All of the systems can accommodate changes to the core wall thicknesses but these should be minimised. Refer to Figures 6.1, 6.2 and 6.3.
With all of the outlined systems, a level of integration is required between temporary and permanent works. The plan layout of the walls, floor-to-floor heights and location of any penetrations needs to be frozen at an early stage.
Although these are standard formwork systems, early engagement can lead to a more refined system. The aim is to cast as many of the structural elements in the initial pass as possible and minimise any return visits or trailing platforms. Further refinements can be made to install precast stairs through the formwork systems. Again, this needs to be fully understood by all parties to ensure that a core being progressed in advance of the rest of the structure has the necessary stability.
Additional pockets and cast-in plates required for any of the systems need to be coordinated, as the preferred location may not be possible due to the design of the core.
Typically these pockets should be considered as penetrations and detailed as such, but there will be a need for additional reinforcement to accommodate concentrated loads.
Detailing needs to reflect the sequence of core construction, as some walls and slabs may be cast from a trailing platform. The structural connection between these elements needs to be fully designed to allow accurate detailing. Beams within the core walls need to be considered early to allow efficient placement of rebar through layering and orientation of bars.
Figure 6.3 An example of a crane-lifted jump-form.
Image: Peri Ltd.
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Figure 6.4 An example of a slip-formed core
Photo: Laing O’Rourke Plc.
Standardisation is of key importance to increase repetition, quality and, subsequently, output. Use of an experienced reinforcement detailing specialist for the cores is highly recommended. Discrete precast components, vertical or horizontal elements may be incorporated into the in-situ core construction as it proceeds. This can assist buildability and improve cycle times.
Consultation between structural engineers and contractors must take place to ensure contractors can ‘pre-set’ building levels to account for any settlement and axial shortening of structures over the construction phase. Sequence and loading at every point within the programme must be fully understood and calculated.
Element levels at each floor will need to be cast higher than the desired final level by a variable amount predicted by theoretical calculation. Contractors will need to monitor all element levels at each floor regularly as construction advances, and report these to designers for comparison with theoretical and target levels and potential adjustment based on actual movements. If conducted collaboratively, using the same approach as the original design, an improved result can be achieved. Further guidance on presetting and monitoring the structure during construction is provided in Chapter 11.
6.2 Column and wall
In addition to vertical elements within core construction, the construction methodologyconstruction
for any columns and additional walls should be determined. The advantage of precast concrete here is that it minimises hook time and risk from adverse weather, with the associated time savings likely to outweigh any potential increase in costs.However, if precast is not a viable solution, an on-site solution can be used. To choose the appropriate methodology, impact on cranage and lay-down must be considered.
6 Buildability
It may be beneficial to prefabricate the rebar and then lift it into place, possibly using double height cages to minimise the impact on cranage, while slab-to-column connections should be considered to ensure buildability.
For a double-height pour, the formwork would require more hook time as it would not be light enough for manual placement. If single height is utilised, then a lightweight formwork solution would be appropriate but due to cranage and economies of scale, it may be more prudent to fabricate bespoke shutters offering the advantage of being a single-lift item to the exact project specifications.
It is important to achieve standardisation in column section sizes, with changes minimised to ensure formwork re-use rates are high. A preference would be to reduce rebar content or concrete strength and then have the changes in section on set floors pre-agreed with the contractor. The walls should be treated in a similar manner, with precasting, prefabrication and formwork solutions all considered.
Reinforcement cages can be prefabricated offsite and brought to site on a just-in-time basis, and detailing should reflect this, wherever possible.
6.3 Slab construction
Consideration should be given to the structural form of the slabs, and its impact on construction. General guidance, irrespective of structural form, includes: In-situ floor construction has commonly been adopted for high rise concrete structures.
However precast or precast/in-situ hybrid floor construction systems should be evaluated for their potential benefits in terms of programme cycle times, safety and quality. The implications in terms of crane-hook times must be assessed.
For in-situ floor construction, flat slab design has buildability advantages with drop/
edge beams and thickenings minimised. Precambers should be avoided if possible but if necessary, should be standardised so falsework can be designed to suit.
Pour sizes should be as large as possible with maximum pour size determined by rate of concrete supply and the finish required. It may also be informed by working-hour restrictions.
Slab depth influences material usage, cost and weight. For example achieving a 25mm saving in slab thickness over 50 storeys also equates to a 1.25m saving in height.
For in-situ slabs, options include reinforced concrete (RC) or post-tensioned (PT) construction. If PT is chosen, particular care should be given to:
Temporary movement joints and pour strips
Tensile shortening of slabs inducing restraint cracking in edge columns
Slab deflections
Fixing points for the faÇade and any cast-in inserts which will need to be coordinated with stressing anchors.
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Contractors may prefer a PT option as it can provide quicker cycle times for the falsework and minimise the weight of reinforcement within the slab.
Alternatively, a hybrid in-situ/precast system may be adopted. Common products include:
Lattice girder planks for the entire slab can provide a high quality finished soffit. It is not suitable, from an architectural perspective, for all end uses. Although it provides permanent falsework, propping is still required at a reduced quantity. It may be that these loads inform the permanent works design.
Lattice girder with in-situ beams can be more economical than an entire lattice plank slab but requires more falsework.
Hollow core slabs’ finish is generally inferior to a RC slab and may need to be made good or covered. Speed of installation is high with little temporary works
requirement, although the connection to vertical elements is restrictive and may require additional items (steel angles) or for cores to be left open to allow for tying in.
Double T Units provide a fast solution, able to cope with large spans with little temporary works requirement. It is necessary to ensure the slabs are tied into the core efficiently and diaphragm action has been accounted for.
An early decision is required on which reinforcement type will be used to inform the detailing, whether loose bars, rollmat or specialist mesh. Standardisation needs to be investigated to ensure flexibility, quality and speed, and prevent the fabrication process and delivery from holding up site progress.
Each option will require different amounts of crane time, with a hook time analysis to be carried out in combination with a cost/benefit analysis of the various options. For example, if rollmat proves to be expensive, standardisation of loose bars and the associated L and U bars may be preferable.
6.4 Slab formwork for
Working within an enclosure system determines the formwork solution. Verticalin-situ concrete slabs
movement of the formwork will be by load-out platform or an integrated formwork hoist; the sizes of which determine the maximum formwork table size. Therefore, either a mobile table solution or panelised soffit system would be applicable.A further determining factor in choosing the formwork solution would be material movement paths. Vertical elements within the permanent works may mean mobile tables cannot be used. Panelised soffit systems are more robust and require little work at height, as they can be erected from below.
Formwork systems often include a cantilever section of formwork/falsework at the perimeter to provide access beyond the slab edge. The associated falsework frames will generally need to be tied down to the preceding floor slab to prevent them from over-turning and falling out of the structure.