Stretch task

Post-tensioned concrete is not a popular choice for construction in the UK today, despite a wide use of the technology in the US and the Far East.

But the City of London now has an eaxample in the shape of AIG Global's new headquarters.

According to James Middling, engineer at consultancy Connell Mott MacDonald (CMM), and one of the building's design engineers: "AIG Global decided to go with a post-tensioned concrete frame to generate an extra floor within the height restrictions imposed by planning regulations. Without it, the project would not have been financially viable because we could not squeeze enough floor space into the building envelope within the set budget."

The client (one of the biggest insurance businesses in the world) was happy that a big 10-storey block on the Fenchurch Street site would generate the required 180,000m2 of floor area, but when it came to scrutinising the planning application, the Corporation of London had other ideas.

It wanted the building to be tiered to reduce the effect of shadows falling on adjacent buildings on Fenchurch Street. So the architect and engineer reworked the scheme: dividing the building footprint into three zones; introducing set backs at the eighth, 12th and 15th floors and reducing the floor area on the way up by a third each time.

"We skinned the floor depth down to 400mm using a 1,700mm wide post-tensioned band beam to span 13m in the primary direction and a 225mm post-tensioned flat slab to span 8.6m in the other direction to make it work," says Middling.

The alternative lean steel scheme would have resulted in a floor depth of approximately 600mm, which meant breaking the height restrictions.

Does this mean that structural concrete is the slimmest form of construction for long-span, high-rise buildings? If that is the case, why in Canary Wharf Contractors not specifying post-tensioned floors instead of millions of square metres of composite metal decking and steel beam?

"There still appears to be a lack of understanding about the technology of post-tensioned concrete as well as a lack of confidence among institutional investors and building developers in its adaptability and long-term integrity, but that is slowly changing," says Middling.

The bad press that post-tensioning received in recent years, with corrosion found in bridge tendons in poorly grouted bonded ducts, has only served to fuel this fear.

Failure to grasp the benefits

It is ironic that the UK construction industry, which boasts of being a world leader in lean construction and fast assembly, has singularly failed to grasp and recognise the benefits of pre-tensioned concrete that now dominates high-rise construction in Australia, Hong Kong, South Korea, China, Malaysia and Singapore.

CMM has some pedigree in post-tensioned design since its merger with consultancy Connell Wagner of Australia, where the majority of high-rise buildings are designed with post-tensioned bonded floors.

"There is a preference for bonded construction now because, should a tendon be cut, the pre-stress in the rest of the tendon is not lost," says Middling. "There are better and more sophisticated mortars for tendon grouting, which leave no air pockets in the ducts and ensures the integrity of the construction."

It has helped that AIG Global was familiar with the technology, as was the architect Kohn Pedersen Fox, with its US background.

When Mowlem won the contract the post-tensioned route came as no surprise, as project director Ray Cutts explains: "We were conversant with post-tensioned construction through our civil engineering experience on bridges."

The contract was let under JCT98 conditions without quantities and with a guaranteed maximum price, which enabled the client to put the risk back on to the contractor.

"The biggest risk we would carry on this scheme was the programme. In the client's scheme we had to demolish two levels of existing basement. This included breaking out the existing raft and cutting out sections of the existing retaining walls to accommodate the new substructure design," says Cutts. "This meant we would be excavating into virgin ground outside the old building footprint, with the strong possibility that we could expose artefacts of Roman origin that would stop the project for an indefinite period to allow an archaeological dig."

Further along Fenchurch Street, the Plantation House project had been held up for two years to allow archaeologists to uncover and record findings that were buried under it. Mowlem looked at the scheme very carefully to see how it could avoid excavating outside the existing retaining walls in the double-storey-deep basement.

"CMM had no details on the structural loadings and design of the original building to enable the engineers to make an accurate assessment of the loading that the existing raft would carry. This meant we could not use the existing raft and retaining walls as a foundation for the new building. In the end we proposed quite a change to the basement design, to avoid penetrating the existing retaining walls,"says Cutts.

What was of greatest concern was excavating into virgin ground when forming the 460mm by 2,000mm deep perimeter columns through the 300mm to 1,000mm thick retaining walls.

"Cutting slots into the existing retaining wall to form the deep columns, then having to prop and brace the entire excavation and what was left of the retaining walls was going to take considerable time," reflects Cutts. "We had the idea of reducing the length and increasing the width of the perimeter columns just slightly, which would then allow us to keep the columns within the existing footprint below ground level."

By introducing a transfer structure and cantilevering the columns above ground level, the original geometry and architectural line of the projecting column features could be retained for the superstructure.

"In approaching the project this way, we would reduce the amount of temporary works, give ourselves greater programme certainty, reduce the risk of an archeological dig and save £500,000 on the original scheme cost," says Cutts.

While still bidding for the job in open tender against other contractors, Mowlem decided to put this idea to the client to see if such a dramatic revision to the substructure design and to the perimeter columns would be considered.

"The client understood the risk we would be taking, liked our lateral thinking with the substructure concept and the £500,000 saving," says Cutts, who admitted Mowlem took a gamble that the client would not pass this information on to the other tenderers.

"The client duly instructed the design team to check the feasibility of our proposal and soon after this was proved to be sound, we were appointed," he continues.

Once the piling work, the raft slab and the new perimeter walls were constructed, the 70m central service and access core was slip-formed from basement level to the roof. The slip-forming took five weeks to complete, working on a 12-hour slip-cycle. Some dividing walls were removed to regularise the core, which measured 20m by 6m in plan and had a wall thickness of 200mm. The dividing walls would be built in blockwork later.

"The core was slipped hydraulically at the rate of 200mm/hr using two cranes for lifting materials and for skipping the concrete into the forms rather than pumping it," says Cutts. "There was no room on the site to position a pump and there were severe restrictions on the use of the pavement or roads on Fenchurch Street to park construction vehicles or plant."

Setting time

The rate of movement of the forms is determined by the setting time of the fresh concrete, which is influenced by ambient temperatures, the cement content and water and the cement ratio of the mix, all of which have to be carefully monitored.

The set of the concrete is established using probes inserted in the bottom 100mm of the concrete within the 3m-high movable shutters. When the concrete in the base of the shutters is sufficiently stiff (usually about two hours after placing), the whole formwork assembly is pulled slowly upwards by a series of hydraulic jacks pushing against 75mm diameter steel tubes embedded through the middle of the core wall.

The post-tensioned floors were relatively quick to construct as there was so little reinforcement to handle. It took just eight days to cycle each of the large floors up to the 8th floor despite using handset demountable table forms.

"Using flying forms over Fenchurch Street was too great a hazard and the 400mm-deep edge beam to the floor slab would have meant a lot of extra work to release the flying forms from under the slab," says Cutts. The dead load and self-weight of the floor are balanced and carried entirely by the pre-stressing tendons, with just a light mesh of 10mm diameter bars at 200mm spacing for distribution reinforcement.

The tendons are sleeved into continuous ducts that are typically spaced 1,150mm apart for the flat slab and draped to follow the bending moment profile, ie close to the bottom of the slab or beam over the mid span section and close to the top of the slab over the column support. The tendons are anchored at each end and the first-stage pre-stress carried out, where 50% of pre-stress is applied when the concrete has hardened and achieved the required compressive strength of 12.5Mpa.

This usually takes two or three days after the slab has been concreted under normal weather conditions. When the first-stage pre-stress has been applied, formwork can be struck and moved on.

The full pre-stress is applied when the concrete has achieved its full design strength of 33Mpa, which is usually after seven days.

"The district surveyor was so alarmed at the lack of reinforcement in the slab when he inspected the first pour that he insisted that additional rebar had to be placed for him to pass it for concreting, " says Cutts.

Little visible reinforcement

Mowlem later approached the district surveyor with the structural engineers to explain the design and why there was so little visible reinforcement to see. Thereafter they did not have to add extra rebar to the floor slabs.

The external curtain walling of powder-coated aluminium and glass panelling that encloses the spaces between large perimeter columns followed the frame just five floors below. The perimeter columns are clad in smooth, glazed terracotta panels - similar to a vitreous enamel - using masonry fixings. All the cladding work was carried out from within the building with the erection crew wearing safety harnesses and without an external scaffold. "Using a mini-mobile crane, the cladding units would be lowered to the floor below and fixed top and bottom to the channel inserts that were cast into the floor slab," says Cutts. "We wanted to eliminate all risk of accidental damage to tendons from site drilling of fixings. We also cast inserts into the soffit of the slab to hang the services from."

Cantilever platforms were used to fix the large, glazed terracotta panels over the external column face. The flat appearance of the curtain walling is broken up by horizontal bands of sun louvres at mid-storey height and vertical glass fins that run up the building face.

On a bright day the sunlight mirrored and refracted by the translucent green glass finials and stainless steel rocker supports of the louvres makes the building shimmer and sparkle.

It begs the question: how long will it be before other developers and contractors take pre-stressed concrete seriously? n