an effective external reinforcement scheme for circular hollow section joints

7/24/2019 An Effective External Reinforcement Scheme for Circular Hollow Section Joints

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Figure 1. Collar plate reinforced CHS X-joint.

4 parts 2 parts (parallel) 2 parts (perpendicular)

A

C

B

D

Figure 2. Arrangement of collar plate parts.

This paper presents results of numerical studies on the behaviour of CHS T- and X-jointswith collar plate reinforcement. The accuracy of the numerical results is verified against theT-joint tests reported by Choo et al. (1, 2). The results show that significant strengthenhancement for collar reinforced joints can be achieved through proper proportioning of the

reinforcement plate. Selected plots are presented to demonstrate the strength enhancementof X-joints under brace axial compression, in-plane and out-of-plane moments.

COMPARISON WITH REFERENCE TEST RESULTS

Reference information on T-joint Tests

Choo et al. (1) presented results from an experimental programme investigating the strengthenhancement to a simple T-joint by provision of reinforcement around the intersectionregion, in the form of a doubler plate or a collar plate. The experimental programmeconsisted of eight tests with brace axial load with four pairs of tests, each pair with brace

compression and tension. The chord length was chosen such that joint failure occurred priorto chord member failure, with particular reference to recommendations by Zettlemoyer (3).

0

d

t

1t

0

l

t

c

c

d1

0d

t 1

l c

d 1

brace

chord

platecollar

= 2l0/d0 = t1/t0

= d1/d0 c = tc/t0

2= d0/t0 lc/d1

2

1

Detail 1 Detail 2

424 Connections in Steel Structures V - Amsterdam - June 3-4, 2004


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Detailed investigations into the behaviour of the test specimens and strength enhancementoffered by the doubler and collar reinforcement for T-joints, are presented by Choo et al. (2)and van der Vegte et al. (4).

In this paper, the experimental result for the collar reinforced T-joint specimen EX-03 and thecalibration of the nonlinear finite element model are provided for illustration. Details can bereferenced in our papers (2, 4).

Mesh densities and element type

For a particular joint subjected to given loading, an analyst can consider the appropriatesymmetry in geometry, loading and boundary conditions to determine the finite element (FE)model for analysis. For a X-joint subjected to brace axial load, only one-eighth of the jointmodelled (as shown in Fig. 3) with appropriate symmetry conditions and load specification isrequired. For each FE model, more refined mesh is generated where stress gradient is morecritical. The automatic mesh generator for reinforced joints in this study is an extension ofthat presented by Qian et al. (5).

For the present FE models, two layers of 20-noded solid elements, type C3D20R withreduced integration in ABAQUS (6), are specified through the thickness of all members toprovide good description of possible non-linearity in the thickness direction. Depending onthe actual joint geometry, 500 to 1000 elements are created to represent one-eighth of a

whole joint. Such mesh density has been proven to be able to produce results with goodaccuracy (7).

Weld geometries

As three-dimensional solid elements are used in the FE models, it is possible to simulate theweld geometries with high accuracy. The actual geometric definition of the welds is includedin all FE models. The geometry of the penetration weld between the brace and the chord ismodelled following the American Welding Society (8) recommendations. The depth of thefillet welds between the reinforcing plate and the chord surface is taken the same as thethickness of the plate, with two layers of finite elements specified. The welds connecting thecollar plate and chord (along the chord circumferential or longitudinal directions) are notexplicitly modeled. These are reflected in the FE model by specifying the appropriatespatially common nodes to be tied.

Figure 3. FE model for one eighth of a collar plate reinforced CHS X-joint

2 = 50.8, = 0.64, lc/d1 = 1.50 and c= 1.0.

Y

X

Z

Connections in Steel Structures V - Amsterdam - June 3-4, 2004 425


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Geometric and material specifications

The geometrical non-linearity is included to predict possible buckling in the chord wallthrough the NLGEOM parameter in the *STEP option in ABAQUS input file. The materialnonlinearity is specified using the true stress and associated logarithmic strain to define theplasticity with isotropic hardening (6).

Contact interaction

When a collar plate reinforced joint is loaded, contact may occur between the bottom of thecollar parts and the chord outer surface. The contact interaction plays an important role inthe load transferring mechanism of plate reinforced joints and thus non-linear contactanalysis is required. Since both of the reinforcing plate and the chord wall are deformablebodies, a deformable-deformable contact interaction was defined using a master-slavealgorithm in the numerical analysis (6).

Comparison between test and FE results

Fig. 4a shows the cut-section of the collar-reinforced Specimen EX-03 after completion ofthe test. It can be observed that the collar reinforcement has relocated the chord plastichinges away from the brace-chord intersection, and that the brace has deformed extensivelyadjacent to the intersection. Fig. 4b shows the deformed shape predicted by the nonlinearFE analysis, and very good agreement with the experimental result is observed.

Figure 4. Comparison between test and FE results, (a) Cut-section of EX-03 after test, and(b) FE prediction.

The load-ovalisation curves (in which ovalisation at particular load level is based on thechange in diameter of the chord section) for Specimen EX-03 are shown in Fig. 5. Thenumerical prediction is found to correspond very closely with the experimental curve, and

this serves to verify the accuracy of the numerical method.

Programme set-up

Parametric studies to investigate the static strength of collar plate reinforced X-joints havebeen conducted by the authors. The chord diameter of all joints was taken as do=508 mm,

with varying from 0.25 to 0.80 (= 0.25, 0.43, 0.64 and 0.80), = 12, and 2= 31.8 and

50.8. The brace-to-chord thickness ratio =1.0, and the brace length was kept at 4d1. Thethickness of the reinforcing plate was assumed equal or larger than the chord wall thickness

t0. For each combination of 2 and ratios, three values of plate thickness parameter

(c=1.00, 1.25 and 1.60) and five values of plate length parameter (lc /d1=1.25, 1.5, 2.0, 2.5

and 3.0) have been considered. The corresponding un-reinforced joints were also includedto provide the appropriate reference strength. A total of 8 un-reinforced joints and 120 collar

a b



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reinforced joints were analyzed, with each joint subjected to brace axial compression, in-plane moment or out-of-plane moment separately.

The un-bent collar plate was assumed to be square in shape, except for large cases,where the plate width exceeded half the perimeter of the chord section, and for this case, the

plate width was limited to half the chord perimeter with welds along its edges.

In the following sections, selected results shown for the various loading conditions are

focussed on joints with 2= 50.8 and = 0.25 and 0.64.

Figure 5. Experimental and numerical load-ovalisation curves for EX-03.

STRENGTH OF REINFORCED X-JOINT UNDER AXIAL COMPRESSION

Failure mechanisms and load-indentation curves

Fig. 6a to 6b show the deformed shapes of two collar reinforced X-joints subjected to axialbrace compression. Due to the weld at the brace-chord intersection, and the collar-chordsegments along the longitudinal (crown) and circumferential (saddle) segments, the collarplate is effective in stiffening the chord and enhancing the load transfer from the brace.

Figure 6. Deformed shapes of collar reinforced X-joints with lc=2.0d1and 2=50.8 with(a) = 0.25, (b) = 0.64.

0 20 40 60 80 100 1200

100

200

300

400

500

Load[kN]

Ovalisation [mm]

=0.54, 2=50.6, Collar, Compression

Experimental

Numerical

= 0.64

= 1.00lc = 2.00d1c= 1.00

= 0.25

= 1.00lc = 2.00d1c= 1.00



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In Fig. 7a and 7b, the non-dimensionalised loads F/fy0t02for the joint with = 0.25 and 0.64

and plate sizes lc=1.25d1to 2.5d1are plotted against the displacement /d0, where is theindentation of the chord wall at the crown position. It can be seen that significant strength

enhancement is achievable for plate reinforced joints. For a joint with = 0.64 and lc=2.5d1,a jump in joint strength can be observed when the collar plate width reaches half of the

chord section perimeter due to a more direct and effective load transfer mechanism throughthe welds.

0.00 0.02 0.04 0.06 0.080

5

10

15

F/fy0*t0

2

/d0

Unreinforced joint

lc/d1=1.25 =12.0

lc/d1=1.50 2=50.8

lc/d1=2.00 c=1.00

lc/d1=2.50 =0.25

0.00 0.02 0.04 0.06 0.080

10

20

30

40

F/fy0*t0

2

/d0

Unreinforced joint

lc/d1=1.25 =12.0

lc/d1=1.50 2=50.8

lc/d1=2.00 c=1.00

lc/d1=2.50 =0.64

Figure 7. Normalised load-indentation curves for collar reinforced X-joints with different plate

width to brace diameter ratios (a) = 0.25 (b) = 0.64.

The deformation limit proposed by Yura et al. (9), which is defined as 60fyd1/E, is adopted todetermine the ultimate strength of a joint without a pronounced peak value in the load-displacement curve. It is noted that the collar plate reinforcement can provide substantialstrength enhancement to the joint.

Effects of cand lc/d1

Fig. 8a and 8b present the strength enhancement due to provision of collar plate for joints

with 2=50.8 and =0.25 and 0.64, with the corresponding un-reinforced joint strength as

reference strength. Each of the strength ratios is plotted against the plate parameters cand

lc/d1 in a three-dimensional diagram for each . As noted in Fig. 8b, the reinforced joint

strength, obtained by the provision of an appropriately dimensioned collar plate, can be up to3 times of the strength of an un-reinforced joint. The strength of a collar plate reinforced jointmay be improved either by increasing the collar plate length or by using a thicker plate. Forjoints with small values of lc/d1, the effect of the plate thickness is insignificant. The effect ofplate thickness becomes more important as the collar plate length increases.

BEHAVIOUR OF REINFORCED X-JOINTS UNDER IN-PLANE BENDING

In this section, the failure mechanisms for un-reinforced and collar reinforced X-joints underin-plane bending are presented to highlight the differences. The geometric parameters of the

joints considered are 2 =50.8 and = 0.25 and 0.64. More details are reported by Choo etal. (10).



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1.01.2

1.41.6

1.0

1.2

1.4

1.6

1.8

2.0

2.2

1.5

2.0

2.5

3.0

1.25

2=50.8 =0.25

Fu,c

/Fu,u

l c/d

1

c

1.01.2

1.41.6

1.0

1.5

2.0

2.5

3.0

3.5

1.5

2.0

2.5

3.0

1.25

2=50.8 =0.64

Fu,c

/Fu,u

l d/d

1

c

Figure 8. The effects of dand ld/d1 on the strength of axially loaded collar plate reinforced

X-joints with 2 = 50.8 (a) = 0.25 (b) = 0.64.

Failure mechanisms

Fig. 9a and 9d show the deformed shapes of collar plate reinforced joints with different

combination of and lc/d1. The collar plate reinforced joint is observed to fail with relativelylarge plastic zones formed near the brace-chord intersection. Because of the welds betweenthe collar plate parts and the chord surface parallel to the chord axis, the collar plate actsclosely with the chord wall on both compressive and tensile sides.

For joints with short collar plates (Fig. 9a and 9c), plastic hinges are observed near thewelds between the collar plate and the chord. The strength enhancement due to the short

collar plate may be regarded as an equivalent increase in . No obvious plastic hinge is

found for a joint with long collar plates (Fig. 9b and 9d).

Figure 9. Deformed shapes of collar plate reinforced X-joints under in-plane bending.

a = 0.25lc = 1.25d1

b = 0.25lc = 2.00d1

c = 0.64lc = 1.25d1

d = 0.64lc = 2.00d1



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Fig. 10a and 10b present the strength enhancement due to provision of collar plate for joints

with 2=50.8 and =0.25 and 0.64. As noted in Fig. 10b, the reinforced joint strength,obtained by the provision of an appropriately dimensioned collar plate can be up to 2.8 times

of the strength of an un-reinforced joint. For joints with small values of lc/d1, the effect of theplate thickness is insignificant. The effect of plate thickness becomes more important as thecollar plate length increases and more deformation of the collar plate takes place.

1.01.2

1.41.6

1.0

1.5

2.0

2.5

3.0

1.5

2.0

2.5

3.0

1.25

2=50.8 =0.25

Mi,u,c

/Mi,u,u

l c/d

1

c

1.01.2

1.41.6

1.0

1.5

2.0

2.5

3.0

1.5

2.0

2.5

3.0

1.25

2=50.8 =0.64

Mi,u,c

/Mi,u,u

l d/d

1

c

Figure 10. The effects of cand lc /d1 on the strength of collar plate reinforced X-joints under

IPB with 2 = 50.8 (a) = 0.25 (b) = 0.64.

STRENGTH OF REINFORCED X-JOINTS UNDER OUT-OF-PLANE BENDING

Failure mechanisms

Fig. 11a and 11b show the deformed shapes of collar plate reinforced joints loaded by out-of- plane bending. It can be observed that the weld connecting the collar plate to the chord,from the saddle positions along the chord circumferential direction is effective in transferringthe brace moment.

Figure 11. Deformed shapes of collar plate reinforced X-joints under out-of-plane bending

with 2 =50.8 (a) = 0.25 (b) =0.64.



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Fig. 12a and 12b show the potential strength enhancement for collar plate reinforced X-joints. It can be seen that the strength ratio of the reinforced joint to the corresponding un-

reinforced joint varies from 1.6 to 3.6. The plate thickness parameter c and length parameter

lc /d1have significant effects on the strength of the reinforced joints for cases with large lc/d1ratios. Equivalent strength enhancement can be obtained by either increasing the platelength or by using a thicker collar plate.

1.01.2

1.41.6

1.0

2.0

3.0

4.0

1.5

2.0

2.5

3.0

1.25

2=50.8 =0.25

Mo,

u,c

/Mo,u,u

l c/d

1

c

1.01.2

1.41.6

1.0

2.0

3.0

4.0

1.5

2.0

2.5

3.0

1.25

2=50.8 =0.64

Mo,

u,c

/Mo,u,u

l d/d

1

c

Figure 12. The effect of dand ld/d1on the strength of collar plate reinforced X-joints under

OPB with 2 = 50.8 (a) = 0.25 (b) = 0.64.

SUMMARY AND CONCLUSIONS

Extensive numerical studies have been conducted to evaluate the behaviour of circularhollow section (CHS) X-joint reinforced with a collar plate, subjected to axial bracecompression, in-plane bending or out-of-plane bending respectively. From the presentedresults of un-reinforced and collar plate reinforced CHS T- and X-joints, the following may beconcluded:

1. The collar plate is an effective reinforcement scheme, and can improve the static strengthof CHS T- and X-joints considerably.

2. Each of the parameters: the brace-to-chord diameter ratio , the plate-to-chord wall

thickness ratio c, and the plate length-to-brace diameter ratio lc/d1 have significantinfluence on the strength of collar plate reinforced joints.

3. For a reinforced joint with fixed brace and chord dimensions, equivalent strengthenhancement can be obtained by either appropriately increasing the plate length or usinga thicker reinforcement plate.

RECOMMENDATIONS FOR FUTURE RESEARCH

Based on the present studies, the following are possible recommendations for futureresearch on the collar plate reinforced joints:

1. Since only part of the geometric parameters and loading conditions have been covered inthe current study, more extensive parametric studies will provide a comprehensive

understanding of collar plate reinforced joints.



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2. Fatigue analyses of collar plate reinforced joints are proposed. The current studyconcentrated on the static strength of plate reinforced joints. It is important to investigatethe behavior of plate reinforced joints under fatigue loading.

3. Experimental investigations on plate reinforced joints subjected to different loading caseswill provide reliable reference results for parametric numerical investigation. Due to lack ofexperimental data on plate reinforced joints, this study used available test results onreinforced T-joints and published numerical results to verify the numerical methods.

ACKNOWLEDGEMENTS

The authors wish to record their appreciation to Dr Nick Zettlemoyer of ExxonMobilUpstream Research (USA) for initiating the research on reinforced joints in the NationalUniversity of Singapore. They like to thank Professor Jaap Wardenier in Delft University ofTechnology and Professor Richard Liew in National University of Singapore for theircontributions towards the studies.

REFERENCES

1. Choo, Y.S., B.H. Li, G.J. van der Vegte, N. Zettlemoyer & J.Y.R. Liew (1998). Staticstrength of T-joints reinforced with doubler plate or collar plate. Tubular Structures VIII:Proceedings Eighth International Symposium on Tubular Structures, Singapore, pp. 139-145.

2. Choo, Y.S., G.J. van der Vegte, B.H. Li, N. Zettlemoyer & J.Y.R. Liew (2005). Staticstrength of T-joints reinforced with doubler or collar plates - Part I: Experimentalinvestigations. Journal of Structural Engineering, ASCE, Vol. 131, No. 1, pp. 119-128.

3. Zettlemoyer, N. (1988). Developments in ultimate strength technology for simple tubularjoints. Proc. Offshore Tubular Joints Conference (OTJ88), Surrey, UK.

4. van der Vegte, G.J., Y.S. Choo, J.X. Liang, N. Zettlemoyer and J.Y.R.Liew (2004). Staticstrength of T-joints reinforced with doubler or collar plates - Part II: Numericalsimulations. Journal of Structural Engineering, ASCE (accepted for publication).

5. Qian X.D., Romeijn A., Wardenier J. and Choo Y.S. (2002). An automatic FE meshgenerator for CHS tubular joints. Proc. 12thInternational Offshore and Polar EngineeringConference. Kita-Kyushu, Japan.

6. Abaqus/Standard Users Manual Version 6.2 (2001). Hibbitt, Karlsson and SorensenInc., Rhode Island, USA.

7. van der Vegte, G.J. (1995). The static strength of uniplanar and multiplanar tubular T-and X-joints. PhD thesis. Delft University Press.

8. A.W.S. (1996). Structural Welding Code, AWS D1.1-96. American Welding Society Inc.,Miami, USA.

9. Yura, J.A., N. Zettlemoyer & I.F. Edwards (1980). Ultimate capacity equations for tubularjoints. Proc. Offshore Technology Conference, Paper OTC 3690, Houston, U.S.A.

10. Choo Y.S., Liang J.X., van der Vegte G.J., Liew J.Y.R. (2004). Static strength of collarplate reinforced CHS X-joints loaded by in-plane bending. Journal of ConstructionalSteel Research, Vol. 60, No. 12, pp. 1745-1760.


an effective external reinforcement scheme for circular hollow section joints

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