Factors influencing choice of form design steel building

Environmental

There are a number of factors which influence the choice of structural form that are particular to the site location. These can have a dominant effect on the framing arrangement for the structure.

The most obvious site-dependent factors are related to the ground conditions. 

A steel-framed building is likely to be about 60% of the weight of a comparable reinforced concrete building. This difference will result in smaller foundations with a consequent reduction on costs. In some cases this difference in weight enables simple pad foundations to be used for the steel frame where the equivalent reinforced concrete building would require a more complex and expensive solution.

For non-uniformly loaded structures it will also reduce the magnitude of differential settlements and for heavily loaded structures may make possible the use of a simple raft foundation in preference to a large capacity piled solution.

Difficult ground conditions may dictate the column grid. Long spans may be required to bridge obstructions in the ground. Such obstructions could include, for example, buried services, underground railways or archaeological remains. Generally, a widely spaced column grid is desirable since it reduces the number of foundations and increases the simplicity of construction in the ground.

Other site-dependent constraints are more subtle. In urban areas they relate to the physical constraints offered by the surrounding street plan, and the rights of light of adjoining owners. They also relate to the planning and architectural objectives for specific sites. The rights of light issues or planning considerations may dictate that upper floors are set back from the perimeter resulting in stepped construction of the upper levels. Invariably the resulting framing plan is not rectilinear and may have skew grids, cantilevers and re-entrant corners.

These constraints need to be identified early in the design in order that they are accommodated efficiently into the framing. For example, wherever possible, stepped-back façades should be arranged so that steps take place on the column grid and hence avoid the need for heavy bridging structures. In other situations the designer should always investigate ways in which the impact of lack of uniformity in building form can be contained within a simple structural framing system which generates a minimum of element variations and produces simple detailing.

Design Steel Building

Design aims

For the full potential of the advantages of steel-frame construction to be realized, the design of multi-storey buildings requires a considered and disciplined approach by the architects, engineers and contractors involved in the project. They must be aware of the constraints imposed on the design programme by the lead time between placing a contract for the supply of the steel frame and the erection of the first pieces on site. The programme should include such critical dates on information release as are necessary to ensure that material order and fabrication can progress smoothly.

The designer must recognize that the framework is the skeleton around which every other element of the building will be constructed. The design encompasses not only the structure but also the building envelope, services and internal finishes.

All these elements must be co-ordinated by a firm dimensional discipline, which recognizes the modular nature of the components, to ensure maximum repetition and standardization. Consequently it is impossible to consider the design of the framework in isolation. It is vital to see the frame as part of an integrated building design from the outset: the most efficient solution for the structure may not be effective

in achieving a satisfactory solution for the total building.

In principle, the design aims can be considered under three headings:

• Technical

• Architectural

• Financial.

Technical aims

The designer must ensure that the framework, its elements and connections are strong enough to withstand the applied loads to which the framework will be subjected throughout its design life.The system chosen on this basis must be sufficiently robust to prevent the progressive collapse of the building or a significant part of it under accidental loading. This is the primary technical aim. However, as issues related to strength have become better understood and techniques for the strength design of frameworks have been formalized, designers have progressively used lighter and stronger materials. This has generated a greater need to consider serviceability, including dynamic floor response, as part of the development of the structural concept.

Other important considerations are to ensure adequate resistance to fire and corrosion. The design should aim to minimize the cost, requirements and intrusion of the protection systems on the efficiency of the overall building.

Architectural aims

For the vast majority of buildings the most effective structural steel frame is the one which is least obtrusive. In this way it imposes least constraint on internal planning,

and produces maximum usable floor area, particularly for open-plan offices. It also provides minimal obstruction to the routeing of building services. This is an important consideration, particularly since building services are becoming more extensive and demanding on space and hence on the building framework.

Occasionally the structure is an essential feature of the architectural expression of the building. Under these circumstances the frame must achieve, among other aims, a balance between internal planning efficiency and an expressed structural form. However, these buildings are special, not appropriate to this manual, and will not be considered in more detail, except to give a number of references.

Financial aims

The design of a steel frame should aim to achieve minimum overall cost. This is a balance between the capital cost of the frame and the improved revenue from early occupation of the building through fast erection of the steel frame: a more expensive framework may be quicker to build and for certain uses would be more economic to a client in overall terms. Commercial office developments are a good example of this balance. Figure 2.1 shows a breakdown of construction costs for a typical development.

Introduction : The Advantages of Steel

The advantages of steel

In recent years the development of steel-framed buildings with composite metal deck floors has transformed the construction of multi-storey buildings in the World.

During this time, with the growth of increasingly sophisticated requirements for building services, the very efficiency of the design has led to the steady decline of the cost of the structure as a proportion of the overall cost of the building, yet the choice of the structural system remains a key factor in the design of successful buildings.

The principal reasons for the appeal of steel for multi-storey buildings are noted below.

• Steel frames are fast to erect.

• The construction is lightweight, particularly in comparison with traditional concrete construction.

• The elements of the framework are prefabricated and manufactured under controlled, factory conditions to established quality procedures.

• The accuracy implicit in the manufacturing process by which the elements are produced enables the designer to take a confident view of the geometric properties of the erected framework.

• The dryness of the form of construction results in less on-site activities, plant, materials and labour.

• The framework is not susceptible to drying-out movement or delays due to slow strength gain.

• Steel frames have potential for adaptability inherent in their construction. Later modification to a building can be achieved relatively easily by unbolting a connection; with traditional concrete construction such modifications would be expensive, and more extensive and disruptive.

• The use of steel makes possible the creation of large, column-free internal spaces which can be divided by partitions and, by eliminating the external wall as a loadbearing element, allows the development of large window areas incorporated in prefabricated cladding systems.