CASE STUDY
St. Mary Axes
For the St Mary Axes, Norman Foster wanted to increase the depth of the building, light shall be able to penetrate through, stack effect shall be induced to regulate the indoor environment, a twisted atrium is suggested. To achieve so, a stiff, rigid external tube is required, such that floor slabs can be hung between inner and outer tube.
To do so, a double spiral diagrid structure is used, and the penalization as a mixture of triangle and rhombus shapes are used. The fully dynamic shape can be panelized that only one panel, which is the tip, need to be curved glass, rest are all planar glass, while the shape and the logic of penalization can easily be described by formula, this makes the building becomes one of the textbook examples of digital architecture.
The disadvantage of the system is the high rigidity and heavy self-weight. Since the load transfer is heavily dependent on the triangulated shell structure, we can imagine that if the building is scaled up, the diameter of the shell would increase exponentially with the height, which would heavily affect the opacity of the outer façade. It means that the structural scheme may not be scalable.
Shanghai Tower
For the Shanghai Tower, the architect described as, ‘The most efficient core shape, which is the rec-tangle, covered by the most efficient façade profile, which is circle, protected by a dynamic shield minimizing the wind load.’ The most eye-catching part of the tower is his profile, a round-edged, triangular shape plan with a taper as wind-guide, rotate and scale down with the height, resulted as a twisted and tapered profile.
With the collaboration of the wind engineers, they can choose the most optimized shape in wind-performance, profile, and floor area. The twisted profile contradicts with the need of the rigid floor plan, the architect decided to hang the façade outside the inner skin, which raised the potential of increasing the flexibility of the façade, each groups of the façade, which is composed by 13 to 15 stories, can move up and down within 250mm, which reduced great amount of wind load.
By all the methods mentioned, architects can hence reduce the structure load and hence, structure sizing, which increased the usable floor area and sky atrium can be created even under high land price.
Commerzbank
The challenge taken by the Commerzbank is creating large void in mega-tower. To do so, the archi-tect used the side-core scheme, the core is split into three mega-columns, while the floor-slabs are hanged between the cores, the system is mainly under suspension. This move allows the architect cre-ate a set of dynamic atriums by simply rotating one set of floor plan.
The disadvantage of the system is the concentration of loading, we can imagine that once we scale up the building, the deflection difference of the three cores will increase, so as the span of the floor slab in between, the connection for the slab and core, needed to take the load of stabilizing the three cores, the suspension loading and the self-deflection, which means the building may tear himself apart un-der strong wind. It means that the structural scheme may not be scalable.
