Steel structures
3.2 The architecture of steel the factors which affect the decision to
3.2.2 The technical performance of steel as a structural material
3.2.2.1 Introduction
Steel is the strongest of the four commonly used structural materials but has a strength-to-weight
high). It is ideally suited to the form of skeleton frameworks, where the principal alternative in multi-storey buildings is reinforced concrete. Its high ratio of strength to weight also makes it suitable for lightweight frameworks such as are used in roof structures. In this application the principal alternative is timber. The advantages and disadvantages of steel in relation to these materials is reviewed briefly here.
3.2.2.2 Advantages of steel
Strength
The high strength of steel, and its high ratio of strength to weight, makes it suitable for use in single- and multi-storey skeleton frames, over a large range of spans and building heights. The material therefore offers the advantages of skeleton-frame construction in all of its manifestations. In addition to freedom from the restrictions of loadbearing walls in the planning of both the interior and the exterior of the building, as mentioned above, these include the subsequent flexibility to alter plans and elevations when required.
Considerable flexibility is also possible in the planning of the building services and in their subsequent maintenance and replace- ment when this becomes necessary. These advantages are, of course, also present with reinforced concrete frameworks but because the structural elements in steel frames are more slender and less obtrusive than those in equivalent reinforced concrete frames, the use of steel allows interiors with lighter and more open aspects to be created.
Ratio of strength to weight
Steel frames are lighter than reinforced concrete frames of equivalent strength, partic- ularly if efficient types of element such as triangulated girders are used. This makes them more suitable than reinforced concrete frames for use in single-storey buildings and the roof structures of multi-storey buildings. In this role the principal alternative to steel is timber. Quality control
Steel is manufactured under conditions of strict quality control and its properties can be
relied upon to be within narrow specified limits; this allows relatively small factors of safety to be adopted in the structural design calculations and is a further reason why light and slender elements are possible.
Appearance
Due to the strict quality control which is exercised during its manufacture and to the methods which are used in the final shaping of steel components, the finished structure has a distinctive appearance which is characterised by slender elements, smooth surfaces and straight, sharp edges.
Prefabrication
Steel structures are assembled from prefabri- cated components which are produced off-site and this allows their dimensions and general quality to be carefully controlled. It also results in fast erection of the structure on site and enables a relatively simple erection process to be adopted, even on difficult sites.
Prefabrication with site-jointing also means that it is relatively easy for the designer to exercise control over whether or not the struc- ture is statically determinate.
3.2.2.3 Disadvantages of steel
Intractability
Steel is a very tough material which is difficult to work and shape in the solid form and this has a number of consequences. It means that, in most steelwork design, it is necessary to specify elements from a standard range of components which are produced by steel manufacturers and to carry out the minimum amount of modification to these. The standard elements, which are produced by a hot-rolling or cold-forming process (see Section 3.4), are straight and parallel sided with the result that steel frameworks must normally have a regular, rectilinear, or at least straight-edged geometry.
The production of 'tailor-made' cross- sections, or of geometries which are curvilin- ear, is difficult and the use of steelwork tends therefore to place more restrictions on the overall forms of structures than does the use of reinforced concrete. Also, the final adjustment
of the dimensions of elements on site is diffi- cult, if not impossible, so that a much higher standard of quality control is required in such processes as the initial shaping and cutting to length, than is necessary with, for example, timber.
Weight
The density of steel is high and this makes individual components fairly heavy. Elements such as beams and columns are difficult to move around on site and cranes are normally required for the assembly of steel structures. Cost
The basic cost of a steel structure is normally greater than that of its timber or reinforced concrete equivalent. The shorter on-site construction time can be a compensatory factor, however.
Durability
Most steels are relatively unstable chemically and a corrosion-protection scheme is normally required for a steel structure.
Performance in fire
Steel loses its ability to carry load at a
relatively low temperature (around 500 °C) and this means that, while a steel structure does not actually burn, it will collapse in fire unless it is kept cool. This is normally achieved by protecting the steelwork with a suitably thick layer of fire resistant insulating material but sometimes more sophisticated methods, such as water cooling systems, are used. The trad- itional fireproofing material was concrete - the elements of a steel frame were simply encased in concrete - but much lighter materials which are easier to apply have been developed. The need to provide fireproofing for steelwork nevertheless increases the complexity of a steel-frame building and adds to the cost of the structure.
The need to provide fire protection is obviously particularly problematic if there is an intention to expose the structure as part of its architectural expression. A prominent example already mentioned is the Centre Pompidou in Paris where an elaborate system of shutters, which would operate automatically in the event
of a fire, had to be installed adjacent to the glass external wall in order to protect the steel- work on the exterior of the building should a fire occur. The steel trusses in the interior were encased in fireproofing material which was wrapped in a thin skin of sheet metal to preserve the appearance of a steel structure. A similar architectural language was proposed for the Lloyd's Headquarters Building in London. In this case, however, no satisfactory way of meeting the fire-resistance requirements could be found and reinforced concrete was finally adopted as the structural material.
3.2.2.4 Conclusion
Steel is a material whose properties make it particularly suitable for skeleton-frame struc- tures. The principal advantage which results from its adoption is that it releases the designer from the restrictions on internal planning and the aesthetic treatment of the exterior which are imposed by the use of loadbearing walls. In addition, the high strength of steel makes possible a very wide range of frame types; both single-storey and multi-storey frames can be constructed over a very wide range of spans and very tall multi- storey structures are also possible. In all cases the structures will be lighter than equivalent reinforced concrete frames. Other advantages stem from the fact that steel structures are prefabricated: these include speed of erection, ease of assembly on difficult sites and, if required, statical determinacy. Restrictions on the overall form of a building must normally be accepted, however, due to the limited range of components which are readily available, and the detailing of the structure is likely to be complicated by the need to provide fire protec- tion. In addition, the cost of the steel-frame structure is likely to be higher than that of an equivalent reinforced concrete frame.