Description of the numerical model

Due to the high concentration of stresses near welds, in the heat affected zone (HAZ), which caused unexpected fracture of the beam flange of RBS-L3 specimen, a numerical simulation investigation was carried out, in order to better understand the cause of the failure. On this purpose, a numerical model, able to simulate the post-elastic large strain cyclic deformation was calibrated for specimens RBS-S3 and RBS-L3. All the components were modelled using solid elements. In order to have a uniform and structured mesh, some components with a complex geometry were partitioned into simple shapes. The engineering stress-strain curves of the steel grades obtained from tensile tests were computed into true stress-true plastic strain and used further in the numerical model. The modulus of elasticity was considered equal to

Connections in Steel Structures VII / Timisoara, Romania / May 30 - June 2, 2012 271 272 Connections in Steel Structures VII / Timisoara, Romania / May 30 - June 2, 2012

210000 N/mm2 and the Poisson coefficient equal to 0.3. For the cyclic analysis, a combined isotropic/kinematic hardening model was used for the material, containing the cyclic hardening parameters from Dutta et al. (2010). A dynamic explicit type of analysis was used. The load was applied through displacement control at the top of the columns. 3.2. Results

Numerical simulation was first used to evaluate the solution with shear slip resistant splice connection, after the problems revealed after tests of bolted flush- end plate splice connection specimens. The second goal of numerical simulation was to optimise the shape of the cut-out in the beam flanges in the reduced zone. Figure 10. shows the hysteretic curves of tested specimens RBS-S3 and RBS-L3, having a splice type connection. As expected, this type of connection prevented the bolt slippage and therefore a continuous beam was taken into account within the numerical simulations. It can be observed that the behaviour anticipated by the numerical simulation is confirmed by the tests.

-800 -600 -400 -200 0 200 400 600 800 -200 -100 0 100 200 300 400 B ase s h ear f o rce, k N Top Displacement, mm RBS-S3 FEM a) -800 -600 -400 -200 0 200 400 600 800 -300 -200 -100 0 100 200 300 400 Ba se s h ea r fo rc e, k N Top Displacement, mm RBS-L3 FEM b)

Figure 10. Hysteresis curves: a) RBS-S3; b) RBS-L3

Based on the observed results it was proposed a reconfiguration of the reduced beam section where the length of the RBS was reduced from 450mm to 300 mm (as for short beams) (Figure 11.b, c). This new solution did not affect the stiffness but decreased the amount of shear force (Figure 11a).

Figure 11. Comparison between RBS-L3 and RBS-L3_MOD (FEM): a) cyclic behaviour; b) beam configuration; c) dimensions of the flange cut-out

As it can be seen in Figure 12b, the plastic deformation are developed only in the reduced beam section, in comparison with the initial configuration (RBS-L3) shown in Figure 12a. The von Misses stress distribution for the two cases is presented in Figure 13.

Figure 12. Equivalent plastic strain: (a) RBS-L3; (b) RBS-L3_MOD.

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Figure 13. Von-Misses equivalent stress: (a) RBS-L3; (b) RBS-L3_MOD. 5. CONCLUSIONS AND GENERAL RECOMMENDATIONS

Main conclusions on the experimental tests and numerical analysis on reduced beam sections connections of short coupling beams can be summarizes as follows:

i) Experimental test on two types of short beams of RBS connections confirmed the design procedure. The specimens exhibited excellent ductility and rotation capacity up to 60 mrad before failure. However, the flush end plate connection exhibited significant slippages which lead to reduction of the stiffness. Based on these observations, the splice connection was redesigned using a detail that is more appropriate for the predominant shear stress state at the mid-length of the beams. This new connection detail consists of gusset plates on web and flanges and preloaded high strength bolts. This new configuration can prevent the bolt slippage, and therefore both the stiffness and axial straightness of the assembly will not be altered.

ii) For very short beams, the interaction between the shear and normal stresses causes an inclination of the buckled shape in the web. The plastic rotation capacity has two major components, i.e. rotation of the beam (reduced beam section) and distortion of the web panel in the reduced region. Due to the large stiffness of the columns, the contribution of the column web panel can be neglected.

iii) Numerical simulation allowed to calibrate a modified RBS configuration in order to eliminate the stress concentration near the beam to column welding. The results showed the concentration of stresses takes place in the reduced beam section.

ACKNOWLEDGMENTS

Funding of the project was made in the frame of the contract 76/2011 “Numerical simulations and experimental tests on beam-column subassemblies from the structure of a 17 story steel building in Bucharest, Romania”.

C. Vulcu was partially supported by the strategic grant POSDRU/88/1.5/S/50783, Project ID50783 (2009), co-financed by the European Social Fund - Investing in People, within the Sectoral Operational Programme Human Resources Development 2007-2013.

REFERENCES

[1] AISC 341-05 (2005). Seismic provisions for structural steel buildings. American Institute for Steel Construction.

[2] B. Johansson, R. Maquoi, G. Sedlacek, C. Müller, D. Beg (2007). Commentary and worked examples to EN 1993-1-5, JRC – ECCS cooperation agreement for the evolution of Eurocode 3, European Commision.

[3] Dutta A., Dhar S., Acharyya S. K. (2010), Material characterization of SS 316 in low-cycle fatigue loading, Journal of Materials Science, Vol. 45, Issue 7, pp. 1782-1789.

[4] ECCS 1986. European Convention for Constructional Steelwork, Technical Committee 1, Structural Safety and Loadings; Working Group 1.3, Seismic Design. Recommended Testing Procedure for Assessing the Behavior of Structural Steel Elements under Cyclic Loads, First Edition.

[5] EN 1993-1-1 (2005). Eurocode 3. Design of steel structures. General rules and rules for buildings, CEN, EN 1993-1-1.

[6] EN 1993-1-5 (2006). Eurocode 3. Design of steel structures. Part 1-5: General rules - Plated structural elements, CEN, EN 1993-1-5.

[7] EN 1998-1-2004. European Committee for Standardization – CEN. Eurocode 8: Design provisions for earthquake resistance of structures. Part 1.1: General rules. Seismic actions and general requirements for structures, Brussels.

[8] Hibbit. D., Karlson, B. and Sorenso, P (2007), ABAQUS User’s Manual, Version 6.9. [9] Plumier, A. (1990), “New Idea for Safe Structures in seismic Zones”, IABSE Symposium, Mixed structures including new materials, Brussels 1990 (pp.431- 436).

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SEISMIC RESISTANT WELDED CONNECTIONS