Design Of Connection Of Diagrid Of Hollow Square Section

6.2 Some examples of bolted connections to tubular members welded plate with a projecting fin, often in the form of a T-section, which permits a conventional spliced bolted connection to be made welded fin that is welded to, or cut into the section, permitting a conventional spliced connection to be made through bolts or pins with internal ferrules to avoid local crushing of the walls of the hollow section flattened ends (of a CHS), permitting a spliced connection to be made welding to intermediary sections, such as angles or C-sections.

Bolted connections are desirable for site assembly, and large welded sub-assemblies that are prefabricated and bolted together on site at suitable locations. The practical aspects of installation should be considered in the design process. For example, Figure 6.3 shows

Rhs ConnectionsDiagrid Connection Details

6.4 Some examples of tubular connections with pinned ends

Hollow Section Connection Design

possible end-connection details for long-span tubular trusses of various types.

Simple pinned connections can be made in a similar manner to bolted connections using welded end-plates and fin plates. Alternative pinned details for smaller tubular sections are shown in Figure 6.4.

6.3 Welded flange or end-plates and bolted connections

6.3.1 Projecting flange-plates

Welded flange-plates with projecting sides (see Figure 6.2 (a)) are the simplest but potentially one of the least attractive forms of connection, and can be used with any size and shape of member. The flange plates may be either solid or of 'ring-form' with an opening. The opening may be required for the passage of internal pipes, or for concrete infilling, or for galvanizing internally. The external projection of the flange plate should be kept as small as possible, but the plate should be of sufficient thickness to resist the tensile force transferred by the connecting bolts and to avoid distortion during welding.

Similar types of flange connections may be used for CHS or SHS sections. In close-up these connections may look bulky, but in overall perspective, their effect is diminished. In multi-storey construction, connections of this type to tubular columns can normally be accommodated within the floor depth (or within the raised floor depth).

6.3.2 Welded plate with projecting fin plate

This form of connection is an adaptation of the above type using a welded fin attached to the flange plate (see Figures 6.2(b) and 6.2(c)). The flange plate can be welded flush with the section by careful chamfering of the ends of the hollow section. The connecting bolts are then loaded principally in shear, as in a conventional splice connection.

6.3.3 Welded fin cut into the section

A fin plate may be welded into a slot cut through or into the end of the section (see Figure 6.2(d)). In this application, the ends of the section may be sealed with a further semi-round plate or, in some cases, left partly exposed, where the risk of corrosion is small (i.e. in internal applications). The connecting bolts form part of a splice connection.

An interesting variant of this connection used to connect a CHS section to an I-beam is illustrated in Figure 6.5. Here the splice plate is curved at its end to enhance the visual effect. The four bolts transfer the required axial and shear forces.

Tubular Section Splice Joint
6.5 Spliced connection between CHS and an I-beam

6.3.4 Through bolts with internal ferrules

Bolts or solid pins may be passed through holes in the walls of the hollow section (see Figure 6.2(e)). Welded ferrules are located within the section to avoid local distortion on tightening of the bolts. These ferrules need only be tack-welded in place. The end of the section may be either capped or left exposed where the risk of corrosion is small.

6.3.5 Sections with flattened ends

Connections between sections with flattened ends are only appropriate in smaller CHS, such as in space trusses (see Figure 6.2(f)). Pairs of small diameter bolts are generally used in these splice connections which are often connected to prefabricated nodes or to similar sections with flattened ends.

6.3.6 Welded fins

Fins or brackets may be welded to the side of CHS or SHS/RHS sections to provide direct attachment of secondary members such as purlins (see Figure 6.6). Connections of this type require careful design because of the possible local distortion of the walls of larger hollow sections. Alternatively, welded threaded studs with extended washers may be used to attach the purlins to the section.

The attachment of tension-ties or rod-bracing members requires similar details. High local forces from ties may also be transferred by 'patch-type' connections, which may be profiled around the circular

Tie Rod Bracing Connection
6.7 Connection of ties to tube at Cologne Airport (architect: Murphy Jahn Architects)

section so that weld forces are transferred smoothly to the walls of the section. Multiple welded fin connections have been used successfully on a number of major projects, such as at the column bases at the Cologne Airport terminal, as shown in Figure 6.7.

6.4 In-line connections

Connections along the length of a member can be made by welding tubes end to end, or by a variety of bolted splices, as described below.

6.4.1 Welded connections

Welded in-line connections (see Figure 6.8(a)) are by far the neatest solution, particularly if the welds are ground back after fabrication. Welded connections can be designed to achieve the full strength of the tube, but they should normally only be made in the fabricator's shop in order to achieve correct alignment of the tubes. Changes in the thickness of the tubes can be accommodated at this point.

6.4.2 Flange plates

Flanged connections (see Figure 6.8(b)) are simple to make but are not aesthetically pleasing. They are suitable for compression but are less efficient for tension because of bending in the end plate, requiring thicker plates and more bolts. Fillet welding around the section could cause distortion of thin flange plates. Table 6.1 gives guidance on the flange-plate connections that achieve the full tensile-resistance of the given tube size. Fewer bolts or thinner plates may be used for lighter loadings and, in this case, the connection will be weaker than the tube.

(b) Flange plates

(d) End plates

6.8 Examples of tube to tube splices

Table 6.1 Standard details for flanged connections (full-strength connections)

Max tube dimensions d x t Thickness of Nominal diameter Minimum no. Edge

(mm) flange plate of bolt (mm) of bolts distance tf (mm) (mm)

60.5 x 4.0 to 89.1 x 4.0





101.6 x 4.0 to 114.3 x 3.6





114.3 x 5.6 to 139.8 x 4.5





165.2 x 5.0





190.7 x 5.0





216.3 x 6.0





216.3 x 8.0





267.4 x 9.0





318.5 x 7.0





355.6 x 12.0





406.4 x 9.0





6.9 Splice connection
6.10 Connection with cover plate

6.4.3 Splice plates

Splice plate connections (see Figure 6.8(c)) can be made between tubes but the joint must be considered as pinned, making this an unsuitable connection for the middle of a member in bending or in compression. The splice plates can be either left exposed or used with a cover plate to give a smooth external appearance. An example of a splice connection is shown in Figure 6.9, and with its cover plate in Figure 6.10.

6.4.4 Partial end-plates

Figure 6.8(d) shows the detail of a very neat joint using a partial end-or side-plate, particularly if the open side of the connection can be arranged away from view. The number of bolts which can be located inside the section is limited and the lever arm is small, so that the connection should be regarded as pinned. It is unsuitable for members subject to bending or high-tension forces.

6.5 Welded nodes to columns and masts

In tension-tie structures, it is often necessary to attach the ties in the form of rods or cables to steel columns or masts. Multiple ties may be connected by fabricated nodes that are welded to the columns at the top, or at intermediate points along the columns, as shown in Figure 1.2. Connections in tension-tie structures may also take the form of saddles at the top of the columns over which the ties run. These saddles are fabricated and welded to the ends of the columns.

A striking example of the use of welded nodes is the tension structure at Darling Harbour, Sydney (see Figure 6.11). Four columns are placed together and the nodes are grouped in order to connect to the ties which support the long-span trusses. Another example of the innovative use of column clusters is shown in Colour Plate 21.

The 30 St Mary Axe building is constructed using a diagrid of intersecting tubular members with welded steel nodes acting largely in compression (Colour Plate 1). The nodes also support the perimeter ties and the internal beams, and an example is shown in Figure 6.12.

6.6 Pinned connections to tubular sections

Tubular connections provide the opportunity for true 'expressed' pins, as follows.

6.6.1 Column bases

Bases to tubular columns take two basic forms: pinned and rigid (or moment-resisting). The details employed reflect the transfer of forces and moments. A genuine pinned connection can be achieved by a single pin from a projecting plate, as shown in Figure 6.13. A

Architecture Steel

moment-resisting connection is achieved by a welded end-plate with four or more bolts. The thickness of the end plate depends on the moment to be transferred (see Section 5.7).

Tubular Pinned Base Connection
6.13 Typical pinned connection to a foundation

6.6.2 Expressed pinned connections

Connections using true 'pins' provide much scope for the literal interpretation of a rotationally flexible connection between members in a 'pin-jointed' assembly. Pins are usually made from two or three components. A central pin connects two ends or heads by passing through a hole in the connecting plates. The pin can be made from mild or stainless steel, and is generally smooth internally and threaded at its ends. If it is made from stainless steel, neoprene washers must be inserted to prevent bimetallic corrosion taking place with any attached mild steel elements. True pinned connections are shown in Figure 6.13 and Figure 6.14. Interesting details can be created using cast iron or cast steel nodes in a pinned connection.

Steel Bracing Connection Offset Members
6.14 Pinned connections at Ponds Forge Swimming Pool, Sheffield — see also Colour Plate 20 (architect: FaulknerBrowns)

6.7 Welded tube to tube connections

The form of welded connections between tubular members depends on the:

• shape and relative size of the members to the connections

• angle of intersection of the members

• number of members to be connected at one location.

Some fabricators are specialists in tubular construction and can advise on costs and details at the planning stage. Additional aspects, such as the need for the grinding of welds and any special connection details should be identified at this stage.

In terms of fabrication cost, a lattice girder using CHSs would require about 30 to 45 hours' work per tonne, and a similar lattice girder of triangular cross-section would require about 70 hours' work per tonne. When using larger CHS, fabricators with specialist profiling equipment can make the connections between the chords and web members efficiently. The alternative may be to use SHS sections, which only require cutting the ends of the chord members at the correct angle rather than profiling the cut ends.

6.7.1 Typical welded connection configurations

Welded connections which are standard throughout the industry are known as X joints, T and/or Y joints, N and/or K joints, with or without overlaps, as illustrated in Figure 6.15. The precise form of these connections depends on the size and shape of the members. Gaps or overlaps between the bracing or incoming members can be detailed, and influence the load capacity of the connection (see Section 6.8).

(c) N and K joints with gap

(c) N and K joints with gap

(c) N and K joints with overlap

6.15 Connection designations in welded tubular construction

6.7.2 Connections between square or rectangular sections

Welded connections may be formed relatively easily between the ends of one member and the flat wall of a larger SHS or RHS section. The structural engineer should check the local capacity due to distortion of the wall of the main member or chord when section sizes differ considerably and high forces are to be transferred. It should be noted that the resistance of the connection will be dependent on the size of the members rather than the strength of the weld.

The connection design should therefore be carried out at an early stage to avoid costly and potentially unsightly changes at a later stage in the design process. Welds may be formed by fillet welds externally, or by partial penetration welds to the prepared ends of the incoming section. The second detail is more attractive visually. Welds may be ground down where visually important.

Bracing members are generally aligned so that the centre-lines of the bracing members meet at the centre-line of the main chord in order to minimise secondary bending effects in these members.

The minimum angle of intersection of SHS or RHS members for welding is 30° to the axis of the main member, although, in practice,

6.16 Welded connection of CHS bracing members to CHS chord these connections should be made at an angle close to 45°, so that access for welding is less difficult.

6.7.3 Connections between circular sections

Welded connections between CHS require careful cutting and preparation to form the correct profile at the end of the incoming member. Profiling should also take account of the location and size of other intersection members. Severely overlapping member-connections increases the difficulty of profiling and welding. The minimum angle of intersection of CHS members for welding is 20° to the axis of the main member. Advice should be sought regarding welding of different sizes of members at shallow angles.

Three-dimensional welded nodes can be extremely complex, as seen in offshore construction. These nodes may be prefabricated, and the chord and bracing members are welded to the prefabricated nodes. It may be economic to consider the use of prefabricated cast steel nodes where the repetition of details can be achieved.

6.8 Connections in trusses and lattice construction 6.8.1 Two-dimensional trusses

Tubular sections are commonly used in long-span trusses for reasons of aesthetics and structural efficiency. Generally, CHS members are used for both the chords and bracing members, and a typical welded connection is illustrated in Figure 6.16. However, the top and bottom chords may use RHS rather than CHS members in order to facilitate

Lattice Steel Detail

the connection with the roof or floor slab and other cross-members (an example of this type of detail is shown in Figure 6.16).

Lattice trusses have traditionally been designed as pin-jointed assemblies in which the members are in tension or compression and the forces between them are transferred at the connections. It is usual practice to arrange the connections so that the centre-lines of the bracing members (branches) intersect on the centre-line of the main member (chords), as shown in Figure 6.17. This is known as 'noding'.

Whilst 'noding' is common practice, for ease of fabrication it is sometimes required to provide a small degree of eccentricity of the nodes (as illustrated in Figure 6.18). A node with negative eccentricity may be architecturally more interesting, although a node with a total overlap is less so. The structural engineer or steel fabricator will advise on specific details.

Other connections between the elements of a truss can be made in various ways. Figure 6.19 shows various forms of right-angle connection at the end of the truss. Figure 6.20 shows connections of the inclined bracing members to the bottom chord). An example of the above detail is shown in Figure 6.21

Simpler connections in shorter span trusses can be made by bolted connections using gusset plates welded to the main member or chord. A simple detail in which the CHS bracing members have flattened ends is shown in Figure 6.22. A more architectural example of a bolted splice connection with a curved gusset-plate is illustrated in Figure 6.23.


End cap assists achieving rigidity of joint

(a) Overshooting right-angle connection

(a) Overshooting right-angle connection

(b) Flush right-angle connection

Infill plates provide greater capacity for load transfer


Gap joint noding

Gap joint noding

6.17 Illustration of the alignment of centre-lines of tubular members in a welded connection

(b) Partial overlap with negative eccentricity
(c) Total overlap joint with negative eccentricity

6.18 Examples of noding with modest eccentricity

(c) Flush right-angle connection with infill plate

6.19 Right-angle connections between tubular members

6.20 Inclined connections in a lattice truss

6.21 Example of welded CHS connection in a truss

(b) Over shooting bottom member (K or N connection)

(c) RHS cranked chord connection -with vertical bracing member

Pinned Bolted Tubular ConnectionsPipe Diagrid Connection
6.23 Architectural use of a bolted gusset-plate connection in a lattice truss for a railway bridge

6.8.2 Connections in multi-planar trusses

Trusses can also be designed in triangular cross-section along their length, as shown in Figures 6.24 and 4.42. These triangular section trusses have several advantages over plane trusses, because of:

• the increased stability offered by the twin compression chords — they are frequently used as exposed structures with long spans

• the simplification of bracing requirements in roof structures, in which in-plane forces have to be transferred along the roof

• their ability to resist torsional effects from incoming beams or trusses.

In this form of construction, there are various possibilities for the alignment of the chord and bracing members, as shown in Figure 6.25. Overlaps of the intersecting bracings from both planes may occur where the chords are smaller in diameter than 1.4 x bracing member diameter. This may occur in an offset connection as shown in Figure 6.25(c). Where many members come together at one node, this is known as a 'multi-planar' connection.

Two alternative bracing arrangements in triangular lattice trusses are illustrated in Figure 6.25. The configuration in Figure 6.26(a) requires more complex welding of the nodes at the bottom chord. The simpler connection detail in Figure 6.26(b) facilitates welding by arranging for a greater symmetry in the bracing arrangement. For

RHS bottom chord

RHS bottom chord

^ Offset

^ Offset

(b) Offset

(b) Offset

(c) Overlapped diagonals 6.25 Connection types used in triangular section trusses

(a) A relatively complex member requiring precise cutting, welding and grinding of the joints (see Colour Plate 23)

(a) A relatively complex member requiring precise cutting, welding and grinding of the joints (see Colour Plate 23)

(b) Simplified connection detail

6.26 Alternative bracing patterns for triangular lattice girders

RHS chords, it may be necessary to increase the wall thickness to provide more resistance to forces transferred from the bracing members.

6.8.3 Reinforcement of connections

For maximum resistance of the members, it is usually more efficient to select larger tubular sections with thin walls. However, when designing the connections, it is more advantageous to use chord members that are thicker and smaller in section (provided that they are not smaller than the bracing members). Therefore, a compromise is necessary for overall design and fabrication efficiency.

In some cases, connections may have to be strengthened locally to resist the applied forces, if it is not possible to increase the member size or thickness. This can be achieved by welding plates to the chord face (see Figure 6.27(a)). It should also be noted that overlaps will also increase the connection resistance, especially for RHS members. When a third member is required at the intersection, a 'T' piece can also be used (see Figure 6.27(b)).

Other non-standard stiffened K connections can be used to increase the load capacity of the connection, as illustrated in Figure 6.28.

For multiple-bracing connections, the intersections can be moved back from the node point. This can be achieved by introducing a

Plate Nodes Metal Structure

6.27 (a) Adding plate to chord section; and (b) adding a T-plate to facilitate connection

6.28 Additional stiffening plates to create non-standard K connections short length of CHS, or by employing hollow spheres. Spheres have the advantage of the same cut at the end of the member, connecting the member's end regardless of the intersection angle. However, the source of this type of node is limited.

Saddle reinforcement can be used to locally increase the chord thickness and local compression resistance, as illustrated in Figure 6.29.

At the headquarters of Royal Life in Peterborough, one of the features is a glazed-screen elevation sweeping in a curve from one block to another. The façade was located 700 mm away from the primary structure, and it required its own support structure. The designers increased the stiffness of the section by welding four pairs of longitudinal steel fins, which, in turn, match the metal fins of the cladding (see Figure 6.30).


6.29 Reinforcement to tubular sections to increase their local resistance to forces from the bracing members: (a) saddle reinforcement; and (b) flange plate reinforcement

6.29 Reinforcement to tubular sections to increase their local resistance to forces from the bracing members: (a) saddle reinforcement; and (b) flange plate reinforcement

6.30 Royal Life UK headquarters — steel tubular column with four pairs of fins (architect: Arup Associates)

6.8.4 Connections in Vierendeel trusses

Vierendeel trusses comprise members connected at right angles and resist shear loads primarily by bending in the members. In this way, bracing members are eliminated but the chords are much heavier because they resist bending as well as axial forces. Vierendeel trusses employ only rigid or full-moment connections, unlike triangulated trusses in which the connections are designed as pinned. SHS or RHS sections are generally used in Vierendeel trusses, rather than CHS sections, because of their better bending resistance.

Vierendeel trusses are relatively inefficient at resisting high shear-forces because of the lack of diagonal bracing and, therefore, it is necessary to use thicker or larger chord members than in triangulated trusses. Ideally, the chord and vertical members should be the same external size. If not, stiffening elements are generally inserted to increase the local bending resistance of the connections. Figure 6.31 shows various ways in which nominally pinned connections can be strengthened in Vierendeel trusses. Visually, some of these details are not preferred, except when the trusses or these connections are hidden. An example of a 'hybrid' welded and bolted connection between RHS sections is shown in Figure 6.32.

6.9 Beam to column connections in tubular construction

The configuration of beam to column connections depends on the type and size of members to be joined. Three generic types of connection exist:

Vierendeel Connection

(a) Unreinforced

(b) Spliced

(a) Unreinforced

(b) Spliced

Vierendeel Steel Columns

6.31 Types of Vierendeel connections between SHS/RHS members

(c) Bracing plate stiffeners

(d) Chord plate stiffener

Bolted Shs Connections

6.32 'Hybrid' welded and bolted connection

• RHS beams to I-section columns

• I-section beams to CHS or SHS columns

Beams and columns are usually connected on site by bolting. In the case of an RHS beam connection to an I-section column, a welded extended end-plate to the RHS beam permits the use of a conventional bolted connection to the column flange or web (see Figure 6.33). The bolts may be countersunk into the thick end-plate if the connection is important visually.

In the case of bolted connections to SHS or RHS columns, special forms of bolts are required, which can be located from one side. 'Flowdrill' and 'Hollo-Bolt' are two particular forms of bolt suitable for use with SHS or RHS sections (see Section 6.10). Alternatively, brackets or fins can be welded to the RHS column to provide direct bolted connections.

A number of typical simple connections using cleats welded to an RHS column are shown in Figures 6.34 to 6.36. Figure 6.34(a) shows a fin plate welded to the face of the column. The supporting bracket in Figure 6.34(b) can be detailed to be visually interesting. Figure 6.35 shows the use of channels welded at the tips of their flanges to

Rhs Connection

6.33 Connection of RHS beam to I-section column

Rhs Column
6.34 Conventional cleats welded to an RHS column
Welded And Bolted Steel Hollow Sections
6.35 C-sections welded to an RHS column to facilitate the use of a bolted connection


6.36 Studs or seating plate and cleat welded to an RHS column

6.37 Details of RHS beams connected to RHS columns permit access for bolting on site. Figure 6.36(a) shows the use of welded threaded studs, but these must be protected during transit to prevent damage. Figure 6.36(b) shows an I-section with a partial depth end-plate connection supported by a shear block welded to the column. The welded block must be sufficiently thick to allow for all site tolerances. Also, the single bolted connection may not be acceptable for 'robustness' requirements in multi-storey buildings.

Figure 6.37 shows other typical connections of an RHS beam to an RHS column. For lightly loaded connections, the T-section shown in Figure 6.37(a) may be replaced with a fin plate. Where through bolting is used (as in Figure 6.37(b) and 6.37(c)), spacer tubes

Fin Plate TrussRhs Shs Connection

improve the local bending resistance of the wall of the incoming section.

For the connection of tubular trusses to RHS columns, typical bolted details are shown in Figure 6.38. High shear-forces may require the use of more bolts than shown. The sharing of load between the upper and lower chords in the connection depends on the presence of a vertical bracing member at the end of the truss. In the detail of Figure 6.38(a), the upper connection will resist all of the applied shear-force. In Figure 6.38(b), the upper and lower parts of the connections may be assumed to resist equal shear-force.

Whilst the above details may not be the most visually appropriate for exposed applications, they illustrate the general principles of support conditions to trusses using tubular sections of all types.

A good example of a simple and elegant detail of a connection between a CHS column and an I-section roof beam is shown in Figure 6.39.

Gusset Plate For Section Gable FrameFlowdrill Bolt

i stage

6.40 Flowdrill bolt i stage e

6.40 Flowdrill bolt

6.41 Illustration of the stages of forming and making a Flowdrill connection using a fully threaded bolt

6.10 Special bolted connections to SHS and RHS 6.10.1 Flowdrill connections

The 'Flowdrill' method of bolting may be used where an architecturally 'clean' connection to an RHS member is required.

Flowdrill is a form of connection that does not require access from both sides of the connection. It is a thermal drilling process that makes a hole through the wall of a hollow section without the removal of the metal normally associated with drilling. The hole is then threaded in a second operation. The threaded hole will then accept a fully threaded bolt (see Figures 6.40 and 6.41). At present, the application of the Flowdrill process is limited to steel thicknesses up to 12.5 mm. It is mainly used for connecting end plates of beams to RHS connections.

Flowdrill requires the use of a high-speed drill, as the normal drill speed is not sufficient. Because of this, the RHS section may have to be taken out of the main production line, which adds both cost and time. Therefore, Flowdrill connections tend to be used for specialist applications. Further information can be obtained from Corus, Tubes & Pipes, and guidance on connection design is given in Joints in Simple Construction.

6.10.2 Hollo-Bolt connections

Lindapter has recently developed the 'Hollo-Bolt', which is another type of bolt used to connect hollow sections to other members, and where the connection is accessible from one side only.

The Hollo-Bolt features three parts (supplied pre-assembled) — a body, cone and central setscrew. The entire product is inserted through both the fixture and steelwork, and the central set screw is tightened whilst gripping the collar. As the set screw tightens, the cone is drawn into the body, spreading the legs and forming a secure fixing. The Hollo-Bolt principle is illustrated in Figure 6.42. The principal advantages of Hollo-Bolt connections are:

• there is no need for welding

• it is quick and simple to install

Location flats SY TV" Hexagonal head

Body Cone knurling

Location flats SY TV" Hexagonal head

Body Cone knurling

Fixture RHS

Central bolt

Fixture RHS

Central bolt

6.42 Hollo-Bolt connection

• it is fully tested in both tensile and shear applications

• no special tools are necessary, it can be installed using two spanners

• there is no need to provide close tolerance holes

• access is needed from one side only

• it is available in mild steel or stainless steel

• it can be used with a threaded rod or a central bolt

The principal disadvantage of this connection is that the bolt hole is considerably larger than in normal bolted connections (approximately 1.7 times the bolt diameter), which may affect the local resistance of the wall of the RHS when subject to bending or tension forces. Furthermore, its capacity in shear and tension is low compared to normal grade 8.8 bolts. Also, the bolt cannot be undone after it is tightened because of the expansion of the rear of the cone.

6.10.3 Huck 'blind' fasteners

The Huck Bom Blind fastening system uses fasteners between 3/16" (4.8 mm) and 3/4" (19 mm) diameter, which can be connected from one side only.

Resistances in shear and tension compare favourably with other kinds of connection, and the appearance is more attractive. The main disadvantage is that the fastener cannot be undone, and the connection appears to be more like a riveted than a bolted connection.

Tension structures refer to suspended or 'tent-type' structures in which the 'ties', i.e. members designed to carry tension, are major elements in the overall structure. Tension structures differ from conventional framed-structures in two important respects: the structural concept is explicit in the architecture, and the detailing of the connection between the tension and compression elements can be more complex. The design of a tension structure requires careful thought about load paths, stability, flexibility of the system, cladding interfaces and foundation design.

In most framed buildings, the building itself defines the form of the structure to a large extent: columns, walls, beams and slabs are arranged and sized to suit the application using basic rules which are dictated by the plan form and structural efficiency. However, in a tension structure there is more freedom in the choice of the form of the structure, which is mostly external to the building envelope. Tension structures include various forms of suspended structures and cable-stayed roofs. Figures 2.9 to 2.11 and Figures 7.1 to 7.10 illustrate some well-known examples of these types of structure.

Tension structures are most commonly used in long-span roof structures, but they can be employed in a wide variety of applications, including canopies, glazed façades, and even staircases.

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