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Connections

Below is an introduction to the topic Connections. You can also search for your specific topic using the Search box at the top of the page or click on any of the following keywords and phrases: Simple connections; Moment connections; Greenbooks.

Introduction

In a typical braced multi-storey frame the connections may account for less than 5% of the frame by weight, and yet probably 30% or more of the total cost. Efficient connections will therefore have the lowest detailing, fabrication and erection labour content - they will not necessarily be the lightest.

Standardisation

A problem facing the connection designer is the bewildering range of options he has in selecting:

  • The type of connection
  • Grades and sizes of fittings
  • Bolt grades, sizes and lengths
  • Weld types and sizes
  • The geometry to adopt

Modern connection design is therefore based on a rationalisation of the above options and this procedure leads to a standard connection where the fittings, bolts, welds and geometry are fully defined. The benefits of this approach are:

  • A reduction in the number of connection types which leads to a better understanding of their cost and performance by all sides of the industry and encourages the development of deign aids and computer software.
  • The use of few standard flats or angles for fittings which improves availability, leads to a reduction in material costs and reduces buying, storage and handling time.
  • The use of one grade and diameter of bolt in a limited range of lengths which saves time changing drills or punches in the shop, leads to faster erection and fewer errors on site.
  • The use of small, single pass fillet welds which avoids the need for any edge preparation and reduces the amount of non-destructive testing required.

Steel structures can be complex and there will be times when standard connections are not suitable. However even in these cases it will still be possible to adopt some of the general principles of standardisation such as limiting the range of fittings, sections and bolt sizes.

A summary of the recommended components is given in the table below.
ComponentPreferred OptionNotes
Fittings Material of grade S275 Limited range of standard flats and angles
Bolts M20 grade 8.8 bolts Some heavily loaded connections may need larger diameter bolts
Foundation bolts M24 grade 4.6
Holes 22 mm diameter punched or drilled, or 22 mm x 26 mm slotted holes made by:
- Punching in one operation
- Formed by drilling two holes and completed by cutting
- Machine operated flame cutting
26 mm dia for M24 bolts
6mm oversize for Foundation bolts
Welds Fillet welds with E35 electrodes 6 mm or 8 mm leg length Larger welds may be needed for some column bases

Connection design

Simple connections

Design procedures for simple connections are given in the publication Joints in Steel Construction - Simple Connection (1). This publication provides procedures for desiging connections in steel-framed structures in accordance with the recommendations given in B=S 5950-1: 2000(2). It uses the simple design method described in clause 2.1.2.2 of BS 5950-1: 2000. Connections between universal beams and universal columns are included and between universal beams and hot finished structural hollow section columns using Flowdrill and Hollo-Bolt systems Design are provided for:

  1. Beam-to-beam and beam-to-column connections
    1. Double angle web cleats
    2. Flexible end-plates
    3. Fin plates
  2. Column splices
  3. Column bases
  4. Bracing connections
  5. Special connections

The procedures given in this publication are suitable for either hand calculation or for the preparation of computer software.

Designing connections by hand is a laborious process and so a full set of capacity tables is included in Joints in Steel Construction - Simple Connection (1). Checking the strength of a simple connection involves three stages:

  1. Ensuring that the connection is detailed so that it develops only nominal; moments which do not adversely affect the members or the connection itself
  2. Identifying the load path through the connection i.e. from the beam to the supporting member.
  3. Checking the strength of each component, ensuring each is capable of transferring the load from one part to the next.

Moment connections

Design procedures for moment connections are given in the publication Joints in Steel construction - Moment Connections (3). This publication provides methods for designing the following types of moment resisting connections in steel-framed structures:

  1. Beam-to-column
    • Bolted end-plates
    • Wind-moment connections
    • Shop and site-welded connections
  2. Beam-to-beam
    • Bolted splices
    • Welded splices
  3. Columns
    • Bolted splices
    • Welded splices
    • Bases

The design procedures given in this publication are based on a combination of design models to established methods used both in the UK and overseas and capacity checks on bolts, welds and sections based on BS 5950-1: 2000. The methods are generally not suitable for hand calculation.

References

  1. Joints in Steel Construction - Simple Connections, published by the Steel Construction Institute and the British Constructional Steelwork Association
  2. BS 5950-1: 2000. Structural use of steelwork in building Part 1: Code of practice fro design - Rolled and welded sections
  3. Joints in Steel Construction - Moment Connections, published by the Steel Construction Institute and the British Constructional Steelwork Association