Table Of ContentWELDING OF TUBULAR STRUCTURES
Proceedings of the Second International Conference held in Boston. Massachusetts.
USA. 16-17 July 1984 under the auspices of the International Institute of Welding
SOUDAGE DES STRUCTURES TUBULAIRES
Communications presentees ä la Deuxieme Conference Internationale tenue ä Boston.
Massachusetts, les Etats-Unis. les 16 et 17 Juillet 1984 sous les auspices de I'Institut In
ternational de la Soudure
1984
Published on behalf of the
INTERNATIONAL INSTITUTE OF WELDING
BY
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Copyright © 1984 International Institute of Welding
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Other Pergamon Titles of Interest
ALLSOP a KENNEDY Pressure Diecasting. Part 2
ASHBY S BROWN Perspectives in Creep Fracture
ASHWORTH Ion Implantation into Metals
ATTERAS Underwater Technology: Offshore
Petroleum
BARRETT a MASSALSKI Structure of Metals. 3rd Edition
CARLSSON Mechanical Behaviour of Materials.
2 Vols. (ICM 4)
COMINSaCLARK Specialty Steels and Hard Materials
FROST a ASHBY Deformation-Mechanism Maps
GIFKINS Strength of Metals and Alloys (ICSMA 6)
HENLEY Anodic Oxidation of Aluminium and its
Alloys
HULL Introduction to Dislocations
IIW The Physics of Welding
IIW Underwater Welding
MANN Bibliography on the Fatigue of
Materials. Components and Structures.
Volume 3
MASUBUCHI Analysis of Welded Structures
NIKU-LARI Advances in Surface Treatment 1
OSGOOD Fatigue Design. 2nd Edition
PARKIN a FLOOD Welding Craft Practice
UPTON Pressure Diecasting. Part 1
Pergamon Journals of Related Interest
Acta Metallurgica
Canadian Metallurgical Quarterly
Engineering Fracture Mechanics
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International Journal of Engineering Science
International Journal of Mechanical Sciences
Materials Research Bulletin
Metals Forum
Ocean Engineering
Scripta Metallurgica
Welding in the World
PREFACE
This volume contains the collection of papers to be presented at the Second Inter
national Conference organized by the International Institute of Welding (IIW) on 16
and 17 July 1984 at the IIW Annual Assembly in Boston (United States). The theme
of this year's conference is Welding of Tubular Structures, and sessions on Applica
tion, Fabrication and Design of Tubular Structures have been developed. Abstracts
were submitted from various authors around the world from which forty-six were
accepted for publication.
The theme is introduced in the Houdremont Lecture by Mr. Peter W. Marshall
(United States) in his presentation "Connections for Welded Tubular Structures"
and is developed by the Session Chairmen, Mr. Amiot (France)—Application, Mr.
Crick (United States)—Fabrication, Dr. Wardenier (Netherlands)—Design, the ses
sion moderators and individual speakers.
This conference has been arranged by an Organizing Committee comprising
members of IIW Commissions XIII, "Fatigue of Welded Components and
Structures," and XV, "Fundamentals of Design and Fabrication for Welding," and
various industry people knowledgeable on the subject. The aim of the Organizing
Committee is to assemble the maximum amount of scientific and technical infor
mation on welding of tubular structures, to place this information at the disposal
of interested parties, and to promote within the IIW and member countries the
studies required to ensure progress in the relevant subjects.
The holding of the conference in the United States, where welding of tubular
structures is of prime importance, demonstrates the concern of IIW to concentrate
its activities on contemporary problems whose urgency justifies an appeal for inter
national cooperation.
BRUNO L. ALIA
Chairman
Organizing Committee
PREFACE
Le present recueil contient 1'ensemble des contributions qui doivent etre
presentees ä la deuxieme Conference Internationale organisee par I'lnstitut Interna
tional de la Soudure (IIS) les 16 et 17 juillet 1984 ä l'occasion de l'Assemblee an-
nuelle de l'IIS ä Boston (Etats-Unis). Le theme de cette Conference est le soudage
des structures tubulaires et des sessions sur les applications, la fabrication et la
conception des structures tubulaires ont ete prevues. Des resumes ont ete pro
poses par divers auteurs de par le monde, dont 46 ont ete acceptes pour etre
publies.
Le theme est introduit par la Conference Houdremont faite par M. Peter W. Mar
shall (Etats-Unis) sous le titre "Assemblages pour structures tubulaires soudees" et
est developpe par les Presidents des sessions, M. Amiot (France) pour les applica
tions, M. Crick (Etats-Unis) pour la fabrication et le Dr. Warnenier (Pays-Bas) pour la
conception, ainsi que par les animateurs des sessions et les auteurs eux-memes.
La preparation de cette Conference a ete assuree par un Comite d'Organisation
comprenant des membres de la Commission XIII "Fatigue des elements et construc
tions soudes" et de la Commission XV "Conception et execution des constructions
et calcul des assemblages" de l'IIS et diverses personnalites competentes de In
dustrie. L'intention du Comite d'Organisation a ete de rassembler le plus possible
d'informations scientifiques et techniques sur le soudage des structures tubulaires,
de mettre ces informations ä la disposition des milieux interesses et de promouvoir,
au sein de l'IIS et des pays membres, les etudes necessaires pour faire progresser
les sujets qui s'y rattachent.
Le fait que cette Conference se tienne aux Etats-Unis oü le soudage des structures
tubulaires est de premiere importance demontre le souci de l'IIS de concentrer ses
activites sur des problemes actuels dont l'urgence justifie un appel ä la cooperation
internationale.
Bruno L. Alia
President
Comite d'Organisation
1984 HOUDREMONT LECTURE
CONNECTIONS FOR WELDED TUBULAR STRUCTURES
by
Peter W. Marshall
Shell Oil Company
USA
Professor Dr.-Ing. Eduard Houdremont was a metallurgist at the Fried.
Krupp Co. in Germany during the 1930*s, 40*s and 50fs. His competence
in the broad fields of steel production and steel processing was recog
nized by experts from many countries. He was frequently called upon
when difficult problems had to be solved, and took pleasure in putting
his knowledge at the disposal of the welding community.
Today's lecture is being given in Prof. Houdemont's honor by a struc
tural engineer whose knowledge of welding metallurgy has been gained
mostly from experts with Armco Steel Company. These men, who followed
Prof. Houdremont's example, and whose contributions to the technology
of tubular connections, particularly for applications to offshore
structures, should also be acknowledged.
Prof. Houdremont's special place in the pantheon of IIW derives from
his many lectures and publications and his role in the German Welding
Society, DVS, in gaining acceptance for DVS as a member of the Inter
national Institute of Welding. As Vice President of IIW, he prepared
the way for the 1957 Annual Assembly to be held in Essen, Germany;
where as chairman of Comm. IX he also conducted a session on welding
metallurgy. Although he died of a sudden heart attack shortly there
after, the record of his tireless effort serves as an example for all
of us who tend to volunteer for too much.
APPLICATIONS OF TUBULAR STRUCTURES
Tubular members enjoy an efficient distribution of their material,
particularly in regard to beam bending or column buckling about
multiple axes. For exposed applications, the clean lines of a closed
section are aesthetically pleasing, and minimize the amount of surface
area for dirt, corrosion, or other fouling. With circular tubes, re
duced forces also apply for loadings due to wind and waves.
Tubular columns are widely used in high-clearance single story build
ings, such as shopping malls and warehouses, where radius of gyration
is more important than section area.
1
Tubular designs are also widely used for such long span structures as
expressway overhead signs, pedestrian bridges, booms for construction
cranes and mining draglines, drilling derricks, radio masts, and the
like.
Tubular space frames are used for such dramatic architectural applica
tions as long-span roofs, atrium skylights, radio-telescope dish
antennas, Olympic ski-jumps, and wildly looping amusement park rides.
Here square and rectangular tubes offer simple welded connections in
orthogonal planes, while circular tubes offer simple welded connections
in any plane desired. Unfortunately, the potential elegance of these
structures is often ruined because of problems with the connections.
The designer may lack confidence in simple direct welded connections,
and devise an awkward gusseted joint to do the same job. The fabri
cator may unwittingly take on such a job, unprepared for the special
ized layout, cutting, fitting, welding, and inspection tasks involved.
The erector may insist on bolted field connections. Finally, the job
may get bogged down in a dispute with officials who do not understand
the technology, either.
Thousands of tubular structures have been built for offshore oil
drilling and production since 1947. Typically, these consist of a
tubular space frame, or jacket, which extends from the sea floor to
just above the sea surface. This is usually fabricated in one piece
at an onshore site, transported by barge, launched at sea, and upended
on site by partial flooding. Tubular piling are driven through the
jacket legs to resist vertical service loads and lateral storm loads.
To complete the structure, a deck section is added, usually a composite
of tubular members and conventional rolled sections.
Tubular construction is also used for the lattice legs of jack-up
drilling units, and for the interconnecting space frame of column
stabilized floating units.
Early development of offshore technology was on a trial and error
basis. Braces were simply welded to the jacket legs, which served as
the main member at the tubular connection without any reinforcement.
After several hurricanes, some recurring failure modes became apparent,
concentrated in these simple connections. As will be discussed in the
section on design, these include local punching-shear/pullout failure
in the main member, general collapse of the main member, unzipping of
the weld, and lamellar tearing. Materials problems also included poor
weldability and brittle fracture. Although fatigue failure has been an
ongoing concern of research over the last 20 years, this geriatric mode
of failure has only recently been observed in actual structures.
These problems are covered by the "Tubular Structures" section of the
American Welding Society Dl.l Structural Welding Code - Steel. Much of
the technology from which this part of code evolved was developed in
recent decades by the offshore oil industry. This is also the author's
background.
DESIGN OF CONNECTIONS
Even today, despite the availability of codes of practice like AWS
Dl.l, welded tubular connections have a certain mystique about them.
This is no doubt enhanced by a number of spectacular failures which
have occurred.
2
Some result in structural collapse, while others spell financial ruin
for the contractor involved. Often, when a welded tubular connection
fails, the fracture is at the toe of the weld, in the heat affected
zone. The designer involved may sieze upon this fact to attribute the
failure to faulty materials or welding. Sometimes elaborate metallur
gical witch hunts are staged to bolster this claim. Never mind that
the weld toe is also the site of stress concentrations which are so
high that most practical connections experience localized plastic
straining before reaching the design load. The lawyers and their
witnesses will get rich, and the mystique grows.
Perhaps to a larger degree than with other types of structure, welded
tubular connections require an integrated approach to fracture control.
Design, material selection, fabrication, welding, and inspection must
be considered together. Responsible design includes more than using
stress analysis calculations to dimension the structural elements.
The designer must understand the demands he implicitly places on the
materials to be used, beyond yield strength and availability; and he
must anticipate the methods of fabrication and welding, their limita
tions, and their effects on service performance. The designer who
does not look beyond the code formulas is a failure waiting to happen.
If only to protect themselves, the practical materials and welding
people who follow in executing his design should also understand what
demands are being placed upon their specialty as part of the overall
fracture control picture.
Although many different schemes for stiffening tubular connections have
been devised, the simplest is to simply weld the branch member to the
outside surface of the main member (or chord). Where the main member
is relatively compact (D/T less than 15 or 20), and the branch member
thickness is limited to 50% or 60% of the main member thickness, the
connection will be sufficiently strong to develop the full static capa
city of the members joined, without performing detailed engineering
calculations (Sherman, 1976). Where these conditions are not met, e.g.
with large diameter tubes, a short length of heavier material (or joint
can) is inserted into the chord to locally reinforce the connection
area. Here, the design problem reduces to selecting the right combina
tion of thickness, yield strength, and toughness for the joint can.
Elastic Analysis
An example of the very high local stresses which can exist in tubular
connections can be found in Fig. 1. Despite the fact that the joint
can here is twice the branch member thickness, the peak local stress
is 7.4 times as high as the nominal axial stress in the branch member.
At first yield, this connection only develops 14% of the branch member
capacity.
In this T-connection, the peak local stress is also 14.7 times as high
as the nominal acting punching shear stress given by:
t
acting V = f * * sin Θ (eqn. 1)
P a
T
Stated differently, localized yielding will occur when the punching
shear stress is only 7% of the material yield strength, for this parti
cular connection geometry. With cyclic loading at higher levels of
3
load, repeated plasticity in the area of peak localized stress may
actually generate heat; hence the term "hot spot".
The results shown are from the analytical shell theory solution of
Dundrova (1966). They are confirmed by finite element solutions (e.g.
Greste, 1969, and Reimer, 1976), and by experimental stress analyses
(e.g. Toprac, 1968, and Grigory, 1969).
Several features of the stress distribution in the connection of Fig. 1
are worth noting. Locally, chord stress peaks at the intersection of
tubes, where the branch member delivers its line load to the chord.
Around the circumference which is at the branch member centerline, the
stress pattern is remarkably like that of a closed ring (Roark, 1954)
to which a concentrated load is applied at its intersection with the
branch member. Indeed, due to the ovalizing flexibility of the chord,
the branch member delivers most of its load at the hot spot saddle
position rather than at the lower stressed crown position, causing an
uneven distribution in load transfer across the weld. Finally, the
ovalizing tendency persists for a considerable distance along the
chord. At one diameter from the branch member centerline (the 4th
ring), the stresses have a sinusoidal pattern, with positive values in
the plane of the member centerlines, negative values in the plane at 90
degrees rotation, and magnitudes up to 40-60% that of the most highly
loaded ring at the same angular positions.
Figure 2 gives the results of a parameter study on the elastic strength
of tubular T-connections (Caulkins, 1968). Connection geometry is ex
pressed in terms of dimenslonless parameters beta (d/D) and gamma (R/T),
Capacity is expressed as punching shear stress at first localized
yielding in the chord, normalized by the material yield strength (e.g.
0.07, or 7% efficiency, for the connection of Fig. 1, having d/D=0.5
and R/T=20).
In all of these connections, localized yielding (primarily shell bend
ing at the hot spot) occurs well before the punching shear stress
reaches the actual shear strength of the material (57% of yield). In
the mid-range of betas, 0.3 to 0.7 as often used in space frame struc
tures, the influence of beta is rather flat, and the punching shear
capacity is dependent mainly on gamma, with high R/T producing weaker
connections. As beta approaches unity (equal diameter members), the
branch member delivers much of its load tangentally onto the chord,
rather in punching action, and the relative capacity of the connection
increases dramatically. For very small beta, the capacity approaches a
constant value as for a point loaded plate or shell, corresponding to
higher nominal punching shear values as the perimeter decreases.
Although punching shear may oversimplify the complex pattern of shell
bending and membrane stresses in tubular connections, eqn. 1 directly
expresses the primary importance of thickness ratio t/T on their
efficiency, as well as the fact that it is loads perpendicular to the
chord wall that cause most of the local stress problems.
One should be careful not to generalize the behavior of tubular connec
tions too much based on the behavior of only T-connections. Figure 3
shows two plots of longitudinal and circumferential chord stress in a
K-connection having the same member sizes and the same applied branch
member nominal stress as the T-connection shown earlier in Fig. 1.
Results for all the methods of analysis cited earlier are also shown.
4
The pattern of circumferential stress, section C-D, retains the same
characteristic shape as seen in the T-connection; however, the peak
hot spot stress is only 35% as high. In addition to the high circum
ferential stresses at the chord saddle position, there are now equally
high longitudinal stresses in the chord at the crown position, in the
gap region between the two branch members (section A-B, near end A).
Since punching shear is proportional to the gradient of shell bending
stresses in the chord, we see indications that much of the load is
being transferred in the gap region. This load transfer across the
gap, with the inward punching load of brace B partly offsetting the
outward punching load of brace A, is responsible for the reduced chord
ovalizing tendendy, lower chord stress, and greater efficiency of the
K-connection. The other crown position, near point B, has lower chord
stresses and stress gradients, with less load being transferred across
the weld, more like the T-connection.
Comparison of the various methods of analysis is also of interest.
They all give essentially the same picture of circumferential chord
stresses, which arise locally due to the overall geometry of the
connection and the loading pattern imposed on the branch members.
Indeed, the scatter between replicate experimental measurements is
larger than the scatter between various methods. Experimental stress
analyses can be very sensitive to small eccentricities and indeter-
minancies in the loading arrangements.
For longitudinal stress, the Dundrova shell theory has some difficulty
reproducing the complex pattern and sharp peaks with its trigonometric
series expansion. Including additional boundary element terms which
peak sharply at the intersection would probably improve this approach.
Shell theory retains an advantage over finite element methods in that
the time consuming effort of mesh generation is eliminated; this effort
becomes particularly burdensome for complex multiplanar connections
where automated mesh generation schemes have not yet been developed.
Both thin shell and thick shell finite element analyses reproduce the
overall pattern of stresses in the chord. However, near the weld,
which is the region of interest for hot spot stress, the thick shell
modelling is more realistic. A typical thick shell mesh (for the K-
connection of Fig. 4) is shown in Fig. 4. Section A-A details
modelling near the weld. With judicious choice of element size adja
cent to the weld, the Gauss-point surface stress (GPSS) corresponds to
the location of the strain gage which measures hot spot stress
(American definition). The European definition of hot spot stress
(linear extrapolation to the toe of the weld) is also entirely consis
tent with this kind of modelling. In neither case does the finite
element analysis reflect local or microscopic notch effects at the toe
of the weld; these will be discussed later under the subject of
Fatigue.
Ultimate Strength
The elastic stress analyses just described are useful in understanding
the general behavior of tubular connections, and are indispensible in
fatigue analysis. However, they do not provide a practical measure of
ulimate strength for static design, as most tubular connections have a
tremendous reserve strength beyond first yield.
5
