1. Manzano 1
Societal Impact of the Seismic Retrofit Project of the Dumbarton Bridge, CA
The science of seismic retrofitting has experienced unparalleled progress in the past
decades. The demand for strengthening bridge structures has been increasing, and the southern
tip of the San Francisco Bay area is no exception. The Dumbarton Bridge was opened to traffic
there in 1982, after the State of California had bought it in 1951 for $2.26 million. The
Dumbarton Bridge seismic retrofit project (October 2010-January 2013), contracted to Shimmick
Construction Co. Inc., based in Oakland, CA, consisted primarily of superstructure and deck
modifications, and installation of isolation bearings. These design changes allowed, for instance,
a lateral movement of 42 inches, previously constricted to 20 inches. The 1.6 miles long structure
connects the cities of Menlo Park and Fremont and is used by 61,000 vehicles per day, carrying
three lanes of traffic in each direction. The retrofit project cost was $55,404,000
(http://www.shimmick.com/projects/?i=48) and was supervised by Caltrans and the Bay Area
Toll Authority (BATA), among others. The seismic retrofit of the Dumbarton Bridge had
profound unintended consequences for motorists as well as to tax payers from socio-economic
and environmental standpoints.
To begin with, a major detrimental effect of seismic activity on a highway
transportation network such as the Dumbarton Bridge is the delay experienced by drivers. Y.
Zhou et al. (2010) designed an analytical framework to assess the cost-effectiveness of seismic
retrofitting of bridges in California by simulating various probabilistic earthquake scenarios.
Drivers’ delay, together with loss of opportunity (being unable to reach their workplaces or the
shopping malls due to an earthquake), was concluded to be the major social cost, or “degradation
of system performance after an earthquake”. Generally speaking, when an individual is unable to
perform an economic activity, a time valued cost is associated to the forgone action. The
summation of all drivers’ delay in the event of an earthquake severely damages the economy
because millions of hours of labor are wasted and, subsequently, inefficiencies prevent the
economy from operating at a full employment level.
In the research conducted by Y. Zhou et al. (2010), the drivers’ delay is quantified
as a comparative index between the total time spent by an average driver on an intact highway
network and that in a damaged highway network. Additionally, a highway network system is
characterized by the possibility of rerouting traffic in the case of link damage (“the worst state of
bridge damage within a link”), or link inaccessibility. Therefore, alternative routes are
considered to be substitutes for links damaged by seismic activity. The ability to handle traffic
through alternative routes is defined as the residual traffic capacity of a link. Results show that
for retrofitted bridges, the drivers’ delay over time after an earthquake is lower compared to non-
retrofitted ones, and this effect is larger for low-link residual capacity than for high-link residual
capacity.
Although retrofitting a bridge provides structural advantages, the process may not
always be cost effective. Y. Zhou et al. (2010) defined cost-effectiveness of seismic retrofitting
as a ratio of total benefit to retrofit cost. The ratio between the bridge repair cost and its
replacement value, the damage ratio, were taken into account when the bridge retrofit cost was
computed. Other factors such as “bridge type, material properties, importance, bridge location
and retrofit measures implemented” were primordial in estimating the bridge retrofit cost. The

2. benefits from a project of this nature are the reduction in failure risk and reduction in post-event
losses stemming from repair and restoration costs. Results show that the two most cost-effective
100% retrofit projects are the ones with 3% discount rate and low-link residual capacity, and the
ones with 5% discount rate and low-link residual capacity. The only retrofit project with
moderate-link residual capacity that was cost effective was the one with a 7% discount rate. In
addition, Zhou et al. (2010)’s model predicts that no retrofit project with high-link residual
capacity is cost-effective.
Furthermore, a two-step increase in tolls was implemented in early 2010 by BATA
for multi-axle vehicles. The main purpose of the toll increase was to aid financing seismic
retrofits of Antioch and Dumbarton bridges. Moreover, the toll increase helped countering the
increased cost of debt financing rooted in the financial market turmoil (2007-2099 Financial
Crisis), as well as to offset revenue losses due to plummeting toll-paying traffic on the bridges
since fiscal year 2003-2004 (http://www.mtc.ca.gov/news/press_releases/rel567.htm). A
decadent economic health of the global economy had devastating effects on the US economy,
and the recessed economic activity led to the paralysis of major construction projects. From an
economic standpoint, it can be argued that, on the one hand, the increase in the relative risk of
bonds increased, leading to a decreased demand for bonds. On the other hand, an increase in
government borrowing (in this case by the State of California) led to an increase in bond supply.
Consequently, a downward pressure in the bond prices, tied with an increased interest rate, made
it more difficult for firms such as Shimmick Construction Co. Inc. to borrow funds.
Environmentally, the Dumbarton Bridge retrofit had potentially adverse effects on
the population of local bird species such as the California Clapper Rail, an endangered species as
specified by the USFWS in October 1970 (Caltrans, 2009). The clapper rail inhabits the southern
part of the Dumbarton Bridge, in the salt and brackish water marshes
(http://www.epa.gov/espp/factsheets/ca-clapper-rail.pdf). An engineer’s duty is to serve society
by providing technical solutions at all times, and this includes the preservation of and respect for
local ecosystems. According to a Caltrans report on the Seismic Retrofit Project of the
Dumbarton Bridge (2009), several measures were taken during the project execution in order to
minimize the effect of air and noise pollution so that the population of Clapper Rails, among
other bird species, did not significantly diminish or disappear. As preventive measures to the
extinction of the California Clapper Rail, the “implementation of biological monitoring,
preconstruction surveys, and seasonal avoidance and exclusion techniques” ensured that no
disruptive effects took place. The current existence of California Clapper Rails reflects the
success of such preventive measures. Currently, the major two threats to the Clapper Rail
survival are a further reduction in the marshland area and the existence of nonnative predators
such as Norway rats and red foxes (http://wildequity.org/species/4). Since retrofitting does not
imply either of the two threats and due to the preventive measures mentioned above, it can be
argued that the bridge retrofit project was not harmful to the California Clapper Rail population
status.
In conclusion, the evaluation of the socio-economic and environmental effects of
the Dumbarton Bridge Retrofit illustrates a fundamental key to a successful engineering project:
the incorporation of the human factor into every idea or innovation. To minimize driver’s delay
after an earthquake, the design of alternative routes to replace damaged links and the residual

3. traffic capacity of links were taken into account. Additionally, Y. Zhou et al. (2010) concluded
that, among 100% retrofit projects, the 3% discount rate and low-link residual capacity project,
followed by the 5% discount rate and low-link residual capacity project were the most cost-
effective. However, two main limitations interfered in Y. Zhou et al. (2010)’s research. First, the
analytical estimation of seismic risk associated with a highway transportation network and its
impact on societal economy are extremely difficult. Second, the analysis is carried out from the
Caltrans standpoint, meaning that the risk definition could differ among stakeholders. In
addition, toll increase and bond rates were closely related, where both investors and contractors
struggled to escape the decadent US economic panorama in 2007-2009. Also, the engineers in
charge of the retrofit project demonstrated environmental awareness by trying to minimize the
adverse effects of noise and air pollution on the ecosystem of local bird species. Retrofitting
clearly benefits individuals by improving safety and reducing the risk of bearing the cost of
replacing a collapsed bridge, but ultimately, a balance between social, economic and
environmental criteria has to be satisfied to generate a successful engineering solution.
Bibliography
 Youwei Zhou , Swagata Banerjee & Masanobu Shinozuka (2010) Socio-economic
effect of seismic retrofit of bridges for highway transportation networks: a pilot study,
Structure and Infrastructure Engineering: Maintenance, Management, Life-Cycle
Design and Performance, 6:1-2, 145-157, DOI: 10.1080/15732470802663862
 DUMBARTON BRIDGE (#35-0038) SEISMIC RETROFIT PROJECT, Initial Study
with Mitigated Negative Declaration (CEQA) and Environmental Assessment (NEPA)
with Findings of No Significant Impact (FONSI), State of California Department of
Transportation (Caltrans), September 2009.
Figure 1 Dumbarton Bridge
(From: http://bata.mtc.ca.gov/bridges/dumbarton.htm)