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Last edited 02 Jan 2020
As materials technology has advanced, and steel and concrete have been able to achieve higher and higher compressive strengths, so the height of skyscrapers has increased to heights that were previously deemed unrealistic.
A skyscraper is essentially a cantilevered steel beam but its rigidity and resistance to lateral and vertical forces is made possible by the core that runs up its entire length. In effect, the core is a hollow tube within a hollow tube, and the structure usually comprises an outer frame and an inner core.
Whether a steel- or concrete-framed building, the core is usually a square or rectangular reinforced concrete tube located inside the building. It comprises concrete walls that will have openings for access and egress and is typically placed centrally in the floor plan with usable space – for apartments, offices etc – arranged around it. But it can also be offset which can give a better use of floor space.
- Aids the building’s structural stability.
- Provides access and escape e.g via lifts and stairs
- Allows a convenient clustering of services such as toilets, storage and fire services.
- Creates a protected fire compartment.
Today’s skyscrapers are usually hybrid structures, comprising an outer relatively light steel frame linked to an internal concrete core. The cladding to the building is usually non-loadbearing. Steel is typically used for the frame as it can be built to higher levels than reinforce concrete. The typical arrangement seen in many of the world’s tall buildings involves an external steel frame and inner concrete core.
The core’s walls are typically called shear walls and achieve a lateral stiffness that is greatly in excess of that of the outer frame. This allows the core to resist the lateral forces arising from wind and earthquakes that can act on buildings. Used in conjunction with an outer steel frame, the core resists the horizontal loads while the lighter steel columns resist the vertical load.
As an alternative to a concrete core, steel-framed buildings may also have steel cores which give the advantage of rapid construction without having to wait for concrete to cure. In such cases, stiffness may be imparted to the steel core by diagonal steel bracing or by prefabricated concrete panels inserted in bay as construction progresses.
Lateral loads are transferred from the outer steel structure to the core through the floor structure, typically comprising long-span beams, their secondary counterparts and concrete slabs or steel decks.
The combination of frame and core shear-wall action allows skyscrapers to resist huge lateral forces while suffering only mild deflection: prior to their destruction, the twin towers of the World Trade Centre in New York would have a top sway (or drift) in the order of around 900mm.
Concrete cores in high-rise buildings can also be stabilised further by the use of outriggers – trusses inserted to brace the core and which may be supported by super tall (or mega) columns. Outriggers can be one or two storeys deep and connect the core to the perimeter columns.
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