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Engineers push ahead with offsite construction

As it passes its 10th anniversary, Hewson Consulting Engineers has assembled a substantial portfolio of offsite construction work.

It is almost universally acknowledged in the sector that offsite construction can deliver significant time, safety and cost benefits in large and environmentally sensitive infrastructure projects.

Add in the fact that prefabrication techniques can also translate into considerable improvements in the quality of the finished building or structure and it is a wonder that more major schemes are not delivered in this way.

As the industry prepares to deliver gargantuan projects like High Speed 2 (HS2) and Crossrail 2, Hewson Consulting Engineers director Andrew Hodgkinson makes a compelling case for clients, contractors and suppliers to think more strategically about creating, using and assembling precast and prefabricated components.

Kuala Lumpur Mass Transit

Kuala Lumpur mass transit: Precasting allowed maximum repetition and standardisation

The firm has employed offsite construction on a series of local and international projects and he thinks the sooner the whole supply chain is engaged, the better.

“The earlier you consider and embed the idea of offsite construction into a project, the more likely it is that you will allow bold strategic decisions to be made about procurement strategies,” he says. “I think we’ve got to understand the benefits of it, and that’s not just people like us, it’s the procurement side of things, the major clients or overseas promoters.”

Hewson certainly appears to have had some success in persuading international clients to embrace the concept. In one of its more celebrated projects, the consultant led the development of 60km of viaducts for the new mass rapid transit system above the crowded heart of Malaysia’s capital, Kuala Lumpur.

Pont Briwet, Wales

Pont Briwet: Precasting eased safety and environmental issues

“We developed schemes for precasting the viaduct superstructure and substructure, including the piers and portals,” says Hewson director Nigel Hewson. “In the final solution, precasting was used for all the typical viaduct decks and special crossings with spans up to 80m, and for the parapets that line both sides of the viaduct.”

Hewson pioneered the use of dry-jointed precast segmental construction in Asia in the 1990s on the Bangkok Second Stage Expressway. The lessons from this project were combined with more recent experience of precasting projects to develop the dry-jointed deck and external tendon prestressing design that was finally adopted on the Kuala Lumpur scheme.

“Advanced planning also resulted in two precasting yards being set up to provide the segments to the individual viaduct packages, one for the north side of the city and one for the south side,” says Hewson.

Subiyah causeway, Kuwait

Big stuff: Precast element sizes on the Subiyah Causeway are large to minimise journeys between the precasting yard and the final installation position

“This allowed maximum repetition and standardisation throughout the whole project.”

Hodgkinson wonders whether the same principles could apply to HS2. “For me that is the lesson we could learn,” he says. “Maybe there’s a potential for setting up project-based, offsite fabrication and project-based offsite pre-casting facilities that effectively then supply a project like HS2.”

Hewson associate director Richard Scantlebury says the company’s experience of offsite construction in Asia is also helping it to advise on another international project, this time in Kuwait.

Klang Valley viaduct

Quality product: Factory environment produces consistent output

For the £1.7bn, 36km Subiyah Causeway Bridge between Shuwaikh Port and Subiyah across Kuwait Bay, the firm has been supporting contractor Hyundai’s efforts to maximise the use of precast elements.

“Precast element sizes on this project are large in order to minimise the number of journeys between the precasting yard and the final installation position,” he explains.

“We have designed temporary works and developed methodology for lifting, moving and installing precast shell units for pile caps, with each unit weighing up to 1750t.”

The shell units are being installed using a massive floating sheerleg crane with a 2,000t capacity.

Jakarta Gantry

Urban site: Prefabrication makes construction easier in congested areas

They are even pre-fitted with temporary steel cofferdams to minimise insitu work after they are placed underwater.

So far, a number of units have been installed successfully, with the rest scheduled to go in, in the coming months.

“We have also designed major elements of temporary works, including lifting frames for these units, and a separate frame for the 1,800t precast deck girder units which are due to be installed from 2016 onwards,” adds Scantlebury.

“By working very closely with precasters, it’s quite amazing what they can do and the quality they can achieve”

Andrew Hodgkinson, Hewson Consulting Engineers

He thinks a close collaboration with Hyundai has helped deliver the best solutions for each requirement. “For example, the precast pile cap shells were installed using a series of self-equalising tackle blocks and lifting anchorages, in combination with an adjustable lifting frame,” he says. “By contrast, the deck units are installed using a simple lifting frame with standard wires.”

Whereas in the past, precasting only tended to be used on a project where there was a significant amount of repetition, Hodgkinson says precasters are increasingly willing to cast bespoke items in limited runs.

“We often still find it is economic to cast small numbers of bespoke items if it avoids complications of doing in-situ construction over water, rail or highways.”

“Precast element sizes on the Subiyah Causeway are large to minimise journeys between the precasting yard and the final installation position”

Richard Scantlebury, Hewson Consulting Engineers

A good example is the 136m-long Pont Briwet viaduct for road and rail across the Dwyryd estuary near Penrhyndeudraeth in North Wales. Here, the consultant advocated precasting the beams and crosshead shells for the viaduct.

“There were huge benefits in reducing the work over the estuary in terms of programme, health and safety, and minimising the impact on the environment,” says Hodgkinson.

The 22t precast shells for each crosshead were lifted onto simple steel supports previously cast into the columns. Precast U-beam installation then followed a simple stitching pour to permanently connect the crossheads to the columns. This limited the need to transport concrete from a batching plant for pouring insitu, minimising impact of work on the surrounding area.

Hewson associate director Jeremy Barnes, who worked as the lead designer on the viaduct, says: “The precast crossheads were designed to be installed in two sections to match the phased replacement of the existing rail and road bridge decks. The temporary works concepts developed enabled level adjustments to be made to ensure visual continuity at the joint.”

Hodgkinson thinks offsite construction allowed for a higher quality finish to the crossheads which were architecturally-shaped to resemble a boat hull.
“Precasting allowed a consistently high quality finish to be achieved with a complex shape and for the contractor to minimise the temporary workneeded for construction.It’s much easier to set that up in factory conditions rather than trying to set it up above an estuary.”

“Precasting allowed a consistently high quality finish to be achieved with a complex shape”

Andrew Hodgkinson, Hewson Consulting Engineers

Dwelling on the quality theme, Hodgkinson adds that concrete precasters in particular can now deliver a consistently high standard of work.

“Precasters are now very good at doing bespoke precasting,” he says. “We recognise that by working very closely with precasters, it’s quite amazing what they can do and the quality they can achieve.”

However, he thinks more research into prefabrication techniques and the associated details is needed. “If we are to advance offsite technology, more research is needed to support this, particularly on the subject of connection details between steel and precast concrete or precast concrete and insitu concrete,” he says.

He cites the “excellent” SCI design guide into full-depth, precast panels, which highlights the need for further research on the fatigue performance of transverse joint details.

“I think designers have been shy about using that technology because there hasn’t been adequate research into the effects of fatigue,” he says.

“Without this and other similar research taking place, wider adoption of this very important offsite technique is not going to happen, but who will fund this? It’s a question we have to address as there simply isn’t enough research going on in the UK at the moment.”

Equally, Hodgkinson implores the sector to learn from the international market. “Overseas there’s an awful lot of offsite construction going on. I think there’s a huge benefit in cross-fertilising those ideas with what we do back here.”

 

Produced in association with

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