Civil Engineering Reference
In-Depth Information
Elastic analysis for a uniform beam shows that the end moments are
wL
2
/15. They would be
wL
2
/12 if the joints were truly rigid, so their
flexibility causes a 20% redistribution of hogging moment. The situation
for a composite beam in practice is more complex because
E
a
I
b
is not
uniform along the span, and the columns are not rigid.
A
semi-rigid
joint has an initial rotational stiffness between these two
limits, Fig. 5.5.
The classification of joints by strength is as follows.
A joint with design resistance
M
j, Rd
is classified as
nominally pinned
if
M
j,Rd
is less than 25% of the bending resistance of the weaker of the
members joined, and if it has sufficient rotation capacity. It is not difficult
to design connections that satisfy these conditions. An example is given
in Section 5.10.
A
full-strength
joint has a design resistance (to bending, taking account
of co-existing shear) at least equal to
M
pl,Rd
for the members joined. There
is a separate requirement to check that the rotation capacity of the connec-
tion is sufficient. This can be difficult. It is waived if
M
j,Rd
≥
1.2
M
pl,Rd
(5.3)
so in practice a 'full-strength' connection may be designed to satisfy
Condition 5.3. It can then be assumed that inelastic rotation occurs in the
beam adjacent to the connection. The rotation capacity is then assured by
the classification system for steel elements in compression.
A
partial-strength
joint has a resistance less than that of the members
joined; but must have sufficient rotation capacity, if it is at the location of
a plastic hinge, to enable all the necessary plastic hinges to develop under
the design loads.
5.4
Design of non-sway composite frames
5.4.1
Imperfections
The scope of this Section is limited to multi-storey structures of the type
shown in Fig. 5.1, modelled as two sets of plane frames as explained in
Section 5.1. It is assumed that the layout of the beams and columns and
the design ultimate gravity loads on the beams are known.
The first step is to define the imperfections of the frame. These arise
mainly from lack of verticality of columns, but also have to take account
of lack of fit between members, effects of residual stresses in steelwork,
and other minor influences, such as non-uniform temperature of the struc-
ture. The term 'column' is here used to mean a member that may extend
