PRESS RELEASE
Vancouver Skybridge – Cable tension forces measured with non-contact
technology.
Cable Stay bridges gain more and more popularity in the structural engineering world.
The reason behind this great push for these structures is the cost efficiency of
construction and the pleasant appearance of a cable stay bridge. However, the main
structural elements holding the deck in place are the stay cables which are in most
cases not thoroughly inspected over the years. Especially older structures with cable
systems containing grouted elements have been proven not to be as corrosion resistant
as hoped.
Due to corrosion, slippage, settlement of the structure or part of the structure, load
imbalances may occur which in reverse may have a negative impact to the live span of
the structure. Other events as impacts, fire or seismic movement can have a significant
impact on cable force distribution and therefore an influence on the fatigue livespan of
the cables. Expanding the live span is a major objective for all bridge owners due to the
ongoing constraint of funding and the significant costs to replace such a big asset.
Repairing a structure of this size especially when significant structural damages have
occurred by lack of maintenance is very costly and could potentially lead to a total loss of
the same. Maintaining the structure and fixing the issues when they arise is more cost
efficient in the long run.
Metro Testing Laboratories has developed a non-contact Laser based vibrational
analysis tool with which we are able to determine the tension forces acting on each
individual cable of the structure.
Since our testing method is non-contact and non-invasive, we do not need to attach an
instrument to the cable and we do not need to have direct access to the cable we are
measuring.
This allows us to measure with speeds up to 6 minutes/cable which results in 10 cables
per hour under good weather conditions.
As an end result, you will receive an complete overview of the tension forces of all
cables, which allows us in cooperation with the structural engineering firm or the bridge
designer to evaluate the structure.
In the case that irregularities were
found, Metro Testing is capable of
providing
long
term
monitoring
systems for the cables in questions
and evaluates the data 24/7 on our
servers. Corrective actions and further
investigations can be performed at the
time when frequency spectrums
measured provide evidence of crack
growth or partial cable strand failure.
Laser based non-contact vibrational
measurements is the most cost
effective
solution
for
dynamic
structural analysis.
Compared to traditional methods, this technology does not require a lift off of the cable
of attachment of a sensor on the cable.
It is recognized throughout the research community and provides equal results to
traditional methods (accelerometer based). (SPIE Vol. 3400, p.194-204)
In addition to the non-contact
advantage, no rigging equipment is
required to measure at 1/3 or midpoint
of a cable. Simple line of sight with
being approximate perpendicular to
the measurement point are the only
requirements to the technology.
The Laser based non contact tension
force measurement of stay cables on
cable stay bridges is the most
advanced and most cost efficient
solution to determine the structural integrity of you most important elements on your
bridge – the stay cables.
The equipment used is highly portable and can be used virtually anywhere around the
world.
Metro Testing has successfully executed vibrational measurements on the Vancouver
Skybridge, an automated light rapid transit bridge crossing the Fraser River and is now
offering these advanced testing services to structural engineering firms and bridge
owners.
MMM as the Consulting Engineering firm for the BC Rapid Transit Company managed
the logistics and the data interpretation of the project.
The Skybridge Vancouver – A Case Study
The Skybridge is a cable-stayed bridge in Metro Vancouver, British Columbia, Canada.
Built between 1987 and 1989, it spans the Fraser River and connects New
Westminster with Surrey. It holds the record as the longest cable-supported transit-only
bridge in the world.
Skybridge Specs
Tower height
Max deck elev.
Main span length
Total length
# of cables
123 m
45 m
340 m
616 m
120
Metro Testing has been awarded with
the contract from the MMM Group to
measure all 120 cables of the bridge in a
period of 3 weeks during the
maintenance hours of the Skytrain
service.
Such a unique bridge brings unique
challenges. Maintenance access is very
limited: there is as little as 100 minutes
of available time each night to perform
the measurements. This must include
transport, set-up and takedown of all
equipment, as nothing is to be left
impeding the guideway or emergency
access.
By using state-of-the-art equipment such as a Laser-based Scanning Vibrometer
provided by Vancouver’s IE-Consulting, Metro was able to conduct the measurements
with a compact set of equipment that would easily fit in even the smallest of Skytrain’s
maintenance vehicles. The Laser Vibrometer system is housed in durable cases that are
ready for airline transport around the world.
The primary advantage of the Laser Vibrometer is that it is
a remote measurement device, able to scan from over 50
metres away. With the laser, it is possible to take
measurements at the midpoint of each cable, which
produces optimal results. There are no signal wires to be
strung, nor aerial lifts required to reach the cables. This was
critical to keep on the tight schedule which only allowed for
6 minutes per cable, including the move to the next cable.
The team of Marcus Schmieder (IE-Consulting) and Alex
Taylor Noonan (Metro Testing Laboratories) was able to set
up, obtain data from up to 10 cables, and teardown in a 2
hour shift.
The measurements gave a good representation of the
tension force of each cable and helped the MMM Group
and its team members, Jan Kocaba and Brian Counihan, to
evaluate the health of the structure.
Measurements of stay cables provide real world data from which tension forces can be
estimated. Research and case studies have confirmed that these estimated tension
force calculations are within 2-3% range of the real forces occurring. Therefore, Laserbased remote measurements of stay cables provide a valuable and cost effective form of
cable tension measurement to estimate the structural behaviour of a cable stay bridge.
In addition to that, repeated
measurements
over
a
period
of
time
allow
structural
engineers
to
evaluate the long term
behaviour of the cables and
the structure. This requires
a baseline measurement
after the structure has been
erected and/or repeated
measurements over several
years while the structure is
in service.
Older structures can be quickly assessed and their integrity could be evaluated by
comparing the calculated forces to the current.
For further information, please contact:
Marcus Schmieder, NDT Consultant
Metro Testing Laboratories Ltd.
6991 Curragh Ave.
Burnaby, BC, V5J4V6
Phone: 604 – 436 9111