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Steel Designers' Manual - 6th Edition (2003)
756 Design of connections
struction of buildings in seismic areas. Such connections can provide full moment
continuity but can be expensive to produce. This type of connection is rarely used
in the UK but with careful planning there is no reason why they cannot be used for
a number of framing systems. A procedure for the design of both site welded and
shop welded beam-to-column connections is given in the SCI/BCSA design guide
Joints in Steel Construction – Moment Connections. 2
The wind-moment method 10 for the design of unbraced frames has been extensively
used to design many multi-storey steel framed buildings. In this design method
the connections are assumed to behave as pins under gravity loads and the beams
they support are designed assuming simple supports. However, under lateral wind
loads the connections are assumed to behave rigidly. Connections which behave in
this manner are called wind-moment connections. These connections generally
consist of flush or extended end-plates and have little or no stiffening in the columns.
They are easy to fabricate and provide a cost-effective solution for low-rise
unbraced buildings. The main requirement for wind-moment connections is that
they should be ductile. That is, they must be able to rotate as plastic hinges under
gravity loading and still have sufficient strength to resist the moments from wind
loads. To ensure this type of behaviour it is important to design the connection in
such a way that the end-plate is thin enough to be the weak link and that it will fail
before either the bolts or the welds. The end-plate must also have sufficient strength
to resist the moments from the wind loads. The size of the end-plate in relation to
the size and strength of the bolts must, therefore, be selected with great care. If it
is too thick the bolts will fail first, making the connection brittle. If it is too thin, the
strength of the connection will be too small to support the wind moments. When
using grade 8.8 bolts the appropriate end-plate thickness is about 60% of the bolt
diameter.
The design philosophy and design methods described in the next section can be
used to design either a flush, extended or wind-moment connection.
26.3.2 Design philosophy
BS 5950: Part 1 gives little detailed information on the design of moment connections
and therefore the method used in this publication is based on a combination
of the capacity checks given in BS 5950: Part 1 4 and the design model given in Annex
J of Amendment No. 1 to ENV Eurocode 3: Part 1.1. 11 The capacity checks for the
bolts, welds and sections are all based on BS 5950: Part 1. However, the checks for
calculating the capacity of either the end-plate or column flange in the tension
region of a bolted end-plate connection are based on the design model given in
Annex J of ENV Eurocode 3: Part 1.1/A1.
An end-plate connection transmits moment by coupling tension in the bolts with
compression in the bottom flange. The bolt row furthest from the compression
(bottom) flange may therefore attract the most tension, and traditional practice in
the UK has been to assume a triangular distribution of bolt forces. However, the