Table Of ContentThe International Institute of Welding
Fatigue
Design Procedure
for Welded
Hollow Section
Joints
IIW Document XIII-IS04-99 and IIW Document XV-I035-99
Recommendations ofIIW Subcommission XV-E
Edited by
Xiao-Ling Zhao and Jeffrey A Packer
ABINGTON PUBLISHING
Woodhead Publishing Ltd in association with TWI Ltd
Cambridge England
Published by Abington Publishing,
Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington,
Cambridge CB21 6AH, England
www.woodheadpublishing.com
First published 2000, Abington Publishing
© 2000, The International Institute of Welding
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British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library.
ISBN-13: 978-1-85573-522-4
ISBN-10: 1-85573-522-9
Printed by Victoire Press, Cambridge, England.
Preface
These recommendations for the fatigue design of directly-welded, unstiffened, joints (or
connections) between structural steel hollow sections, in planar or multi-planar truss-type
structural systems, have been compiled by the volunteer membership of Subcommission
XV-E of the International Institute of Welding (IIW). The recommendations represent a
current consensus of international "best practice", with the fatigue design procedure being
based on the "hot spot stress approach". This IIW Document XIII-1804-99/XV-1035-99
represents the second edition of recommended fatigue design procedures for hollow section
joints, and as such it supersedes the first edition IIW Document XIII-l 15 8-85/XV-582-85
published in 1985. The purpose of this current IIW document is to serve both as an
International Standards Organisation (ISO) draft specification as well as a model standard
for national and regional steel structures specifications worldwide.
The following represent the current membership of IIW Subcommission XV -E:
Y.S. Choo National University of Singapore, Singapore
G. Davies (Secretary) University of Nottingham, U.K.
J. Farkas University ofMiskolc, Hungary
P. Grundy Monash University, Australia
K. Jarmai University ofMiskolc, Hungary
Y. Kurobane Kumamoto Institute of Technology, Japan
M. LeFranc Offshore-Design, Norway
P .W. Marshall MHP Systems Engineering, U.S.A.
E. Niemi Lappeenranta University, Finland
J.A Packer (Chairman) University of Toronto, Canada
R. Puthli University of Karlsruhe, Germany
J. Wardenier Delft University of Technology. Netherlands
AM. van Wingerde Delft University of Technology, Netherlands
N.F. Yeomans Corns Tubes, U.K.
N. Zettlemoyer Exxon Production Research Company, U.S.A.
x.-L. Zhao Monash University, Australia
Much of the work on which these IIW recommendations are based was initiated by CIDECT
and financed by CIDECT, ECSC and numerous national and regional public sector funding
agencies. Their contributions are all gratefully acknowledged.
The Recommendations (Part I) and Commentary (Part II) were edited by Dr. X.-L. Zhao at
Monash University (Australian Delegate to IIW Commission XV) and Professor J. A
Packer at University of Toronto (Canadian Delegate to IIW Commissions XIII and XV). The
nw
documents were reviewed by members of Subcommission XV -E, IIW Commission XIII
and IIW Commission XV.
1. Scope and General
1.1 Scope
The recommendations deal with the design and analysis of unstiffened, welded, nodal
joints in braced structures composed of hollow sections of circular or square shape (with
or without rectangular chord) under fatigue loading which meet the following conditions:
1.1.1 steel hollow sections that fulfill the requirements given in Appendix A.
1.1.2 weld details given in Appendix B.
1.1.3 structures using steel grades permitted in Clause 1.5.
1.1.4 hollow section joints defined in Clause 1.6.
1.1.5 square or rectangular hollow sections with a thickness between 4 mm and 16 mm.
1.1.6 circular hollow sections with a thickness between 4 mm and 50 mm.
1.1.7 the stress range is the range of "hot spot" stress.
1.1.8 identical brace (branch) members
1.2 Referenced Documents
The following standards or design recommendations are referred to in the
recommendations:
• DEn: (1993): Background to new fatigue design guidance for steel joints in offshore
structures, Internal Report, Department of Energy, London, UK
• Dijkstra, O.D., van Foeken, R.J., Romeijn, A., Karamanos, S.A., van Wingerde, A.M.,
Puthli, R.S., Herion, S. and Wardenier, J. (1996): Fatigue design guide for circular
and rectangular hollow section multiplanar joints, TNO-Report, 91-CON-R1331,
Delft, The Netherlands
• DEn (1984): New fatigue design guidance for steel welded joints in offshore
structures, Recommendations of the Department of Energy, UK
• Dutta, D., Wardenier, J., Yeomans, N., Sakae, K., Bucak, 6 and Packer, J.A. (1998):
Design guide for fabrication, assembly and erection of hollow section structures,
CIDECT Series "Construction with Hollow Steel Sections", Serial No.7, TUV
Verlag, KOln
• ECCS-TC-6 (1985): Recommendations for the fatigue design of structures, European
Convention for Constructional Steelwork
• EC3 (1992): Design of steel structures - Eurocode 3 Part 1.1: General rules and rules
for buildings, ENV 1993-1-1, European Committee for Standardization (CEN),
London, UK
• Hobbacher, A. (1996): Fatigue Design of Welded Joints and Components, Abington
Publishing, Cambridge, England
• nw (1985): Recommended fatigue design procedure for hollow section joints - part 1
hot spot stress method for nodal joints, nw Doc. XV -582-85 and nw Doc. XIll-
1158-85, nw Annual Assembly, Strasbourg, France
• ISO 630 (1980): Structural steel. First Edition, International Organization for
Standardization
• Niemi, E. (1999): Designer's guide for hot spot fatigue analysis, IIW Doc. XIII-WG3-
06-99, nw Annual Assembly, Lisbon, Portugal
• Packer, J.A., Wardenier, J., Kurobane, Y., Dutta, D. and Yeomans, N. (1992): Design
guide for rectangular hollow section (RHS) joints under predominantly static loading,
3
CIDECT S~ries "Construction with Honow Steel Sections", Serial No.3, TOv
Verlag, Koln, Germany
• Wardenier, J., Kurobane, Y., Packer, J.A., Dutta, D. and Yeomans, N. (1991): Design
guide for circular hollow section (CHS) joints under predominantly static loading,
CIDECT Series "Construction with Hollow Steel Sections", Serial No.1, TOV
Verlag, Koln
• Wardenier, J., Dutta, D., Yeomans, N., Packer, J.A. and Bucak, 6. (1995): Design
guide for structural hollow sections in mechanical applications, CIDECT - Series
"Construction with Honow Steel Sections", Serial No.6, TOV-Verlag, KOln
• Zhao, X.L., Herion, S., Packer, J.A., Puthli, R.S., Sedlacek, G., Wardenier, J.,
Weynand, K., van Wingerde, A.M. and Yeomans, N. (1999): Design guide for
circular and rectangular hollow section welded joints under fatigue loading, CIDECT
- Series "Construction with Hollow Steel Sections", Serial No.8, TOv -Verlag, Koln
1.3 Definitions
For the purpose of the recommendations, the definitions below apply. Definitions
peculiar to a particular Clause are also given in that Clause.
1.3.1 Fatigue: Deterioration of a component due to the initiation and growth of cracks
under fluctuating loads.
1.3.2 Fatigue Life: The fatigue life is generally specified as the number of cycles of
stress or strain ranges of a specified character, that a given joint sustains, before
failure of a specified nature occurs. In these recommendations crack growth
through the wall thickness is considered as failure.
1.3.3 Nominal Stress: The nominal stress is specified as the maximum stress in a cross
section calculated on the actual cross section by simple elastic theory without
taking into account the effect of geometrical discontinuities due to the joint
configuration on the stress.
1.3.4 Sr - N curve: A Sr - N curve gives the relation between the stress range and the
number of cycles to failure. Conventionally the range of stress is plotted on the
vertical axis and the number of cycles on the horizontal axis using logarithmic
scales for both axes. The Sr - N curves given in the recommendations have been
derived from a statistical analysis of relevant experimental data and represent lives
that are less than the mean life by two standard deviations of log N.
1.3.5 Constant Amplitude Fatigue Limit: The stress range for a specific Sr-N curve
when the number of cycles (N) is 5 million or greater.
1.3.6 Cut-Off Limit: The stress range for a specific Sr-N curve when the number of
cycles (N) is 100 million or greater, used in the assessment of fatigue under
variable amplitude loading.
1.3.7 Hot Spot Stress: The "hot spot" is defined as the point along the weld toe where
the extrapolated principal stress has its maximum value. The extrapolation must
be carried out from the region outside the influence of the effects of the weld
geometry and discontinuities at the weld toe, but close enough to fall inside the
zone of the stress gradient caused by the global geometrical effects of the
connection. The extrapolation is to be carried out on the branch or brace (cut and
welded member) side and the chord (continuous member) side of each weld (see
Figure 1.1). In the recommendations the hot spot stress can be determined by
considering the stress normal to the weld toe since the orientation of the maximum
principal stress is normal or almost normal to the weld toe.
1.3.8 Brace or Branch: The term "brace" is interchangeable with the term "branch" in
the recommendations.
4
1.3.9 Stress Concentration Factor (SCF): The stress concentration factor (SCF) is the
ratio between the hot spot stress at the joint and the nominal stress in the member
due to a basic member load which causes this hot spot stress. In joints with more
than one branch each branch has to be considered. Generally stress concentration
factors are calculated for the chord and branch.
1.3.10 Stress Range: The stress range Sr is the algebraic difference between the
maximum and minimum stresses in a stress cycle (see Figure 1.2). The nominal
stress range is based on the nominal stresses while the hot spot stress range is
based on hot spot stresses.
1.3.11 Stress Ratio (R): The stress ratio (R) is the ratio of the algebraic minimum and
maximum stresses in a cycle (see Figure 1.2). Tension is taken as positive and
compression as negative.
Load applied in
the branch
Branch
~--- Saddle point
Weld
Joint nomenclature
Increase in stress due
to overall joint geometry
-r.,lnm,i. .. ,,1 stress
Stress in branch
Extrapolation of nt:>r,mc.t. .. ,..
stress distribution to
weld toe
Chord wall
Stress increase due _--.t:Tt,---I
to weld geometry Brace hot spot stress
Stress distribution in branch
Extrapolation of geometric Branch wall
stress distribution to weld toe
Stress increase due
to weld geometry
Increase in stress
due to overall
Chord hot spot stress
joint geometry
Nominal stress
Chordwali
Stress distribution in chord
Figure 1.1 Hot spot stress definition in nodal joints
5
Tensile t!
stress
R>O
I
R=-1
IWV~r\IR\=rO!~
~ L_ \_~r\ f\ f\ fT
Sr
I VV
Compressive V 1.::
stress ,
Figure 1.2 Stress range Sr and stress ratio R
1.4 Notation
Symbols used in the recommendations are listed below. Where non-dimensional ratios are
involved, both the numerator and denominator are expressed in identical units. The
dimensional units for length and stress in all expressions or equations are to be taken as
millimetres and megapascals (N/mm2) respectively, unless specifically noted otherwise.
=
A cross-sectional area of a member .
=
C factor of chord-end fixity (see Clause 4.1.4)
=
CHS circular hollow section
=
COV short form for Carry-OVer (see Figures D.9 and D.lO)
=
D damage accumulation index
=
L chord length between simple supports or points of contraflexure (also see Figure
D.1)
=
Lr distance from weld toe (Figure C3.1)
Mch =b ending moment in chord member
=
Mipb in-plane bending moment as defined in Figure D.2
Mopb = out-of-plane bending moment as defined in Figure D.2
MCF = Multiplanar Correction Factor (see Tables 4.1 and 5.1)
=
MF Magnification Factor (see Table 3.1)
=
N number of cycles
Nf = number of cycles to failure
=
Ov overlapping percentage of braces (q/p in % shown in Figure 1.5)
Pax = axial force in brace
P axial-force = axial force in brace or chord
=
P ch axial force in chord
R = ratio of minimum to maximum algebraic stress in a cycle (see Figure 1.2)
=
REF short form for REFerence (see Figures D.9 and D.lO)
RHS = rectangular hollow section
=
SCF stress concentration factor
=
SCFK SCF for uniplanar K-joints
=
SCFKK SCF for multiplanar KK-joints
SCFaxial-force-in-brace = SCF for load condition "axial force in brace"
=
SCFaxial-force-in-chord SCF for load condition "axial force in chord"
=
SCFaxial-force-in-COV-brace SCF for load condition "axial force in carry-over brace"
=
SCFaxial-force-in-REF-brace SCF for load condition "axial force in reference brace"
=
SCFb,ax SCF for brace under basic balanced axial loading
=
SCFb,ch SCF for brace under chord loading
=
SCFch,ax SCF for chord under basic balanced axial loading
6
SCFch,ch = SCF for chord under chord loading
=
SCFi,ch = SCF for location i under axial balanced chord loading (i 1, 2, 3, 4)
=
SCFi,cov,ax = SCF for location i under axial loads in COY braces (i 1,2,3,4)
=
SCFi,cov,ipb = SCF for location i under in-plane bending in COY braces (i 1,2,3,4)
= =
SCFi,cov,opb SCF for location i under out-of-plane bending in COY braces (i 1,2,3,4)
= =
SCFj,ref,ax SCF for location i under axial loads in REF braces (i 1,2,3,4)
SCFi,ref,ipb = SCF for location i under in-plane bending in REF braces (i = 1,2,3,4)
= =
SCFi,ref,opb SCF for location i under out-of-plane bending in REF braces (i 1,2,3,4)
SCFipb-in-brace = SCF for load condition "in-plane bending in brace"
=
SCFipb-in-chord SCF for load condition "in-plane bending in chord"
=
SCFipb-in-REF-brace SCF for load condition "in-plane bending in reference brace"
=
SCFj,ax = SCF for location j under axial load in brace (j A, B, C, D, E)
=
SCFj,ipb SCF for location j under in-plane bending in brace (j = A, B, C, D, E)
SCFj,ch = SCF for location j under chord loading (j = A, B, C, D, E)
= =
SCFk ax SCF for location k under axial load in brace ( k brace crown, brace saddle,
chord crown, chord saddle)
= =
SCFk,ipb SCF for location k under in-plane bending in brace( k brace crown, brace
saddle, chord crown, chord saddle)
SCFk,opb = SCF for location k under out-of-plane bending in brace( k = brace crown, brace
saddle, chord crown, chord saddle)
SCFo,b,ax = reference SCF for brace under basic balanced axial load
=
SCFo ch ax reference SCF for chord under basic balanced axial load
=
SCFopb-in-brace SCF for load condition "out-of-plane bending in brace"
SCFopb-in-COV-brace = SCF for load condition "out-of-plane bending in carry-over brace"
=
SCFopb-in-REF-brace SCF for load condition "out-of-plane bending in reference brace"
SNCF = StraiN Concentration Factor
=
Sr stress range, or nominal stress range
=
Sr,axial-force nominal stress range due to axial force
Sr,ipb = nominal stress range due to in-plane bending
=
Sr,opb nominal stress range due to out-of-plane bending
Srhs = hot spot stress range
Wipb = elastic section modulus of a member for in-plane bending
=
Wopb elastic section modulus of a member for out-of-plane bending
=
a throat thickness of fillet weld
=
bo chord width of RHS
do = chord diameter of CHS
=
b width of brace i (RHS)
i
d = diameter of brace i (CHS)
j
=
e joint eccentricity
g = gap length
=
g' g/to
=
ho chord depth of RHS
=
hi brace depth of RHS
=
m ratio of the brace axial load in carry-over plane to that in reference plane (see
Figure 4.7)
p = projected connecting length to chord of overlapping brace (see Figure 1.5)
=
q overlap length (see Figure 1.5)
=
r internal comer radius of RHS
=
to chord wall thickness
=
tj brace wall thickness
7
a = relative chord length (2Udo or 2Ubo)
~ = diameter or width ratio (d/do or b/bo)
'Y = the chord slenderness (d/21o or b/21o)
= angle between planes with braces in multiplanar joints (see Figure D.9)
<!>
'YFf = partial safety factor for fatigue loading
'YMf = partial safety factor for fatigue strength
9 = acute angle between brace and chord axes (in Y-, X- and K-joints)
't = wall thickness ratio (1/10)
=
'V circumferential gap parameter (<1>-2 . arcsin(~))
~ = relative gap (gldo or glbo)
1.5 Materials
The recommendations are valid for both hot-finished and cold-formed steel hollow
sections that fulfil the requirements given in Appendix A. The manufactured hollow
sections should comply with the applicable national manufacturing specification for
structural hollow sections.
1.6 Types of Joints
1.6.1 The joints covered in the recommendations consist of circular or rectangular
hollow sections as used in uniplanar or multiplanar trusses or girders, such as T-,
Y-, X-, K-, XX- and KK-joints (see Figures 1.3 and 1.4). For the definition of gap
and overlap, see Clause 1.6.2.
1.6.2 Definition of Gap and Overlap: The gap (g) is defined as the distance measured
along the length of the connecting face of the chord between the toes of the
adjacent brace members. The overlap (Ov) is expressed as (q/p) x 100% as shown
= = = =
in Figure 1.5 where bl = hI bz h2, tl hand 91 92-
1.6.3 Recommended weld details for hollow section joints are given in Appendix B.
(a) CHS T-Joints (b) CHS Y-Joints (d) CHS K-Joints with gap
(c) CHS X-Joints
(e) RHS T-Joints (g) RHS K-Joints with gap (h) RHS K-Joints with overlap
(f) RHS X-Joints
Figure 1.3 Types ofuniplanar joints covered by the recommendations (RHS are assumed
to be square, although the recommendations are likely applicable to rectangular chord
members, welded to square branch members).
8
(a) CHS XX-Joints
(b) CHS KK-Joints with gap (c) RHS KK· Joints with gap
Figure 1.4 Types of multip lanar joints covered by the recommendations (RHS are
assumed to be square, although the recommendations are likely applicable to rectangular
chord members, welded to square branch members),
/
: do I
JO.
+e
i ::: 1 or 2 (overlapping member)
j ::: overlapped member
=~
Overlap x 100%
p
Figure 1.5 Definition of gap and overlap
9
