SLAB SYSTEM
The three most common floor systems used for building structures such as offices, shopping centers, and car parks are the flat plate, flat slab, and banded slab. For high-rise construction, a fourth system is widely used which consists of band beams at relatively close spacing spanning from the building perimeter to the service core.
Although the economy of each of these depends primarily on the span and applied load, it is generally true to say that a band beam scheme is cheaper than a flat slab which in turn is cheaper than a flat plate.
Flat Slab
This system is commonly used in Sydney for high-rise residential construction where the span is usually 7 to 8 meters, most attractive feature of this floor system is its flush soffit which requires simple formwork and greatly simplifies construction. The depth of a flat plate is often dictated by shear requirements. Thinner slabs or longer spans can be constructed if column capitals or shear heads are employed. Used where spans are similar in both directions.
| Economic Span Range | 7.0 to 9.0 m |
| Imposed Loads up to | 7.5 kPa |
Flat Plate
This system is used for structures where spans in one direction are predominant. The sides of the band can be either square or tapered for a more attractive result. The band beam has a relatively wide, shallow cross-section which reduces the overall depth of the floor while permitting longer spans. This concrete section simplifies the formwork and permits services to easily pass under the beams. The post-tensioned tendons are not interwoven leading to fast installation and decreased cycle time.
| Used | Span predominant in one direction |
| Economic Span Range | Band Beam: 8.0 to 15.0 m Slab: 6.0 to 10.0 m |
| Imposed Loads | Up to 15.0 kPa |
Banded Slab
The economical span range over a flat plate is increased by the addition of drop panels. The drop panels increase the flexural stiffness of the floor as well as improve its punching shear strength.
This system provides the thinnest floors and can lead to height reductions and substantial savings in facade costs.
| Used | Where spans are similar in both directions |
| Economic Span Range | Up to 13.0 m |
| Imposed Loads | Up to 10.0 kPa |
High Rise Banded Slab
This system has gained favor over the past 10 to 15 years for high-rise construction and consists of band beams at relatively close centers spanning between a perimeter beam and the service core. The system suits system formwork due to the amount of re-use in high-rise construction. Services may either pass under the shallow bands or, alternatively, pass under service ‘notches’ in the band soffit. For clear slab spans in excess of 4.5m, the use of post-tensioning is economical perpendicular to the bands and assists in reducing the weight of the slab carried by the bands.
| Used | Long span high rise construction |
| Economic Span Range | Band Beam: 9.0 to 15.0 m |
| Imposed Loads | Up to 7.5 kPa |