AN EXPERIMENTAL STUDY ON MTMD TO IMPROVE STRUCTURAL PERFORMANCE UNDER DYNAMIC LOADING

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1 AN EXPERIMENTAL STUDY ON MTMD TO IMPROVE STRUCTURAL PERFORMANCE UNDER DYNAMIC LOADING N. Handayani 1, D. Kusuastuti 1, and Rildova 1 1 Faculty of Civil and Environental Engineering, Institut Teknologi Bandung, Bandung, Indonesia Eail: dkusuastuti@si.itb.ac.id ABSTRACT : Studies confired that using optiu paraeters (ass ratio, daping ratio, and frequency range), MTMD can iprove structural perforance under dynaic loading. This study aied to better understand the behavior of structural syste with MTMD experientally, and verify nuerical approaches. Two scaled odels were used in the experiental study, a SDOF and a 3-DOF syste. The odels represent regular structures with doinant first ode. MTMD were then attached to the ain structures, with the total ass ratio of approxiately ten percent of the story ass. Three different nubers of TMD were used, i.e. 1TMD, 3TMD, and 5TMD. For MDOF structure, the locations and distributions of MTMD were also varied fro. The odels without any TMD were used as a benchark odel to evaluate the effectiveness of MTMD. Prior to experiental studies, identification procedures were carried out to each structural odel and dapers to deterine the dynaic properties. The experiental study was conducted using shake table with haronic otions. The responses were recorded using acceleroeters and LVDT. The results show that MTMD can reduce the axiu displaceent of structures under haronic otions, thus confiring nuerical results. Having MTMD instead of TMD also increased the robustness of the systes, deonstrated by the reduction of structural responses for various input otions. It should be noted that for ost cases, using all MTMD located on the top story produced best results copared to other placeents or distributions. KEYWORDS: Multiple tuned ass dapers, experiental study, benchark odel, passive control, dynaic load. 1. INTRODUCTION The behavior of structures subjected to dynaic loads is ore difficult to predict due to the coplexity of the probles. Analysis can be very coplicated and usually requires sophisticated tools. Controlling the structural responses under dynaic loading has been an objective to any researches. Various approaches have been developed to control or reduce the structural responses under dynaic loads. One of the devices used for controlling dynaic response is Multiple Tuned Mass Dapers (MTMD), which can be odeled as several additional asses attached to the ain structure. Studies revealed that the perforance of MTMD syste is deterined by its paraeters, i.e. ass ratio, daping ratio, frequency range (defined as half of the range between the lowest frequency and the highest frequency of TMD), locations, and distributions. Using an optiu cobination of paraeters, MTMD can effectively iprove structural perforance under dynaic loading (Rana and Soong, 1998; Soong and Dargush, 1997; Abe and Igusa, 1995; Kusuastuti and Rildova, 2006). Most of these studies were conducted nuerically to obtain the effectiveness of MTMD syste. Few experiental studies were conducted on the subject to validate the nuerical odels. Therefore, this experiental study aied to better understand the behavior of structural syste with MTMD, as well as to verify the nuerical approaches. 2. EXPERIMENTAL MODELS Two physical odels were used in the study, a SDOF and a 3-DOF syste, which were approxiately of one tenth scale (about 30 c height for SDOF and 100 c height for 3DOF), and were designed within the liitations of the testing facilities at the Structural Mechanics Laboratory, Inter University Center, ITB.

2 The odels were 2D steel structures and constructed such that they acted as shear buildings with only one translational oveent occurred. The coluns were ade of steel plate, and the beas acted as the story asses were ade of steel blocks, thus liiting the oveent laterally in the bea longitudinal axis. The odels were designed to be versatile with reovable parts and easy access for energy dissipation devices. The story asses are siilar, and they can be adjusted along the height of the ain coluns. The odels without any dapers were used as the benchark odels. For structures with MTMD or TMD, dapers were attached to the floor asses using additional trays fastened to the floor asses. Figure 1 shows the structural odels used in the experients. (a) (b) Figure 1 Physical odels of (a) SDOF structure and (b) 3DOF structure. Each daper used for TMD and MTMD syste was odeled and constructed as an SDOF syste. The total ass ratio of a TMD/MTMD syste was approxiately 10 percent of the story ass of the odel. Three different nubers of TMDs were used for each structural odel, i.e. 1TMD, 3TMD, and 5TMD. Tuning of the daper frequencies were done using Den Hartog s solution, with the iddle frequency of the daper in MTMD syste was 0.9 of the frequency of the structure (Den Hartog, 1984). For 3DOF structure, locations and distributions of MTMD were also varied in addition to variation of the nuber of TMDs. Figure 2 shows all dapers used for TMD and MTMD systes in the experiental work. Figure 2 Dapers used for TMD and MTMD.

3 Using variations of structural odels and dapers, 18 configurations of structural syste with MTMD were obtained. They include 4 SDOF odels and 14 MDOF odels, and illustrated in Figure Figure 3 Variations of structural systes with MTMD. The odels were then placed on top of the shake table The test set up for both odels are given in Figures 4 and 5. The responses were recorded using acceleroeters and LVDT. A series of haronic otions in ters of sinusoidal were then selected to be used as dynaic loadings for the odels. The selection for the input otions was based on the structural properties and the shake table characteristics and liitations. LVDT Acc 1 Acc 0 LVDT Shaking table Figure 4 Test set-up of SDOF odel.

4 LVDT 3 Acc 3 LVDT 2 Acc 2 LVDT 1 Acc 1 LVDT 0 Acc 0 Shaking table Figure 5 Test set-up of 3DOF odel. 3. EXPERIMENTAL RESULTS Prior to conducting experiental studies with haronic otions, identification procedures were carried out for each structural odel and the TMDs to deterine their dynaic properties using free vibration tests. Accelerogras were obtained and by utilizing Fast Fourier Transfor to produce the Transfer Functions, the frequencies, ode shapes, stiffness, and daping characteristics were calculated. The results show that the predoinant periods of SDOF and 3DOF structures are approxiately 0.37 sec. and 0.61 sec., respectively. The periods of the dapers were found close to the designed periods. Dynaic loads were then applied to the structure in ters of sinusoidal displaceent tie histories. The axiu displaceent of the shake table ranged fro 2 percent to 6 percent of the axiu stroke, while the frequencies of the input otion were varied fro 1 Hz. to 5 Hz. with 0.5 Hz. increents. Responses were easured using Dynaic Magnification Factor () which is the ratio of axiu roof displaceent of structures with dapers to the axiu roof displaceent of structures without. Dapers. The saller values are, the ore effective dapers reduced structural responses. Table 1 shows results for SDOF structures with 2 percent axiu stroke of the shake table. Both TMD and MTMD see to be sensitive to tuning, shown fro large values. For input frequency that is close to the natural frequency of the structure, resonance was apparent and could not be countered with dapers. Results fro SDOF structure were used to refine the 3DOF odels and dapers used with these odels. Input Frequency Maxiu Displaceent() Table 1 Results of experients on SDOF odel. 1TMD 3TMD 5TMD Maxiu Maxiu Displaceent() Displaceent() 1TMD No TMD 3TMD No TMD 5TMD No TMD Table 2 present results for experients with 3DOF structures by varying the nuber of dapers, the location of MTMD syste, and the distribution of MTMD syste. Soe of tests scheduled were unable to conduct due to

5 large displaceent of structural odels. Table stroke aplitude () Table 2 Results of experients on 3DOF odel. Input Nuber of dapers Frequency Location of MTMD Distribution of MTMD (Hz) 3TMD at 3TMD at 3TMD at 1 TMD 3TMD 5TMD Triangular Unifor 3rd Floor 2nd Floor 1st Floor

6 The experiental results reveal that MTMD can reduce the axiu displaceent of structures under haronic otions. The effectiveness of MTMD varies, depending on the paraeters of MTMD, structural properties, and the input frequencies. Using 5TMDs sees to produce best results, while in general, having MTMD instead of TMD also increased the robustness of the systes, deonstrated by the reduction of structural responses for various input otions. Variation of location and distribution revealed that placing all TMDs at the top floor give axiu reduction in structural response. 4. EXPERIMENTAL VS NUMERICAL RESULTS Nuerical odels were developed for SDOF and 3DOF structures based on the experiental study. Figure 5 shows the results fro both experiental and nuerical studies of SDOF structure with various nubers of dapers. The nuerical analysis was able to estiate the experiental studies, although inor discrepancies still occurred due to iperfections of the odels. The structural response at input frequency of 2 Hz for 3TMD and 5TMD see to be high due to resonance with the frequency of SDOF structure with 5TMD, while 1TMD which was tuned to the frequency of the structure worked very well to avoid this proble. For other input frequencies, MTMD was as effective as TMD, if not slightly better. Max. displaceent, TMD - exp. 3TMD - exp. 5TMD - exp. 1TMD - nu. 3TMD - nu. 5TMD - nu Input Frequency, Hz Figure 6 Coparison of experiental and nuerical results for SDOF structure with 1TMD, 3TMD, and 5TMD. Results for 3DOF odels show that the coparison between nuerical and experiental studies is not as good as SDOF odel. Figure 7 shows that while nuerical analysis predicted that MTMD syste can reduce the axiu top floor displaceents, the experiental study found that the response of systes with MTMD was uch higher. Various iperfections in the experiental odel are likely to contribute to the discrepancies. For exaple, the frequencies of dapers in the experiental odel were not evenly spaced as in the nuerical one. Also, the connections of the structural syste and dapers were prone to oveents of the table. The shaking ight have unfastened the bolts on these connections. However, in the case where the input frequencies were very close to the frequencies of the structure, MTMD systes were able to reduce the structural response. With ore than one DOF, MTMD systes were able to control the response for the first ode or higher.

7 N 3TMD at 1st Floor E 3TMD at 1st Floor N 3TMD Triangular E 3TMD Triangular N 3TMD Unifor E 3TMD Unifor N 3TMD at 3rd Floor E 3TMD at 3rd Floor N 3TMD at 2nd Floor E 3TMD at 2nd Floor Input Frequency, Hz Figure 7 Coparison of experiental and nuerical results for 3DOF structures. 4. CONCLUSIONS MTMD has been introduced as a device used to control dynaic responses of structures. Studies confired that using optiu cobination of its paraeters (ass ratio, daping ratio, and frequency range), MTMD can effectively iprove structural perforance under rando dynaic loading. Therefore, an experiental study was conducted to better understand the behavior of structural syste with MTMD, as well as a verification of nuerical approaches. Two scaled structural steel odels were used in the experiental study, a SDOF and a 3-DOF syste, designed such that these odels acted as shear buildings with one translational oveent. The odels without any TMD attached were also used as a benchark odel. For structural systes with MTMD, MTMD were attached to the ain structures and the total ass ratio of MTMD syste was taken as approxiately ten percent of the story ass of the odel. Three different nubers of TMD were used, i.e. 1TMD, 3TMD, and 5TMD. For MDOF structure, the locations of TMD were also varied fro the first story to the third story, with different distribution of MTMD ass ratio on each floor. Identification procedures were carried out to each structural odel and the TMDs to deterine their dynaic properties. The odels were then placed on top of a shake table and subjected to haronic otions, and the structural responses for odels with MTMD were then copared to the odels without MTMD. Experiental studies revealed that MTMD was able to control structural responses, and the effectiveness of the syste is coparable to TMD syste. In the case of istuning, MTMD was able to overcoe this proble, thus confiring nuerical results. Having MTMD instead of TMD also increased the robustness of the systes, deonstrated by the reduction of structural responses for various input otions. Nuerical and experiental results are coparable, especially for SDOF structure. For 3 DOF odel, the agreeent between nuerical and experiental odels is not as good as expected, due to iperfections found in the physical odel. Further studies need to be conducted to analyze and odel the iperfections and include the in nuerical analysis. It should be noted that both experiental and nuerical studies reveal that for ost cases, using all MTMD located on the top story produced best results copared to other placeents or distributions.

8 ACKNOWLEDGMENT This research was partially funded by Riset ITB No. 0018/K /PL2.1.5/I/2006 and Riset ITB No. 0018/K /PL2.1.5/I/2007. The support is gratefully acknowledged. REFERENCES Abe, M. and Fujino, Y. (1994). Dynaic Characterization of Multiple Tuned Mass Dapers and Soe Design Forulas. Earthquake Engineering and Structural Dynaics 23, Abe, M. and Igusa, T. (1995). Tuned Mass Dapers with Closely Spaced Natural Frequencies. Earthquake Engineering and Structural Dynaics 24, Den Hartog, J. P. (1984) Mechanical Vibrations, Dover Publications, Inc. Igusa, T. and Xu, K. (1994). Vibration Control Using Multiple Tuned Mass Dapers. Journal of Sound and Vibration 175, Karee, A. and Kline, S. (1995). Perforance of Multiple Mass Dapers under Rando Loading. Journal of Structural Engineering 121, Kusuastuti, D. and Rildova (2006a). Seisic Perforance of Structures with Multiple Tuned Mass Dapers. Proceedings of 10th East Asia Pacific Conference on Structural Engineering and Construction, Bangkok, August 2006, 7, 89-94, AIT Thailand. Kusuastuti, D. and Rildova (2006b). Paraetric Study of Multiple Tuned Mass Dapers in Reducing Seisic Response of Building Structures. Technical Report of ITB Research Grant 2006 No. 0018/K /PL2.1.5/I/2006, LPPM-ITB, Bandung, Indonesia. Rana R. and Soong, T.T. (1998). Paraetric Study and Siplified Design of Tuned Mass Dapers. Engineering Structures 20, Rildova, Kusuastuti, D., and Handayani, N. (2007). Optial placeent of ultiple tuned ass dapers to iprove structural perforance of buildings under earthquake excitation. Proceedings of the 1st European Asian Conference on Civil Engineering (EACEF), Lippo Karawaci, Septeber 2007, C123-C130. Soong, T.T. and Dargush, G.F. (1997). Passive Energy Dissipation Systes in Structural Engineering, John Wiley and Sons. Yaaguchi, H. and Harnpornchai, N. (1992). Fundaental Characteristics of Multiple Tuned Mass Dapers for Suppressing Haronically Forced Oscillations. Earthquake Engineering and Structural Dynaics 22,