Kilnbridge covers all angles with Triangle Bridge

The story began with the ageing 1930s structure the new bridge was destined to replace.

Decades of service had left the original reinforced concrete crossing inadequate for Stratford’s fast-evolving landscape. Replacing it required not only removing the old, but doing so while protecting the environment and keeping essential services running.

That task was anything but straightforward. “We had to come up with new and innovative ideas to take this bridge out,” recalls Conor Hogan, project manager with Kilnbridge. “The environmental aspect meant ensuring no demolition debris entered the river. It was a significant research and development effort.”

Complex demolition in tight windows

The demolition programme was restricted to a fixed winter closure window specified by the Canal and River Trust. Missing it would have delayed the project by up to nine months. Complicating matters further, asbestos was discovered during the temporary bridge construction, requiring specialist removal without disrupting progress.

Kilnbridge deployed a mix of high-spec plant to execute the removal safely and efficiently. Brokk 400 robotic machines with breaker attachments were used to break out sections of the bridge decks, while 20-tonne excavators handled the heavier demolition.

For the beams and piles, WS30 wire saws were deployed, with marine divers installing pulleys underwater to allow cutting below bed level. Every cut was meticulously planned, bridge sections lifted were taken away by barge and waste management carefully controlled.

The total volume of waste removed was significant, with recycling and reuse data still under final review, but the strategy was always geared toward minimising landfill and maximising recovery.

For Hogan, this was where Kilnbridge’s holistic capability shone: “This project uses nearly every part of our business: demolition, marine works, reinforced concrete, steel fabrication, waste management, lifting, temporary works design.”

Keeping Stratford connected

Before demolition could proceed, a temporary bridge had to be installed to carry live utilities: gas, water, electricity and district heating. This essential lifeline ensured services remained uninterrupted during the works.

The prefabricated utility bridge, installed ahead of schedule, bridged the gap while giving Kilnbridge the freedom to focus on the main programme. Its dimensions and load capacity were designed specifically for the utility networks, demonstrating the project’s complexity even in its temporary phases.

With the old bridge cleared, attention turned to the new structure: a striking design by William Matthews Associates, working closely with the London Legacy Development Corporation (LLDC) and developers Ballymore.

At 44 metres in length and 12.5 metres wide, the new bridge required 322 tonnes of structural steel at installation. The most substantial elements were the main plate girder bottom chords, each 22 metres long and weighing 20 tonnes. Delivered in two halves, they were lifted into place with precision.

In total, the bridge was fabricated into 233 assemblies. To reduce time spent on the constrained site, the main footway was prefabricated into four 20-metre-long modules at K FAB, Kilnbridge’s own steel fabrication facility in Northampton. These were transported to site and bolted into place using tension control bolts, allowing each module to be installed in a single day.

Having in-house fabrication capability gave Kilnbridge a critical edge. “[It] meant we managed the production risks,” says Hogan. “That’s a huge benefit in terms of time and control.”

Welding precision

Perhaps the most technically demanding element was the welding of the top chords and roadway hangers. These were fabricated from S460 high-strength steel, a material rarely used in bridge projects of this type. S460 was specified by the designers to allow for slimmer, more elegant hangers, achieving both structural efficiency and architectural intent.

But this material introduced new challenges. Unlike conventional S355 steel, S460 required preheating to around 150°C and the use of ceramic heat pads to maintain that temperature during welding. This controlled process was vital to prevent rapid cooling and hardening, which could lead to brittle fractures. Supplied in K2 subgrade, the steel offered excellent resistance to low-temperature fracture, but only if strict quality procedures were followed.

“It’s very intricate,” explains Kilnbridge’s technical lead for fabrication, Paul Watson. “The complexity of the geometry is unusual and unlike most structures that we build.

“Fabricating a tapering pentagonal top chord requires a strong technical ability and careful planning. You’ve got to thread pieces through each other and weld internally and externally. Just planning how to make something like this is a challenge in itself.”

Intricate welds sometimes took up to a week per linear metre, with constant correction and straightening to counteract heat warping. Many welds resembled additive manufacturing in their intricacy.

The design team carried out detailed stress analysis and specified a FAT 56 quantified service category to meet fatigue requirements in line with Series 1800 of the Design Manual for Roads and Bridges (DMRB).

“This decision, made in close collaboration with the contractor, ensured compliance while avoiding unnecessary over-specification,” says Watson. “It delivered best value for the client and streamlined the testing process, reducing programme time, all while maintaining full compliance with Execution Class 3 under EN 1090-2."

Marine-based methodology

One of the project’s defining characteristics was its marine-based construction method.

With the site hemmed in by narrow roads and no space for large cranes, conventional lifting approaches were impossible. The solution was to assemble the bridge on a pontoon, float it into position and jack it down onto its abutments.

This complex manoeuvre drew on Kilnbridge’s previous marine experience and required bespoke lifting beams, reinforced modules and carefully designed formwork. The precision of prefabrication meant there was no trial assembly, tolerances had to be right first time.

“We had to make the design work in practice, not just on paper,” says Watson. “There was a robust feedback loop between design intent and construction reality.”

Ben White, Kilnbridge’s head of engineering, feels the Triangle Bridge is a flagship project not only for its engineering but also because of how it showcases the company’s integrated model, including demolition, marine works, reinforced concrete, steel fabrication, waste management, lifting and temporary works design.

“This is a landmark project for Kilnbridge,” he says. “It’s really complex and showcases everything we can do all in one place.”

The Triangle Bridge is also a landmark for Stratford, its sleek architectural form now part of the area’s ongoing regeneration, helping to connect communities more effectively.