Feature

Core Values

Construction of the core is one of the most important aspects of the construction of a tall building. It provides the stability, vertical transportation in the form of lifts and stairs, and the main service runs up and down the building.

There are three standard ways of constructing the concrete core for a tall building: slipform, jumpform and precast. Each has its merits and considerations when designing. However, the choice of construction type is normally down to the contractor so the designer may not be sure in the initial design stages which option will be used.

The main drivers for contractors are:

  • Cost-effectiveness
  • Safety of site operatives and the public
  • Crane use
  • Incoming logistics, lay-down and storage areas
  • Vertical movement of operatives and materials
  • Standardisation of structural elements and components, which facilitates repeatability.

Slipform

Slipform is the default option for commercial buildings as it is quicker than jumpform and so the passenger lifts, frequently on the critical path because of the number required in a commercial building, can be fitted earlier in the overall programme. However, the finish that is possible when slipforming is not normally good enough to be left exposed as visual concrete.

Slipforming is a continuous process, where the formwork is slowly and gradually moved up the core using jacks placed within the slipform rig. The rate of climb is about 250-300mm per hour and it is best used when there can be 24-hour working. This is not normally allowed in the centre of cities, so the slipforming is done for the allowable time during the day and the concrete is kept workable at the end of the day with a retarder in the mix.

But even with 12-hour working, slipforming will create about a floor of core a day. This will mean that the core is normally well ahead of the rest of the floors and the designer should consider its stability in the temporary condition.

20 Farringdon Street, London (2018)

20 Farringdon Street is a new speculative office development on the edge of the City of London. The decision to expose the internal surfaces of the concrete core was taken early on for economic, practical and aesthetic benefits.

The design team and concrete frame contractor worked closely, pre-tender, to marry the logistics of constructing a jumpformed core with the aesthetic aspirations. A formal pre-construction report included concrete mix parameters, selection of formwork and release agent, day-joint detailing, tie rod and lifting eye aesthetics and final architectural concrete details as well as photographs of acceptable and non-acceptable details on exposed surfaces.

A pragmatic board layout was developed to achieve greatest cost and programme efficiency without pausing the cycle to adapt shutters or reline boards. Remarkably the team used the same MDO-faced ply boards for all 11 jumps or pours, assisted by a thorough grout washing regime prior to their first use and strict control of the release agent.

The jumpform system was based on a fully enclosed automatic, climb-as-one rig, customised to meet the aesthetic aspiration. The internal climber was fixed through the regular pattern of tie holes rather than the more conventional system of needing larger pockets or holes in the wall, thereby avoiding areas of patched-up concrete. Bespoke mitred corner panels were also used to avoid unsightly striking pieces or infill strips at the corners.

The stairs and landings were precast by Byrne Bros; the flights cast on edge so that the three formed sides would be visible, with the unformed, floated side hidden against the core wall. Telescopic structural connectors between landings and walls, and flights and landings provided a neat shadow gap detail. As the stair was to serve as access and circulation during construction for follow-on trades, the finished stair had a “Rolls-Royce” level of robust factory-applied temporary protection.


PROJECT TEAM
Architect Denton Corker Marshall Architects
Engineer Waterman Structures
Concrete frame contractor Byrne Bros (Formwork)

Slipforming requires greater management and quality control by the contractor than the other two forms of construction. A fairly new innovation is the use of GPS to help position the slipforming rig, but even so the tolerances are about 25mm in any direction. This is because the continually moving jacks can cause the rig to twist or skew.

Another fairly new innovation is to install a crane on top of the rig so that it is lifted with the rig. This makes the vertical logistics of getting personnel and materials up the building easier. The core under the rig can be used for access if the stairs are precast and lifted into place through the rig.

Since the jacks are connected to the reinforcement, the rebar requirements for slipformed cores may be dictated by the needs of the slipform rather than the permanent design. The concrete required for a slipformed core depends heavily on the temperature conditions while it is being poured as it needs to be fairly quick-setting and, for the higher levels, it also needs to be pumped. This means that the concrete tends to need quite a high percentage of cement. For the Shard in London, 36 different concrete mixes were established before work began on the slipformed core to allow for different weather conditions and times of day.

Jumpform

Jumpform is another method for forming in-situ concrete cores. In this case the walls are cast in storey lifts rather than continuously as is the case with slipforming. The jumpform rig is a complete system which includes the formwork and access platforms for fixing the reinforcement, concreting and any post-concreting work. Jumpform is the default option for high-rise residential buildings as fewer passenger lifts are typically needed compared with offices and therefore fitting out the lifts is less significant for the critical path. The jumpforms are normally a storey high and therefore require one lift per storey, taking about five-to-seven working days per cycle.

Jumpforms can be either top hung or side hung. If top hung, this is normally done in the form of a closed box which makes for a safe working space. If the forms are side hung, these are clamped to the side of the core walls and when moved, pushed up from the bottom on rams pushing against climbing shoes fixed to cast-in anchors. All the load from the jumpform rig goes through these cast-in anchors, so if there is a clash between the anchors and an opening, the rig will be more difficult to design. Openings at corners should be avoided if possible because of the clash with the cast-in anchors.

As the core goes up more slowly than with slipform, the jumpform rig tends to be just ahead of the rest of the floor plate. This means that access is via the working deck and therefore no hoist is required specifically for the core access. If big cast-in plates or anchorage points are required within the core walls, then jumpform tends to be a better solution than slipform as they are easier to fix in place due to the greater time in the programme. One of the benefits of using jumpform is that high-quality finishes are possible (see case study, above), as the concrete is poured against a normal static formwork shutter rather than one that is moving.

Precast options

There are two main precast options for concrete cores in current use in the UK. These are twinwall and volumetric precast core units. Twinwall is a hybrid concrete solution where the precast element consists of two skins of concrete containing the main reinforcement spaced apart with lattice girder rebar. The twinwall elements are brought to site and lapping rebar fixed to provide continuity. In-situ concrete is then poured between the two precast outer skins to form the solid wall. As it is a partially precast solution, this is very fast on site, with cycle times similar to those for slipform construction.

Volumetric precast concrete core units are a fully precast solution where the core is constructed offsite and brought to site in a just-in-time manner and lifted into position from the lorry. The core units are then bolted together to provide the required robustness. Each core unit is normally half the storey height so two core units are required for each level.

Both systems can provide a high-quality finish that can be left exposed, or with very little additional finishing. Both require fewer personnel on site so have safety advantages. However, they cannot be used at great height as there would be too much wind for the crane to lift them into place. Both methods been used up to about 22 storeys.

Conclusion

As with all forms of construction, the best outcome is provided when there is early discussion with the contractor. The construction of the core will affect the design and the design will affect the construction. While there are standard defaults for using one of these options, contractors have preferred methods of working and there are always innovations that will inform the decision. The finish and the speed of construction required are other important factors, so should be discussed with the contractor as soon as possible.

Jenny Burridge is head of structural engineering at The Concrete Centre. This article was prepared with help from Don Houston, director at Byrne Bros

Photo: HB Reavis

FURTHER READING
Tall Buildings, The Concrete Centre and the Fédération internationale du béton (fib), 2014
Concrete Tall Buildings, free to download from www.concretecentre.com, 2018