Henley Gate Vehicle Bridge – an overnight success!

The Henley Gate residential development forms the latest part of the Ipswich Garden Suburb which is aimed to provide up to 4,500 new homes for the city.

The development site is divided by a cutting accommodating the main Ipswich to Lowestoft railway line. Railway possession availability to allow the construction of the bridge was extremely restrictive.

The design for the proposed bridge would need to provide the 45-metre span required but be able to be installed during a single Christmas Day railway closure.

A traditional in-situ cast solution had been explored and discarded due to site access difficulties, construction time and rail closure limitations. An alternative design had then been developed using a four steel beam ladder construction with 18 precast deck units each providing the full 14 metre deck width and weighing over 30 tonnes each.

Working closely with the main contractor and designer, ABM brought their experience of supplying numerous similar structures and quickly identified that the proposed full deck with panels would not only present lifting issues due to their weight, but would also require impractically accurate manufacture and placement of the steel beams to ensure that all the connections between the steel work and precast units aligned correctly during the time critical railway closure.

An alternative precast unit layout was suggested, developed and implemented which split each deck section into three precast panels. Whilst this increased the number of units to install, the benefits were clear.

• The revised design had significantly higher capacity to accommodate possible tolerances in the construction an avoid time consuming adjustments during installation.
• The lower weight of the precast units impacted on the crane size required and also simplified delivery of the units to the access restricted site.
• Incorporating the deck’s stringcourse detail into the outer precast units eliminated a further on-site process and the inclusion of cast-in parapet anchor bolt clusters meant that edge protection could be pre-installed to quickly provide a safe working environment on the new bridge deck.
• The small panel size also helped reduce the risk of the installation being affected by wind conditions.

The 36 precast deck units were manufactured and delivered to site in advance of the Christmas Day railway closure and were all successfully installed during the available time.

By working closely with the scheme’s designer and main contractor, ABM were able to contribute to developing a bridge deck design which met the sites construction restrictions in a practical and cost-effective manner and provided the client with the structure they needed to continue development of the Garden Suburb scheme.