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Back to the future with sliding connections

Could a simple connection detail help take the precast concrete sector back to the future? Construction News talks to a man who thinks so.

As construction continues to leave the recession behind, thoughts have shifted from winning work to how to staff the newly won projects.

There are concerns the downturn has put off many school-leavers from starting a career in construction, while those skilled workers that left are proving difficult to lure back.

Simplifying construction techniques and taking the focus from a labour-intensive onsite environment to a more offsite one has long been trumpeted by the precast sector as a better way of using resources and addressing the skills shortage.

One method coming to the fore that can also optimise resource use and reduce the need for skilled onsite labour is the sliding connection.

Sliding connections

Sliding connections that link precast concrete elements such as beams and columns are nothing new to the industry.

Unfortunately they have never really taken off in the UK in the same way as on the continent.

“Designers and specifiers like their simplicity and aesthetics, precast manufacturers like their efficiency and erectors like the ease and speed of use”

Cliff Billington, Invisible Connections

But with systems becoming simpler to use while offering an architecturally aesthetic finish, specifiers and designers are starting to look upon their use more favourably.

They have been used on some flagship projects for precast, such as the Birley Fields Campus at Manchester Metropolitan University and Skanska’s redevelopment of its Bentley Works facility near Doncaster.

“There is no doubt sliding connections are becoming more popular,” says Cliff Billington, technical director at Invisible Connections, which specialises in specifying and supplying these sliding systems.

Simplicity and aesthetics

“Designers and specifiers like their simplicity and aesthetics; precast manufacturers like their efficiency; and erectors like the ease and speed of use.”

The connections consist of two boxes cast into separate precast members at the designed connection point and a sliding ‘knife’. In the case of a beam-to-column connection, the beam box is cast into the beam, which is being supported.

Working with steel reinforcing bars and threaded rods, it transfer loads into the body of the concrete.

A column box is cast into the column with similar shear reinforcement to help transfer loads. The solid steel sliding knife actually carries the load from one member to another.

It is located within the beam box and then partially slid out when in position to bear on the bottom of the column box once the two members are offered up together, in a similar way as a bolt will slide across a gatepost.

The resulting joint is then sealed around its periphery before the whole mechanism is grouted up from the sliding point, producing a fully sealed, homogenous structural joint.

The system can be used at almost all joint connections: beam-to-column, column-to- column and beam-to-beam. It also has the flexibility of enabling members to be placed at an angle to one another, allowing designers to develop complex frame layouts.

Efficiency boost

The connections are actually placed into the mould and cast in during the production process.

This means that the manufacturers do not have to alter their expensive formwork assemblies to incorporate corbels into their precast columns, or cast steps into beams. It boosts efficiency and cuts waste according to Mr Billington.

“There are savings through the amount of material used to cast the corbels - less steel reinforcement and less concrete. It also means precast sections are lighter and more regularly shaped, making them easier to transport and reducing vehicle movements.

With fire protection built into the system, no additional work is required after erection; even the design and specification of sliding connections can offer savings.

The joint connections themselves are available in a range of loading capacities, while the steel reinforcement details around the connections are standardised, too.

“Architects like these systems because the final structure looks neater,” Mr Billington says. “Structural engineers like them because they are easily specified.

Precasters like them because it makes efficient use of moulds and contractors like them because they are easy to install.”

The system can even be used to connect precast frame members to steel sections as precast specialist Creagh did at the Manchester Metropolitan University project (see box).

With such concrete benefits, it’s difficult to argue against the use of sliding connections.

A video of the process can be seen here.

Manchester Metropolitan University, Birley Fields Campus

Creagh Concrete Products secured a £6.1m precast package from main contractor Sir Robert McAlpine for the provision of a new campus building.

It comprises two 96 m-long, 16.5 m-wide, five-storey teaching wings with a central atrium area and a stand-alone three-storey cantilevered feature stair rising through it.

The challenging design, manufacture and construction programmes meant that the whole project was to be delivered in just eight months with construction completed within 21 weeks.

Quality of finish was critical with all internal concrete being visible; no additional paint or other finishes were permitted.

To achieve this all precast concrete used self-compacting concrete mixes with flows from 750 to 780 mm.

Sliding and invisible connection systems were used throughout the scheme at beam to columns, precast beam to beam, steel beam to precast beam and shear wall section connections.

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