To sum up, timber is a material which offers the designers of buildings a combination of
properties that allow the creation of lightweight structures which are simple to
construct. However, its relatively low strength, the small sizes of the basic components and
the difficulties associated with achieving good structural joints tend to limit the size of
structure which is possible, and the majority of timber structures are small in scale with short
spans and a small number of storeys. Currently, its most common application in
architecture is in domestic building where it is used as a primary structural material either to
form the entire structure of a building, as in timber wall-panel construction, or as the
horizontal elements in loadbearing masonry structures.
3.4 Steel
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’
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30
Fig. 3.13 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.
Photo: Pat Hunt
cross-sections see Section 4.3 and longitudinal profiles are adopted.
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 Fig. 3.13, 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 0.5 m ⫻ 0.5 m square, is reduced by
the rolling process to much smaller dimensions and to a particular precise shape
Fig. 3.14. 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 see Appendix 2.3 is high in relation to
the total area Fig. 3.15. 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.
Structural materials
Fig. 3.14 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. Photo: British Steel
Fig. 3.15 Hot-rolled steel elements.
31
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 Fig.
3.16. 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.
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
Figs 3.17 7.7 and the joints in the steelwork of the train shed at Waterloo Station, London
Fig. 7.17.
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 see Figs iv, 1.10 and 7.26. 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.
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32
Fig. 3.16 Cold-
formed sections are formed from thin
steel sheet. A greater variety of cross-
section shapes is possible than with the
hot-rolling process.
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 Fig. 3.18. 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.
33 Structural materials
Fig. 3.17 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. Photo: A. Macdonald
a b
Fig. 3.18 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 Fig. 3.19.
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
Structure and Architecture
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Fig. 3.19 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. Photo: Alastair Hunter
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.
3.5 Concrete