Frames - The Behaviour and Design of Steel Structures to EC3 - page 351

8.3.3 Frames with semi-rigid joints
Semi-rigid joints are those which have dependable moment capacities and which
partially restrain the relative rotations of the members at the joints. The action
of these joints in rectangular frames is to reduce the maximum moments in the
beams, and so the semi-rigid design method offers potential economies over the
simple design method [4-10].An example of a typical semi-rigid joint between a
beam and column is shown in Figure 9.3b.
Amethodofsemi-rigiddesignispermittedbyEC3.Inthismethod,thestiffness,
strength, and rotation capacities of the joints based on experimental evidence are
suggestedin[1],andmaybeusedtoassessthemomentsandforcesinthemembers.
However, this method has not found great favour with designers, and therefore
will not be discussed further.
8.3.4 In-plane behaviour of frames with rigid joints
Arigidjointmaybedefinedasajointwhichhassufficientrigiditytovirtuallypre-
vent relative rotation between the members connected. Properly arranged welded
and high-strength friction grip bolted joints are usually assumed to be rigid (ten-
sioned high strength friction grip bolts are referred to in [1] as preloaded bolts).
An example of a typical rigid joint between a beam and column is shown in
Figure9.3c.Thereareimportantinteractionsbetweenthemembersofframeswith
rigid joints, which are generally stiffer and stronger than frames with simple or
semi-rigid joints. Because of this, rigid frames offer significant economies, while
many difficulties associated with their analysis have been greatly reduced by the
widespread availability of standard computer programs.
The in-plane behaviour of rigid frames is discussed in general terms in
Section 1.4.2, where it is pointed out that, although a rigid frame may behave
in an approximately linear fashion while its service loads are not exceeded, espe-
cially when the axial forces are small, it becomes non-linear near its in-plane
ultimate load because of yielding and buckling effects. When the axial compres-
sionforcesaresmall,thenfailureoccurswhenasufficientnumberofplastichinges
hasdevelopedtocausetheframetoformacollapsemechanism,inwhichcasethe
load resistance of the frame can be determined by plastic analysis of the collapse
mechanism. More generally, however, the in-plane buckling effects associated
withtheaxialcompressionforcessignificantlymodifythebehaviouroftheframe
near its ultimate loads. The in-plane analysis of rigid-jointed frames is discussed
in Section 8.3.5, and their design in 8.3.6.
8.3.5In-plane analysis of frames with rigid joints
8.3.5.1 General
The most common reason for analysing a rigid-jointed frame is to determine the
moments, shears, and axial forces acting on its members and joints. These may