Joints - The Behaviour and Design of Steel Structures to EC3 - page 399

Preloaded friction-
grip joint
High-strength bolts
in clearance holes
Close tolerance bolts
in fitted holes
Ordinary structural bolts
in clearance holes

Joint
force

Slip load

Initial slip

Joint deformation

Figure 9.8 Behaviour of bolted joints.

more of the bolts come into play. At higher forces, the more highly loaded bolts
start to yield, and the joint becomes less stiff. Later, a state may be reached in
which each bolt is loaded to its maximum capacity.
It is not usual to analyse this complex behaviour, and instead it is commonly
assumed that equal size bolts share equally in transferring the force as shown in
Figure9.9b(exceptifthejointislong),evenintheserviceloadrange.Itisshown
in Section 9.9.1 that this is the case if there are no clearances and all the bolts
fit perfectly, and if the members and connection plates act rigidly and the bolts
elastically.If,however,theflexibilitiesofthejointcomponentmembersandplates
are taken into account, then it is found that the forces transferred are highest in
theendboltsofanylineofboltsparalleltothejointforceandlowestinthecentre
bolts, as shown in Figure 9.9c. In long bolted joints, the end bolt forces may be
sohighastoleadtoprematurefailure(beforetheseforcescanberedistributedby
plastic action) and the subsequent 'unbuttoning'of the joint.
Shear and bearing joints using close tolerance bolts in fitted holes behave in a
similar manner to connections with clearance holes, except that the bolt slips are
greatlyreduced.(Itwasnotedearlierthatfittedclosetoleranceboltsarenowrarely
used.)Ontheotherhand,slipisnotreducedinpreloadedfriction-gripboltedshear
joints, but is postponed until the frictional resistance is overcome at the slip load
asshowninFigure9.8.Again,itiscommonlyassumedthatequalsizeboltsshare
equally in transferring the force.
This equal sharing of the force transfer is also often assumed for tension force
joints in which the applied force acts out of the connection plane. It is shown in
Section 9.9.2 that this is the case for joints in which the plates act rigidly and the
bolts act elastically.
Welded force joints do not slip, but behave as if almost rigid. Welds are often
assumed to be uniformly stressed, whether loaded transversely or longitudinally.
However, there may be significantly higher stresses at the ends of long welds
parallel to the joint, just as there are in the case of long bolted joints.