The main vertical elements of the Commerzbank's structure rise directly up from the piled foundations, the cellular substructure of rounded triangular shape that caps the piles. In each rounded corner of the triangle, they consist of a pair of diagonally braced composite steel-and-concrete curved walls and a number of associated columns and lighter walls.
Together, these walls and columns form three stiff towers which, like core structures elsewhere, accommodate lifts and services. The affinity with the framed tube arises from the interconnections of the towers around the remainder of the perimeter. These consist of steel walls, perforated by windows, which are continuous vertically for six, eight, 12, or 14 floors between the four-story openings.
They are described as Vierendeel trusses — composed only of rigidly connected vertical and horizontal members — because they not only link the towers to assist them in resisting wind loads but also partly carry the floors around a central triangular atrium that is open for the whole height of the building. Without sacrificing structural efficiency, they allow natural ventilation at all levels and views of sky gardens from all upper floors.
Alternative Stabilizing Forms
There are several ways in which stability has been maintained, even under considerable side load, when heights increase beyond the point at which stories consisting of simple columns and beams or walls and floors could simply be piled on one another. Up to heights of 20 to 30 stories, this was done by merely exploiting further the stiffness in shear of stronger enclosing and dividing walls and the stiffness in bending developed by new rigid joints between columns and beams.
No other departure from the forms and proportions appropriate to lower buildings was called for. At greater heights, three approaches have been followed. In the first, the same sources of stiffness were drawn upon, but a stricter planning discipline was imposed, most columns were closely spaced around one or more closed peripheries, and beams were made proportionately deeper to create, in effect, perforated tall tubes.
In the second, a compact core containing lifts, stairs, and vertical service runs was made to resist much or all of the side load and much or all of the vertical load. In the third, the column-and-beam structure was stiffened by diagonal bracing, or most of the loads were passed on to a more self-sufficient vertical truss.
In the second and third of these approaches the core or vertical truss can become a megastructure into which the individual floors and rooms are simply filed, to which they were attached, or from which they were hung. This was the case in the four corner towers of the Knights of Columbus Building by Roche-Dinkeloo and the cores of all buildings in which the floors were suspended from cantilevers at the top.
Such structures share a great advantage with structures like deep-sea oil rigs. Their material is better concentrated to carry large loads over considerable heights than is that of a structure in which the load is distributed between large numbers of more slender columns and beams.
The possibility therefore exists of building structurally highly efficient larger megastructures simply as supports which could then be used, like giant coat hangers, to carry floors and walls arranged with as much freedom as the internal partitions of a large open floor.
The idea of doing so has been around for a considerable time and received wide publicity from several groups of architects in the late 1950s and early 1960s. Habitat 67 by Moshe Safdie was an attempt to put the idea into practice on a modest scale.
The Centre Pompidou by Richard Rogers and Renzo Piano — an example of a wide-span structure — also went part way towards realizing the ideal of separating basic structure and internal subdivision and thereby paved the way for further experiment. Although the Centre Pompidou was only six stories high, its basic structure was initially conceived as a support for a highly adaptable interior.
Hongkong and Shanghai Bank
A most successful structure of this kind is the Hong Kong headquarters building of the Hongkong and Shanghai Bank by Foster and Partners. The brief called for both an ability to accommodate possible extensive future changes in banking requirements and a design that would allow construction around and over the existing bank while, for a time, this continued in use.
This wish to be able to build around the existing bank led to the choice of a suspension system, and this choice was retained even when it was later decided to start by clearing the whole site. For a number of reasons including the cramped nature of the site, it was also thought desirable to adopt a considerable measure of prefabrication.
As built, the basic supporting structure consists of four main supporting frames of three different heights, each 17 feet (5.1 meters) wide and separated by clear spaces of 36 feet (11. 1 meters). Each frame consists of two square towers spaced 110 feet (33.6 meters) apart and carrying, according to its height, four or five pin-jointed suspension trusses, each two stories deep and having an overall span of 213 feet (64.8 meters).
The outer halves of the north towers and trusses are seen fully exposed on the exterior, as are the tops of the adjacent interior towers and trusses. The towers each consist of four large-diameter cylindrical steel columns that diminish in wall thickness towards the top and are joined ladder-wise by Vierendeel braces to form highly efficient larger composite columns.
These are then braced in their own plane by the suspension trusses and, out of sight in the transverse direction, by diagonal cross-braces between adjacent towers at the level of each truss and one lower level. So braced, they form a completely self-sufficient megaframe capable of standing up to the worst typhoon.
The floors that accommodate all the activities of the bank are suspended from the three low points of the suspension trusses by thick-walled steel tubular hangers. As originally completed, they filled varying proportions of the available space, leaving the height of two stories completely open at the foot, an open banking-hall atrium extending through another nine stories in the center, and stepped-back openings on the east side to respect the then-current planning restrictions. Outside the composite columns are hung prefabricated service modules.
The megaframe was designed with the intention that it should be possible to change these modules and the floors at will, including filling most of the spaces originally left open. As at Habitat 67 however, nominally identical modules differ too much to be readily interchanged, although there should be no real difficulty in adding to the floor areas.
Again, achievement fell somewhat short of intention. Compromises had to be made. But the result was a fine and memorable building that came closer to the ideal megastructure than anything else yet built.
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Rowland J. Mainstone is a consulting engineer and fellow of the Society of Antiquaries and an honorary fellow of the Royal Institute of British Architects. For 30 years he was responsible, at the UK Building Research Establishment, for a variety of structural and architectural research focused on current design procedures and problems.
This article is excerpted from Developments in Structural Form, copyright © 2001, available from Architectural Press and at Amazon.com.