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FEASIBILITY OF WRAPPED FRP CIRCULAR HOLLOW
SECTION JOINTS
PEI HE1 and MARKO PAVLOVIC2
1Faculty of Civil Engineering and Geosciences, Delft University of Technology, Postbus 5,
2600 AA Delft, the Netherlands
E-mail: P.He@tudelft.nl
2Faculty of Civil Engineering and Geosciences, Delft University of Technology, Postbus 5,
2600 AA Delft, the Netherlands
E-mail: M.Pavlovic@tudelft.nl
The concept of an innovative fiber reinforced polymer (FRP) joining technology by wrapping is
presented as an alternative to traditional welding technology to connect circular hollow section
(CHS) braces (diagonals) to chords. The aim is to considerably enhance fatigue endurance and
corrosion resistance of tubular hollow section joints used in off-shore or hydraulic structures,
and steel bridges. Small-scale wFRP uniaxial and X-joints are tested with monotonic tensile and
cyclic loading. Digital Image Correlation (DIC) technique is used to measure displacements and
strains in order to support hypotheses of joint mechanical behavior. The wFRP joint is created
by glass fiber reinforcement mixed with thermoset resins wrapped around the zone of the joint
between the braces and the chord. The testing results are satisfactory and promising, indicating
that high strength steel (HSS) improves the static behavior of wFRP joints; the challenging
angle does not jeopardize but has a favorable influence on the static and dynamic behavior of
wFRP X-joints. Further research need to be conducted to prove long-term behavior of the
innovative joints under permanent loading and harsh marine environment in off-shore
application.
Keywords: FRP, CHS, wrapped joints, off-shore, fatigue endurance, DIC
1 Introduction
Circular hollow sections (CHS) are extensively used in off-shore structures, hydraulic structures
and steel bridges, as shown in Figure 1, due to excellent durability and high cost efficiency (
Wardenier 2010). However, application of CHS is significantly hampered by the design,
execution and fatigue endurance of joints. In the traditional design method of CHS joints, the
brace and chord members are connected through welding technology and mild steel (MS) is
utilized as fabrication material.
Jacket off-shore supporting structures and steel truss in bridges are subjected to heavy and
cyclic loading, requiring excellent fatigue endurance of the joints. Unluckily, application of
welding reduces fatigue resistance of CHS joints compared to the single member. Main reasons
are reduced fracture toughness of the material in the heat affected zone of the weld, and stress
concentration resulted from weld eccentricity and local weld geometry. Furthermore, welding
costs are predominant in fabrication of off-shore jacket supporting structures.
Utilization of high strength steel (HSS) makes it possible to reduce thickness of joint
members, thus decreasing self-weight of off-shore jacket structures and realizing longer spans of
Proceedings of the 17th International Symposium on Tubular Structures.
Editors:
X.D. Qian and Y.S. Choo
Copyright c
ISTS2019 Editors. All rights reserved.
Published by
Research Publishing, Singapore.
ISBN: 978-981-11-0745-0; doi:10.3850/978-981-11-0745-0 043-cd 292
Proceedings of the 17th International Symposium on Tubular Structures (ISTS 17)
293
steel bridges. However, due to limited fatigue endurance of welded CHS joints, thicker profile is
indispensable and employment of HSS is hindered.
To reach full application potential of CHS, the concept of an innovative fiber reinforced
polymer (FRP) joining technology is proposed as an alternative to traditional welding
technology, as shown in Figure 2. CHS brace members (diagonals) and chord members are
bonded together by FRP wrapping. Superiority of using FRP composite material to create
transition pieces between steel members is that they can be shaped in an optimal manner to
decrease stress concentration at the bonded interface. Furthermore, extremely thin bond lines can
be achieved through direct contact between steel surface and FRP wrapping, reducing the risk of
cohesive failure of wrapped FRP joints. Fatigue endurance of wFRP CHS joints is expected to
improve considerably compared to welded ones due to complete prevention of welding .
Corrosion resistance is also enhanced because corrosion critical region of the joint is protected
by FRP wrapping.
Fig 1. Engineering application of CHS
Fig 2. General configuration of the wrapped FRP joint – an example of K-joint geometry for CHS
members
a) Off-shore wind turbine jacket supporting structures
b) Support structures of hydraulic barriers
c) Truss steel bridges
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2 State of the Art
Fiber reinforced polymers (FRP) consist of continuous high strength fibers embedded in a
polymer matrix (Mallick 2007). The most commonly used fibers are glass fibers, carbon fibers,
and aramid fibers, while thermoset polymers, e.g. unsaturated polyesters, epoxy resins and vinyl
ester resin, are normally selected as matrix material. The strength and stiffness of FRP laminate
are dependent on fiber orientation and fiber volume fraction. Fiber Reinforced polymers (FRP)
have been proved to be effective to rehabilitation, retrofit and upgrade of timber, masonry and
concrete structures in civil engineering field. Interest has increased considerably with regard to
application of FRP on retrofitting of steel structures, e.g. steel girders, pipelines and hollow
sections, etc. (Hollaway 2002, Shaat 2004, Zhao 2007). However, the majority of studies are
limited to strengthening of steel members.
Recently, strengthening of welded tubular joints by FRP have been investigated by several
authors. Jiao and Zhao (2004) investigated the behavior of carbon fiber reinforced plastics
(CFRP) strengthened butt-welded very high strength (VHS) circular steel tubes, and a significant
strength increase was achieved using CFRP–epoxy strengthening technique; Xiao and Zhao
(2012) successfully increased flexural stiffness and fatigue life of cracked RHS-to-RHS T-
connection repaired by CFRP; Aguilera and Fam (2013) successfully utilized bonded FRP plates
to strengthen welded rectangular hollow section T-joints against web buckling induced by
transverse compression; Lesani, Bahaari and Shokrieh (2013, 2014 and 2015) compared
numerical and experimental results of FRP strengthened and un-stiffened tubular welded T/Y
joints under axial compressive loads, showing that FRP wrapping has significant influence on
the ultimate load capacity of the connections and has considerably improved connection
behaviour; Hosseini, Bahaari and Lesani (2019) investigated stress concentration factors (SCF)
in FRP strengthened tubular T-joints subjected to brace axial loading, in-plane and out-of-plane
bending moments, and concluded that FRP strengthening method is effective to reduce the SCFs
and consequently extend the fatigue life cycle of tubular T-joints.
A new idea of non-welded structural hollow section joints adhesively bonded by FRP has
been proposed by Pavlovic (2018), followed by preliminary experiments and comparison of
mechanical behaviour between wrapped FRP CHS uniaxial and X90 joints, and welded ones,
aiming to gain understanding of the new joint and propose future recommendations.
3 Small-scale experiments set-up
The small-scale experiments are conducted in Stevin Lab ϩ of TU Delft - CiTG, to validate
feasibility of the concept of wrapped FRP joints, following the preliminary work conducted by
Pavlovic (2018). This main goal can be further divided into three objectives:
(i) to understand the effect of high strength steel (HSS) on the static behavior of wFRP
uniaxial joints;
(ii) to investigate the influence of angles on the static behavior of wFRP X-joints;
(iii) to further characterize the fatigue behavior of wFRP X-joints and show it’s favorable
performance over welded CHS joints.
The tested CHS joints are mainly two small-scale specimen types: (1) uniaxial joints of HSS
and (MS) hollow section Φ63.5×3.6 mm; (2) X45 and X30 joints of MS Φ60.3×4 mm brace
members and Φ108×5 mm chord members. The joints are connected by wrapping around the
connection area with glass fiber reinforcement and thermoset polymer resin. The series and
geometry of the joints are shown in Figure 3.
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Fig 3. Series and geometry of small-scale wrapped FRP joints
Monotonic static loading and cyclic loading are applied to series of specimens in
displacement and load control regime, respectively. Digital Image Correlation (DIC) method is
utilized to improve displacement and strain measurement feasibility.
4 Discussion of the Results
Firstly the static experimental results of uniaxial joints are presented. Three specimens with HSS
(S690), labelled as Pi 6-1, Pi 6-2, Pi 6-3, and three specimens with (MS) (S355), labelled with Pi
5-1, Pi 5-2, Pi 5-3, are loaded with monotonic tensile force until failure. The corresponding
testing results are shown in Figure 4 and Figure 5.
Figure 4 shows that HSS considerably enhances static behavior of wFRP uniaxial joints,
with both average linear load limit and tensile strength improved by 49% and 75%, respectively,
compared to MS specimens. HSS specimens show more brittle behavior, with the failure
displacement 30% lower than for MS specimens. This is in agreement with the behavior of HSS
vs. MS material. The ductility of the wrapped FRP joints with HSS still shows quite ductile
behavior for an bonded joint.
From the curves of MS specimens, apparent constant load plateau can be observed, with
corresponding tensile stress in CHS equal to 428 MPa (290 kN/677.45 mm2). This stress level
corresponds to yield strength of the tested CHS profiles which is higher than nominal value for
S355 material. Therefore, load plateau of the wrapped FRP joint with MS is caused by yielding
of MS CHS outside the wrapped joint region. This is confirmed by large strain on two sides of
the joint shown in Figure 5a as major strain form DIC results, at the loading stage just before the
final failure. Relatively large plastic yielding strains of approx. 4-5% at the FRP-steel interface
trigger the final debonding failure of the joint, see Figure 5c.
Results of DIC on HSS specimens in Figure 5b show that the yielding of steel outside the
wrapped region does not happen. The major principal strain is distributed gradually over the
region of FRP wrapping right before the failure. The specimen fails due to delamination of the
FRP laminate near the edge of the wrapping area, i.e. the cyan ring at the right-hand side end of
the wrapping in Figure 5b. The failure path is indicated by solid lines in Figure 5d. The
delamination initiates near the edge of the wrapping developing further through the plies toward
the FRP-steel interface at the connection of two CHS profiles in the middle. The ultimate load of
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HSS specimens correspond to stress of 812 MPa in CHS profiles which is close to ultimate
strength of the S690 material.
The conclusion is that for MS and HSS the yielding and ultimate resistance of the CHS profiles,
respectively, can be achieved by uniaxial wrapped FRP joints.
Fig 4. The force-displacement loading curve of wrapped FRP uniaxial joints
Fig 5. Major strain distribution (just before failure) and failure patterns of HSS and MS specimens
It could be expected that smaller angles of wrapped X-joints could jeopardize the
mechanical performance of the joints due to difficult wrapping in the sharp corners. To
investigate the influence of angles on the static behavior of small-scale X-joints, three X45 and
three X30 specimens are loaded with monotonic tensile force until failure. The load-
displacement curves are compared with three X90 specimens tested previously (Pavlovic, 2018)
in Figure 6a. The three series of specimens have the same dimensions of CHS profiles and FRP
wrapping. The conclusion is that the challenging joint angles does not jeopardize but has a
favorable influence on the static behavior of X-joints. Compared to X90 joints, X45 and X30
specimens reach 65% and 135% higher linear load limit, respectively. The tensile strength of
X45 and X30 are 19% and 22% higher compared to X90, respectively. Even the ductility is
a) Major strain distribution of the MS specimen
d) Failure characteristics of the HSS specimen
c) Failure characteristics of the MS specimen
b) Major strain distribution of the HSS specimen
Proceedings of the 17th International Symposium on Tubular Structures (ISTS 17)
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improved by 76% and 34%, respectively. The linear behavior and tensile strength is inversely
proportional to the joint angle.
The distribution of principal major strain for one X45 and one X30 joint from DIC results
are shown in Figure 6b and 6c, just before the failure. Stress concentrations, up to 6% major
strain at the surface of FRP wrapping are dominating in the region of the sharp corner.
Nevertheless, the FRP does not suffer breakage and the final failure in all the tested specimens is
debonding of the brace CHS member from the FRP wrapping. The conclusion is that FRP
wrapped joints with challenging angles can be made to withstand the loads which are even
higher that the yielding resistance of the brace CHS member.
a) Force-displacement curves of X-joints
b) Major strain distribution (the X45 specimen) c) Major strain distribution (the X30 specimen)
Fig 6. Results of static experiments on wrapped X-joints with different angles
In order to characterize the fatigue resistance of wFRP X-joints, three wFPR X45 joints (Pi
1-1, Pi 1-2, Pi 1-3) are tested in cyclic loading regime at different load (stress) ranges (-100~100
kN, 10~110 kN, 15~165 kN). Final failure of joint is not obtained even after 7 mil. cycles for the
loading range corresponding to 143 MPa in the CHS brace member. Instead, a steady stiffness
degradation, up to 40% is observed over the cycles. Therefore, the number of cycles shown in
preliminary S-N curve in Figure 7 is obtained at 10% stiffness degradation of the joint. The
stress used in the S-N curve is calculated as a nominal stress in the brace member, i.e. the load
range divided by the cross sectional area of CHS60.3x4mm profile. Results of the wrapped X45
joint in the S-N curve are compared to an estimated curve for a welded CHS joint of same
dimensions. The stress concentration factor (SCF) and the S-N curve of welded X45 joints are
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calculated based on DNV-RP-C203. The S-N curves indicate superior fatigue endurance of the
wrapped FRP joint over the equivalent welded CHS joint. Cyclic loading on wrapped joints is
stopped at reaching 40% of the joint stiffness degradation. Afterwards, the monotonic static load
is applied to check the residual static resistance of the joint. Approximately 340 kN is reached,
showing no degradation of static resistance of the wrapped X45 joint.
Fig 7. Comparison of preliminary S-N curve of wrapped X45 joints vs. equivalent welded joint
5 Conclusions and Outlook
Innovative wrapped FRP small-scale uniaxial and X-joints of CHS are tested with monotonic
tensile and cyclic loading, to evaluate feasibility of this connection concept. Very satisfactory
and promising testing results are obtained and concluded below:
n HSS significantly improves both linear behavior (49%) and tensile strength (75%) of uniaxial
specimens. It is shown that tensile resistance of the wrapped FRP joint can reach the full
resistance of the steel profile (brace or chord member) even in case of high strength steel CHS.
n The challenging angle does not jeopardize but has a favorable influence on the static behavior
of wFRP X-joints. Compared to X90 joints, X45 and X30 specimens have, 19% and 22% higher
of tensile resistance.
n Wrapped FRP joints show promising fatigue endurance over equivalent welded CHS joints.
Preliminary S-N curve for X-joints under 45 degree angle shows an order magnitude larger
number of cycles to failure. Wrapped FRP joints show steady stiffness degradation with no
degradation of residual static resistance even after 40% of stiffness degradation at 7 mil. cycles.
Further testing is indispensable to prove validation at real scale, and long -term behavior
under permanent loading, harsh marine environmental conditions and UV-radiation. The
ongoing validation by Finite Element Modelling will provide better insight on stress
concentrations, failure modes, with the aim to give input for optimized design for the wFRP
joints.
Acknowledgments
This research is supported by NWO (Netherlands Organization for Scientific Research) under
Demonstrator project “Fatigue resistant Wrapped FRP joints of structural hollow sections”, proj. no. 16949.
The authors grateful acknowledge provision and fabrication of the wrapped FRP joints by Versteden b.v.
and Ask Romein b.v. The authors are very grateful for the assistance of technicians from Steven Lab ϩ of
TU Delft.
References
Proceedings of the 17th International Symposium on Tubular Structures (ISTS 17)
299
Aguilera J. and Fam A., Bonded FRP Plates for Strengthening Rectangular Hollow Steel Section T-Joints
Against Web Buckling Induced by Transverse Compression, Journal of Composites for Construction,
17(4), 421-432, August, 2013.
Aguilera J. and Fam A., Retrofitting Tubular Steel T-Joints Subjected to Axial Compression in Chord and
Brace Members Using Bonded FRP Plates or Through-Wall Steel Bolts, Engineering Structures, 48,
602-610, March, 2013.
Hollaway L. C. and Cadei J., Progress in the Technique of Upgrading Metallic Structures with Advanced
Polymer Composites, Progress in Structural Engineering and Materials, 4(2), 131-148, June, 2002.
Hosseini A. S., Bahaari M. R. and Lesani M., Stress Concentration Factors in FRP-Strengthened Offshore
Steel Tubular T-Joints under Various Brace Loadings, Structures, 20, 779-793, August, 2019.
Jiao H. and Zhao X. L., CFRP Strengthened Butt-Welded Very High Strength (VHS) Circular Steel Tubes,
Thin-Walled Structures, 42(7), 963-978, July, 2004.
Lesani M., Bahaari M. R. and Shokrieh M. M., Detail Investigation on Un-Stiffened T/Y Tubular Joints
Behavior under Axial Compressive Loads, Journal of Constructional Steel Research, 80, 91-99,
January, 2013.
Lesani M., Bahaari M. R. and Shokrieh M. M., Numerical Investigation of FRP-Strengthened Tubular T-
Joints under Axial Compressive Loads, Composite Structures, 100, 71-78, June, 2013.
Lesani M., Bahaari M. R. and Shokrieh M. M., Experimental Investigation of FRP-Strengthened Tubular
T-Joints under Axial Compressive Loads, Construction and Building Materials, 53, 243-252, February,
2014.
Lesani M., Bahaari M. R. and Shokrieh M. M., FRP Wrapping for the Rehabilitation of Circular Hollow
Section (CHS) Tubular Steel Connections, Thin-Walled Structures, 90, 216-234, May, 2015.
Mallick P. K., Fiber-Reinforced Composites: Materials, Manufacturing, and Design, CRC press, 2007.
Pavlovic M., Veljkovic M. and Liatzouras M., Wrapped FRP Joints of Steel Hollow Sections, Eighth
International Conference on Thin-Walled Structures, Lisbon, Portugal, July 24-27, 2018.
Shaat A., et al., Retrofit of Steel Structures Using Fiber-Reinforced Polymers (FRP): State-of-the-Art,
Transportation Research Board (TRB) Annual Meeting, 2004.
Veritas D. N., Fatigue Design of Offshore Steel Structures, DNV Recommended Practice DNV-RP-C203,
2010.
Wardenier J., Hollow Sections in Structural Applications, CIDECT, Geneva, 2010.
Xiao Z. G. and Zhao X. L., CFRP Repaired Welded Thin-Walled Cross-Beam Connections Subject to In-
Plane Fatigue Loading, International Journal of Structural Stability and Dynamics, 12(01), 195-211,
January, 2012.
Zhao X. L. and Zhang L., State-of-the-Art Review on FRP Strengthened Steel Structures, Engineering
Structures, 29(8), 1808-1823, August, 2007.
