The internal core comprises of a stiff rigid frame which may be either braced or framed, this core is effectively a vertical cantilever truss, anchored at the base by means of the foundation. The outer, or perimeter structure may be straightforward beam to column connections without the need for moment resisting connections. The outer columns on the windward side or the structure would effectively be in tension, while on the leeward side they would be in compression, maintaining balance.

The core is usually utilized as the elevator shaft, emergency stairway, and/or service well.

Generally, buildings with core systems are effective up to 60 – 70 storeys, beyond that; the resistance of the core becomes increasingly inefficient due to bending. The necessity of stiffening the core by excessively robust welded or bolted connections and splices has an impact on the economic viability.

The outrigger system however enables core structures to be built up to 150 storeys or more, the outrigger works like the mast on a sailing ship, with the central core being the mast, the outriggers, the spars, and the outer columns simulating the stays. The outriggers help to reduce the overturning moment of the core that would otherwise act as a pure cantilever, transferring the reduced moment to the outer columns

Up to now we have only discussed steel cores; the more common practice today is to use reinforced concrete, not so much because it’s stronger, but because it tends to be easier and quicker to construct by using a self-jacking slide former. In such cases the horizontal steel components are fixed directly to the concrete core by means of cleats which have anchors embedded into the concrete as the slide forming advances.

One of the many advantages of core based high-rise buildings is that the outer surfaces of the structure are relatively un-cluttered allowing the architects more freedom with regard to external aesthetics, it allows for the glass curtain walls which are a feature of many buildings today. The disadvantage is that the core can take up valuable floor space. Examples or core structures listed are:

Braced Hinge Frame

Here, the inner core consists of a simple Braced Frame which makes it relatively easy to construct. Such construction is usually limited to buildings of no more than 10 storeys

Rigid Frame Core Construction

Here, the inner core is constructed from a Rigid, Moment Resisting Frame allowing the structure to be constructed up to 30 storeys. This method was very common until the advent of super high-rise structures

Braced Rigid Frame

This is essentially a Rigid Frame which is reinforced by bracing, such construction was used widely in seismically active zones. This method of construction allows buildings to be erected up to 40 storeys high, but it does tend to be expensive due to the complexity of the construction, which though modular is still quite difficult to erect due to the tight tolerances involved

Rigid Frame with Concrete/Steel Shear Wall

The additional Concrete or Steel Shear Wall increases shear resistance of the structure, enabling it to reach up to 60 storeys

Rigid Outrigger Frame

The outriggers themselves usually comprise of a ‘belt truss’ which extends from the core in both directions. When the belt trusses are supported at each end by ‘mega-columns’ structures may extend to 150 storeys, or more. Though such construction is not strictly a core-based construction it offers a wider perimeter thus resisting the pushing effects of the lateral loads, the mega-columns principally are towards the center of the structure and can easily be incorporated into the building’s aesthetic.

The disadvantage with the belt-truss is that it will occupy essentially an entire floor thereby reducing the usable space, but with careful architectural planning, these disadvantages can be overcome by utilizing the space for ‘mechanical’ floors housing essential services.

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