1. September/October 2015 construction business 29
Public Works
U
rbanization is making its mark on
many large Canadian cities. With
populations in major metropolitan
areas consistently growing, increasing
traffic demands put an added strain on transpor-
tation infrastructure. With existing transporta-
tion routes built for very different infrastructure
needs and requirements, conditions are deterio-
rating and cannot be sustained for much longer.
Whether to rehabilitate or construct new
infrastructure is a dilemma that many owners
face, and both options come with their own
special challenges. However, when demolishing
an old structure and constructing a new one is
not an available option, owners must look for
long-term rehabilitation solutions. This situa-
tion especially rings true for the aging bridges
across the nation.
For this reason, developing advanced rehabili-
tation techniques and optimizing service life de-
sign has become exponentially more important
as outdated infrastructure becomes more and
more commonplace.
ENGINEERING INNOVATIONS
Buckland & Taylor | COWI (B&T) has been
heavily involved in major bridge rehabilitation
projects since 1972. With this involvement have
come multiple advancements in detailed reha-
bilitation design technologies including seismic
innovations.
In order to address the challenge of optimiz-
ing and maintaining the service life of bridges
in the most cost-effective way possible, B&T
has developed unprecedented design and erec-
tion engineering schemes to replace the en-
tire suspended structure of major suspension
bridges. This complex procedure has become
revolutionary throughout the industry for
its technical, aesthetic and holistic solution
towards deck rehabilitation. It involves re-
placing the entire suspended structure of the
bridge in segments during nighttime closures,
and re-opening the bridge again to the com-
muter traffic rush each morning.
Our first experience with such a complex re-
decking project was in 2001 on the Lions’ Gate
Bridge in Vancouver, B.C., where we carried out
the final bridge design and acted as the owner’s
engineer during construction, reviewing the
contractor’s erection scheme. On the Macdon-
ald Bridge project, we not only designed the fi-
nal bridge, but also developed the sequences to
be followed by the contractor during erection.
BUILDINGANOVASCOTIAN LEGACY
Angus L. Macdonald Bridge or the “Old Bridge,”
as it is locally known, is a lifeline of the metro-
politan City of Halifax, Nova Scotia, providing
vehicles, cyclists and pedestrians with access to
the downtown core since 1955. The design of
the bridge redecking addressed the major chal-
lenges associated with extending the life of the
bridge, and in doing so improved the transporta-
tion route for the local community.
The suspended portion (including 46 deck
segments, stiffening trusses and hangers) of the
Macdonald Bridge underwent a full deck re-
placement during 10.5-hour nighttime closures,
and was re-opened each morning to traffic.
The severe Halifax wind climate, the urban
location, and the proximity to Department of
National Defense — Forces buildings directly
below the construction area have posed signifi-
cant challenges throughout the design process.
During a typical installation cycle, existing
20 m (65 ft) long deck segments (weighing
approximately 150 tons each) are cut free and
lowered onto a barge. A new, pre-fabricated
segment is lifted into place and is connected,
first to the old and then to the previous new
deck segment. In the case of the Macdonald
side spans on the Halifax end, where the bridge
is above Department of National Defense —
Forces property, 10 m (33 ft) long segments
were supplied and removed by a transporter on
the bridge, rather than being supplied and re-
moved from below.
Upon completion, in a unique operation, the
bridge will be gradually lifted to result in an in-
creased shipping clearance of 2.9 m (10 ft) at
mid span. This will allow larger ships to pass un-
derneath to access nearby port facilities, thereby
increasing port business and competiveness.
A very important feature of the project is the
benefits that come with the focus on increasing
service life design. Our durability methods are
giving old bridges a second chance at life. In the
case of Macdonald Bridge, our design reduces
maintenance and enhances sustainability in
four ways. These include: using smooth welded
joints to replace the existing riveted ones (re-
ducing costly crevice corrosion); reducing the
area to be painted by half; ensuring that almost
all the steel to be painted is under the deck,
protected from deicing salt spray; and install-
ing a main cable dehumidification system to ef-
fectively protect them from corrosion.
This type of rehabilitation project also brings
a number of benefits to the economy and sur-
rounding community.
Together, the travelling public, stakeholders,
owners and contractors share in the benefits.
These include: minimizing effects on traffic
with no interruption during peak demand hours
Monday to Friday; providing an entire, brand
new suspended structure; reducing the demands
on the main cables and towers due to a lighter re-
placement deck (resulting in a greater safety fac-
tor for the major existing bridge elements); pro-
viding wider roadway lanes, and improved and
more durable pavement; increasing the vertical
clearance for ships; providing a quieter route due
to new, smoother expansion joints; and provid-
ing greater aerodynamic stability with a stronger
deck structure.
Such a complex feat of engineering also at-
tracts attention from around the world that
is sure to have a lasting effect on the economy
through increased business and additional en-
gineering and construction jobs. In addition,
long-span bridge engineering technology is be-
ing transferred to the region, which fuels innova-
tion in the profession.
CONTRUCTING FOR THE FUTURE
Aging transportation infrastructure may be
challenging our nation, but by continuously de-
veloping innovative and cost effective engineer-
ing techniques, owners can be assured that using
careful, creative engineering to arrive at suitable
rehabilitation options can provide the long-term
solutions that they require.
Keith Kirkwood, P.Eng., is chief project
manager at Buckland & Taylor | COWI in
North Vancouver, BC.
Rehabilitating Aging Bridges
By Keith Kirkwood