Tension members - The Behaviour and Design of Steel Structures to EC3 - page 41

concentrationscausesafavourableredistributionofstress.However,insituations
for which it is doubtful whether the steel will behave in a ductile manner, as
when there is a possibility of brittle fracture of a tension member under dynamic
loads (see Section 1.3.3), or when repeated load applications may lead to fatigue
failure (see Section 1.3.2), stress concentrations become very significant. It is
usual to try to avoid or minimise stress concentrations by providing suitable joint
and member details, but this is not always possible, in which case some estimate
of the magnitudes of the local stresses must be made.
Stress concentrations in tension members occur at holes in the member, and
wheretherearechangesorverylocalreductionsinthecross-section,andatpoints
where concentrated forces act. The effects of a hole in a tension member (of
net width b net and thickness t ) are shown by the uppermost curve in Figure 2.9,
which is a plot of the variation of the stress concentration factor (the ratio of
themaximumtensilestresstothenominalstressaveragedoverthereducedcross-
section b net t ) withtheratiooftheholeradius r tothenetwidth b net .Themaximum
stressapproachesthreetimesthenominalstresswhentheplatewidthisverylarge
( r / b net → 0).
This curve may also be used for plates with a series of holes spaced equally
across the plate width, provided b net is taken as the minimum width between
adjacentholes.Theeffectsofchangesinthecross-sectionofaplateintensionare
shownbythelowercurvesinFigure2.9,whiletheeffectsofsomenotchesorvery
local reductions in the cross-section are given in [5]. Methods of analysing these
and other stress concentrations are discussed in [6].
Largeconcentratedforcesareusuallytransmittedtostructuralmembersbylocal
bearing, and while this produces high local compressive stresses, any subsequent

3 . 5

N o m i n al s t r e s s i s
c a l cu la t e d f o r n e t
area

3 . 0

b net × t

b
2 r b net /2

2 . 5

2 . 0

b

r b net

1 . 5

1 . 0

0 .2 0 . 4 0 . 6 0 . 8 1. 0

0

Ratio r/b net

Figure 2.9 Stress concentrations in tension members.