT has long been working with:
First, the document explains how the previously discussed finding was reached, that nothing suggested the bridge wasn’t fixable. It also notes:
As information is received in real time through the biweekly in-person inspections and 24-hour monitoring system, we learn more about the bridge and how it is behaving, allowing us to make more informed decisions. What we have learned to date has yielded confidence that the bridge behavior is predictable and that it is technically feasible to rehabilitate the bridge and restore its remaining service life, or more. However, we will continue to perform reconnaissance work to see if trends change as we finalize stabilization measures this fall.
Here’s how the studied possibilities were decided on:
The CBA began with five alternatives, all of which were on-alignment and fit the same grade/profile as the current bridge. In response to community feedback, the study adopted a sixth alternative – an immersed tube tunnel (ITT). This is the only alternative that deviates from the current alignment and grade/profile. During this process, we also eliminated the third alternative, a partial superstructure replacement, from consideration as it was determined that it was not the apparent low-cost option for a rehabilitation alternative. Section 2 of this report includes full alternative descriptions and details of each concept. WSP chose five representative alternatives based on apparent low cost to fit within the scope and schedule confines of the CBA. There exist other options within each category, many conceptually mentioned within the appendice that could and should be further explored or developed.
Since one giant question is “how much would (any option) cost?” it should be noted that the CBA says, “For each alternative, WSP developed ROM estimates for initial capital cost (inclusive of monetized risk items) and for life cycle costs. Life cycle costs include allowances for initial capital investments, operation and maintenance costs, inspection costs, and future repair/rehabilitation costs.”
The document goes into details of each type of option evaluated – including repair, which gets the formal title “direct strengthening to restore live load.” Here are the options, as screencapped from the CBA and as previously reported, with estimated durations:
Repairs are projected to last 40 years and to be able to reopen the bridge to traffic in 2022, with some additional seismic work required in the 10 years after that, and the bridge needing replacement by 2062.
Another duration-related note – how much closure time is estimated for each option. From the CBA:
The continued closure causes increased vehicular traffic using detour routes through marginalized communities. Because the overall closure duration was similar for Alternatives 2, 4, and 5 (even if in two phases), these were rated as better than Alternative 1, which would require two closures totaling 7.5 years, or Alternative 6, which would require one closure totaling 9 years.
For this measurable, rehabilitate and replace (except Alternative 6) have similar outcomes – the only difference is that part of the rehabilitation alternative’s duration is projected to occur in 40 years.
One question that has arisen often in these seven months – could a new bridge hold West Seattle light rail too? The CBA suggests it could:
Sound Transit’s current plan is to build its own structure for the West Seattle to Ballard Light Rail Extension (WSBLE) However, noting the opportunity for synergy between agencies, this measurable examined the technical feasibility of adding light rail to the WSHB structure, now or before 2032, when it is assumed that the WSBLE would be complete. Coordination between Sound Transit and the City has commenced to determine if a joint structure is desired. To date, Sound Transit and SDOT have discussed:
— Design life
— Capital costs
— Sound Transit O&M cost data
— Geometric considerations (vertical curves)
— Seismic design spectra
— Design loads (any plans for future increase?)
In addition to these discussion topics, SDOT has requested the following items from Sound Transit to support the planning of a replacement structure:
— USCG Navigation Impact Report (NIR)
— Topographic surveys and mapping of the ground and waterway
— Geotechnical information logs and borings
— Environmental reports/studies
— Traffic data, forecasting, and modeling used for the WSBLE DEIS
— Preferred alignment drawings and cross sections
— Anticipated construction schedule
— List of planned temporary and permanent property acquisitions
For the purpose of the CBA, WSP assessed which alternatives could accommodate light rail. Alternatives 1 and 2 do not reconstruct the bridge superstructure until after 2032, while Alternatives 4, 5, and 6 would be able to\ accommodate light rail prior to 2032.
The document reviews a long list of other factors – from broad, such as business and worker impact, noting that businesses may benefit from a captive audience as well as be harmed by the cross-river travel challenges, to technical, such as easements and utilities.
And now, the numbers. Note that one column shows the basic construction costs – the other columns take into account cost factors such as maintenance over whatever the ensuing lifespan is:
Everything above is the result of a quick readthrough, so we could get the document out to you. We will continue reading and add any other toplines. SDOT, meantime, (4:39 pm update) just published its own explanatory post; they write that Options 2 and 4 – repair and superstructure replacement – “rise to the top.” Meantime, tomorrow’s Community Task Force meeting (noon Wednesday; watch here – UPDATED LINK) will include a topline review and discussion; the document was sent to CTF members late last night, and they are expected to finalize their thoughts on it by month’s end. The mayor is now expected to make her recommendation sometime nextd month.