On-Site Magazine
Article

A tale of two bridges


Print this page

November 1, 2011 by David Godkin

Understanding how bridges come into existence in Canada requires a perspective embracing both the technical and political dimensions of infrastructure development. This is the story of two structures that have generated enormous debate about the importance of bridges in Canada, what happens when we ignore them and how innovations in engineering and construction are transforming both their look and stability.

THE PORT MANN – A SERIES OF FIRSTS
When plans for twinning the 2,100-metre Port Mann Bridge over the Fraser River were announced on Jan. 31, 2006, the howls of protest could be heard across Metro Vancouver. “How can you propose another major infrastructure project when what we really need is a thorough overhaul of our transit system?” they demanded to know. “You might reduce congestion for a few years, but what will you do when congestion returns?” others asked. “Build another bridge? And what about all those greenhouse gases?” David Suzuki charged the bridge plan violated Metro Vancouver’s Livable Region Strategic Plan by ignoring its environmental impact.

Undeterred, the province pressed on with plans for a P3 design-build contract in 2009 under the aegis of a newly formed consortium, the Connect BC Development Group, which sought financing for the project at the very moment the global economy was shrinking. This time the government was forced to re-think its plans for the bridge after failing to come to terms with the four companies that made up the consortium. Suddenly gone was the original idea for a second bridge adjacent to the original. In its place was a 10-lane replacement bridge and highway-widening project worth nearly $2.5 billion.

Even the stripped down version of the Port Mann is impressive: a link 2.2-kilometres long, 65-metres wide carrying 10 lanes-the widest in North America-and a principle span 470 metres long, making it the second-longest cable-stayed span in the western hemisphere. Unpack those dimensions a little more, says Scott Hoodenpyle, project manager for the Port Mann Bridge, and you discover achievements in bridge engineering and erection, notably the pile testing. “We believe that we performed the largest pile test in North America, at 53 MN, to confirm the design capacity of the project’s driven pile.”

According to another engineer, Ontario’s head standards engineer Nicolas Theodor, such testing is vital because of the “huge loads” large bridges like this are expected to carry. “When you have very large units that are expected to carry very large loads your whole set up has to be such that it can transfer all that load into the pile.”

In a city already struggling with road and highway congestion, the Fraser River and the barges that course up and down its length, have come to play a key role in local construction. For example, once the exterior shell of the Port Mann’s primary marine footing was precast, it was towed by tug up the Fraser River, says Hoodenpyle, where it was ballasted down over previously driven pile. “The shell was used as a cofferdam to allow for resteel and concrete operations to be performed in dry conditions, essentially creating a hole in the water.”

Another innovative construction technique, says Hoodenpyle, involved concrete slips at the bridge’s two pylon towers. “Each pylon is 160-metres tall, 76 metres of which has been slipped-the first time, such an operation has been performed in North America,” he says. Key to the bridge’s integrity has been configuration of its geometry so the design is as efficient as possible, while accommodating the environment, physical restrictions of the existing Port Mann Bridge, a major CN switching yard, marine users and neighboring businesses.

By far the biggest and most interesting attraction at the Port Mann has been the use of a massive 720-tonne gantry to set in place 90-tonne, pre-made concrete bridge deck sections. More than 288 deck sections have been placed on the Surrey side to date, with 831 pre-made concrete sections to be installed in the Coquitlam side’s north approach to the bridge. The work couldn’t have been done as quickly or as efficiently without the gantry, says Hoodenpyle.
“Configuration of the bridge required the use of two different types of precast segmental constructions. The gantry we are using is a combo-truss, which allows us to accommodate both types of construction methods with a single gantry.”
This modular approach to construction where components are fabricated off-site, trucked and installed on-site is part of a growing trend, according to Kevin Baskin, chief bridge engineer with B.C.’s Ministry of Transportation and Infrastructure. “It makes for more rapid construction on-site and there’s less labour on-site,” he says. Use of a gantry this size plainly depends upon the configuration and span length. A shorter spanned bridge doesn’t provide the economy of scale to justify its use.
“To gear up for that kind of structure you need a long span, a lot of repetitiveness and a lot of components,” says Baskin. “That’s where that kind of structure is cost competitive. That’s where you see that kind of gantry.”

PICKING UP THE PIECES…
To hear Quebec transportation officials tell it, Sept. 30, 2006 was a day like any other: lovely fall weather, bright clear driving conditions-and yes, reports of concrete falling from an overpass onto Autoroute 19, but nothing that hadn’t been heard before or that appeared too much out of the ordinary. A technician dispatched to do a sight-and-sound test quickly determined the road posed no immediate hazard and need not be closed. Instead, he had the fallen concrete removed and left the site.

Thirty minutes later, in what sounded to area residents like an earthquake, three lanes of the overpass collapsed, crushing two vehicles, killing their five occupants, and seriously injuring six others whose vehicles plunged over the side. Stunned, authorities moved quickly to rescue survivors, pull bodies from the wreckage and remove debris. How, the evening TV forecasts asked later on that evening, could this have happened?

Unravelling what happened fell to Pierre Marc Johnson, a Montreal attorney who for more than a year conducted an exhaustive enquiry into the overpass’s collapse. His conclusions were stark: poor quality concrete that deteriorated faster under freezing conditions, improper rebar installation at the time of construction, lack of shear reinforcement and inadequate water protection in the overpass thick slab. All this, coupled with repeated vehicle impact at the overpass expansion joint, Johnson concluded, contributed to the tragedy at the Concorde Overpass that day in September.

Quebec’s Ministry of Transportation (MTQ) responded quickly, pointing out that this particular structure was built in the 1960s, long before it began using high-performance concrete for all its road structure slabs. In an email interview for this article, ministry officials who declined to be identified said these materials are already integrated into the departmental standards. “Since the beginning of the 2000s, the MTQ has begun to use ternary cement for its concrete. The properties of this are clearly superior to those recommended in [previous] codes in terms of durability and implementation.” As for the high concentration of rebars in the upper part of the overpass abutment that caused horizontal cracking and weakened the bridge, the MTQ avoided this altogether by opting for a different design.

“The new built structure is completely different from the existing structure. It is a conventional two-span overpass with a steel girder deck and concrete slab. Thus, there is no problem in terms of potential weakness created by rebars in a thick slab overpass.”

According to experts, shear reinforcement would have intercepted the zone of weakness in the Concorde’s abutment and controlled the internal cracking. This is done, says Kevin Baskin, by
designing the reinforcing so that it crosses the plane of the potential failure cracks. “So that when the concrete does crack, there’s rebar there to pick up that load and you don’t get a sudden and brittle failure.” Accordingly, in October 2007, nearly a year since the overpass collapsed, the MTQ ordered that all thick slabs have at least one shear reinforcement to withstand shear stresses, even if the capacity of the concrete is sufficient.

The Johnson report was also highly critical of the slab’s thickness, complaining about its lack of water tightness, something the MTQ spokesman said could not occur in bridge structures today. “All our new structures have a high-performance prefabricated membrane to ensure the water tightness of the concrete deck, in addition to hot-mix asphalt.” That bridge deck top is the structure’s most vulnerable component, says Bala Tharmabala, bridge office manager with Ontario’s Ministry of Transportation, because of salt water that forms during winter de-icing operations and can corrode the re-enforcement. “Traditionally we have used something called a hot rubberized asphalt and have gone back and checked its performance,” says Tharmabala. “For us it seems to be performing fairly well.”

For her part, Maud Cohen, president of the Ordre des ingénieurs du Québec (OIQ) says a central issue about Quebec bridges today and one connected to alleged collusion in Montreal’s construction industry is “a loss of expertise” in the MTQ itself since the 1980s. “The actual infrastructures that were built already weren’t being maintained as they were supposed to,” says Cohen. “Investments that were supposed to be made were not made.” In other words, it’s not enough to find the cash to pay for a bridge’s construction; resources for repair and maintenance are also critical.

“That’s what Ontario is doing,” says Ontario’s Tharmabala. “It’s not only building, but timely maintenance that is important for keeping the value of the structure intact and keeping it safe, too.”

The MTQ seems to have got the message. Since 2008, it has extended a training course in supervision of structural work to all stakeholders responsible for supervision, i.e. the supervisors and their technical representatives. It has invested in a new structure management system that integrates an improved inspection system for thick slab overpasses like the Concorde. The inspection frequency has also been increased and a cracking notice process instituted to help inspectors assess the incidence of defects that may affect concrete structure.

Whether it’s an overpass like the one in Montreal, or a massive structure like the Port Mann in Vancouver, two things are clear. First, when we fail to adhere to and periodically update existing standards in bridge design, construction and inspection, natural physics will intervene, sometimes with deadly results. The second is that reliable standards are the bedrock upon which innovation in bridge design and construction rest. We can’t move forward designing and building new structures unless we have rock solid confidence in the standards that underpin public safety.

David Godkin is a B.C.-based freelance writer. Send comments to editor@on-sitemag.com


Print this page



Related




Have your say:

Your email address will not be published. Required fields are marked *

*