SPECIALIST SERVICES INFRASTRUCTURE
WSP SELLS
THE SECRET LIFE OF BRIDGES
“We don’t build Brooklyn Bridge every day,” says Satrajit Das, “but we do have to maintain it.” Das, Southeast Structures Manager at WSP SELLS in Cary, North Carolina, is explaining the key role played by its fast-growing structural diagnostics service. “Only a handful of companies can do this. You need specialised equipment, hardware and software, and highly trained personnel.”
Das leads a team of expert engineers, scientists and inspectors who specialise in assessing and repairing bridge structures, extending their lives and avoiding inconvenient and expensive closures. It’s not surprising then that the service has proved so popular with clients. Under contracts worth $1.2m, WSP SELLS is load testing bridges and culverts across North Carolina, and evaluating the corrosion condition of a
4,000ft post-tensioned segmental concrete box girder section of the state’s Albemarle Sound Bridge. It is also carrying out in- depth investigation on the 2,725ft Robert F Kennedy suspension bridge in New York, and may soon extend the service to New Jersey too. The WSP SELLS team have worked on a wide variety of concrete and steel bridge types, but Das says that their techniques can be applied to any structure.
This fast-growing structural diagnostics service is a mix of highly complex engineering – and detective work
the 1930s, there often aren’t many plans available now. Even without the plans, we can find out its actual capacity and help identify potential weak spots, if any, by analysing what happens when we drive different types of truck over it, with different weights and axle spacings.” In the absence of evidence, a blanket weight restriction is often imposed, potentially disrupting the local economy. Mitigating this is
ONLY A HANDFUL OF COMPANIES
HAVE THE ABILITY TO DO THIS. YOU NEED SPECIALISED EQUIPMENT, HARDWARE AND SOFTWARE, AND HIGHLY TRAINED PERSONNEL.
The problem with existing bridges is that once they are built, it is often difficult to tell how the structure is deteriorating and identify weak points. “Many bridges experience much greater truck loads than they were originally designed for, but they can’t always be replaced,” says Das. “We work on some that are registered with the National Register of Historic Places. If a bridge was built in
Robert F Kennedy bridge, New York. Photo: Nicola Evans, WSP Group
one of the service’s greatest selling points. “The client will identify bridges that have the greatest economic impact, out of hundreds in a state or county, and we will carry out more refined analysis.” The team attaches sensors to key points of the bridge and collect extensive data, undertaking advanced finite element analysis back at the office. “We don’t have to make any conservative assumptions, we can find out how
the structure is actually behaving,” says Das. “Typically we find that a bridge does not need such a low weight restriction – we might have put signs up restricting loads to 30 tons, but after analysis that changes to 45 tons.”
Das’ team can also evaluate the level of corrosion throughout long-span segmental bridges, made of concrete with steel tendons running through rubber or metal ducts filled with grout. “Once a bridge is built, there’s usually no way of knowing what’s going on in those ducts, or the corrosion state of the grout and tendons.” WSP SELLS carries out non-destructive tests using ground-penetrating radar and an ultrasonic device called a tomographer to locate potential hot spots for corrosion. Then they select a small number of sites to excavate, and subject samples of grout to a battery of lab tests to determine the remaining service life of the bridge. “When a bridge of this type fails, you don’t necessarily see any cracks beforehand, it just collapses. Clients need to be proactive, so these tests are invaluable.”
satrajit.das@wspsells.com SOLUTIONS 15
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