Connections between Isections

5.1 Introduction to connections

In a typical steel structure, the detailed design of the connections, the preparation of the production drawings, the fabrication and the erection accounts for some two-thirds of the total cost of the framework. Most of the cost is absorbed in the detailing and fabrication of the connections between the members.

In most projects, steel fabricators undertake the design and detailing of all connections according to their preferred method of fabrication. Because of this, there has tended to be a diversity both in connection types and design methods. Therefore, designers should have an awareness of the range and types of typical details in order to assess the suitability and adequacy of the proposed details.

The provision of industry standards for connection design is developing rapidly. In 1991, the SCI and the British Constructional Steelwork Association (BCSA) first published a design guide, Joints in simple construction, which presents design procedures for connections for use in buildings designed with braced frames. A further publication on moment-resisting connections was produced in 1995. The publication on simple connections was revised in 2002 and now includes tubular connections.

Various forms of connections may be identified in regular steel frames. These are connections of:

• beams to column flanges

• beams to column webs

• column to column splices

• column bases at foundations

• bracing connections.

Their detailing depends on the forces and moments to be transferred, and on the chosen member sizes. However, some common detailing rules apply, which are discussed in the following sections in order to gain an appreciation of the form of connections in regular frames.

5.2 Benefits of standardisation

In a typical braced multi-storey frame, the elements in the connections may account for less than 3% of the frame weight and yet probably 30% or more of the total cost. Efficient connections will therefore have the lowest detailing, fabrication and erection cost, and are standardised over a broad range of application. Parameters relevant to standardisation include the:

• type of connection

• grade of steel used in the connecting parts

• bolt grades, sizes and lengths

• member sizes and geometry.

The benefits of this approach include:

• a reduction in the number of connection types, making fabrication easier and cheaper

• the development of design aids and associated computer software

• savings in buying, storage and handling time, which leads to a reduction in overhead costs

• reduced design costs, and fewer difficulties in Building Regulation approval

• the use of one grade and diameter of bolt in a limited range of lengths saves time changing drills or punches in the shop, and leads to faster erection and fewer errors on site.

In a particular project, it is important to first define a series of connections of 'common' form, whether pre-existing standard connections or those determined as 'standard' for the member types, sizes and arrangements that are encountered in the building structure. This should also extend to the interfaces with other key components, such as cladding and roofing.

5.3 Industry-standard connections

Standards have been established for connections between hot-rolled steel sections in portal frames and in rectilinear frameworks used in multi-storey buildings. Details of the standard connections adopted in the UK are found in the publications by the SCI and the BCSA. , Their adoption makes fabrication more straightforward, promotes better communication, and reduces the chance of design and site errors.

Table 5.1 presents a summary of the preferred elements in standard connections. Fully threaded bolts have grown in popularity because they can be used with a wide range of thicknesses of the connected parts. However, they are not usually appropriate for exposed connections, unless the projecting length of the bolt is hidden.

Table 5.1

Preferred sizes of elements in standard connections

Element

Preferred option

Notes

Bolts

M20 Grade 8.8 bolts

Some heavily loaded connections may need larger diameter bolts Foundation bolts — M24 Grade 4.6

Fully threaded bolts may be used where the thickness of the connected parts is variable

Holes

22 mm diameter punched or drilled, or 22 mm x 26 mm slot punched

26 mm diameter for M24 bolts 6 mm oversize holes for foundation bolts

Welds

Fillet welds with E43 electrodes, 6 mm or 8 mm leg length

Larger welds may be needed for some column bases

Limited range of standard flats and angles

Refer to the SCI/BCSA publications for further information

Some general detailing rules, as described in the SCI/BCSA publications, ' are as follows. For nominally pin-jointed connections:

• the top of the top flange will generally be used as the setting out point

• the tops of all fittings (e.g. cleats) are placed 50 mm below the top of the beam

• the first bolt row is located at a constant 90 mm from the top of the beam, independent of the flange thickness

• the bolt rows are then set at 70 mm intervals below the row above. Therefore, the final row will be at a variable position above the bottom flange, depending on the section size.

For moment-resisting connections using end-plate type details:6

'flush' end-plates should be extended 15 mm above the top flange of the beam to allow for welding to the top flange of the beam 'extended' end-plates should be extended 90 mm above the top flange of the beam to accommodate an additional pair of bolts the first row of bolts below the flange is located at a constant 60 mm from the top of the beam, independent of the flange thickness the bolt rows are set at 90 mm intervals below the bolt row above, as for nominally pin-jointed connections for extended end-plates, the upper row of bolts should be located 40 mm above the top flange.

The following section deals with the typical range of connections for traditional applications.

5.4 Beam to column connections

The most common types of beam to columns used in buildings are described as follows:

Double angle web cleats

A double angle web cleat connection (Figure 5.1(a)) consists of a pair of angle cleats that are usually bolted to the beam web in the shop. The beam assembly is then bolted to the column on site. Single angles are much weaker as the bolts are loaded in single shear. The minimum size and thickness of angle section for a standard connection is 80 mm x 6 mm thick. The preferred size of angle is 90 mm x 10 mm thick for most applications. Stool or seating cleats

A stool or seating cleat is sometimes placed under the end of the supporting beam, which provides a safe and positive landing position for the beam during erection. A web cleat is always used with a seating cleat in order to stabilise the member laterally (see Figure 5.1(b)). Fin plates

Fin plates are most commonly associated with beam to beam connections, but may also be used for beam to column connections. Projecting plates may be welded to the column flange or web to which the incoming beam is bolted (see Figure 5.2). Bracket connections

Other forms of bracket connections may be made to the side of columns. A good example is in the parallel beam approach, as illustrated in Figure 5.3 and as described in Section 4.2.5. Projecting channel sections are welded externally to the tips of the column flanges and extend outwards to connect to the beam web. The bolted connection through the end plate is made on site. This connection is not appropriate for small columns because of the difficulty of access for welding the bracket. The weld size is also determined by the torsional effects due to the continuous beam spanning over the main beams.

(b) Web cleat (single or double) and seating cleat

5.1 Web cleated connections

(b) Web cleat (single or double) and seating cleat

Welded fin plates 5.2 Fin plate connection (welded to column flange web)

Parallel beam connection (secondary beam shown dotted)

Parallel beam connection (secondary beam shown dotted)

5.3 Welded bracket to connect pairs of beams

• Flexible end-plates

'Flexible' end-plates consist of a thin plate welded to the beam in the workshop. The plate is typically 8 to 12 mm thick, depending on the size of bolts used. The beam is then bolted to the supporting member on site. End plates are probably the most popular of the beam-column connections used in the UK. They are versatile in that they can be used with skewed beams and can tolerate moderate offsets in beam to column alignments.

They are termed 'flexible' because they are thin and are not necessarily welded to the beam flanges, and therefore do not transfer significant moments (see Figure 5.4(a)). The SCI/BCSA publication gives guidance on typical details for these connections.

• Welded shear blocks

Welded shear blocks, as shown in Figure 5.4(b), are widely used in continental Europe. The upper bolts are only used for location and to provide for tying forces. The welded shear block resists the vertical load transferred from the beam. However, the

(a) Flexible or partial depth end plate connection

•T'—. -T-

i p

• :

• •

1 ^

(b) Partial depth end plate with welded shear block

(b) Partial depth end plate with welded shear block

5.4 Flexible or partial depth end-plate connections

VFlush' end plate extends 15 mm ^ above top llange to allow top flange weld

(a) Flush end plate connection

5.5 Moment resisting end-plate connections

(a) Flush end plate connection

- • || • -* il *

I t

(b) Extended end plate connection

(b) Extended end plate connection block must be thick enough to allow for all tolerances in beam placement.

Thick end-plates

Moment-resisting connections between beam and columns can be fabricated by welding thicker end-plates to the beams. The end plate is typically 15 to 20 mm thick. Flush end-plates are welded to the flanges and web of the beam so that there is minimal (15 mm) projection of the end plate (see Figure 5.5(a)). Extended end-plates project above or below the beam depth (see Figure 5.5(b)) and achieve greater bending resistance by having the facility for bolts above and below the tension flange.

The SCI/BCSA publication Joints in Steel Construction — Moment Connections provides guidance on the practical application of these moment-resisting connections. Importantly, the number and size of the bolts is designed primarily to resist the tension forces caused by the applied moment. The lower bolts resist the applied shear-forces. Connections to the webs of columns require careful detailing, as the end plate to the beam must fit between the depth of the root radii of the column section. (The root radius can be up to 25 mm per flange.) Haunched connection

A haunched connection, shown in Figure 5.6, is a further example of a moment connection that is typically used in single or multi-span portal frames. However, haunched composite beams have been used to create longer spans of minimum depth (see Section 4.2.3). The haunch is designed so that the connection is not the 'weak link' in the failure mechanism of the frame. It is usually created by cutting and welding a portion of the same beam-section in order to minimise wastage. Welded connections

Fully welded connections are rarely used in building construction in the UK because of the potential difficulty in

Steelwork Haunch
Cutting profile for haunch

5.6 Haunched connection of a beam to a column achieving good-quality welds on site. However, it may be possible to provide bolted splice connections elsewhere in the beam to facilitate transport and lifting, and to fully weld the main connections in the factory (as column 'trees').

Stiffened connections

Where the connection between the beam and column requires additional load-bearing capacity, or where the loading may be eccentric to the member axes, the connection may be stiffened in the form of welded plates of typically 6 to 12 mm thickness. Welding of stiffeners is relatively expensive and should be avoided in regular beam and column construction. It may be cost-effective to increase the column weight (size) to avoid the need for stiffeners. Seated connections

In some low-rise buildings, seated connections may be used. Beams, or more usually, trusses are seated on end plates welded to the tops of the columns. Pairs of bolts provide for shear and uplift resistance. These connections are treated as pinned. Usually the supported members are restrained laterally by some other means, for example, a perimeter tie or eaves beam.

5.5 Beam to beam connections

The SCI/BCSA publication Joints in Simple Construction provides guidance on the practical applications of beam to beam connections, which are generally treated as pinned. Practical conditions to be addressed are as follows:

• Relative sizes of beams

A common feature of beam to beam connections is that the top flanges of the beams should be at the same level. Therefore, the ends of the secondary beams are often 'notched' so that they can be attached to the web of the primary beams (see below). Most of the previously described connection types may be used, and some are illustrated in Figure 5.7.

Welded end plates (partial depth) Welded fin plate (partial depth)

5.7 Typical beam to beam connections

Bolted cleats

(on one or both sides of web)

Welded side plate

Welded fin plates avoid the need for notching, but extend well outside the beam width and cause bending of the fin plate. Welded side-plates cause local bending of the flanges, and are not recommended for heavily loaded applications unless the web is stiffened to resist these local forces. A Tee section welded to the web provides this stiffening function. Notching of beams

It is often necessary to notch or cut-back beams when connecting to other beams or columns. The detail in Figure 5.8 shows the amount of cutting back that is required in standard connections. Splicing of beams

Spliced connections are rarely used in building construction except in very long-span beams where transportation or erection requirements necessitate the supply of shorter members. Spliced connections require web plates to transfer shear, and often top and bottom plates to transfer moment applied to the beam. Preferably, these splices are not made in the regions of high moment. Splice connections generally use high-strength friction grip bolts acting in shear to avoid the effects of bolt slip on deflections.

Connections of beams at different levels

In some building types, it is possible to align the beam at different levels, in which cases connections below the top flange may be made by end plates or web cleats, as in conventional connections. Beams suspended below beams may make use of special connectors, such as by Lindapter (see Figure 5.9). These connections may be of particular interest where drilling or welding is not permitted on site.

Stiffening Beam Web

Standard notching of beam-beam connection b_ I (from face of web)

Beam Plan View Connection

Plan view on beam-column connection

Column

Flange trim (mm)

size

a

b

356 UC

240

190

305 UC

195

160

254 UC

150

135

203 UC

110

110

152 UC

70

85

Standard trimming for various column sizes

5.8 Typical detailing requirements for beam to beam connections

5.9 Suspended beam to beam connections by Lindapter

5.6 Column splices

Column splices in multi-storey construction are usually provided every two or three storeys, and are located about 500 mm above floor level. This results in convenient column lengths for fabrication, transport and erection. The splicing operation is safer and easier to perform if it is done at a reasonable working height. Section sizes for the upper levels can be reduced at splice positions, but the provision of splices at each floor level is seldom economic, since any saving in column weight is generally far outweighed by the additional costs of the fabrication and erection. Figure 5.10(a) illustrates typical column splices in columns of the same size, and Figure 5.10(b) illustrates splices at a change of column size.

There are two basic types of column splice: bearing and non-bearing. In the bearing type, the loads are transferred from the upper to lower columns directly, or through a division plate. To ensure efficient fit at the splices, the ends of the columns should be finished square. For lightly loaded columns, a sawn end is sufficiently accurate so that bearing surfaces do not have to be machined to achieve good contact. For larger, heavily loaded columns, the ends

Steel Beam Splice Welding

Holds column in position prior to welding

Splice plates may be inside, outside, or in pairs each side of the flange

(i) Bolted splice plates

(ii) Site welded (used only when visual appearance is very important)

Holds column in position prior to welding

Splice plates may be inside, outside, or in pairs each side of the flange

(i) Bolted splice plates

(ii) Site welded (used only when visual appearance is very important)

5.10 Various forms of column splices: (a) typical connections between column sections of the same size; and (b) typical connections between column sections of different sizes

Column Splice

(iv) Extended end plates

(v) Bolted splice plates (with packs)

(iv) Extended end plates

(v) Bolted splice plates (with packs)

should be machined in order to achieve good-bearing contact. In non-bearing splices, the loads are transferred by way of bolts and splice plates, and any bearing between the members is often ignored. Countersunk bolts may be used in the splice plates to avoid protruding bolt heads, which may otherwise interfere with finishes and fire protection, and may be less visually acceptable. However, this is a more expensive option than using conventional bolts.

5.7 Column bases

Column bases can be designed as nominally pinned (simple) or moment resisting (rigid). Nominally pinned bases are only required to transmit axial and shear forces into the foundation, and are provided in braced structures and in portal frames. They are generally preferred to moment-resisting base connections for reasons of cost and practicality. Uplift due to internal wind pressure and external wind suction may have to be considered in single-storey structures, which leads to a minimum size of foundation for a given building size.

Moment-resisting bases may be required in rigid-frame structures in order to reduce the effects of sway and deflections. These bases and their foundations are considerably larger than for nominally pinned column bases.

Holding down (HD) systems are designed to satisfy the following requirements:

• In service, they must transmit shear from the column to the foundation; if HD bolts are fitted using oversize holes in the base-plate, then shear must be resisted by other means.

• During erection, they must be capable of stabilising the column and other structural elements. Thus, four bolts are provided, even in a nominally pinned connection.

• They must resist uplift, depending on the design condition.

The base plate should be of sufficient size, stiffness and strength to transmit the compressive force and bending moment from the column to the foundation through the bedding material, without exceeding the local bearing capacity of the foundation.

Usually the force transfer from the column to the base plate is by direct bearing, and the welds between them are designed to resist shear only. Where required, the plate is designed for bending due to over-turning or uplift effects, which may cause tension in the HD bolts. Typical details for a column base using UC or SHS columns are shown in Figure 5.11.

Generally, the thickness of the base plate is chosen so that it does not require additional stiffening. However, there may be architectural merit in using shaped stiffeners in exposed applications.

To allow for tolerances in the concrete foundation, the top surface of the concrete is designed to be 30 to 50 mm below the bottom of the base plate. The column is temporarily supported on steel packs and wedges which permit vertical adjustment of the column. High-strength grout is then injected under the plate, and the wedges are

Elastic Grout Base Plate

Optional '30 dia.

holes depending on size of base

M20 bolts in 24 mm holes

Universal Column

Optional '30 dia.

holes depending on size of base

M20 bolts in 24 mm holes

Universal Column

Rectangular Hollow Section

5.11 Typical simple column bases

Rectangular Hollow Section

Foundation Base Grouting

removed when the grout has gained sufficient strength. Where column bases are required to be concealed, an allowance for this gap and for the end-plate and the projecting bolts must be made when determining the covering to this detail (typically 100 to 120 mm should be allowed). This may increase to 300 or 450 mm where rainwater downpipes are also located in the column zone.

5.8 Connections in trusses

Various forms of truss or lattice girder may be defined depending on the span and load configuration (see Section 4.5). Lattice girders have parallel top and bottom chords and are used as beams, whereas trusses may have inclined top chords for use in roofs. In both cases, the connections between the members may be bolted or welded. Welded connections are often preferred in tubular construction, or where the cumulative effect of bolt slip is critical to the design of the truss. Nevertheless, it may be necessary to introduce splices in the chord members if the trusses are too long for transportation. These splices should be located and detailed carefully if they are architecturally important.

5.8.1 Trusses comprising angle sections

Traditionally, roof trusses used angles, with bolted and gusseted connections (see Figure 5.12(a)). However, deeper T-sections for the

Bolted Connection Inclination
5.12 Traditional bolted connections in trusses

main chords avoid the use of gusset plates, provided the bolts can be accommodated (see Figure 5.12(b)). The projection lines of the bolt setting-out lines are detailed in such a way that eccentricities in the forces transmitted by the bolt groups are minimised.

In welded connections, the depth of the T-section is chosen so that the centroidal axis of the sections can be arranged to eliminate eccentricity (see Figure 5.13). Top and bottom chords are usually continuous, except at changes in direction or where splices are necessary for erection purposes. Pairs of angles, bolted or welded periodically along their length, are preferred, as they are much more resistant to buckling than single angles.

Lighter lattice girders used as secondary beams may be connected to continuous columns at their top chord only. This forms an effective 'pin' connection for design purposes (see Figure 5.14). However, heavier lattice girders supporting secondary beams should be connected to the columns at both their top and bottom chords.

Flange Connection Truss
5.13 Typical bracing — chord welded connection

5.14 Typical truss-column flange connection

Hollow Section Splice Length
Gusset plate of same thickness as top chord

5.8.2 Lattice girders comprising heavier sections

Long-span lattice girders often comprise UC sections or tubular sections rather than angles in order to increase their compression resistance (see Figure 4.29). Heavy members may be required in special applications, such as transfer structures between floors which support point loads from columns above. In some cases, they are designed as storey-high assemblies. Deflection control is particularly important in long-span applications, and welded or friction grip bolted connections may be preferred to avoid the cumulative effects of bolt slip.

5.15 Examples of bracing connections in frames using angle sections

Connection Between Truss And Column

5.9 Bracing and tie-members

Vertical and horizontal bracing members resist wind and other horizontal loads applied to the building or structure, and transfer the loads to the foundations or other stabilising elements, e.g. concrete cores. In general, there are five forms of bracing and tie-members that may be considered: angles, flats, cables, rods and tubes. Some of them are only suitable for resisting tension, which dictates the form of construction in which they can be used.

The simplest form of bracing member is the steel angle, either placed singly or in pairs back to back. Single angles are less efficient in compression than double angles. Various forms of bracing assemblies may be used, such as X- and K-bracing, in which the members may be designed to resist tension or compression (see Section 3.6). Typical bracing connections using angle sections are shown in Figure 5.15. Angles designed for tension only will be more slender than those designed to resist both tension and compression.

Tubular connections are often preferred for bracing connections because of their good compression resistance (see Chapter 6).

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