Study of TMD-controlled Reinforced Concrete Framed Structure under Code-based Artificial Earthquake

Size: px

Start display at page:

Download "Study of TMD-controlled Reinforced Concrete Framed Structure under Code-based Artificial Earthquake"

Margaret Cameron
6 years ago
Views:

1 Rev. Téc. Ing. Univ. Zulia. Vol. 37, Nº 1, 42-48, 214 Study of TMD-controlled Reinforced Concrete Framed Structure under Code-based Artificial Earthquake Min-Ho Chey School of Architecture & Art, Yanbian University of Science & Technology (YUST), Yanji City, Jilin Province, China 133 Abstract - This study explores the adoption of the optimized tuned mass damper (TMD) system on a well designed stiff reinforced concrete framed structure and its performance control effectiveness. Through the application of the previously examined optimal TMD parameters, the controlled seismic performances of the target structure are investigated under the suggested code-based (NZS423) artificial earthquake excitation. Using the specified and suggested optimal parameters, the effectiveness of the TMD is examined from the view point of the displacement response properties of the framed structure and TMD itself. From the response results of the structure and its TMD, the mass ratios affecting the effectiveness of the TMD are found. Finally, the results of a series of time history analyses indicate the reasonably acceptable TMD effectiveness as a passive damping device to control the vibration of the structures to artificial earthquakes. For a given level of critical damping in the structure, increasing the mass ratio makes the TMD more effective in reducing structural response. This implies that a system with higher intrinsic damping requires a TMD with a larger mass ratio to provide similar reduction to that needed for a system with lower damping. Keywords - tuned mass damper, reinforced concrete, artificial earthquake, displacement, interstory drift 1 INTRODUCTION The structural control by using newly developed damping devices offer the possibility of extending the applications and improving the efficiency of the devices. For the control of medium and highrise buildings, some types of auxiliary damping devices have been successfully employed for safety and serviceability purposes. One of the classical dynamic vibration damping devices is the Tuned Mass Damper (TMD), consisting of a supporting sub-mass, located at the top of the building and connected through a passive spring and damper to attenuate any undesirable vibrations. It is well known that the TMD will minimize the vibration if the TMD frequency is set equal to the frequency of the disturbing force (Den Hartog, 1956). The TMD is relatively easy to implement in new buildings and in the retrofit of existing ones. It offers the advantages of portability and ease of installation (because of the small size of an individual damper), which makes it attractive not only for new installation, but also for temporary use during construction or for retrofit in existing structures. The TMD does not require an external power source to operate and does not interfere with vertical and horizontal load paths as do some other passive devices. Because of the above advantages of the TMD, the TMD system has been thought as a suitable damping device for many structures. However, most TMD applications have been made to mitigate wind-induced motion (Kwok and Samali 1995; Ricciardelli et al. 23; Varadarajan and Nagarajaiah 24; Yan et al. 1999; Yang et al. 24), whereas the seismic effectiveness of TMD still remains an important issue for future study (Bernal, 1996; Soto-Brito and Ruiz, 1999; Murudi and Mane, 24). This situation and these facts have provided motivation for this study. In this study, the optimal TMD parameters are derived and applied to the vibration control of 12-story concrete structural frames under a code-based artificial earthquake excitation generated to match the New Zealand Standard (NZS423, 1992) spectra. From the results of a number of case studies, some seismic responses with respect to the structure and TMD are examined. These results indicate the effectiveness of the TMD as a passive damping device to control the vibration of a benchmarkable concrete framed structure, reflecting the reliability of using optimal TMD system to artificial earthquake excitations. 2 PROTOTYPE STRUCTURAL FRAME USED In the current study, a 12-story, two-bay reinforced concrete framed structure is examined to demonstrate the effects of the TMD. This structure was designed to earlier codes where the seismic coefficients at longer periods were governed by its minimum value. This building represents a well designed one, where the reserved strength might exist. This is strong (and stiff) such that the P-delta effects seen to be ignored towards a given earthquake. According to the New Zealand Loadings Code (NZS423, 1976), Jury (1978) originally designed the reinforced concrete frames and then its strengths were revised following the changes to NZS423 (1984). It is assumed that the frame would be required to resist the component of earthquake motion in the plane of the frame only. No torsional effects for the building as a whole are taken into account. Fundamentally, this structure was designed on the basis of the equivalent static analysis. The computed base shears and deflection at top of the frame are 1,1kN and.33m, respectively. 42

2 Rev. Téc. Ing. Univ. Zulia. Vol. 37, Nº 1, 42-48, 214 The gravity load moments were calculated by a simplified sub-framing system. For the beam design, bending moment envelopes were constructed by the combination of factored gravity loads and equivalent seismic lateral loads, then moment redistribution was carried out using the method developed by Paulay (1976). An effort was made during moment redistribution to allow the full utilization of beam sections by equalizing, if impossible, the demand for top and bottom flexural steel at the column face. For the column design moments and design axial loads, the method set out in an article by Paulay (1976) was used. The over-strength factor, ϕ, approximate at the ground floor, where column was taken as 1.4 and at roof level where column hinging is acceptable, ϕ was taken as 1.1. The dynamic magnification factor, ω, was calculated form ω=.6t+.85 but not less than 1.2 nor more than 1.8, where T was the period of the structure in its first mode of vibration. The design column sectional properties were based on ideal strength since the design method used already incorporates capacity factor. For the modulus of elasticity E c =25Gpa for f c '=28Mpa was used for all members. The beam-column members which were used for the base columns in the frame were modeled to allow for interaction between the axial force and bending moment yield states governed by an interaction diagram as shown in Fig. 2. Thomson (1991) made some modification to Jury s frames. The column dimensions were increased according to data reported by Paulay (1979), who redesigned the columns because the original Jury s design called for rather high reinforcement ratios. The building dimensions and member sizes adopted in this study are shown as in Table 1. Table 1 Frame sizes used Members Level Dimensions (mm) Fig story reinforced concrete target structure Beams Exterior Columns Interior Column Fig. 2 Concrete beam-column yield interaction surface 3 TMD DESIGN To represent the effects of the TMD stiffness coefficient, the spring member which is incorporated to the program Ruaumoko (Carr, 24). In Ruaumoko, this member may be used to model special effects in the structure or to represent members acting out of the plane of the frame but representing forces that act in the plane of the frame. A couple of the explicit formulae for reduction-based optimal frequency tuning ratio (f 2opt ) 43

3 Rev. Téc. Ing. Univ. Zulia. Vol. 37, Nº 1, 42-48, 214 and optimal TMD stiffness (k 2opt ), which were obtained from the previous parametric study of the Table 1 (Chey and Kim, 212), are driven by curve-fitting method as shown in Fig. 3(a) and Fig. 3(c), respectively. The later optimal parameter of k 2opt is used to the value of the TMD spring stiffness option in the transverse direction. For the hysteresis rule for the spring member, a linear elastic hysteresis is used to represent the elastic properties of the TMD behavior. The other component, the TMD damping which may be added to a structure, can be modeled using the damping or dash-pot member in the program Ruaumoko. This model represents the action of a local viscous energy dissipater that may exist in the structure and contribute to the damping matrix of the structure. An optimal TMD damping coefficient(c 2opt ), which is transferred from the optimal TMD damping coefficient (ξ 2opt ), is used as the transverse damping coefficient as shown in Fig. 3(b) and Fig. 3(d). Also, the linear elastic model is used as the hysteresis rule. Table 2 Optimal TMD parameters and RMS responses with 5% critical damping (Chey and Kim, 212) f 2 opt 2opt k 2 opt c 2 opt y y 1 y y2 y 2 y f 2opt.9 ξ 2opt.1.85 y = 3.615x x y = x x (a) (b) k 2opt y = -37,2x ,86x c 2opt 1 5 y = 5,838.6x x (c) (d) Fig. 3 Optimal TMD parameters and the derived explicit formulae: (a) frequency tuning ratio; (b) TMD damping ratio; (c) TMD stiffness; (d) TMD damping coefficient 4 SEISMIC EXCITATION 4.1 Earthquake Records Used Since no natural earthquake has a response spectrum that is likely to match the design spectra for a given site, it is usual to require for time history analyses that a suite of accelerograms be used to cover the uncertainties in the matching of the design spectra to that of the accelerograms. As a simple approach, it is commonly assumed that over a small range of periods the natural earthquake can be scaled to match the design 44

4 Acceleration (g) Rev. Téc. Ing. Univ. Zulia. Vol. 37, Nº 1, 42-48, 214 spectra. However, there is still the question as to which accelerogram should be selected. Another method is to generate artificial or synthetic accelerograms to fit, to some degree of precision, the prescribed response spectrum. This technique starts by generating a white noise accelerograms which is then enveloped to give it the desired timewise envelope of accelerations and then the frequency content is mapped to best-fit that of the target spectrum. In this study, an artificial accelerogram generated by the SIMQKE program (Vanmarke, 1976) was used to produce an artificial accelerogram according to the spectra specified in the New Zealand Standard (NZS423, 1976) as shown in Fig 4. The effective length of the record was 2 seconds long in which a trapezoidal envelope was used with a 2 seconds rise time and 1 seconds level time accelerations. The target response spectrum was taken from the New Zealand Standard for a structural ductility factor of 1. in an intermediate soil site. It was specified that the target maximum ground acceleration (PGA) was.5g, the time step size was.1 and the number of spectral points was 2. Then the record was multiplied by a zone factor of 1.2 so that it represents the highest zone factor value in the New Zealand Loadings Code of NZS423: Response Spectra The elastic response spectra, which have been based on 5% of critical damping for the used artificial earthquake are shown in Fig. 5. The spectral acceleration describes the maximum total acceleration of the structure. The design spectrum for New Zealand Standard (NZS423:1992) is based on a 5% damped uniform hazard spectrum representing structural ductility, =1, for an intermediate soil condition. The shapes of the spectral acceleration (SA) indicate that peak accelerations are irregularly distributed over the period range but decrease very significantly for the long structural periods. The code specified response spectrum, NZS423:1992, presents a smoothed response spectrum based on an earthquake with a specified probability of exceedance during the expected design life of the structure. The maximum displacement of the structure with a given natural period of free-vibration can be read from the displacement spectrum (SD). The spectral displacements for a long period structure are larger than for short periods. It can be seen that the used artificial earthquake ground motions produce relatively large maximum displacements and give rise to continuous increases of the maximum displacements at long natural periods, especially after the given natural period of 2 seconds. For the comparison, the spectral velocity (SV) is shown in Fig. 5 too Time (Sec) Fig. 4 Artificial earthquake record generated to match the New Zealand Standard NZS423:

5 Spectral Data Rev. Téc. Ing. Univ. Zulia. Vol. 37, Nº 1, 42-48, NZS(g) SA(g) SD(m) SV(m/s) Natural Period (sec) Fig. 5 Response spectra with 5% of critical damping for the used artificial earthquake (b) 5 PERFORMANCE RESULTS 5.1 Seismic responses interested In the design of building structures, the lateral displacement, or drift of a structural system under earthquake forces is important from several perspectives, such as structural stability, architectural integrity and potential damage to various non-structural components. This is an issue which should be addressed in the early stages of design development. This reduction in displacement-based response is very important in the structural design because many important design quantities, such as the base shear and moments, are directly proportional to the pseudo-accelerations which, in turn, are proportional to the displacements of the structure. In this study, maximum story displacement and maximum interstory drift were selected as the indices of the structural integrity. In addition, the travel of TMD relative to the top floor (the stroke) is also selected. The optimal damping is not the only constraint in the selection of the TMD. Available space will limit the travel of the TMD relative to the structure. The designer may wish to select a TMD damping larger than the optimal value to reduce the TMD travel. Hence, other control parameters in terms of the TMD behavior were required and can be used. The large displacement of the TMD may contribute substantially to the costs of the TMD itself and to the costs of accommodating the displacements of the structures. Some TMDs require large strokes to be effective. 5.2 Final seismic performances For the artificial earthquake excitation, which matches the New Zealand Standard NZS423(1992) spectra, the maximum story displacements without and with 2% TMD and 5% TMD are presented as shown in Fig. 6(a). These profiles show that the effectiveness of the TMD controlling the seismic response of the structures subjected to the considered excitation. Basically, the envelopes increase steadily and uniformly by story level and the displacement reduction becomes more prominent as the mass ratio increases (2% 5%). It is observed that the displacement reductions with 5% TMD is larger than with 2% TMD especially over 7 th floor. It is noted that the TMD effectiveness for the excitation is quite significant. Fig. 6(b) shows the maximum interstory drifts without and with 2% and 5% TMD based on the results from the maximum displacement. It is observed that at some stories the higher reductions do not exactly correspond to the higher mass ratio. From the figure, the higher mass ratio seems to provide satisfactory reductions over the 7 th floor of the height of the frame. The time-history behavior of the TMD (the stroke the relative displacement of the TMD to the top floor) is given in Fig. 7. For the artificial earthquake motion adopted, increasing the mass ratio decreases the TMD stroke. With a larger mass ratio, the TMD inertia increases, causing the stroke to reduce. Also, with the TMD absorbing most of the energy of the excitation, its displacement is much larger than that of the top floor. Hence, it is noted that the stroke is dominated by the mass ratio and a designer can use the TMD response property as the important design factor for optimal TMD design. 46

6 TMD Displacement (m) Story Story Rev. Téc. Ing. Univ. Zulia. Vol. 37, Nº 1, 42-48, No TMD 2% TMD 5% TMD No TMD 2% TMD 5% TMD Maximum Displacement (m) (a) Interstory Drift (m/m) (b) Fig. 6 Displacement and story drift performances of the 12-story target structure % TMD 5% TMD Time (sec) Fig. 7 TMD displacement responses on the top of the 12-story target structure 6 CONCLUSION In order to investigate the effectiveness of the optimal TMD, a series of the structural analysis were conducted using the structural analysis program, Ruaumoko. As a case study, a benchmarkable 12-storey reinforced concrete framed structure which was designed according to the New Zealand Loadings Code (NZS 423) was adopted to demonstrate the effects of the TMD. As an input excitation, an artificially generated earthquake record according the spectra specified in the New Zealand Standard NZS 423), is used. With the specified conditions, some important information was derived from a series of time-history responses of the structure and the practical valuable points obtained are summarized as follows; Basically, from the structural analysis for the 12-story frame, the results with respect to the parameters such as mass ratio, frequency tuning ratio and TMD damping ratio, correspond to the results from the previous parametric studies on the single degree of freedom system (Chey and Kim, 212). Although there is some uncertainty as to the effectiveness of the TMD for the individual earthquake, a proposed optimal TMD reduces the displacement story responses reasonably, and the TMD is effective in reducing the story displacements of the structure subjected to seismic excitations and the effectiveness is increased by increasing the mass ratio. This implies that a system with higher intrinsic damping requires a TMD with a larger mass ratio to provide similar reduction to that needed for a system with lower damping. The stroke of the TMD is dominated by the TMD mass ratio. From this TMD stroke-based property, as an important design consideration, a designer therefore can use the TMD response property as the important design factor for optimal TMD design giving good information to help design an appropriate TMD system. Additional study is recommended to study the effect of the TMD on a wider variety of response quantities such as local deformation, energy dissipation rate, plastic hinge rotation, curvature ductility and damage indices. From these various response quantities, the effectiveness of the TMD can be compared with the results from this study. 47

7 Rev. Téc. Ing. Univ. Zulia. Vol. 37, Nº 1, 42-48, 214 References Bernal, D., Influence of Ground Motion Characteristics on the Effectiveness of Tuned Mass Dampers, 11 th World Conference on Earthquake Engineering, Paper No. 1455, Carr, A.J. Ruaumoko - Computer Program Library, Department of Civil Engineering, University of Canterbury, Christchurch, New Zealand, 24. Chey, M.H. and Kim J.U. Parametric Control of Structural Responses using an Optimal Passive Tuned Mass Damper under Stationary Gaussian White Noise Excitations, Frontiers of Structural and Civil Engineering, Vol. 6(3), pp , ISSN: , 212. Den Hartog, J.P. Mechanical vibrations, McGraw-Hill, Jury, R.D. Seismic load demands on columns of reinforced concrete multistorey frames, ME Thesis, University of Canterbury, Christchurch, New Zealand, Kwok, K.C.S., and Samali, B. Performance of Tuned Mass Dampers under Wind Loads, Engineering Structures, Vol. 17(9), pp , Murudi M.M, Mane S.M. Seismic Effectiveness of Tuned Mass Damper (TMD) for Different Ground Motion Parameters, 13 th World Conference on Earthquake Engineering, Paper No. 2325, 24. NZS423 New Zealand Standard; Code of Practice for General Structural Design and Design Loadings for Buildings, Standards Association of New Zealand (SANZ), NZS423 New Zealand Standard; Code of Practice for General Structural Design and Design Loadings for Buildings, Standards Association of New Zealand (SANZ), NZS423 New Zealand Standard; Code of Practice for General Structural Design and Design Loadings for Buildings, Standards Association of New Zealand (SANZ), Paulay, T. Moment redistribution in continuous beams of eqrthquake resistant multistorey reinforced concrete frames, Bulletin of the New Zealand National Society of Earthquake Engineering, Vol. 9(4), , Paulay, T. Developments in the Design of Ductile Reinforced Concrete Frames, Proceedings of the 2nd South Pacific Regional Conference on Earthquake Engineering, Wellington, New Zealand, , Ricciardelli, F., Pizzimenti, A.D. and Mattei, M. Passive and active mass damper control of the response of tall buildings to wind gustiness, Engineering Structures, Vol. 25(9), pp , 23. Soto-Brito R, Ruiz S.E. Influence of Ground Motion Intensity on the Effectiveness of Tuned Mass Dampers, Earthquake Engineering and Structural Dynamics, Vol. 28, pp , Thomson, E. D. P-delta effects in ductile reinforced concrete frames under seismic loading, ME Thesis, University of Canterbury, Christchurch, New Zealand, Vanmarke, E.H. SIMQKE A Program for Artificial Motion Generation, Civil Engineering Department, Massachusett Institute of Technology, Varadarajan, N. and Nagarajaiah, S. Wind response control of building with variable stiffness tuned mass damper using empirical mode decomposition/hilbert transform, Journal of Engineering Mechanics, Vol. 13(4), pp , 24. Yan, N., Wang, C.M. and Balendra, T. Optimal damper characteristics of ATMD for buildings under wind loads, Journal of Structural Engineering, Vol. 125(12), pp , Yang, J.N., Agrawal, A.K., Samali, B. and Wu, J.C. Benchmark problem for response control of wind-excited tall buildings, Journal of Engineering Mechanics, Vol. 13(4), pp ,

EFFECTS OF STRONG-MOTION DURATION ON THE RESPONSE OF REINFORCED CONCRETE FRAME BUILDINGS ABSTRACT

EFFECTS OF STRONG-MOTION DURATION ON THE RESPONSE OF REINFORCED CONCRETE FRAME BUILDINGS ABSTRACT Proceedings of the 9th U.S. National and 1th Canadian Conference on Earthquake Engineering Compte Rendu de la 9ième Conférence Nationale Américaine et 1ième Conférence Canadienne de Génie Parasismique