numerical vs. experimental behaviour of bolted dual · pdf filenumerical vs. experimental...

Numerical vs. experimental behaviour of bolted dual-steel T-stub connections

ANA-MARIA POP, DANIEL GRECEA, ADRIAN CIUTINA

Department of Steel Structures and Structural Mechanics Politehnica University of Timisoara

1, Ioan Curea, 300224 Timisoara ROMANIA

[email protected], [email protected], [email protected]

Abstract: - The present research represents the numerical investigation used to characterize the behaviour of bolted dual-steel T-stub connections. The study is based on existing experimental investigation on T-stub components performed at the Politehnica University of Timisoara, using mild and high strength steel. First a calibration of numerical results is performed based on the monotonic experimental response. The challenge of the numerical investigation is represented by the calibration of cyclic curves, using adequate FEM techniques. The third step of the analysis would be the parametric investigation on monotonic response, by changing different characteristics of the T-stub components.

Key-Words: - Numerical behaviour, experimental behaviour, bolted dual-steel, T-stub, monotonic and cyclic loading

1 Introduction The seismic resistant buildings located in seismic zones are designed on the principle of their dissipative behaviour in case of earthquake. This means that the dissipative elements chosen to absorb the seismic input will develop plastic deformations thereby reducing the demand on non-dissipative members. To avoid increasing the size of sections in dissipative and non-dissipative members because it also changes their stiffness, the best way to accomplish this demand is to realize them of MCS (Mild Carbon Steel) and HSS (High Strength Steel) respectively. The structures made from two different steel grades are called Dual – Steel (DS) structure. In this type of structure, Mild Carbon Steel (MCS) – S235 is use in dissipative elements (beams) and High Strength Steel (HSS) – S460, S690, in non-dissipative elements (columns, end-plate). In this way, MCS members have to work like fuses, dissipating the seismic energy through plastic deformation, while the HSS ones, had to remain predominantly elastic, or with limited damage, being responsible for robustness of the structure. Dual Steel (DS) concept is extended to connections, too, on the same philosophy related to ductile and brittle components, in order to achieve both ductility and robustness criteria. In fact, when connect MCS

beams to HSS columns will result a DS beam-to-column joint. Base on Eurocode 8 rules, the dissipative zones could be located either in elements or in beam-to-column joints. In extended end-plate bolted connection, besides the column web, the end-plate in bending becomes very important for ductility. So, end-plate connections could prove adequate rotation capacity if special measures are taken, e.g. use of relatively thin end-plates, avoiding brittle failure of welds and bolts etc. From this point of view, T-stubs are basic components of the component method used in EN 1993-1.8 (Fig.1) for evaluation of strength and stiffness of bolted end-plate beam to column joints. According to Eurocode 3, T-stub macro-component failed down by 3 types of failure mode, named 1, 2 and 3 (Fig.2 and Table 1).

Fig.1. End-plate connection: T-stub element

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Fig.2. Types of T-stub failure modes Table 1. Classification of joints according to T-stub

failure mode Failure mode Ductility Classification

Mode 1 Ductile Partial-strength / Semi-rigid

Mode 2 Semi-ductile

Full strength / Rigid

Mode 3 Fragile Full strength / Rigid

Mode 1 is represents generally a very ductile behaviour and is characteristic to thin end-plates, but cannot be considered a real solution to strength demands in case of seismic resistant structures. On Mode 3 leads usually to a good T-stub resistance, while the failure mode is fragile by bolt rupture. In consequence, the Mode 2 could answer well to both strength and ductility demands. In order to study the performance of dual-steel configuration for beam-to-column joints and starting from the above considerations, a large experimental research program was carried out at the CEMSIG Research Center of the "Politehnica" University of Timisoara. The experimental study was considering full joint specimens, T- stub and weld detail specimens, under monotonic and cyclic loading [1], [2], [3]. The present research focuses on the numerical investigation (by FEM) vs. experimental results of the tested T-stub elements. Validation of the numerical model is followed by a parametric study for monotonic loading and for the cyclic loading the results assets in terms of dissipate energy. 2 Results of experimental program 2.1 Summary of testing program The main objective of the experimental program was to study the performance of welded and bolted end-plate beam to column joints realized from two different steel grades. To accomplish this purpose, the experimental program consisted in experimental investigation on materials, welded components, T-stub components, and beam to column joints. Although the entire research is much larger, this

paragraph describes only the investigations performed on T-stub components, chosen for numerical study. Both monotonic and alternating cyclic tests were performed on T-stub components obtained by welding S235 web plates to S235, S460 and S690 end-plates, using K beveled full-penetration welds (Fig.3 and Table 2). Loading was applied in displacement control under tension and force control under compression.

Fig.3. The geometry of beam-to-column joint Previous papers by the same authors already summarized the results on materials, welded components, weld details and beam-to-column joints [1], [2], [3].

Table 2. T-stub characteristics

Type A

Label t1/ steel grade

t2/ steel grade Welding material

TST-8A-S690 15/S235 8/S690 ER 100S-G/AWS

A5.28/LNM Moniva TST-10A-

S460 15/S235 10/S460 ER 100S-G/AWS A5.28/LNM Moniva

TST-12A-S235 15/S235 12/S235 G3Si/EN 440

Type B

Label t1/ steel grade

t2/ steel grade Welding material

TST-8B-S690 15/S235 8/S690 ER 100S-G/AWS

A5.28/LNM Moniva TST-10B-

S460 15/S235 10/S460 ER 100S-G/AWS A5.28/LNM Moniva

TST-12B-S235 15/S235 12/S235 G3Si/EN 440


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2.2 Results of testing program The measured values of yield stress fy, tensile strength fu and elongation at rupture A are shown in Table 3. Bolts were tested in tension as well, showing an average ultimate strength of 862.6 N/mm2.

Table 3. Material properties

Nominal steel grade

fy, N/mm2

fu, N/mm2

A, %

S235 266 414 38 S460 458 545 25 S690 831 859 13

Fig.4 shows the experimental results under monotonic and cyclic loading by overlapping their response in terms of Force-

Deformation curves. No significant differences in force values between failure modes of monotonic and cyclic specimens were recorded.

Fig.4. Experimental T-stubs results under monotonic an cyclic loading

Also, both results agreed with the analytical predictions computed according to EN 1993-1.8 [4], [5]. In general a good ductility was observed for all specimens. However, thicker end-plate specimens, even for S235 show a smaller ductility. Cyclic loading reduced the maximum resistance of the T-stub specimens, though the reduction was not significant. The ductility of the T-stub specimens was quantified through the ultimate displacement Du. It is to be emphasized that specimens realized from high-strength end plates (S460 and S690, with lower elongation at rupture), had a ductility comparable with the one of specimens realized from mild carbon steel (S235). The parameters governing the ductility of T-stubs were type of loading (monotonic / cyclic) and failure mode (end-plate or bolts) [6]. The choice of thickness associated with steel grade is important in the conception of a proper connection, for obtaining a good balance between strength, stiffness and ductility of components. The conclusions of the experimental study proves that fact. 3 Numerical analysis 3.1 Description of the numerical model Using ABAQUS computer program and starting from idea that FE mesh must be sufficiently refined to produce accurate results and that the number of elements and nodes must be kept as small as possible in order to limit the analysis time, have been defined the following characteristics for the model: • for all T-stub components the type of finite element used was linear 8-nodes solid elements reduced integration (brick C3D8R element); • for material uni-axial response was used a true stress-strain constitutive law (σtrue – εtrue), based on σ-ε engineering law through the fallowing relationship:

σtrue = σ (1+ε) and εtrue = ln (1+ε) (1)

• for analysis was used dynamic explicit type; • the general contact type was used between elements: the tangential component is defined by frictionless formulation while the normal component is defined by a “hard” contact pressure-overclosure;


ISBN: 978-960-474-337-7 194

• load was applied in displacement control at the top of the T-stub; • the mesh of the elements was done using linear hexahedral elements. The overall response of the T-stub is greatly influenced by the bolt behaviour and the geometry of welding. The bolt was modeled with the real geometrical and mechanical characteristics: the shank diameter was corresponding to the threaded part and the pre-tensioning effect was introduced in the bolt shank through an elongation under an axial force corresponding to 50% pre-tension force. Reason for choosing this method is that: in dynamic explicit analysis the option who allows loading the bolt isn't possible, as in dynamic implicit procedure. Fig.5a and Fig.5b shows the results of pre-tensioning effect in terms of Von Mises stress distribution, both for implicit and explicit procedure.

a) Implicit

procedure b) Explicit

procedure Fig.5. Pre-tensioning effect

Based on observation on failures of the double bevel welding during the experimental tests performed in the CEMSIG Laboratory in Timisoara , a special shape of welding geometry was considered. It was noticed that in most cases the welds do not fully penetrate on the full beveled plate thickness (Fig.6b), even if the theoretical recommendation is as full penetration (Fig.6a). Table 4 offers the geometrical values used for the numerical models.

Table 4. The welding geometry

Label Length 1 Length 2

TST-8A-S690 6.5 mm 2.5 mm

TST-10A-S460 6.5 mm 5 mm

TST-12A-S235 4.5 mm 2.5 mm

TST-8B-S690 6.5 mm 2.5 mm

TST-10B-S460 6.5 mm 5 mm

TST-12B-S235 4.5 mm 2.5 mm

a) weld requirement b) FEM weld dimensions

c) FEM dispositions of welds

Fig.6. Double bevel welding of elements

3.2 Analysis results Fig.7 presents the comparison between numerical and experimental results under the form of axial force –deformation

Fig.7. FEM response under monotonic loading


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Base on this comparison, it could be concluded that the FE response follows with high accuracy the monotonic response of experimental specimens. Even the failure modes obtained were identical to the ones obtained experimentally, namely failure mode 2. The calibration serves as basis for the parametrical study which purpose is optimisation of T-stub behaviour. The calibration of cyclic behaviour is a main objective of finding accurate FEM models which take to account specific parameters in cyclic loading: pinching effect, degradation of resistance and so on. The results assets in terms of dissipate energy (Fig.8 and Fig.9) and the numerical analysis is in progress.

Specimen: TST-8A-S690 Nr. of cycles: 35

Energy experimental: 17593

Energy numerical: 14246 [%]: 81.0

Fig.8. T-stub with 8mm thickness of end-plate: FEM response under cyclic loading and energy

dissipated

Specimen:TST-10A-S460 Nr. of cycles: 45

Energy experimental: 34426

Energy numerical: 34582 [%]: 100.5

Fig.9. T-stub with 10mm thickness of end-plate: FEM response under cyclic loading and energy

dissipated 4 Parametric study 4.1 Overview In order to optimize the behaviour of T-stub elements a parametric study was developed using different steel grades: S690, S460 and S235, different distance between bolt rows (Fig.10), different thickness of end-plate: 8, 10, 12, 16 and 20

mm and different dimensions for end-plate (Fig.11). This parametric study was performed only on T-stubs of type B, because this type of geometric configuration is the most common

configuration of the beam-to-column

joint (see Fig.3).

Initial case Case 1 Case 2

Dimensions [mm] Longitudinal:

90 Longitudinal:

80 Longitudinal:

110 Transversal: 120 Transversal: 110 Transversal: 130 Fig.10. Values for distance between bolts used for

parametric study

Initial case Case 1 Case 2

Dimensions [mm] Longitudinal:

180 Longitudinal:

140 Longitudinal:

190 Transversal: 190 Transversal: 170 Transversal: 220

Fig.11. Values for end-plate dimensions used for parametric study

4.2 Results of parametric study 4.2.1 Influence of end-plate thickness Fig.12 shows the numerical response under the form of characteristics force-deformation curves for the new analysis cases, in comparison with the initial case. The initial case means following thickness of end-plate: for steel grade S235, end-plate thickness is 12 mm, for steel grade S460, end-plate thickness is 10 mm while for steel grade S690, the end-plate thickness is 8 mm. It could be observed that the thickness of end-plate in association with the steel grade can change the failure mode of the T-stubs considered in this study. For the steel grade S235, the failure Mode 2 is


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obtained starting from end-plate with 12mm thickness , while for the steel grade S460, this failure mode is obtained starting from end-plate with 10mm thickness. In terms of resistance, it could be observed that for the same end-plate thickness, the using a High Strength Steel (HSS) increase the value of Fy with up to 28% (this is a average value of yield limit resistance when comparing the steel grade S235 with S460). In terms of stiffness, for the same end-plate thickness, the using of High Strength Steel (HSS) increase this value with up to 4% (this is a average value of elastic stiffness when comparing the steel grade S235 with S460). Table 5 offers this values both for resulted yield limit resistance (Fy), as well as for the elastic stiffness (Sj,ini) and yield displacement Δy.

Fig.12. Characteristics Force-Displacement curves for different end-plate thickness and different steel

grades Table 5. Elastic resistance, elastic stiffness and yield

displacement for different end-plate thickness

*Forces Fy are given in kN, elastic stiffness is in kN/mm while yield displacement is in mm Fig.13 shows very clearly that the changing in steel resistance influences in great extent the resistance, the stiffness and the element ductility. To a proper understanding, the thickness of the end-plate was kept constant (for this example, the end-plate has 8mm thickness). It could be observed very clear that steel grade increase the resistance of the connection (compared to the steel grade S235, the value of yield limit resistance , Fy, increases with 32% , while for steel grade S690, the Fy increases with 112%).

Fig.13. End-plate whit 8mm thickness:

Characteristics curves for different steel grades

End-plate [mm]

Sj,ini S235 S460 S690

8 240.27 255.89 286.51 10 535.62 549.92 566.68 12 640.48 684.34 693.65 16 907.32 929.29 952.85 20 1063.95 1092.91 1123.44

End-plate [mm]

Fy S235 S460 S690

8 162.60 215.21 345.11 10 280.95 359.72 555.67 12 330.09 480.76 593.56 16 482.67 592.06 654.44 20 622.09 676.23 684.19

End-plate [mm]

Δy S235 S460 S690

8 0.68 0.85 1.24 10 0.54 0.68 0.68 12 0.52 0.72 0.88 16 0.54 0.65 0.71 20 0.60 0.63 0.62


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Table 5 presents the results from both numerical and analytical approach. This comparison is the way whereby the numerical results are validates by analytical method. Table 5 shows also the good corelation between the results, both for numerical and analytical method. Table 5. End-plate whit 8mm thickness: values for different steel grades

4.2.2 Influence of distance between bolts The results of numerical and analytical approach shows very clearly that the change in distance between the bolts influencing in great extent the resistance, stiffness and the element ductility (Fig.14). It was used also the comparison between the numerical and analytical approach to validate the numerical results. Table 6 presents the good corelation between the results, both for numerical and analytical method.


Characteristics curves for distance between bolts Table 6. End-plate whit 8mm thickness: values for

different distance between bolts

If considering the case 1 of geometrical disposition (decreasing the distance between bolts), an increase in the resistance (up to 20%) and stiffness is

obtained. On contrary, the use of larger distance, case 2 in this study, the resistance and elastic stiffness are significantly reduced. 4.2.3 Influence of end-plate dimensions Fig.15 and Table 7 shows very clearly that resistance and stiffness are directly proportional influenced by the end-plate dimensions. Based on this, a better response is obtained in study case 2 by extending the end-plate dimensions, thereby improving the resistance and stiffness of the beam-to-column joint.


Characteristics curves for end-plate dimensions

Table 7. End-plate whit 8mm thickness: values for different end-plate dimensions

The conclusions of this study is that: the using of a steel grade with higher resistance together with the disposition of the bolts closer to the beam web and beam flange and also using an extended end-plate, ensure a better connection response in terms of stiffness and ductility. 5 Conclusions The strength and ductility of the bolted end-plate beam-to-column joint is highly influenced by the T-stub behavior. The FE model of some previously tested T-stubs show good agreement response under the form of Force-Displacement curves (monotonic). The initial stiffness, resistance and failure mode of a T-stub element could be controlled through: material quality of the components (end-

Fy Sj,ini Numerical Analytical Numerical Analytical

S235 162.60 158.20 240.27 262.50 S460 215.21 232.40 255.89 262.50 S690 345.11 330.55 286.51 262.50

[mm] Fy Sj,ini Numerical Analytical Numerical Analytical

Case 1: 80x110 423.63 369.75 395.54 352.80 Initial case:

90x120 345.11 330.55 286.51 262.50

Case 2: 110x130 274.95 283.81 167.01 168.00

[mm] Fy Sj,ini Numerical Analytical Numerical Analytical

Case 1: 140x170 297.34 330.40 264.15 315.00

Initial case: 180x190 345.11 330.55 286.51 262.50

Case 2: 190x220 395.67 360.10 303.66 302.40


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plate, elements); bolt arrangement and end-plate dimensions. The control of the cyclic response (work in progress) can give additional information on the behaviour of T-stub macro-component in olygocyclic fatigue. Acknowledgments

This work was partially supported by the strategic grant POSDRU 107/1.5/S/77265, inside POSDRU Romania 2007-2013 co-financed by the European Social Fund – Investing in People References: [1] Dubina, D, Stratan, A, Muntean, N, Grecea, D,

Dual-steel T-stub behaviour un-der monotonic and cyclic loading, ECCS/AISC Workshop: Connections in Steel Structures VI, Chicago, Illinois, USA, 23-55, 2008.

[2] Dubina, D, Stratan, A, Muntean, N, Dinu, F, Experimental program for evaluation of Moment Beam-to-Column Joints of High Strength Steel Components, ECCS/AISC Work-shop: Connections in Steel Structures VI, Chicago, Illinois, USA, June 23-55, 2008.

[3] Dubina, D, Muntean, N, Stratan, A, Grecea, D, Zaharia, R, Testing program to evaluate behaviour of dual steel connections under monotonic and cyclic loading, Proc. of 5th European Conference on Steel and Composite Structures - Eurosteel 2008, 3-5 September, Graz, Austria, 609-614, 2008.

[4] Dubina, D, Grecea, D, Stratan, A, Muntean, A., Performance of dual-steel connections of high strength components under monotonic and cyclic loading, STESSA 2009, Behaviour of Steel Structures in Seismic Areas, Taylor & Francis Group, London, 16-20 Aug. 2009, Philadelphia, USA, 437-442, 2009.

[5] Dubina, D, Dual-steel frames for multistory buildings in seismic areas”, Keynote lecture, Proceedings of SDSS’Rio 2010 International Colloquium Stability and Ductility of Steel Structures, 8-10 September, Rio de Janeiro, Brazil, 59-80, 2010.

[6] Muntean, N, Grecea, D, Dogariu, A, Dubina, D, Strength and ductility of bolted T-Stub macro-components under mono-tonic and cyclic loading, Proceedings of SDSS’Rio 2010 International Colloquium Stability and Ductility of Steel Structures, 8-10 Sept, Rio de Janeiro, Brazil, 223-230, 2010.


ISBN: 978-960-474-337-7 199

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