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The use of steel as a primary structural material dates from the late nineteenth century when cheap methods for manufacturing it on a large scale were developed. It is a material that has good structural properties. It has high strength and equal strength in tension and compression and is therefore suitable for the full range of structural elements and will resist axial tension, axial compression and bending-type load with almost equal facility. Its density is high, but the ratio of strength to weight is also high so that steel components are not excessively heavy in relation to their load carrying capacity, so long as structural forms are used which ensure that the material is used efficiently. Therefore, where bending loads are carried it is essential that ‘improved’ cross-sections and longitudinal profiles are adopted.

 

 

Pic. 1. Hopkins House, London, UK; Michael Hopkins, architect ;

Anthony Hunt Associates, structural engineers. The floor structure

here consists of profiled steel sheeting which will support a timber deck.

A more common configuration is for the profiled steel deck to act compositely

with an in situ concrete slab for which it serves as permanent formwork.

 

 

The high strength and high density of steel favours its use in skeleton frame type structures in which the volume of the structure is low in relation to the total volume of the building which is supported, but a limited range of slab-type formats is also used. An example of a structural slab-type element is the profiled floor deck in which a profiled steel deck is used in conjunction with concrete, or exceptionally timber (Pic.1), to form a composite structure. These have ‘improved’ corrugated cross-sections to ensure that adequate levels of efficiency are achieved. Deck units consisting of flat steel plate are uncommon.

 

The shapes of steel elements are greatly influenced by the process which is used to form them. Most are shaped either by hot-rolling or by cold-forming. Hot-rolling is a primary shaping process in which massive red-hot billets of steel are rolled between several sets of profiled rollers. The cross-section of the original billet, which is normally cast from freshly manufactured steel and is usually around 0.5m x 0.5m square, is reduced by the rolling process to much smaller dimensions and to a particular precise shape (Pic. 2). The range of cross-section shapes which are produced is very large and each requires its own set of finishing rollers. Elements that are intended for structural use have shapes in which the second moment of area is high in relation to the total area (Pic. 3). I- and H- shapes of cross-section are common for the large elements which form the beams and columns of structural frameworks. Channel and angle shapes are suitable for smaller elements such as secondary cladding supports and sub-elements in triangulated frameworks. Square,circular and rectangular hollow sections are produced in a wide range of sizes as are flat plates and solid bars of various thicknesses. Details of the dimensions and geometric properties of all the standard sections are listed in tables of section properties produced by steelwork manufacturers.

 

 

Pic. 2 The heaviest steel sections are produced by a
hot-rolling process in which billets of steel are shaped by
profiled rollers. This results in elements which are straight,
parallel sided and of constant cross-section. These features

must be taken into account by the designer when steel is

used in building and the resulting restrictions in form accepted

 

 

Pic. 3  Hot-rolled steel elements

 

 

The other method by which large quantities of steel components are manufactured is cold-forming. In this process thin, flat sheets of steel, which have been produced by the hot-rolling process, are folded or bent in the cold state to form structural cross-sections (Pic. 4). The elements which result have similar characteristics to hot-rolled sections, in that they are parallel sided with constant cross-sections, but the thickness of the metal is much less so that they are both much lighter and, of course, have lower load carrying capacities. The process allows more complicated shapes of cross-section to be achieved, however. Another difference from hot-rolling is that the manufacturing equipment for cold-forming is much simpler and can be used to produce tailor-made cross-sections for specific applications. Due to their lower carrying capacities cold-formed sections are used principally for secondary elements in roof structures, such as purlins, and for cladding support systems. Their potential for future development is enormous.

 

 

Pic. 4 Cold formed sections are
formed from thin steel sheet.

A greater variety of cross section
shapes is possible than with the
hot-rolling process

 

 

Structural steel components can also be produced by casting, in which case very complex tailor-made shapes are possible. The technique is problematic when used for structural components, however, due to the difficulty of ensuring that the castings are sound and of consistent quality throughout. In the early years of ferrous metal structures in the nineteenth century, when casting was widely used, many structural failures occurred– most notably that of the Tay Railway Bridge in Scotland in 1879. The technique was rarely used for most of the twentieth century but technical advances made possible its re-introduction. Prominent recent examples are the ‘gerberettes’ at the Centre Pompidou, Paris (Pic 5) and the joints in the steel work of the train shed at Waterloo Station, London.

 

Most of the structural steelwork used in building consists of elements of the hot-rolled type and this has important consequences for the layout and overall form of the structures.An obvious consequence of the rolling process is that the constituent elements are prismatic:they are parallel-sided with constant cross-sections and they are straight – this tends to impose a regular, straight-sided format on the structure. In recent years, however, methods have been developed for bending hot-rolled structural steel elements into curved profiles and this has extended the range of forms for which steel can be used. The manufacturing process does,however, still impose quite severe restrictions on the overall shape of structure for which steel can be used.

 

The manufacturing process also affects the level of efficiency which can be achieved in steel structures, for several reasons. Firstly, it is not normally possible to produce specific tailor-made cross-sections for particular applications because special rolling equipment would be required to produce them and the capital cost of this would normally be well beyond the budget of an individual project. Standard sections must normally be adopted in the interests of economy, and efficiency is compromised as a result. An alternative is the use of tailor-made elements built up by welding together standard components, such as I-sections built up from flat plate. This involves higher manufacturing costs than the use of standard rolled sections.

 

 

Pic. 4 The so-called ‘gerberettes’ at the Centre Pompidou in Paris,
France, are cast steel components. No other process could have
produced elements of this size and shape in steel

 

 

A second disadvantage of using an ‘off-the-peg’ item is that the standard section has a constant cross-section and therefore constant strength along its length. Most structural elements are subjected to internal forces which vary from cross-section to cross-section and therefore have a requirement for varying strength along their length. It is, of course,possible to vary the size of cross-section which is provided to a limited extent. The depth of an I-section element, for example, can be varied by cutting one or both flanges from the web,cutting the web to a tapered profile and then welding the flanges back on again. The same type of tapered I-beam can also be produced by welding together three separate flat plates to form an I-shaped cross-section, as described above.

 

Because steel structures are pre-fabricated, the design of the joints between the elements is an important aspect of the overall design which affects both the structural performance and the appearance of the frame. Joints are made either by bolting or by welding (Pic. 5). Bolted joints are less effective for the transmission of load because bolt holes reduce the effective sizes of element cross-sections and give rise to stress concentrations. Bolted connections can also be unsightly unless carefully detailed. Welded joints are neater and transmit load more effectively, but the welding process is a highly skilled operation and requires that the components concerned be very carefully prepared and precisely aligned prior to the joint being made. For these reasons welding on building sites is normally avoided and steel structures are normally pre-fabricated by welding and bolted together on site. The need to transport elements to the site restricts both the size and shape of individual components.

 

 

Pic. 5. Joints in steelwork are normally made by a
combination of bolting and welding. The welding is usually
carried out in the fabricating workshop and the site joint is
made by bolting.

 

 

Steel is manufactured in conditions of very high quality control and therefore has dependable properties which allow the use of low factors of safety in structural design. This,together with its high strength, results in slender elements of lightweight appearance.The basic shapes of both hot- and cold-formed components are controlled within small tolerances and the metal lends itself to very fine machining and welding with the result that joints of neat appearance can be made. The overall visual effect is of a structure which has been made with great precision (Pic. 5).

 

 

Pic. 6. Renault Sales Headquarters, Swindon, UK, 1983;

Foster Associates, architects; Ove Arup & Partners, structural
engineers. Joints in steelwork can be detailed to look very neat and

to convey a feeling of great precision

 

 

Two problems associated with steel are its poor performance in fire, due to the loss of mechanical properties at relatively low temperatures, and its high chemical instability, which makes it susceptible to corrosion. Both of these have been overcome to some extent by the development of fireproof and corrosion protection materials, especially paints, but the exposure of steel structures, either internally,where fire must be considered, or externally,where durability is an issue, is always problematic.

 

To sum up, steel is a very strong material with dependable properties. It is used principally in skeleton frame types of structure in which the components are hot-rolled. It allows the production of structures of a light, slender appearance and a feeling of neatness and high precision. It is also capable of producing very long span structures, and structures of great height. The manufacturing process imposes certain restrictions on the forms of steel frames. Regular overall shapes produced from straight, parallel-sided elements are the most favoured.