SEISMIC BEHAVIOR AND DUCTILITY OF CONCENTRIC CROSS BRACING WITH THE USE OF SLIDING-TENSION ACTION

Transcription

1 SEISMIC BEHAVIOR AND DUCTILITY OF CONCENTRIC CROSS BRACING WITH THE USE OF SLIDING-TENSION ACTION R. Ahmady Jazany, B.H. Hosseini Hashemi, H. Khobe, H. Kayhani ABSTRACT A type of frictional damper is suggested here that can be used in the cross bracing resistance system. This damper may be designed to adjust sliding force under tension and compression in order to slide under a specific force. In addition, some stoppers will be provided to prevent excessive sliding. The final goal is to provide a system which is able to bear more cycles of loading without decrease in resistance and prevent the system from buckling caused by compression force. The system is also able to act in different phases for different seismic levels. It is shown that this system possesses a stable hysteretic behavior with regular form without decrease in stiffness and resistance with the use of maximum member capacity. Moreover, hysteretic cycles are not sensitive to loading range, frequency or the number of load cycles and the system base reaction will be stationary. Keywords: Frictional damper, Concentric bracing, Bracing ductility, Sliding Action 1. INTRODUCTION Mass casualties caused by severe earthquakes resulted in high interest in energy absorption instruments in order to reduce earthquake effects on buildings. Techniques used for reducing effects of earthquake force without increasing resistance and rigidity of structure are desirable ( Law,SS, and Wu,ZM, Chan SL(26)). First tests about SBC (Sliding Bolt Connection) dampers were provided by Venuti(1975) in San Jose University. Fitzgerald et al (1989). Considered frictional joints in concentric bracing systems. Reinhorn and Constantinou (1995) produced an instrument to be used in bridge sliding system (a rustproof iron in contact with bronze that was mixed with graphite)(reinhorn et.al(1995) Constantinou, M.C et.al(27)).popov,e.g et al(1995) and Clifton(2) performed some extra tests on previous samples. Butterworth(1999),(2),(22) (199)considered different kind of frictional sliding joints in two types of structures including linear slide and revolving slide Oldfield(25) et al. did some experiments on bolted joint under harmonic loading. Pall and Marsh(1982) designed a frictional damper for sectional bracings. Meanwhile, Law et al.(26) did analytical modeling about performance of elements with two sliding steel plates under cyclic base excitation.performance of this structure depends on three main factors: resistance, rigidity and ductility. There could be three variations of setting up the bolts in frictional dampers: the first one and somewhat the more general use of this device is the mid-length installation of bolts in the shank, the second is end-length installation of bolt shown in Fig. 1 (closer to brace and column conjunction) and third is the beginning of the shank which could make compression forces in the braces therefore it could be uneconomic since they must be designed considering buckling phenomenon; moreover, when the buckling design is ignored the pinching will be observed in hysteretic behavior of this third variation. This article aims at considering the advantages of end-length bolt location. When a horizontal displacement act on the roof, the bracing which will be under compression will continue to achieve F=μ N (N equal to Normal force resulted by fastened bolts). The force of F is designed so that F=μN<F Cr in order to prevent buckling. Despite Bracing A which would slide if F>Fcr and it carries compression force, bracing B which is under tension would not slide and its full tension capacity may be used. When earthquake force changes its direction, before bolts returns to initial condition, both bracings will slide and later they will act as previously described.

2 Figure 1. schematic placing of sliding joints and type of their installationss 2. VERIFICATION OF ANALYTICAL MODEL Some analytical modeling was performed to evaluate the effects of the bolt location on the behavior of braces. First, the result of an experimental study was chosen to verify the Finite element modeling. Tehranizade M, Khaleghian (27) performed some testes to study the effects of surface material on cyclic behavior and dynamic load bearing capacity of frictional dampers. Fig. 2 shows the test specimen. Figure 2. Experimental specimen [1] A universal equipment with 6 KN capacity was used for monotonic loading and totally 36 specimenss with various material were studied. In this test, performance of damper in different loading ranges was investigated. Loading is shown in Fig 3. and Fig. 4 demonstrates damper reaction in monotonic incremental loading. The result of the tests indicated that the reaction Force levels in different imposed displacement ranges were stable and did not change with increasing loading range. In fact, independency of sliding load with respect to fluctuation range was clear. Fig. 5 shows seismic curve, which indicate the optimal performance curve of this damper in range of 5 cycles of loading (Tehranizade,M. and Khaleghian,F.(27)). The applied frequencies for test were.7 and..8 Hz. Displacement ( sec) Figure 3. Experiment loading pattern Figure 4. Experiment force level diagram

3 Figure 5. Experiment seismic behavior of studied damper (Tehranizade,M.and Khaleghian,F.(27)) The difference of damper s hysteretic behavior in compression and tension sliding force is due to manufacturing procedure. It means that sliding force of equipment in compression are higher than tension which is a result of damper manufacturing procedure ( Tehranizade,,M.and Khaleghian,F.(27)) ). Test Result indicated that required force for sliding was constant and also there were no degradations; moreover, no changes were observed in damper s performance by increasing frequency and loading range in different tests. It could be concluded that damper s response does not dependd on frequency and loading speed. For studying the proposed model, the specimen #7 of aforementioned test was modeled using ABAQUS finite element program with Solid 3D element No was used to compare result of analytical and the experimental models. Required definitions including friction coefficient, surfaces and conditionn of supports were conducted. Contact element was used for modeling friction effect with.34 friction coefficient according to test result. In addition, the 3 KN pretensionn force for each bolt was applied in FE model. Fig. 6 shows FE model of the damper. Figure 6. Finite element model of Test load pattern was also used for the analysis. Analytical results show that the reaction force level does not change when displacement varies as can be seen in Fig. 7. Therefore, stability of bearing force can be seen and its independency from each parameter is clear. Fig 8 shows seismic behavior of analytical model. Despite experimental result (Fig. 5), the force level of analytical model (Fig. 8) is fixed and it does not change, as it was depicted, it is due to manufacturing defects which was not consideredd in analytical model but the maximumm level of force in analytical and experimental result are the same and the models could simulate the behavior of damper. Figure 7. Base reactions result of FE model Figure 8. Siesmic behavior of the analytical model 3. INTRODUCING SUGGESTED MODEL AND IMPLICATION Main jointing plate is located between two external members and the brake lining is placed between external member and main plate. Two bolts were used for holding brake linings and main plate (channel beam). After fastening bolts, friction between surfaces can be provided by brake linings and

4 perforated plate. When each of two tension or compression forces exceeds frictional force, channel beam willl slide and energy will be absorbed. Double channel ([]) section with size of 8 mm and length of 5 meter was modeled with 3D-solid element as the bracing member. Bracing member with channel section (4x4mm) weree modeled and connected to the frictional damper to consider the effect of this sub-assemblagement (Fig. 1). Steel properties is considered with perfect elasto-plastic form and Von- model. Displacement control loading according to ATC-24 (1994) was used for the analysis. Mises criterion for yielding level and collapse, the test material properties were applied for analytical Figure 9. Finite element model of proposed damper improvement Figure 1. Analyticall model applied load pattern The location of the bolts was chosen at the end-length could be observed in Figs. 11 and 12. Considering Fig. 11, of the long-slotted holes as stated before. The output result of this kind of installation there is an extra base reaction force in comparison with the experimental case (Fig. 7), this stepped shape is originated from yielding of brace in tension that would produce extra capacity. Fig. 12 shows seismic behavior of his system, as it is obvious there is an extra area of force that indicates the possibility of more energy dissipation. 2 Base reaction Tim e Figure 11. Base reaction force vs. time Figure 12. Seismic behavior of proposed model This system provides a stable hysteretic curve and there is no degradation in seismic behavior. Because of tension essence of proposed model, this system has no dependency on severity of loading. Also in tension mode, there is more acceptable resistancee and stiffness when the brace yields.

5 Moreover, in tension force sliding mode, before the brace yields, the force level is μnn and then, step by step, when displacement increases, the yielding flow commences through brace section until reaction stress reaches F y, this phenomenon would happen slowly and it will be without any considerable changes in initial stiffness. In compression mode, the sliding mode prevails and reaction force is equal to μn. This performance could be achieved if reliable performance of bolt slide joint is ensured, otherwise the bolt fracture could result in brittle failure mode in the system. This system possesses some advantages like: avoidance of brittle condition when joint behaves in sliding range; Simple and cheap manufacturing procedure and maintenance. In this research, performance graph of the suggested model will be drawn according to part 2-8 of ASCE/SEI 41-6(27). Fig. 13 shows the backbone curve of this system that has been derived from its seismic behavior. In the next section, it will be employed to evaluate the effect of such improvement on overall behavior of a frame. Figure 13. Backbone curve of the hysteretic behavior 4. IMPLICATION OF IMPROVED MODEL Two common 5-story steel buildings were chosen to investigate the effectiveness of proposed damper as shown in Fig. 14. One was a simple braced frame model with axial hinge properties for braces and column according to FEMA 356, another bulling had improved damper and modified hinge specification for braces as in Fig. 13 and FEMA 356 axial hinge properties were used for column hinges like the first model. The time history analyses were performed using north-southh component of the Imperial Valley earthquake recorded at Elcentro. Figure 14. Structural model used in this study Results of the analysess on two models (simple and with propose dampers) indicate thatt structure with frictional bracing has better and more uniform performance in comparison with simply braced structure. In frictional bracing, most of the hinges plastic rotation are in immediate occupancy range, while in simple bracing; the hinges plastic rotation are higher than collapse prevention performance limit. Roof displacement time history is shown in Fig. 15. As it could be seen, displacements are more regular and do not fluctuate, but in simple model, Fig. 16, displacements are rapidly changing. It

6 means that in tension mode, which has been achieved by means of the improved system, displacement response could be controlled properly. Considering Figs. 17 and 18, it is clear that more base-shear will be absorbed in frictional model due to its high capacity in comparison with simple braced frames. Displacement Figure 15. Fifth story displacement time history with improved damper (cm) Displacement Figure 16. Fifth story displacement time history with simple braces Base shear Figure 17. Base shear time history for improved damper(kg) Base shear Figure 18. Base shear time history for simple braces 5. CONCLUSION: Three effective parameters in earthquake including resistance, rigidity and ductility are used in suggested model. Following conclusions can be made: 1. No buckling happened in compression part of suggested joint and no sleeve or special cover is used, and just the tensional capacity of braces are important if this damper is used. 2. This kind of bracing is applicable in two different performance levels: frictional force and yielding brace forces, 3. Member capacity for tension will increases to the yielding level without any degradation.

7 REFERENCES Venuti,GH.(1975).Sliding bolt connection as a Frictional Dampers. ASCE, Journal.of Structural Division, 12:6, Law,SS, and Wu, ZM, Chan SL. (26). Analytical Model of a Slotted Bolted Connection Element and its Behaviour under Dynamic load. Journal of Sound and Vibration Vol 292, Fitzgerald,T.F.,Goodson,M. and Zsutty,T.(1989).Slotted Bolted. Connections in Seismic Design for Concentrically Braced Connections. Earthquake Spectra, l 5:2, pp Reinhorn,AM.and Constantinou,MC.Li. (1995).Use of Supplemental Damping Devices for Strengthening of Lightly Reinforced Concrete Frames. Department of Civil Engineering State University of New York at Buffalo, NY 1462, NIST workshop. POPOV,E.G.,Grigorian,C.E. and Yang,T-S.(1995).Developments in Seismic Structural Analysis and Design. Engineering Stuctures, Vol.17, NO.3. Butterworth,John.(22). Ductile Concentrically Braced Frames Using Slotted Bolted Joints. SESOC Journal 12:2. Butterworth,John.,Clifton, Charles.G.C.(2).performance of Hierarchical friction dissipating Joints in Moment Resisting, Steel Frames. 12 World Conference of Earthquake Engineering. Butterworth, John.(1999).Seismic Damage Limitation in Steel Frames Using Friction Energy Dissipators. 6th International Conference on STEEL& SPACE STRUCTURES, Singapore. Butterworth, John. (1999).Seismic Response of Steel Frames Containing Hierachical Friction-Dissipating Joints., Mechanics of Structures and Materials, Bradford, Bridge & Foster (eds) Balkema, Rotterdam 12:3, Oldfield,Matthew., Ouyang,Huajiang.and Mottershead,John (25).simplified Models of Bolted Joints Under Harmonic Loading. Computers and Structures 23:2, Applied Technology Council(ATC24)(1994).guidelines for cyclic seismic testing of components of steel structures. American Society of Civil Engineering(27).Seismic Rehabilitation of Existing Buildings.ASCE/SEI Constantinou,M.C., Soong,T.T., Dargush,G.F.(27) Passive Energy Dissipation Systems for Structural Design and Retrofit. MCEER-volum1. Pall,A.S.and Marsh,C.(1982). Response of Friction Damped Brace Frame. ASCE, Journal.of Structural Division, Tehranizade,M.and Khaleghian,F.(27). The Comparison of Friction Damper Laboratory Result with Various Sliding Surfaces. 5th International Conference on Seismology and Earthquake Engineering, Tehran Butterworth John,W. (199).Seismic Response of Moment Resisting Steel Frames Containing Dual-level Friction Dissipating Joints. NZSEE Conference, Rotorua. Clifton.Charles.G. and Butterworth,John.(2).Moment-Resisting Steel Framed Seismic-Resisting Systems with Semi-Rigid Connections. 12Word Conference of Earthquake Engineering.