Introduction

The successful integration of structure is crucial to solving the aesthetic, functional, technical and economic requirements of a building. These requirements must be addressed during the design process.

The choice of structural system is a critical element of the design process, but not one that cannot easily be abstracted. Decisions regarding structure will be part of a problem solving process that will require consideration of various requirements and the choice of structural system may be dictated by a number of these. Although structure may simply be regarded as a means of supporting the building, its contribution is a far more complex and fundamental aspect of building design.

When considering structure, there is a wide choice of structural systems and materials that may be used.

Common systems include:

• planar (or loadbearing) forms e.g. masonry or concrete walls
• frames, e.g. timber, steel or concrete linear elements
• membrane, e.g. fabrics and shells.

Steel has useful properties of strength, versatility and ease of connection which make it appropriate for frame systems. In addition it lends itself to prefabrication.

Steel construction generally uses standard section columns and beams, but also includes arches, trusses and grids, which can be connected together to produce a wide range of shapes and forms. Other sections and shapes can be fabricated to order.

A frame deals primarily with the structural forces in a building, allowing other elements, such as walls, cladding, insulation and weatherproofing, etc to be independent of the main structural requirements of the building.

Whilst other materials can be used to form structural frames, steelwork, with its properties of strength, isotropy and stiffness, also its straight and compact linear elements, easily assembled from prefabricated elements, is particularly appropriate. These elements gather and transfer the major structural loads as directly as possible to the foundations. These qualities provide strong, lightweight structural systems that can support large spans and tall buildings whilst maintaining cost-effectiveness.

There are several points in the design process at which the consideration of structure may become critical, depending on the brief, availability, visual language or cost.

Initial design approaches will necessarily be concerned with the brief development and plan organisation. The exact point at which a decision concerning choice of primary structure can occur at various points, e.g. as part of the brief, or dictated by availability of materials; it may be a particular aesthetic requirement; or it may occur as part of a cost comparison with other structural forms.

A number of early decisions about the form and organisation of the building will affect the choice of structural system. These include:

• size and shape
• volumetric and functional requirements
• spatial organisation
• nature of enclosure and building skin
• integration and relationship to environmental control and servicing
• aesthetics

For a long span single storey building general stability is not a problem and can be solved by cross-bracing or rigid column/foundation connections; the choice of spanning element is dictated by the span, e.g. simple beams, trusses or castellated sections, portal frames, arches.

For multi-storey steel buildings, high strength is required - the net floor area is critical and there is, therefore, a requirement for small columns; resistance to lateral loads (bracing) is critical, and there is a need to consider fire protection.

Roof volumes and span requirements may necessitate high open spaces, eg. atria, sports halls. Deeper structural spanning elements such as trusses may be more useful on single storey buildings as they do not add much to height.

When considering large spaces above cellular accommodation and vice versa it is important to maintain continuity of structural elements such as columns to foundation. Cellular accommodation above clear spaces may require large transfer elements.

The implications of large glazed areas or small openings are quite different. The skin can be independent of primary structure but will require secondary structure to transfer loads to primary structure, e.g. glazing mullions. The structure may need to be unobtrusive. It may be necessary to consider possible positioning of cross bracing elements in elevation.

For a highly serviced building, duct requirements can be critical. The relationship of servicing system to structure needs to be carefully considered – accommodation in the same zone is important in multi-storey buildings. There are other interactions too - for example a simple, cheap structure may allow the use of a more expensive and sophisticated skin, whilst the use of other elements to assist structure e.g. lift shafts/fire stairs acting as shear walls/cores, can help reduce costs.

For an exposed structure, the appearance and form of the building may be partly or wholly dictated by the structural system. In contrast, where a structure is hidden it can be cheap, cost effective and efficient, possibly allowing other areas to be more complex, e.g. straightforward office accommodation to allow an atrium space.

Practical requirements may give rise to particular structural forms, e.g. poor ground requiring piles,

During design stages key requirements can be identified.

A useful early exercise is to identify clearly particular requirements of the brief that must be satisfied by the structural system, eg. a need for unobstructed spaces, floor to floor heights, likely column layouts. At early planning stages probable structural positions can be identified and as far as possible, projected onto other floors. As the building is developed in section, consideration can be given to the nature of the elements and the overall arrangement of the frame.