Kilnbridge - Bridging the gap between precast and cast-in-place
When faced with a hybrid mix of pre-cast, cast-in-place and reinforcement concrete, a tight delivery schedule, a marine environment and complex construction methodologies, Kilnbridge Construction Services turned to Trimble’s Tekla Structures for assistance on the Water Street Bridge project.
A recent addition to London’s Canary Wharf estate, the new Water Street Bridge was designed by Knight Architects with COWI and Eadon Consulting, to provide a road and pedestrian link between Montgomery Square and the new Wood Wharf development. A single-leaf bascule bridge, with an upward swing to allow boats to pass underneath, it spans over 25m across the Bellmouth Passage, between the pivot point within the East Abutment chamber and the elastomeric nose-bearing at the West Abutment.
Kilnbridge, a multidiscipline construction and engineering business, was awarded the £5million project by Canary Wharf Contractors, with the task of designing, fabricating and installing the three primary concrete structures required for the bridge’s construction - the East Abutment, West Abutment and Marine Causeway.
Both the East and West Abutment were to be formed of reinforced concrete, with the surrounding marine environment requiring careful detailing and consideration of the adjacent structures. The West Abutment consisted of a number of vertical structures, notably two feature concrete columns that support the bridge’s elastomeric bearings and approach deck; all of which were supported on bearing piles through the existing promenade. In comparison, the East Abutment is a semi-submerged concrete structure, founded on 1200mm diameter bored marine piles with permanent steel casings sealed into the Lambeth Beds below. Designed to withstand impact from a small ship, the main function of the East Abutment is to support the bridge’s pivot bearing and lifting cylinder, as well as house the other mechanical components and plant room area.
Kilnbridge was also tasked with designing, fabricating and constructing the Marine Causeway, which ran alongside the existing marine deck of the new Wood Wharf development. Consisting of bored marine piles that support transverse precast beams, precast planks then span between the beams, providing permanent formwork for the in-situ concrete deck above.
It was a complex project, which combined pre-cast, cast-in-place and finished concrete, as well as various temporary works. The limited three-month pre-construction lead-in period also required the accelerated design and detailing of all pre-cast elements. Kilnbridge therefore turned to Trimble’s Tekla Structures for assistance.
Alastair Courtney, Senior Engineer at Kilnbridge said: “We have been using Tekla software for concrete reinforcement detailing for around three years now, with the company also having utilised it on steel fabrication works for approximately eight years prior to this. The benefits this software can bring to both small- and large-scale projects are huge and indeed widely evident across the whole construction process. Tekla Structures can effectively aid design development and pre-construction works through its automatic clash detection and allow the 3D models to be used and integrated by the entire project team. It can also help develop efficient construction methodologies and deal promptly with on-site changes.
“On the Water Street Bridge site, given the hybrid nature of the structures and concrete elements, effective project co-ordination and development was crucial. It was here that we were aided immeasurably by the use of BIM and Tekla Structures. As result of some of the elements being highly complex in terms of design and positioning, it was vital that we were able to first model and study them in 3D prior to construction, both to ensure that they were buildable and also to avoid any clashes between the heavy reinforcement elements.”
One such example of this design and construction complexity was the East Abutment. With the intention of reducing the need for marine plant machinery and also improve site safety for workers and operatives, it was concluded that constructing the permanent outer concrete shell structure above the dock water level was the most efficient option, effectively transforming a marine operation into a land-based one.
Alastair explained further: “After much consideration, we decided that the outer concrete shell would be constructed above water, complete with a temporary working platform, before gradually being lowered over five metres into the water below using strand jacking techniques. It would then be sealed and de-watered to provide the working space necessary to complete the remainder of the concrete structure.
“Of course, planning and then carrying out such a complex piece of engineering required a great amount of preparation, which is another area where Tekla Structures came in. Not only did the software greatly assist with the modelling of the temporary works required for such an operation, including temporary reinforcing bars to suit the change in structural loading, but it also aided in detailing the changing reinforcement requirements. We were able to efficiently model, consider and assess the two differing locations of the structure – a key factor considering the installed position was far from the permanent end-location.
“Tekla Structures was also invaluable in allowing us to review the ongoing East Abutment works against the proposed construction sequence. As a result of the export functions, we were even able to use the completed Tekla model and schedule to create an animation of the construction programme, which was used as a visual tool for site briefings so that all workers were aware of the planned agenda.”
In addition to the challenges presented by the marine environment, the team at Kilnbridge also had to consider the proximity of the London Underground Limited (LUL) assets; as well as obeying the tight deadline by which the project’s concrete structures had to be completed. Given this, the decision was made for a large number of concrete elements within the Marine Causeway, as well as the abutments, to be changed from cast-in-place to precast; as Alastair explained:
“By introducing a greater amount of pre-cast elements, we received both large time and cost savings, as well as de-risking the project as a whole – vital considering the tight time-scales. This also allowed for a large portion of the works to be constructed and signed-off prior to the main works commencing on site, enabling us to get ahead of the schedule. Through the use of Tekla Structures, we were able to study the 3D model in detail and review both the potential advantages and constructability of changing the concrete elements to precast. In addition, we were also able to ensure that the joints constructed between the pre-cast elements would not impact on the surrounding structures.”
The benefits of Tekla software and the ease of control allowed with Tekla Structures was further exemplified when, at a crucial point in the project, two weeks prior to a key element being constructed, the permanent works engineer noted that the design calculations hadn’t correctly accounted for the loading from a ship impact. As a result, substantial changes in the reinforcement were required.
“Thanks to Tekla, what could have been a significant set-back and caused serious delays to the project was easily and efficiently solved,” commented Alastair. “The required changes were all promptly incorporated within the model, Tekla Structure’s automatic clash detection confirmed the design was correct and constructible. The corresponding fabrication drawings and schedules were generated within just two days, enabling the new reinforcement to be delivered to site on time. It also allowed us to maximise re-usage of the previously ordered elements from the then obsolete design, subsequently reducing waste.
“Thanks to Tekla Structures, the overall construction programme was not impacted, and we were able to achieve our deadline for the lowering of the East Abutment structure – a real feat of engineering in itself.”
The complex nature of the project has seen Kilnbridge receive praise and recognition from throughout the industry, including being presented with the Infrastructure Project Award in the Tekla UK Awards 2019, where judges praised its integration of both precast and cast-in-place concrete, and also receiving commendation for CONSTRUCT’s Project of the Year Award (2018).