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Altıncı Ulusal Deprem Mühendisliği Konferansı, 16-20 Ekim 2007, İstanbul Sixth National Conference on Earthquake Engineering, 16-20 October 2007, Istanbul, Turkey BİNALARIN DEPREME KARŞI SİSMİK PERFOMASLARININ AYARLANMIŞ KÜTLE SİSTEMLERİ İLE GELİŞTİRİLMESİ TUNED-MASS SYSTEMS TO IMPROVE THE SEISMIC PERFORMANCE OF BUILDINGS Peter NAWROTZKI1 ÖZET Pasif sismik kontrol stratejileri, bir deprem anında yapıyı etkileyebilecek enerjinin azaltılması temeline dayanır. Bazı iyi bilinen yaklaşımlar, özel elemanların sürtünme, plastik şekillendirme veya başka tür enerji yok edici özelliklerinden faydalanırlar. Bu sunum, binaların sismik performansını geliştirmek için, sönümün nasıl arttırılabileceğine dair özel fikirler verecektir. Bu amaç için ilave kütle sistemleri tavsiye edilmekte ve bunların performansı sarsıntı tablalarında da teorik olarak test edilmektedir. Genel bir kanı olarak bu tarz sistemler deprem uygulamaları için uygun görülmemekte iken, belirli tasarım şartları dikkate alındığı takdirde artık bu genel kanı geçerliliğini yitirmiştir. ‘Tuned Mass Control Systems’ (TMCS) – (Dengelenmiş kütleli kontrol sistemleri) yapının deprem sırasındaki deplasmanlarını, ivmelerini ve sistem içindeki gerilimlerini kontrol etmek için kullanılabilmektedir. Yapının çökmeye karşı emniyeti sağlanabilmektedir. Özellikle bina içinde bir müdahale ve değişikliğin mümkün olmadığı durumlarda, bu sistem aynı zamanda mevcut binaların depreme karşı güçlendirilmesi için de uygulanabilmektedir. Bu sayede yapının depreme karşı güçlendirilmesi çalışmaları sırasında, binanın içindeki normal operasyonlar da kesintiye uğramaz. Anahtar kelimeler: Deprem güvenliği, pasif kontrol, Tuned-Mass Systems, sismik güçlendirme ABSTRACT Passive seismic control strategies are based on the reduction of energy, which affects a structure in case of earthquake events. Some well known approaches make use of frictional, plastic or other energy dissipating behaviour of special devices. The following presentation reflects some special ideas for the increase damping in order to improve the seismic performance of buildings. For this purpose additional-mass systems are proposed and their performance is investigated theoretically as well as on the shaking table. Usually these systems are considered as not suitable for seismic applications, but this thesis is no more valid as a general rule, if certain design approaches are kept. Tuned-Mass Control Systems (TMCS) can be used to control the displacements, accelerations and internal stress variables of a structure in case of earthquakes. The safety against collapse and defined states of serviceability of the structures can be achieved. This system can also be used for the seismic retrofit of existing buildings as the inside of the structure is usually not objective to modification. Hence, the usual operation inside the building may go on during the upgrade activities. Keywords: Earthquake Protection, Passive Control, Tuned-Mass Systems, Seismic Retrofit. 1 Dr. P. Nawrotzki, GERB Vibration Control Systems, Berlin/Essen, Germany, [email protected] 327 328 Tuned-Mass Systems to Improve the Seismic Performance of Buildings INTRODUCTION A well accepted strategy in utilizing seismic control systems is based on the increase of structural damping. As a first idea damping devices can be installed solely. Then, they have the task to damp the relative motion between two structures, two parts of the same structure, or the structure and the ‘rigid’ vicinity. The damping effects may be obtained by friction, plastic deformation or viscose behaviour inside the device. The entire improvement of the seismic performance becomes obvious by different national and international standards. Some well known curves are compared, and Fig. 1 provides an idea of possible control effects. Usually 5 % of critical damping can be assumed for buildings, and an increase of the damping ratio causes a reduction of the stress or acceleration response as indicated by the correction factor ξ. As an example the increase from 5 to 20 % of critical damping would cause a reduction of the induced seismic responses by about 50 % according to the Japanese provisions (see Fig. 1). 1,4 Eurocode 8 1,3 Uniform Building Code 97 Correction Factor ξ 1,2 Taiwan Building Code 1,1 Architectural Institute Japan 1,0 IEEE Std 693-1997 0,9 0,8 0,7 0,6 0,5 0,4 0 5 10 15 20 25 30 Damping in % Figure 1. Seismic control effects depending on structural damping Tuned-Mass Damper Systems (TMD) are widely used for the reduction of vibration caused by wind and traffic like pedestrians or railway trains. Typical structures like slender bridges, stacks, high and slender buildings possess low levels of damping and may therefore undergo unacceptable vibration. TMDs cause control effects which are similar to the increase of damping. Depending on the mass ratio, the tuning frequency and the damping capability the amplitude reduction can be very significant and achieve values of about 10 to 20 % of the figures without TMD. The reduction effects in these applications are higher that in case of seismic events because the governing vibration is similar to stationary motions and the TMD gets better adjusted to the motion. Nevertheless significant reduction effects can also be observed for seismic excitation. The ideas of the improvement of seismic performance according to Fig. 1 can be confirmed by theoretical and practical investigations. In order to distinguish between ordinary Tuned-Mass Systems and those for seismic applications the expression Tuned-Mass Control Systems (TMCS) is used. The layout of such systems is slightly different from that for a usual TMD system. Here, the mass and tuning ratio as well as the damping are chosen according to different criteria. A typical situation for structures is shown in Fig. 2. Here, a multi-storey building is equipped with a tuned-mass system on the rooftop. The additional mass consists of reinforced concrete and rests on helical steel spring devices with integrated dampers. P. Nawrotzki 329 Figure 2. Typical tuned-mass system at the top of a building NUMERICAL INVESTIGATIONS Numerical simulations of buildings under earthquake with tuned-mass systems have frequently been performed. In many cases a special building model is taken and the additional mass is connected with the building elastically; sometimes the mass ratio is varied. Then, different recorded earthquakes are run and the responses of the structure with and without tuned mass are compared. The obtained results are usually not showing a unique picture. It can be concluded from this procedure that the tuned mass improves the response behaviour for most of the investigated cases, but there are also models under seismic excitation without significant improvement. In all of the latter cases without significant difference the structural response without TMCS turned out not to be dangerous for the building. The reasons are the induced internal forces and acceleration responses which are at a low level without any need for further reduction. In these cases the governing natural frequencies are not excited. The described steps for the layout of a tuned-mass system do not reflect the required procedure for real projects! For real projects there is a building with columns, beams, frames, walls, floors, and other important members. The structure consists of certain materials, possesses certain dimensions and there is a certain mass or mass distribution, stiffness, ductility and many other mechanical parameters. On the other hand there is the seismic risk which can be described with statistical parameters. The most suitable representation for engineering purposes can be seen in a site specific response spectrum. Here, for instance, we can directly see whether the building is in the dangerous frequency range and furthermore we can derive artificial base-excitation functions which correspond to the project site. Also recorded seismic events can be taken for the layout of the tuned-mass of a real structure, but in these cases the acceleration-time histories have to be scaled according to the site specific response spectrum. In Fig. 3 two examples for the numerical modeling of buildings with TMCS are given. On the left hand side the first mode of an RC structure is shown with a combined lateral force resisting system. The tuned-mass is modeled as a single mass which is elastically connected to the center of the rooftop. A braced steel frame structure is shown on the right hand side of Fig. 3. There are four RC blocks arranged in a manner that the dead load for a single center column is increased by about 6 % of the TMCS dead load only. 330 Tuned-Mass Systems to Improve the Seismic Performance of Buildings Figure 3. FE structures with TMCS on the roof - RC and steel frame building 0,20 Without TMCS With TMCS Hor. Top Displacement [m] 0,15 0,10 0,05 0,00 -0,05 -0,10 -0,15 -0,20 0 5 10 15 20 Tim e [s] Figure 4. Typical time-history of displacement responses with / without TMCS Fig. 4 shows the performance of the structural response improved by a TMCS. The original response is given as a red curve and the induced peak responses are reduced by about 40 % by only activating the tuned mass (blue curve). It becomes obvious that the mass has a significant influence already at an early phase of the ground motion. The TMCS causes an increase of damping for the structure and this can also be seen in the displacement-time history starting at about 10 s. The residual motion after the strong motion phase is significantly damped out as the amplitude becomes nearly zero after about 15 s. The unprotected structure has still amplitudes of about ±100 mm in this time domain. Having a look at Fig. 1 we can conclude an increase of the damping ratio from 5 to 15 % when the AIJ regulation is taken as a basis. Assuming the UBC 97 we can even derive a damping ratio of about 25 % in case of the activated TMCS. The TMCS significantly reduces the top storey displacements, inter-storey drifts, response accelerations and consequently induced internal stress responses due to earthquakes. The corresponding effects of the performance with TMCS can also be shown in modified phase diagrams (Fig. 5). On the left hand side the original response of an RC structure is given assuming a damping ratio of 5 %. By activating a mass of less than 2 % of the total mass of the building, the response curves change significantly. On the right hand side of Fig. 5 the improvement can be described in terms of displacement and acceleration values. Of course, the tuning frequency plays a role in the improvement of the seismic performance as well as the choice of the critical damping. Optimum values can usually be found in the range of 5 to 20 % also depending on the target motion values. The damping broadens the working frequency band of the TMCS. Acceleration [m/s²] Acceleration [m/s²] P. Nawrotzki 12 8 4 0 -4 -8 -12 -100 -75 -50 -25 0 25 50 75 100 331 12 8 4 0 -4 -8 -12 -100 -75 -50 -25 0 25 50 75 100 Displacement [mm] Displacement [mm] Figure 5. Structural behaviour without (left) and with TMCS (right) in the phase plane EXPERIMENTAL INVESTIGATIONS In parallel to numerical investigations it is always important to compare the results with experimental data. For this purpose shaking table tests have been performed at IZIIS, Skopje, Macedonia. The geometry of the tested steel frame model is given in Fig. 6. The columns as well as the beams are made of steel hollow profile appropriately welded at the joints. The structure in the central span has a special bracing substructure, which is used for modeling the stiffness and damping needed for the test. In the orthogonal direction the frame model has only one span of 1.5 m, but by adding a bracing the structural stiffness in that direction is increased several times. The height of each floor is 0.75 m, while all three spans in the longitudinal direction amounts to 1.5m each. The total mass of the tested structure is 19.0 t, and the mass of the TMCS is 260 kg which corresponds to about 1.3 % of the entire mass. The total mass of 19.0 t has been obtained by adding steel blocks on each floor without any influence on the stiffness. SPRINGS & DASHPOTS TMCS LEVEL 05 LEVEL 03 LEVEL 02 DEAD LOAD SG SG SG REFERENT BEAM LEVEL 04 LEVEL 01 SG SG LEVEL 00 BIAXIAL SHAKING TABLE Figure 6. Tested model on the shaking table with position of the TMCS The performance of the frame with and without TMCS has been studied by simulating ten different earthquakes records. Eight of them are recorded during the real earthquake motion, and two are representing artificial earthquake time histories. Among them are Northridge, Kobe, Mexico City, Vrancea (Romania), Izmit (Turkey). The effectiveness of the TMCS has been estimated based on the difference of time responses of the tested model in terms of accelerations, displacements and strain measurements for identification of axial loads and bending moments. At first, the model was tested with unlocked TMCS, and all selected earthquakes were simulated for minimum three different intensities. The same testing has been repeated with locked TMCS and response-time histories were recorded for the same measuring points as for the case with the unlocked TMCS. From the large number of recorded time histories responses of relative displacements at fifth level (top of the model) and bending strains recorded at the bottom of the middle column of the first floor are presented. In Fig. 7 a comparison of time responses of the mentioned quantities for 332 Tuned-Mass Systems to Improve the Seismic Performance of Buildings unlocked (black curve) and locked TMCS (red curve) are presented for the Turkish earthquake (under reduced intensity). For the locked case the peak relative top displacement is recorded to be about 14 mm (Fig. 7, left), while the activated tuned-mass system causes an improvement of performance by nearly 40 %. 15 600 Level 5 Middle 400 Bending strain 1E-6 Relative Displacement (mm) 10 5 0 -5 -10 200 0 -200 -400 -15 -600 0 2.5 5 7.5 Time (s) 10 12.5 15 0 2.5 5 7.5 10 12.5 15 Time (s) Figure 7. Recorded responses with activated (black) and locked TMCS (red) Qualitatively these results correspond very well to the numerical investigation; the corresponding records are shown in Fig. 4. Here, the same base-excitation function has been used for a another building and under different seismic intensity. Having a closer look at the bending strain response of the middle column (Fig. 7, right hand side) the reduction of peak amplitudes with activated TMCS is even higher than 40 %. It becomes obvious from Fig. 7 that the efficiency of the TMCS becomes significantly larger after 10 seconds of ground motion in this case. The dynamics of the tuned mass becomes better adjusted to the motion of the structure, and thus, the damping effect becomes even more evident. More insight into the mechanical behaviour of the structure with activated TMCS can be derived from Fig. 8. Here the axial force response of the bottom of the middle column is shown with locked (red) and activated tuned mass (black curve) under IEEE excitation. It is well known that this artificial earthquake causes the high amplification of responses for structural frequencies between 1.1 and 8 Hz. As the steel frame possesses first frequencies at about 1.9 Hz resonance effects can be expected. The axial force response can well be controlled by the tuned mass as shown in figure 8, left hand side. The peak values are reduced from more than 10 kN to less than 6 kN. The response is also expressed in terms of the corresponding FFT spectrum on the right hand side of Fig. 8. A structural frequency of 1.9 - 2 Hz becomes visible for both cases, and hence, the additional mass does not cause a significant change of the frequency. The enormous reduction of the spectral amplitude can be understood by an effect which corresponds to the increase of the critical damping ratio. Figure 8. Recorded response of axial force of column and corresponding FFT spectrum P. Nawrotzki 333 CONCLUSION Theoretical and experimental investigations have shown that tuned-mass systems can well be applied for the control of seismically induced responses. The quantity of the additional mass has to be chosen according to the target control efficiency; usual values for these purposes can be found in the range of 1.5 - 4 % of the total building mass. With an increase of the tuned mass the control effect becomes higher. Tuned-mass systems are especially suitable for the reduction of seismic drift ratios. Furthermore the damage of important structural members is reduced and thus, the loss of stiffness by cracks, plastic zones, etc., becomes significantly less. Fig. 9, left hand side, shows the typical nonlinear structural behaviour without TMCS in case of an earthquake. The linear load deflection curve exceeds the limit force for the concerned structural member which is given with a value of 2 units in this typical example. The seismic energy which is represented by the remaining triangle (marked in Fig. 9, left) cannot be taken by the member and hence, it causes plastic deformation. The nonlinear deformation (energy) is increased by the corresponding portion in the rectangular section which is also marked in Fig. 9, left side. The resulting total deflection amounts to a value of 7.25 units where only 2 units are related to linear elastic behaviour. On the right hand side of Fig. 9 the same energetic considerations are made for the structure with activated TMCS. The peak forces are reduced by 40 % and hence, the damage causing energy becomes considerably smaller. The corresponding triangle is also marked in the sketch. The resulting overall deflection is this case corresponds to 3.25 units where again 2 units are related to the linear elastic behaviour of this member. The damage causing energy can be reduced from 10.5 to 2.5 units only by activating the TMCS. Consequently, a remarkable decrease of the structural frequencies by seismic effects can be avoided with a proper layout of the tuned-mass system. Fig. 9. Nonlinear seismic effects in structures without (left) and with TMCS (right) A slight drop of the frequencies in case of seismically induced damage would not yet cause the malfunction of the TMCS because a) the tuning of the additional-mass system is usually sub-critical and hence, the structural frequencies would even approach the optimal frequencies of the TMCS, b) usually a high percentage of critical damping is used for the relative motion between structure and additional mass which spreads the working frequency range of the TMCS. For the seismic upgrade of existing buildings it is also possible to take measurements in regard to the most important structural frequencies. In this case damage sustained during the past periods as well as soil-structure interaction effects are already included in the basic layout parameters for the TMCS. Furthermore non-sufficient rigidity or significant loss of stiffness of the fundamental structure would also become evident by measurement. Properly designed tuned-mass control systems can be characterized as follows: a) They reduce seismically induced responses in terms of displacements, accelerations, internal stresses and strains as well as subsoil demands. b) They increase the structural safety. The collapse of a building becomes less probable and hence, human life is protected. 334 Tuned-Mass Systems to Improve the Seismic Performance of Buildings c) They improve the serviceability of structures. Damage and corresponding repair cost in case of seismic events are reduced significantly. d) In comparison to conventional strengthening methods, the building can usually be under operation during the installation of the TMCS (if no additional measures are required). e) Regarding the overall procedure and required material for the installation of a tuned-mass system this strategy can be classified as 'cost effective‘. REFERENCES Nawrotzki P (2002) “Artificial Increase of Elasticity and Damping for Seismically Excited Structures”, Proc. 12th European Conference on Earthquake Engineering, London, Paper No. 58. Nawrotzki P, Jurukovski D and Rakicevic Z (2005) “Shaking Table Testing of a Steel Frame Structure with and without Tuned-Mass Control System”, Proceedings Eurodyn 2005, Paris. Villaverde R (2002) “Roof Isolation System Implemented with Steel Oval Elements: Exploratory Study”, Proc. 3rd World Conference on Structural Control, Como, Italy. Nawrotzki P and Chouw N (2004) “Effectiveness of Tuned-Mass Dampers in Reducing the Response of Soil-Structure Systems to Near-Source Earthquakes”, Proc. The 11th International Conference on Soil Dynamics & Earthquake Engineering, Berkeley, USA.