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Time for tension

Post tensioned concrete beams are helping a new car park straddle Merseyrail tunnels at a tightly constrained site in the middle of Liverpool. NCE reports.

McGee is principal contractor responsible for the design and construction of a £7.3M multi-storey reinforced concrete car park which forms the first phase of Liverpool’s £160M Central Village redevelopment scheme being undertaken for developer Heathfield.

Right on top of the city’s busy Central Station, the Central Village occupies a 2.4ha site and will provide extensive retail, office, residential and leisure facilities when completed in 2013.

Concrete frame

The car park is a 10-storey, split-level reinforced concrete frame sitting on a combination of piled and pad foundations with two lift cores. The car park is bounded by 9m high walls to the north and south with existing properties directly behind.

“The formwork would have to take the full load of the beam before the concrete could support itself”

Ben Ume, Matthews Consultants

The biggest challenge is the existence directly below the site of a tunnel carrying the live underground Merseyrail train service into Central Station and a Network Rail service tunnel running almost parallel to it.

Both tunnels must remain fully operational during the project’s 55-week duration. This has required detailed discussions with Network Rail so work on the existing ground and within the tunnels could take place.

Parts of the tunnel roof consist of brick jack arches supported on plate girders and above the tunnel roof was about 2m of fill. The maximum allowable load permitted at the existing ground level, was 10kN/m2.

Propped podium

McGee, with consulting engineer Walsh Associates, proposed to build a propped podium deck with eight post tensioned transfer beams across the tunnels. The transfer beams span a maximum of 18m and range from 1.8m to 2.4m wide and from 2m to 2.15m in deep.

Post tensioned floor slabs 200mm deep were used to keep the 10-storey car park as light as possible. But even so the loads were massive and one of the biggest challenges was to find a way of supporting the construction load when casting the transfer beams.

“The formwork would have to take the full load of the beam before the concrete could support itself,” says Ben Ume, a director of Walsh Group’s post tensioning subsidiary Matthew Consultants.

Post tensioning

The task of installing the tendons and tensioning the beams was entrusted to post tensioning specialist CCL. The post-tensioning system chosen was CCL’s XM-range, a proprietary system which incorporates high-strength cast steel anchor heads. The number of post-tensioning tendons within each beam
varies, from seven to 19. All eight beams employ 15.7mm bonded multistrand tendons between the CCL anchors.

It was decided to cast each beam in two pours. In this way, the falsework was never subjected to more than half the total weight of the concrete beam.

Beam of two halves

One beam was cast at a time, with the construction joint at the mid-depth of the beam. Extra reinforcement was added to the bottom half to minimise deflections and the top reinforcement was added just below the horizontal joint, with closers added to the permanent shear links. Extra raking props were used to support the five longest beams in the temporary condition.

The construction sequence was therefore to erect the formwork on spreaders, place the permanent reinforcement for the full depth of the beam and place the extra reinforcement for the temporary condition and then install post tensioning tendons for the lower half of the beam before the first concrete pour. Formwork was removed when the concrete had cured sufficiently and the top half was then cast. Post tensioning then took place. Work is now on track for completion in December.

 

Project team

Client Heathfield
Principal contractor McGee Group
Consulting engineer Walsh Group
Post tensioning engineer Matthew Consultants
Post-tensioning CCL
Geotechnical consultant Byrne Looby
Project value £160M

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