Curved tubular members

Curved roofs may be formed using single-curved tubular members, double-layer members connected periodically, or tubular trusses, as illustrated in the following examples.

Table 4.2 Minimum bending radii for common steel sections


Typical radius

Joists and Universal Beams (x-x axis)

Universal Columns (x-x axis)

Universal Columns (x-x axis)

Top Chord 168 Chs

Channels (x-x axis) 127 x 64 x 14 kg/m 203 x 89 x 29 kg/m 254 x 89 x 35 kg/m 305 x 102 x 46 kg/m All sections up to 432 x 102 x 65 kg/m

Joists, beams and columns (y-y axis)

610 x 229 x 140 kg/m

All sections up to 1016 x 455 x 488 kg/m

Castellated and cellular beams (x-x axis)

305 x 133 x 30 kg/m 458 x 165 x 54 kg/m 609 x 178 x 74 kg/m 800 x 210 x 122 kg/m 915 x 305 x 238 kg/m

Castellated and cellular beams (y-y axis)

305 x 133 x 30 kg/m 458 x 165 x 54 kg/m 609 x 178 x 74 kg/m 800 x 210 x 122 kg/m 915 x 305 x 238 kg/m

Circular Hollow Sections

60.3 x 5 mm 114.3 x 6.3 mm 168.3 x 10 mm 219.1 x 12.5 mm

Most sizes up to 610 mm o/d x 35 mm

Square and Rectangular Hollow Sections

50 x 50 x 5 mm 100 x 100 x 6.3 mm 150 x 150 x 10 mm 200 x 200 x 12.5 mm

All sizes up to 400 x 400 x 16 SHS and 500 x 300 x 20 RHS

4.17 Curved enclosure at Cambridge University Law Faculty (architect: Foster and Partners)

The Law Faculty at Cambridge is enclosed by a triangulated Vierendeel structure, which is cylindrical in section, to which the glazing and stainless steel cladding is fixed. This curved element merges between the roof and walls, and provides a sympathetic enclosure to the internal space, as illustrated in Figure 4.17 and see also Colour Plate 1.

The TGV station at Lille extends the concept of a curved tubular roof by using ties periodically to decrease the bending effect in the arch, so as to minimise the required size of the sections (see Figure 4.18). For the Amenity Building of the Saga Headquarters, a series of inclined arches support the fabric roof, as illustrated in Figure 4.19.

A three-dimensional extension of the arch is to a dome structure, where a latticework of welded tubular sections can create an efficient and attractive structural solution to auditoria and large halls, as in Figure 4.20.

The roof of the great glasshouse of the National Botanical Garden of Wales (Colour Plate 2) comprises a series of tubular steel arches in the form of a toroid. The attachment to the glazing is made by projecting fins welded to the tubes.

Curved Latticed BeamMaking Lattice Domes

4.20 Roy Thomson Hall, Toronto, use of welded tubular lattice to create the dome-like roof of the concert hall

Tubular Structures Architecture
4.21 Leipzig Messe — external structure supporting glazing (architect: Von Gerkan Marg & Partners and Ian Ritchie)

At the Leipzig Messe, the curved tubular arches support the tubular latticework which supports the fully glazed façade, as shown in Figure 4.21 and see also Colour Plate 6.

4.4 Columns

In braced frames, columns are designed to resist mainly compression forces. The shape of UC sections is such that they are more efficient in resisting buckling than standard beam sections. Columns used in rigid or sway frames are also designed to resist bending. Where bending effects are dominant, it may be more appropriate to use UBs as columns, such as in portal frames.

Columns may be in the form of UC sections that are spliced at appropriate points (usually every two or three storeys) in tall buildings. In taller buildings, column sizes are generally selected from one serial size with decreasing section weight at upper levels. Beam to column connections are made either to the flanges of the column (major axis connections) or to the web of the column (minor axis connections). Illustrations of typical connections are given later. It may also be necessary to stiffen the columns locally at points of load transfer, such as for beams with moment connections.

Tubular columns

Square or Circular Hollow Sections are very efficient in compression because the material is remote from the axis of the section, therefore increasing the resistance to buckling. Both circular (CHS) sections and square (SHS) sections are widely used as slender columns. The main design issue is the form of connection to the face of the column.

Composite columns

Columns may be designed to achieve greater compression and fire resistance by concrete encasement (in the case of I-sections) and concrete filling (in the case of hollow sections). For example, the in-filling between the flanges of an I-section column without reinforcement can increase its fire resistance up to 60 minutes, whilst retaining the same external dimensions of

the section. The in-filling of tubular sections with concrete can

increase their structural resistance and also their fire resistance to up to 60 minutes without reinforcement, and up to 120 minutes with bar reinforcement.

4.4.1 Exposed tubular columns

Tubular columns are used in applications where the minimum amount of intrusion into the space is sought, or where the external appearance of the column is preserved. They are structurally efficient and can thus be used to advantage in slender columns. Amsterdam's

Schiphol Airport illustrates this principle, as shown in Figure 4.22. The large diameter columns were also used as part of the air-ducting system. Hong Kong's Hung Hom station shows how tubular columns can be used within the tubular spine beams (see Figure 4.23). For another example of exposed tubular columns see Colour Plate 7.

A recent example of clustering columns together in an attractive manner can be seen in the Mediatheque Centre at Sendai (Colour Plate 21).

Circular columns are particularly attractive internally in shopping malls and auditoria. Fire resistance can be achieved by the use of intumescent paints or by concrete filling (see Chapter 13). Other forms of fire protection normally affect the appearance and shape of the section and are not preferred.

Connections to tubular columns are often expressed as part of the overall structure concept. As described in Chapter 6, there is a wide

Hung Hom Station Canopy

4.22 Amsterdam's Schiphol Airport

4.23 Tubular columns and spine beams in Hong Kong's Hung Hom Station (architect: Foster and Partners)

4.24 Tubular columns with pinned or rigid connections

4.25 Tubular struts used to support the roof of Wimbledon No. 1 Court (architect: BDP)

range of structural options, depending on whether the connection is pinned (i.e. resists only shear and tension) or rigid (i.e. also resists moment). Examples of architectural details used in beam to tubular column connections are illustrated in Figure 4.24.

Columns are erected in two or three storey high lengths and splices are usually made by end plates or similar connections just above floor height, in order to avoid intrusion into the floor space.

Tubular sections may also be used as heavily loaded struts to support canopy roofs, as shown in Figure 4.25. This is important also in wind uplift conditions where the reversal of loading may cause tension members to act in compression.

Atria and shopping malls often use tall and slender tubular columns to support long-span roofs, as shown in Figure 4.26.

Pinned connections r

Rigid connections

Rigid connections

Concrete Column That Hold Curved RoofConcrete Column That Hold Curved Roof
4.26 Curved roof at Princes Square, Glasgow (architect: Hugh Martin & Partners)

4.4.2 Concrete-filled columns

Concrete filling improves the compressive resistance and fire resistance of tubular columns. This is because the concrete within the section acts compositely with the steel casing, so that the compressive strengths of the two materials can be mobilised together. Indeed, the strength of the concrete is also enhanced by the confining effect of the tubular section. Often the designer does not utilise the compressive strength of the concrete in normal design, but uses it to enhance the fire resistance of the column on the assumption that the exposed steel section loses all its strength in a severe fire.

A design method for composite columns is presented in an SCI

publication. From the point of view of architectural opportunities, concrete filling can lead to:

• more slender columns

• more heavily loaded columns, where the compressive resistance is increased for a given tube size

• longer fire resistance periods (which can also be enhanced by bar reinforcement)

• excellent impact resistance.

In Australia and the Far East, large diameter concrete-filled tubular sections have been widely used in high-rise commercial developments. In this case, the tubular sections are designed principally to support the framework and floors during construction, and the concrete provides the compressive resistance to subsequent loads. Large tubular sections (of 0.6 to 1.5 m diameter) can be produced from plate, which is bent into a circular form and welded along its seam.

Particular technical issues to be addressed in this form of construction are:

the method of concrete filling, which is normally by pouring from the top of the column in one or two storey heights load transfer from the beams to the columns, which for large diameter columns is achieved by a steel insert with shear connectors embedded into the core of the column fire resistance by bar reinforcement in the concrete. In this case, the column is designed and detailed to standard reinforced-concrete practice. The amount of reinforcement should be minimised (<2% cross-sectional area of the column) in order not to be too congested for concrete filling.

4.4.3 Tubular masts

Tubular members can be used in tall slender masts and can be combined with other sections, depending upon the architectural and structural approach. One excellent early example of the combined use of section types is the Renault Parts Distribution Centre in Swindon, where circular tubular columns supported a framework of tapered UB sections suspended from the column apex and shaft (see Figure 1.2).

Single or clustered tubular columns may themselves form a basic structure with opportunity for architectural expression. Figure 4.27 illustrates a group of tapering tubes arranged to emulate a ship's crane in Genoa, Italy, which support both a fabric membrane roof over a public piazza as well as an elevator ride to provide panoramic views of the city.

A supermarket canopy in Plymouth (Figure 4.28) was designed using tubular columns to express a nautical theme.

Sometimes the separation between column and truss or beam elements is less clear, as in the case where 'tree-like' structures are devised. This kind of expression was also used at Stuttgart Airport (Figure 1.12) and in a more formal manner at Stansted Airport (Figure 4.29), in which 36 column trees act as a rigid framework and support inclined branches which themselves support the entire roof.

Airport Tree LikeTension And Compression Trees

4.5 Trusses and lattice girders

Trusses and lattice girders can be conceived of as triangular or rectangular assemblies of tension and compression elements. The top and bottom chords provide the compression and tension resistance to overall bending, and the web or bracing elements resist the shear forces. A wide variety of forms of trusses can be created. Each can vary in overall geometry and in the choice of the individual elements which comprise them.

Trusses are generally associated with pitched roofs and are designed to follow the roof profile. Shallower roof pitches result in heavier compression chords, whereas steeper roof pitches involve longer and often heavier bracing members.

Lattice girders are generally associated with long-span beams in which the top and bottom chords are usually horizontal. However, for flatter roof pitches, lattice girders with a sloping top chord can also be used efficiently.

4.5.1 Forms of trusses

Trusses or lattice girders may take a number of basic forms, as illustrated in Figure 4.30. The common names for these truss forms are given, together with their typical span range. They are fabricated by bolting or welding standard sections together. For spans of up to 20 m, it is sufficient to use angles, tees and lighter hollow sections. For very long spans, UC or heavier hollow sections may be required. The mixed use of these sections may be appropriate to minimise the visual impact of the bracing members. These alternative section types are shown in Figure 4.31. Trusses are very efficient in the use of steel, but are relatively expensive to fabricate. The bracing members are usually lighter than the chord members.

• Warren or Pratt Lattice Girders

Lattice girders have broadly parallel top and bottom chords in which the bracing (diagonal) members are arranged in a W or N form, respectively. In a Pratt girder (N form), the orientation of the bracing members normally changes at mid-span. The top chord is generally designed to be restrained against out-of-plane buckling by the regular attachment of roof purlins or of the floor slab.

A stricking example of a structure formed from what is essentially a circular three-dimensional Warren girder is the London Eye, designed by Marks Barfield Architects (Colour Plate 23). See also Colour Plate 4.

Pratt girders are a traditional form of construction often using angle and T-sections. They are efficient at supporting vertical loads because all the compression members are short (i.e. the vertical members) and the longer diagonal members are in tension.

Warren girders (W form) are often fabricated from tubular sections as they are efficient as bracing members which act

Steel Roof Truss Element
4.30 Different forms of conventional roof trusses and lattice girders

alternately in tension and compression. In lightweight buildings, wind uplift can be significant and may cause reversal of the forces acting on the truss. Fink, Howe and French trusses

These particular shapes of pitched truss form the shape of the finished roof. The apex and eaves joints between the chords are pinned. They are often used in housing and modest span roof trusses, and generally comprise Tees and angle members. Vierendeel girder

This is a different form of structure in which the diagonal bracing members are eliminated, and the connections between the horizontal and vertical members are made moment-


4.31 Different types of steel section used in trusses resisting. Vierendeel trusses are expensive in the use of steel and in fabrication, and are only appropriate for use in special circumstances, such as when the size of the openings is maximised to permit the passage of services. However, it is possible to design one Vierendeel panel in the centre of an otherwise standard Warren or Pratt girder, especially if the girder achieves composite action with a floor slab. Bowstring truss

One chord of a bowstring truss is curved in elevation and is tied between its supports. Light trusses of this form may also be orientated vertically to support cladding and glazing where architectural expression of the truss is particularly important.

Scissor truss

The scissor truss is a variant of a standard truss form and offers architectural possibilities and greater headroom, but is structurally less efficient because of its shallower depth. North light roof truss

North light trusses are traditionally used for short spans in industrial workshop-type buildings. They allow maximum benefit to be gained from natural lighting by the use of glazing on the steeper pitch which generally faces north or north-east to reduce the solar gain. Developments of roof form

Most of the above lattice girders and trusses can be further developed into more interesting structural and architectural forms. Some possibilities, including curved and mansard roofs, are illustrated in Figure 4.32.

Mansard Roof Truss CurvedNorthlight Roof TrussScissor Trusses

4.32 Development of standard truss and lattice forms

4.33 Lattice girders combine with fabricated steel columns making a hybridportal structure at the Brit School, Croydon (architect: Cassidy Taggart)

Seville Expo Grimshaw Architects
4.34 Lattice bowstring truss of UK pavilion, Expo 1992, Seville (architect: Nicholas Grimshaw & Partners)
Curved Lattice Architecture

Trusses offer an excellent opportunity for architectural expression in a variety of forms, as illustrated in Figures 4.33 and 4.34.

Other smaller-scale components may be considered depending on the form of the truss, such as:

Cables (or steel ropes) are spun from a number of strands or collection of wires. The cables can be impregnated and sheathed with nylon or PVC, and can also be greased and galvanized for corrosion protection. Cables have high-tensile strength but often low ductility. They are suitable only for tensile components in trusses, for example in wind-resisting girders for glazed walls.

Fitments to the ropes provide the coupling mechanism to the adjacent structure. Special consideration is required to the aerodynamic damping of long ties on cables when exposed to wind.

Individual rods are made from solid steel, whose ends are threaded to attach to steel couplers. Rods are linear and more rigid elements, whereas cables will sag naturally. Rods are usually lightly tensioned on erection of the frame. They are only suitable for resisting tension. In a 'wind girder', they can be pretensioned so that the reversal of wind loads does not cause compression.

Steel flats may be considered in X-braced trusses, although they are visually more obtrusive.

4.5.2 Articulation of elements within trusses

The same notions that guide the relationship between members in a frame to give scale, emphasis and articulation to the parts, are equally important to the relationships between elements in an individual member of fixed overall geometry and end conditions. The point is illustrated diagrammatically for a simple planar truss in Figure 4.35 in which the position of the pinned connections between the tension and compression elements, and within the compression elements themselves, can create different details and effects. This principle is generally applicable to any type of member. The particular form assumed by the connections varies depending upon the cross-section of the individual elements.

Tubular Truss Span

4.5.3 Tubular trusses

Trusses using tubular members can provide elegant structural solutions in long-span roofs. They can also be used as 'transfer structures' to support a number of floors above and to create open circulation areas beneath. The span to depth ratio of long-span trusses using tubular sections may be in the range of 20 to 25, reducing to 10 to 15 for heavily loaded applications. Tubular trusses can be very simple in form, as shown in Figure 4.36, which illustrates the use of

4.35 Articulation of elements within the truss to create different effects

4.35 Articulation of elements within the truss to create different effects

Curving Tubular Steel Trusses

4.36 Trusses at Toyota HQ, Swindon (architect: Sheppard Robson)

parallel chord trusses. Inclined tubular trusses may be used in a 'folded plate' form to reflect the shape of the roof, as shown in Figure 4.37. Horizontal forces are resisted by ties (refer to Section 7.5).

More complex roof trusses can be created which are triangular in cross-section, as in Figure 4.38. The bowstring truss in the sports hall shown in Figure 4.39 used a heavy top chord and vertical posts with light bracing and bottom chord members. The apparent depth

4.37 Inclined tubular trusses to create a folded plate roof (architect: Haworth Tompkins Architects)

4.37 Inclined tubular trusses to create a folded plate roof (architect: Haworth Tompkins Architects)

Folded Roof Structure Analysing
4.38 Curved triangular trusses at Swindon's Motorola factory (architect: Sheppard Robson)
Motorola Factory SwindonStratford Market Depot



Stratford Market depot, London (architect: Wilkinson

4.41 Deep curved roof trusses at the TGV terminal at Charles de Gaulle Airport, Paris (architect: Aeroports de Paris)



Stratford Market depot, London (architect: Wilkinson

4.41 Deep curved roof trusses at the TGV terminal at Charles de Gaulle Airport, Paris (architect: Aeroports de Paris)

of the bowstring truss is reduced by the use of these lightweight components.

The long-span trusses at Stratford Market depot are arranged in an intersecting orthogonal pattern and are supported on column trees to minimise the effective span of the trusses (see Figure 4.40). At Ponds Forge, Sheffield, the roof trusses were orientated diagonally across the enclosure and supported on a diagonal grid of tubular members (see Colour Plate 20).

The roof trusses to the TGV terminal at Charles de Gaulle Airport, Paris, used tubular trusses comprising a downward curved bottom chord, which is normally the opposite configuration to that desired for most roofs, but which causes a striking architectural effect. The inclined tubular columns support the upper chords of four trusses, as illustrated in Figure 4.41.

Supermarket Curved Roof Steelwork
4.42 Triangular roof trusses at Hamburg Airport (architect: Von Gerkan Marg and Partners)

At Hamburg Airport the triangular trusses were curved along their length and were supported by inclined tubular struts, as shown in Figure 4.42.

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  • niko
    What is vierendeel girder?
    7 years ago
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    What is lattice steel roof?
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    How to design curved roof trusses from steel?
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  • stefan
    Can purlins be curved outofplane?
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