Introduction
“An engineer is someone who can do for £1 what any fool can do for £5”. Anon
One of the principal objectives of structural design is the achievement of economy of means; the structure should perform its function with minimum input of material and energy. It should also be durable.
Achievement of economy of means requires a knowledge of structural archetypes.
“Engineering is a hard discipline, being concerned with truth”. R N Arnold, Regius Professor of Engineering, University of Edinburgh (1946-63).
There are certain basic realities of engineering that can never be ignored and that can be thought of as “structural truths”. For example, if a structure is not capable of achieving a state of static equilibrium, is unstable or is of insufficient strength, it will collapse. Other types of building failure, such as inadequate fulfilment of programmatic function or rapid deterioration due to inadequate durability, as often occur in much of Modern architecture, can be justified on artistic grounds. The collapse of a building is an encounter with “structural truth” that cannot be explained away in polysyllabic discourse. It must be avoided by adherence to the realities of structural principles.
Another kind of truth that must be accepted in architecture is the relationship between the overall form of a structure and the efficiency with which load can be resisted. As is explained in the presentation, Myths, Realities and Carbon footprint in relationship to Architectural Form (see Ethical Design), this relationship is determined by Newton’s Laws of physics and, in particular, by the effect of gravity on mass.
There is no way in which these laws can be avoided, circumvented or altered by the use of new technologies or methods of design, manufacture or construction. They are facts of Nature.
The relationship between the overall form of a structure and its performance determines the structural archetypes. These are of great relevance to the design of structures, and therefore buildings, that are intended to be of low carbon footprint.
Three other attributes of a well-designed structure are outlined here. These are not so much truths as aspects of a structure that will enhance its performance and reduce its carbon footprint. Simply stated they are:
• keep spans short
• keep load paths short
• concentrate compression – dissipate tension.
Keep spans short – never use a long span if a short span will do.
The strength required of a horizontally spanning structural element is directly proportional to the square of its span. If the span is doubled, the strength must be increased by a factor of four. Tripling the span results in a nine times increase in the strength required. The weight of material required to provide adequate strength is directly proportional to the strength required.
Long spans cause a significant increase in the quantity of structural material that must be provided because the extra material involved is significantly greater than what would be required to provide additional vertical support from columns or walls to keep spans short.
Long spans increase carbon footprint.
The designer of a building should always be called upon to justify the use of a long span.
Sports arenas are examples of building types for which long spans are justified. The game of ice hockey, for example, cannot be played in a space that is interrupted by columns.
The justification of a long span for other types of building (galleries and museums for example) is often questionable.
At the Guggenheim Museum in Bilbao, the architecture is said to overwhelm the art. The large, long-span spaces contribute to this effect and may therefore be inappropriate. They considerably increase the carbon footprint of the structure.
Long spans are a feature of the Centre Pompidou in Paris – the principal floor girders have a span of 48 m. (A more normal span for a multi-storey building would be 10 to 15 m.) Such a large span may have been justified for the creation of a grand entrance space.
Were the long spans justified for the gallery spaces that constitute the major part of the floor area at other levels?
The spaces here are between 10 m and 20 m wide. The triangulated girders seen in the photographs are spanning 48 m.
If internal columns had been used at the Centre Pompidou to limit spans to say 15 m for the majority of spaces, this would not have compromised their programmatic function. It would have reduced the quantity of steel required by a factor of nine, and produced enormous savings in carbon footprint.
The Centre Pompidou consists principally of six very large interior spaces stacked vertically. It was based on ‘big’ architectural ideas concerned with making art democratically accessible, with flexibility in the use of space, and, above all, with a techno-optimist’s vision of how these should be expressed architecturally.
Whether or not these architectural objectives were achieved is a matter of opinion. What is not in doubt is that column-free spaces at all levels, that required the use of a heavy steel structure with a very large carbon footprint, were largely unnecessary.
This observation can be made of many prominent museums and galleries of the Modern period.
The roof structure of the Riverside Museum in Glasgow spans longitudinally along the entire length of its curvilinear plan. This very long span was accomplished by a substantial steel-frame structure of very high carbon footprint. Was it justified?
The question that should always be asked is – is the enormous extravagance of the long-span structure justified by the architectural outcome?
This is a question that architects must be challenged to answer effectively in an age of climate emergency.
For Low-carbon architecture spans should be kept as short as possible. The architectural concept should take account of this requirement.
Keep load paths short – always conduct loads through the structure by the shortest most direct means possible.
The primary purpose of a structure is to conduce loads from the points where they occur to the foundations of the building. This will be done with economy if the route taken by the load is as short and direct as possible.
The longer the route, the greater the amount of structural material involved.
Changing the direction of a load will normally require the provision of an elaborate joint.
Both of these increase carbon footprint.
Here are examples of some unnecessarily long load paths.
The bridging structures at the Hongkong and Shanghi Bank headquarters subdivided the building into zones vertically and created a ‘reconfiguration’ of the skyscraper concept. A conventional skyscraper arrangement with a short-span column grid would have been significantly less costly both economically and in terms of carbon emissions. Was the architectural gesture worth the high environmental cost?
A column under the yellow arrow would have conducted the load more directly to the foundation. Was the creation of a column-free interior absolutely essential? Could its advantages have been achieved in other ways with a less costly short-span structure?
The question that should always be asked is – is the extravagance of a complex load path justified by the architectural outcome?
This is a question that architects must be challenged to answer effectively in an age of climate emergency.
For Low-carbon architecture load paths should be kept as short and direct as possible. The architectural concept should take account of this requirement.
Concentrate compression – dissipate tension
The forces that occur in structures are either tensile or compressive.
They can occur separately or together in single elements.
Columns, piers and load-bearing walls are subjected only to compression.
The sub elements in triangulated trusses carry either pure tension or pure compression.
Beams, and other bending-type elements, have parts that are in tension or compression.
The problems associated with resistance of these two types of force are different and the basic principles are easily demonstrated by experiments with ordinary objects.
Grip the ends of your school ruler. You will not be able to exert sufficient tensile force to break it. Even if you did have sufficient strength (which you would not) you would not be able to grip it hard enough. Your hands would slip off before the ruler broke.
The same ruler can easily be broken by exerting a compressive force – due to the phenomenon of buckling which causes it to bend about the weakest axis of its cross-section.
The ruler therefore has very different strengths in tension and compression despite the fact that, whether it is made from wood or plastic, the constituent material has more or less equal strength in tension and compression.
This simple experiment demonstrates two of the classic problems of engineering.
With tension, the problem is to get the load into and out of its ends – with the joints, in other words.
With compression the problem is buckling, which is greatly affected by the shape of the element’s cross-section.
Buckling is one of the most interesting phenomena of structural engineering and is considered in the section of this website entitled Aspects of Structure that affect Design.
It is sufficient here to note that resistance to buckling depends on slenderness. Elements that are short and thick are stronger in compression than those that are long and thin.
Shape of cross-section is also important. The very best is the circle because it is symmetrical about all axes through its centre. Even better is a circular tube which allows the overall size to be large without excessive quantities of material.
The cardboard core of a roll of toilet paper is quite strong in compression. The same amount of cardboard in a flat sheet has very little strength in compression because it will buckle.
Due to these differences in tensile and compressive behaviour, structural efficiency is enhanced if compression is concentrated into a few short, thick elements and tension is dissipated into a large number of thin elements. This facilitates resistance to buckling in the compression elements and eases the problem of the end joint in the tensile elements.
An example of such an arrangement is the O2 Dome in London which has a highly efficient structure. The dome itself is a form-active cable network. It is supported on a small number of compressive masts to which it is attached by large numbers of straight tensile elements. Compression is concentrated – tension is dissipated.
Concentration of compression and dissipation of tension may conflict with other desirable features of a structural arrangement such as the adoption of short spans. It is not normally advisable, in a rectilinear framework, for example, to reduce the number of columns by increasing the span of the horizontal elements.
So, concentration of compression and dissipation of tension are simply desirable features that should be borne in mind by the designer rather adopted as a matter of course.
In the case of the O2 dome their adoption was highly appropriate.
