A major operation to replace all 64 cable sheaves on the three-year-old Botlek Bridge in Rotterdam was completed last month (October), in an intervention that owner Rijkswaterstaat hopes will finally improve reliability of the beleaguered lifting bridge. The bridge, which opened in November 2015, was built under a design, build, finance and maintain contract by A-Lanes A15 consortium, a joint venture of contractors Ballast Nedam, Strabag, Strukton and John Laing, with structural design by VCE Consult and mechanical and electrical final design and installation by Waagner Biro Bridge Systems. But it has been plagued with a series of operational problems since it opened, earning the bridge a reputation as an unreliable link in the city’s transportation network.

Some of the sheaves could be replaced using cranes on the future railway alignment, but others required road closures (Gerrit Serné)

Problems with the operation of the huge lifting bridge became evident as soon as it opened to traffic, with initial malfunctions relating to the locking mechanism. But in 2016 a major issue with one of the cable sheaves eventually led to all 64 of them being replaced over a seven-month period this year.

The twin lifting bridge is one of the largest lifting bridges in the world, and was built as part of an extension to the A15 motorway in Rotterdam in the Netherlands, replacing the existing bridge and designed to accommodate both highway traffic and a future rail link. Each of the lifting spans crosses a navigation channel 90m wide; the deck provides a clearance of 14.6m in the lowered position and 45.6m in the raised position. The design of the bridge is intended to enable the 5,000t decks to be raised fully in just 90 seconds. Each deck is 100m long and 50m wide and has three steel truss girders as the main structural elements, separating vehicle traffic from the future railway line.

Rijkswaterstaat stakeholder manager Joris Wiers explains how problems with operation became evident right from the start: “Shortly after opening, we found that we were having an unexpected amount of malfunctions,” he says, “which were down to a number of issues, but most of them related to the locking mechanism on the decks.” There are six locking bolts on each bridge deck, and due to the size of the decks, these are huge elements that are essentially massive beams. The accuracy of the locking mechanism is particularly important for the movable spans due to the fact that the bridge is destined to carry rail traffic in the future.

Most of these problems were due to the sensors that were used to detect the presence of the bolts, and the software that controlled the locking system and relayed information to the remote operations room, says Wiers. “Over the last year we have made some improvements both to the hardware and the software of the bridge — essentially a large number of relatively small improvements,” Wiers confirms. “We are still working on modifications to this system, and I can say that this aspect of the operation is still not optimum, but it is satisfactory,” he adds.

However the problem which is currently being addressed by the wholesale replacement of the cable sheaves relates to a failure that took place in 2016 (Bd&e issue no 84). A-Lanes A15 stakeholder manager Peter-Bert van Herwijnen explains that the issue with the cable sheaves came to light when one of them got misaligned during the operation of the bridge, causing a major malfunction of the lifting spans which were stuck in the raised position. “This failure meant that the wheel could not turn, so the bridge was stuck,” he explains. “Our first priority was to get the bridge deck down, and then we were able to replace the cable sheave with a spare one, which allowed us to investigate why it had failed.”

The investigation revealed that the malfunction had been caused by failure of the bearing inside the cable sheave, and this was subsequently found to be due to the design of the system. “The bearing design was not capable of withstanding the forces that it was subjected to in real-life operation,” says van Herwijnen. As it was a design fault, there was a high risk that the same failure would occur in any of the remaining 63 cable sheaves. Consequently A-Lanes A15 has redesigned the cable sheaves and bearings, manufactured new ones, and replaced all of them in a series of operations over the course of the last seven months. The cable sheaves themselves are still the same diameter, to fit with the rest of the structure, but the inner bearings have more than doubled in size. 

The old sheaves (right) and the new ones (left) are visibly different (Gerrit Serné)

“The new design has much bigger bearings,” explains van Herwijnen, “these are 90cm diameter rather than the original 50cm diameter bearings. In addition, the bearing design and bearing concept is very different from the initial design. A major difference is the use of cylindrical rollers rather than spherical rollers”. Replacement of the cable sheaves has been carried out during a series of closures which had to be planned to minimise the impact on highway and waterway traffic. The fact that the bridge does not yet carry any rail traffic made it possible to replace all of the sheaves on that side of the structure without any road closures.

Keeping one of the two lifting spans available for shipping traffic, two mobile cranes could be set up on the rail side of the second span to remove the covers of the 50m-tall towers and the cables on the sheaves, before lifting out each sheave and installing a new one. This work was done in two separate two-week closures, but the replacement of sheaves on the towers on the other side of the bridge have had to be done during weekend closures, just four sheaves at a time. “We can only work during the daylight hours due to the safety restrictions on hoisting,” explains van Herwijnen. The replacement of the final four sheaves took place in mid-October.

As maintenance contractor the cost of the work is being borne by A-Lanes A15, who is also being affected by the availability payment that Rijkswaterstaat makes to the firm under the DBFM contract. “The contract is based on a functional requirement,” says Wiers, “and we make periodic payments to the contractor for availability of the highway. There will be an amount subtracted from this figure to reflect the reduction in availability.” Wiers adds that RWS is not intending to take any legal action over the problems; van Herwijnen was not able to comment on whether A-Lanes A15 was taking any legal action against its subcontractors, neither could he give any indication of how much the work was costing. 

The problems with the bridge have prompted RWS to re-examine the detail of its DBFM contracts and Wiers says that greater oversight by the client was likely to be an outcome. “Ultimately we are responsible for the highway, certainly in the public’s mind. This is why we intensified our cooperation with the contractor, moving from a traditional relationship to a partnership.” Wiers confirms that lessons learned from this DBFM contract are already being used in newer contracts such as one on the A16 in Rotterdam, which is just starting.