When modelling floor elements within an FE model, consideration should be given to the following points:
The node spacing within the FEM mesh must neither be too close nor too tight, particularly in zones where constraints are concentrated e.g. close to supports. Advice should be sought from the FEM software provider as necessary.
Reinforced concrete floors can rarely be fully fixed into the core due to practical considerations, for example, limitations on the diameter/spacing of passive- reinforcement steel projecting from the core. The floor/wall connection should therefore be modelled appropriately so that the stiffness of the wall/floor joint is not over-estimated.
Cracking will occur in the floor elements, which will affect the stiffness of the floor.
Again the material properties used in the FEM model should be set accordingly.
Where floors include beam elements the model must accommodate the eccentricity of the beam and floor centre line. Such effects can induce bending within the floors and beams, particularly where the horizontal elements also transfer significant in-plane forces.
4 Structural elements
4.1.2 Flooring systems
For all of the above options, either reinforced concrete or post-tensioned concrete can be adopted. Post-tensioning of the flooring elements is a popular choice for tall buildings due to the improvements it brings in reduced overall thickness and weight, while speeding up construction. Where post-tensioning is proposed, it is important to consider the following points: Restraint to the pre-stress shortening due to stiff-core walls and columns
Access to undertake the stressing often required from the building perimeter
Method of providing a tie into the core walls when progressed ahead of the floors
Post-tensioning will generally only be designed to resist gravity loadings and traditional reinforcement may be required to cater for any load reversals caused by lateral loading from wind or seismic events.
The use of precast concrete floors systems can offer some advantages in terms of speed of construction and can also offer large-span floors, which are attractive for office and retail uses. Precast solutions also have benefits relating to concrete supply at high level, back propping and strength gain. Where precast floors are used it is essential that the engineer fully considers the robustness of the structure and provides the requisite ties between the individual precast elements to ensure that the structure as a whole can perform
adequately in the event of accidental loading.
Precast systems require more cranage to construct the building. If a precast flooring system is proposed it is essential that the cranage and lifting strategy to be used for the construction is well considered ideally in conjunction with the constructor.
A summary of the typical flooring solutions is provided in Table 4.1.
Structural elements 4
floor plate, which is good for coordination with building services.
Flat slab
Structurally efficient but slower to form andconstruct. Overall depth can be efficient if building services and dropped panels can be coordinated.
Flat slab with drops
Structurally efficient but slower to form and construct. Beams can be useful where flooring system is used to contribute to lateral stability system.
Solid two-way slab with beams
Structurally efficient in terms of material weight but considerably slower to form and construct. Also produces a large overall depth and hence is rarely economic for tall buildings.
Waffle slab
Structurally efficient but can be slower to form and construct. Beams can be useful where flooring system is utilised to contribute to lateral stability system in the direction of the beams. Overall depth can be efficient if building services and beams can be coordinated. This arrangement is suitable for use with precast components
Solid one slab with beams
Structurally efficient but can be slower to form and construct. Beams can be useful where flooring system is used to contribute to lateral stability system in the direction of the beams.
Solid flat slab with band beams
Structurally efficient in terms of material weight but slow to form and construct. Also produces a large overall depth and hence is rarely economic for tall buildings.
Ribbed slab with beams
Key to ratings:- - Poor - Good - Excellent
4 Structural elements
4.2 Columns
The primary purpose of columns is to support the floors and distribute vertical loading to the ground. Columns are generally spaced at regular intervals along the perimeter of the structure but, for larger floor plates, interior columns are frequently needed to reduce the span of the floors.The design of the core benefits from supporting a larger share of the vertical loading, as this assists with resisting overturning from lateral loads. Spacing of columns from the core should, therefore, ideally be maximised. The central core may typically support about 60 % of the vertical loading, with the columns supporting the remaining 40 %.
Columns may be arranged to form part of the stability system, as discussed in Section 4.2.1, and this will make further demands on the performance of the columns.
4.2.1 Performance
Columns are mainly subjected to axial compression. When selecting the columnrequirements
arrangement, engineers should consider the following factors: Reducing column’s geometric footprint to increase façade transparency and create more floor area
Ease of detailing and connection to floors’ structural system
Speed of construction
Robustness of the element and its resistance to fire
Minimum of intrusion into the building’s faÇade.
Concrete is well suited to economically resist high levels of compression stress and can be pumped at high altitudes (614 m from the ground for the Burj Khalifa in Dubai).
High-strength concretes can be used if the type of construction allows it; the gain in usable floor area, resulting from the reduced column cross section, can compensate for any additional cost.