OPERATIONAL MODAL ANALYSIS AND DYNAMIC CHARACTERISTICS OF ASWAN HIGH DAM

Transcription

1 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt OPERATIONAL MODAL ANALYSIS AND DYNAMIC CHARACTERISTICS OF ASWAN HIGH DAM S. M. Abdel-Rahman Mechanical & Electrical Research Institute, National Water Research Center, Ministry of Water Resources and Irrigation, Delta Barrage, Egypt ABSTRACT In practice, nearly all vibration problems are related to structural weakness, associated with resonance behavior. Operational Modal Analysis (OMA) is a complementary technique to traditional modal analysis methods and is based upon measuring only the responses of test structures. The technique enables the testing of large structures such as bridges and dams that are impossible or difficult to excite by external forces, owing to boundary conditions or physical size. Operational Modal Analysis is capable of estimating the modal parameters such as the mode shape, the natural frequency and the damping ratio. For many years, OMA has been the preferred tool in damage detection of large structures. Mode shapes have been used to identify local damages and the modal frequencies has been used to identify global damages. In the present research, OMA was done using the dynamic analyzer system with the finite element software package to determine the dynamic characteristics of the Aswan High Dam (AHD) structure. Aswan High Dam is considered one of the largest dams in the world and has obvious contribution for development in Egypt. Operation, safety, and stability of AHD is vital and it is very important to review operational conditions and to define dynamic characteristics and excitation sources regularly. Test and analysis include the different components of the structure such as the lower tunnels, the upper tunnel, the drainage tunnel, and along the surface of the structure at different stations. Ambient vibration was also measured along the different components of the AHD structure to determine amplitude of vibration at different exciting frequencies. The results show that the dynamic forces affecting the AHD structure is small and safe where the maximum amplitudes of vibration recorded along the different components tested of the AHD structure do not exceed 65 mm/sec 2 as peak acceleration vibration and 1 mm peak to peak displacement. Dynamic characteristics of the different parts of the AHD structure were determined and proved the safety of the structure. Natural frequencies extracted from OMA were in the range of 0.2 Hz up to 18.0 Hz with very small and safe vibration amplitudes. Exciting frequencies were limited and of small range. No coincidence of one of the natural frequencies with the exciting frequencies of the hydroelectric power station (1.67 Hz). The research helps to predict any local damage and improve any structure weakness. The results indicate consistency and durability of the different components of AHD structure and are all dynamically safe. Keywords: Aswan High Dam, modal analysis, ambient vibration, structure damage.

2 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt 1. INTRODUCTION Large complex structures, after some time of operation, are subjected to structural damage which affects their dynamic behavior to some extent, Tayeb [1]. It is possible to detect structural damage by means of full scale testing; however, accurate structural models are needed to determine damage location and severity, Alfolabi [2]. Structural analysis can be performed in either time or frequency domain using FFT analysis. Time domain is usually known as Random Decrement Method and has been used extensively as a dynamic test, Yang [3], where frequency domain identification is more frequently applied and is used for detecting the damage by observing shifts in predicted natural frequencies, Vandier [4]. In recent years a number of procedures (Juang [5], James [6]) have become available to civil engineers for identifying mode shapes, natural frequencies and damping ratios from response only measurements. In such cases the response is to excitation that is unknown but whose force spectrum is usually assumed to be uniform or slowly varying with respect to frequency. For the type of excitation observed during a highway bridge testing (Brownjohn [7]), the forcing was highly dependent on the dynamics of the vehicle(s) crossing the bridge with frequencies of structural response in resonance shifted through participation of heavy vehicles. It was however discovered that by examining short transients of response induced by light vehicles using the natural excitation technique, reasonable estimates of mode shape and frequency could be obtained. The dynamic behavior of structures is a primordial factor determining its structural stability, reliability and lifespan. Operational Modal Analysis provides an interesting manner to investigate the dynamic properties of the structure while in operation. Indeed it allows identifying eigenparameters of structures excited randomly and for which the force inputs cannot be measured. Structure's vibrational properties like natural frequencies, damping ratios and mode shapes measured from the experiments are used to understand the dynamics behavior of the structure to find out the root cause of problems such as resonance and noise often encountered in real life, Mohanty [8]. Structural integrity can be assessed by comparing modal parameters from the current state of a structure with the modal parameters of the structures in nominal conditions (Doebing [9], Parboo [10]). Modal parameters can give indications on the severity of the damage as well as on its location. Some structures experience vibration due to natural excitations. Sometimes, it becomes essential to test the dynamic properties of a structural system while it is functioning in its operational environment in terms of its internal stress state, its coupling with the environment or its thermal condition. When so-called natural excitations are present during operation of a structural system, ambient vibration testing or OMA needs to be considered (Mohanty [11]). The objective of this research is to determine dynamic characteristics of AHD structure. The structure of AHD consists of many components of different materials. The dynamic loads affecting the structure is mainly from hydroelectric power station

3 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt and variation of water level upstream the structure. Ambient vibration tests and OMA are applied to evaluate dynamic condition of AHD structure. 2. APPLICATIONS OF OPERATIONAL MODAL ANALYSIS (OMA) Modal analysis has been widely applied in vibration trouble shooting, structural dynamics modification, analytical model updating, optimal dynamic design, vibration control, as well as vibration-based structural health monitoring in aerospace, mechanical and civil engineering. Since early 1990 s, OMA has drawn great attention in civil engineering community with applications for off-shore platforms, buildings, towers, bridges, etc. Operational Modal Analysis also named as ambient, naturalexcitation or output-only modal analysis, utilizes only response measurements of the structures in operational condition subjected to ambient or natural excitation to identify modal characteristics. It is also called output-only modal analysis, ambient response analysis, ambient modal analysis, in operation modal analysis, and natural input modal analysis. No matter which name that is used the idea is the same: To do modal analysis without knowing and/or controlling the input excitation. This new modal technology is capable of estimating the same modal parameters as the traditional known techniques. The OMA are Multiple Input Multiple Output, MIMO, techniques. This means that the techniques are capable of estimating closely space modes and even repeated modes with a high degree of accuracy. Traditional modal analysis techniques are typically Single Input Multiple Output, SIMO, or Multiple Input Single Output, MISO, or in the worst case even Single Input Single Output, SISO. Such testing procedures will not be able to find repeated poles due to the lack of mode separation. In non-destructive testing, NDT, the objective typically is to monitor the health of a structure over time. For this reason it is also known as Structural Health Monitoring, SHM. Since the structure is observed during service no other modal tool can provide modal information in such a case. Operational Modal Analysis is a technique for extraction of the modal parameters from vibration response signals. The measured responses are governed by the dynamic characteristics of the system and the forces exciting the system. The derived model thus contains information of both the system characteristics as well as the excitation signals. This is one of the challenges in OMA and some understanding of the nature and the characteristics of the excitation forces are therefore very important in order to interpret and understand the results and be able to derive a proper modal model. For many years, OMA has been the preferred tool in damage detection of large structures. Mode shapes have been used to identify local damages that caused curvature changes and e.g. the modal frequencies has been used to identify global damages. Research is still extensive in this area combining technologies such as modal analysis, neural networks or response surfaces, and stochastic decision theory. The main advantages of OMA are: Testing is cheap and fast since no excitation equipment is needed.

4 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt Measurements are done in the actual operation conditions. Identified modal parameters represent the dynamic behavior of the structure as in reality. Testing does not interfere with the operation of the structure meaning for instance that a power generator need not to be laid still to measure its turbine, or a bridge needs not to be closed to traffic when analyzed. The drawback of Operational Modal Analysis has been that the mode shape scaling has been arbitrary causing incorrect modal participation factors. This has caused problems in applications such as response simulation and structural modification. Recently, however, techniques to obtain the right scaling have been developed and they are now being tested on full scale structures. The results of these tests indicate that for even larger structures it is possible to obtain an accurate Frequency Response Function, FRF, from response measurements only. However, one of the major limitation of OMA has been described in the presence of harmonic excitation the procedures, can in many cases, no longer be applied (Mohanty [11]). 3. MODAL PARAMETER ESTIMATION Experimental modal analysis is a method for comprehensive analysis of a structure's dynamic behavior. It involves measuring vibration response due to a known excitation force and processing these data to estimate asset of modal parameters (the modal model), namely natural frequencies, damping, and model shapes, which summaries the structural dynamics in a given frequency range. The elastic dynamic behavior of a structure is assumed to be governed by an n degree of freedom (DOF) linear differential equation: M x.. (t) + D x. (t) + K x(t) = f(t) (1) Which is also known as the physical or spatial model, where f(t) is a vector of forces acting at each DOF and x(t) and its time derivatives correspond to the displacement, velocity, and acceleration at each DOF. M and K are the real, symmetric mass and stiffness matrices and D is the real, symmetric damping matrix that describes the equivalent viscous damping of the system. A transfer function relating the excitation and response vector is established by taking the Laplace transform of Equation (1), assuming zero initial conditions: [M s 2 + Cs + K] {x(s)} = {F(s)} (2) And rearranging; {X(s)} = H(s) {F(s)} = [Ms 2 + Cs+ K] -1 {F(s)} (3) Where H(s) is the transfer function matrix, which can be factorized into:

5 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt n [ Rr ] [ Rr ] H(s) = + * s λ s λ r= 1 r * r (4) The above is a transfer function pole, in which [R r ] is the residue matrix, and (.) * denotes the complex λrconjugate. Frequency and damping information is extracted from the transfer function poles using the relationship: λ * 2 r, λ r = ξ rω nr ± j ω nr 1 ξ nr and r are the unmped natural frequency and the damping ratio of the rth mode respectively. The frequency response function (FRF) matrix is obtained by substituting s = j into equation (4): H(j) = n r = 1 ϕ 1 r L r j ω λ (5) r The modal residue matrix is factorized into the following modal participation factors and mode shape vectors for the participation factor and modal coefficient between points (p) and (q) for mode (r) as follows: pq ϕ =L (r) pq ; or ( r) ( r) pq T R ϕ r [R r ] = L r (6) Mode shapes are defined as φ pr = ϕ p ϕ p ϕ 1 pn 2 (7) Where ϕ pr is the p th column ofϕ r, can be defined in terms of the modal residues: R p 1 R p 2 ϕ pr = (8) R pn

6 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt Equation (8) implies that the modal participation factor for that reference point (excitation or response) is normalized to unity. A range of methods to identify modal parameters from measured data were developed and these can be broadly grouped by the type of the mathematical model, equivalent to Equations (1-5), that is used as a basis for modal parameter estimation. The simplest and perhaps most intuitively attractive method is the peak picking method. 4. FACILITIES AND RESEARCH MEASUREMENTS A Bruel & Kjaer (B&K) Pulse multi-analyzer system with B&K calibrated accelerometers which were used for acquiring the data. The multi-analysis Pulse performs FFT, 1/n-octave (CPB), order, and overall analyses simultaneously. The software package includes Pulse Modal Test Consultant B&K Type 7753 which was used for geometry-driven data acquisition, then transfer the data to Operational Modal Analysis B&K Type 7760 for analysis and validation.me ScopeVES 7754 is used for extracting modal parameters. These facilities were used for OMA generating time signals, frequency response functions, natural frequencies, damping, mode shapes, and animation of the measured components of AHD structure. Overall vibration levels in terms of displacement and acceleration were measured using a single channel analyzer B&K type 2526 with a vibration software B&K type Measurement test facilities are shown in Fig. (1). Test and measurements include inspection and drainage tunnels and AHD surface as follows: o The Upper Inspection Tunnel, o The Lower Inspection Tunnel Upstream (U.S.), o The Lower Inspection Tunnel Downstream (D.S.), o The Drainage Tunnel, o The surface of the AHD structure at St 22+50, and o The surface of the AHD structure at St Figure (2) shows photograph of the Lower Tunnel D.S, where tests were done on the rock and clay parts. Each tunnel has upstream wall and downstream wall, where tests were done on the two walls of each tunnel. Figure (3) shows cross section of AHD structure showing the inspection tunnels.

7 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt Fig. (1) Test facilities and measurements Fig. (2) Photograph of the Lower Tunnel D.S. Stone >550 Compacted dune sand Stone size > 150mm sluiced with Clay blanket Dune sand Rock muck Silt Upper Tunnel Clay core Filters Stone Coarse sand to dune sand Crushed stone Lower Tunnel U.S Lower Tunnel D.S Fine sand Fig. (3) Cross section of AHD structure showing inspection tunnels 5. RESULTS AND ANALYSIS Results of measurements are introduced and analyzed for the Lower Tunnel D.S., Lower Tunnel U.S., upper Tunnel, Drainage Tunnel, surface of AHD structure at St 22+50, and surface of AHD structure at St Dynamic characteristics and modal parameters are derived for different components of AHD structure. Overall vibration levels were measured and compared to the standards (Richart [12], RGBS [13]). The optimum goal of the present research is to ensure safety and predict any defect or AHD structure weakness due to resonance problem.

8 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt 5.1 Lower Tunnel D.S Rock Part Operational Modal Analysis and ambient vibration test were done on the rock part at the beginning of the tunnel from the East. The tunnel has two walls, one towards the upstream (U.S. wall), and the other towards the downstream (D.S. wall). Measurements were done on the two walls of the tunnel. Measurements were done at seven points, three at each wall 40 m apart, and one at the apex of the tunnel (point 4). Points 1, 2, 3 are on the U.S. wall, where points 5, 6, 7 are on the D.S. wall. Figure (4) shows the time signal measured on the rock part of the Lower Tunnel D.S. The signal is periodic signal of 10-3 sec interval and 20 mm/s 2 peak amplitude. Vibration spectrum measured, Fig. (5), shows a decayed signal of small amplitude in the order of 20 m/s 2 in the frequency range 8-24 Hz. However, some peaks were measured at different exciting frequencies in the range up to 4 Hz of higher amplitude up to 600 m/s 2. Figure (6) shows the seven frequency response functions (FRFs) measured. The six signal measured on the walls were similar where the signal measured on the tunnel apex was higher and distinctive than the others. Modal parameters were derived from FRFs and are shown in Table (1). Natural frequency of the first mode was 1.25 Hz of damping ratio 11.6 %, where the natural frequency of the second mode was 3 Hz of smaller damping 1.38 %. Mode shape at 1.25 Hz, Figure (7) shows small deflection pattern at the tunnel walls. Overall vibration level measured in terms of acceleration and displacement is shown in Fig. (8). Acceleration vibration level on the walls was in the order of 20 mm/s 2, where it was on the order of 30 mm/s 2 on the tunnel apex (location 4). On the other hand, displacement vibration level is varied from 0.2 mm at the tunnel apex up to 0.7 mm at the walls. The vibration level measured was very small and safe according to the standards. [m/s²] 40m Time(Signal 2) - Input Working : Input : Input : FFT Analyzer [m/s²] 800u Autospectrum(Signal 2) - Input Working : Input : Input : FFT Analyzer 20m 600u 0 400u -20m 200u -40m 0 40m 80m 120m 160m 200m 240m [s] Fig. (4) Time signal measured on the rock part of the Lower Tunnel D.S [Hz] Fig. (5) Vibration spectrum measured on the rock part of the Lower Tunnel D.S.

9 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt db (1.0 m/s²)² / Hz 40 Frequency Domain Decomposition - Peak Picking Average of the Normalized Singular Values of Spectral Density Matrices of all Data Sets. Table (1) Modal parameters of the rock part of the Lower Tunnel D. S Mode Frequency (Hz) Damping Ratio (%) Mode Mode Mode Mode Frequency [Hz] Fig. (6) Frequency response functions measured on the rock part of the Lower Tunnel D. S Acceleration Vibration (mm/sec 2 ) Vibration Acceleration (mm/sec2) Vibration displacement (mm) Displacement Vibration levels (mm) Measrment Locations 0 Fig. (7) Mode shape at 1.25 Hz for the rock part of the Lower Tunnel D. S. Fig. (8) Vibration level measured on the rockpart of the Lower Tunnel D. S Clay Part Operational Modal Analysis was done on three sections, each of 120 m length, for the clay part of the Lower Tunnel D.S. Measurements were done on the U.S. wall and D.S. wall every 20 m length. Modal parameters were derived from FRFs and tabulated in Table (2) for the three sections of total length 360 m. For Section (1), natural frequency of the first mode was 0.2 Hz and damping ratio was %. Second mode was of natural frequency 1.4 Hz and of a smaller damping ratio 1.38 %. However, modes of the three sections were slightly different and no coincidence of one mode for the tested sections. This was due to non uniformity, and non linearity of the structure. The structure is huge, composes of different materials with different damping and no consistency of the boundary and operating conditions during performing OMA. Acceleration vibration level measured on the U.S.wall and D.S. wall at 15 locations simultaneously are shown in Fig. (9). Vibration characteristics on both walls were

10 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt similar and of average level 25 mm/s 2. Two peaks of levels 48 mm/s 2 and 62 mm/s 2 were measured on U.S. wall. On the other hand, maximum vibration level measured on the D.S. wall was 36 mm/s 2. Table (2) Modal parameters for the clay part of the Lower Tunnel D. S. 70 U. S. Wall D. S. Wall Clay core section Section 1 Section 2 Section 3 Mode Frequency (Hz) Dumping Ratio (%) Mode Mode Mode Mode Mode Mode Mode Mode Mode A c c e le ra tio n v ib ra tio n (m m /s e c 2 ) Measurement locations Fig. (9) Acceleration measured on the walls of the clay part of Lower Tunnel D. S. 5.2 Lower Tunnel U.S. Operational Modal Analysis and ambient vibration test were done on the clay part of the Lower Tunnel U.S. for 560 m total tested section at 14 points 40 m apart. A one reference point was kept during the test while moving from section to section. Five modes were derived for the clay part of the Lower tunnel as shown in Table (3). Frequency of the first mode was 1 Hz and of a small damping ratio 1.24 %, where frequency of the second mode was 2.4 Hz and of damping %. Overall vibration level in terms of acceleration at U.S. and D.S. walls of the Lower Tunnel U.S. is shown in Fig. (10). Vibration level measured on both walls were nearly identical and of average level 24 mm/s 2 and of maximum level 28.5 mm/s 2 on the U.S. wall. Table (3) Modal parameters for the clay part of the Lower Tunnel U. S. Mode Frequency Damping [Hz] Ratio (%) Mode Mode Mode Mode Mode Acceleration Vibration (mm/sec 2 ) Measurement locations U. S. Wall D. S. Wall Fig. (10) Acceleration vibration measured on the walls of the clay part of the Lower Tunnel U. S.

11 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt 5.3 Upper Tunnel Operational Modal Analysis was done on the rock part and clay part of the Upper Tunnel. Tests were done on the walls of the rock part simultaneously, where tests were done on each wall of the clay part separately. Modal parameters of the rock part are listed in Table (4), where modal parameters for different walls of the clay part are listed in Table (5). Modal parameters of the rock part were completely different than those of the clay parts. First mode of the rock part was of frequency 4 Hz and damping 5.3 %, where it was 0.5 Hz of damping 1.24 %, and 1.0 Hz of damping 1.24 for the U.S. wall and D.S. wall respectively of the clay part. Modal parameters for the walls of the clay part were of slightly different frequencies and identical damping ratio. Mode shape of the U.S. wall of the clay part at 0.5 Hz is shown in Fig. (11), where mode shape of the D.S. wall of the clay part is shown in Fig. (12). Deflection at 1 Hz was higher than that at 0.5 Hz. Vibration level measured in terms of acceleration and displacement at four locations for the rock and clay parts of the Upper Tunnel is listed in Table (6). Vibration level measured at the rock part was higher than that at clay part. Maximum acceleration at rock part was 42.7 mm/s 2, where maximum acceleration at clay part was 26.4 mm/s 2. On the other hand, maximum displacement at rock part was mm, where it is mm at clay part. However, vibration levels measured were checked safe. Table (4) Modal parameters of the rock part of the Upper Tunnel Mode Frequency [Hz] Damping Ratio (%) Table (5) Modal parameters of the wall of the clay part of the Upper Tunnel Mode Frequency [Hz] U. S. wall D. S. wall Damping Frequency Ratio [Hz] (%) Damping Ratio (%) Fig. (11) Mode shape at 0.5 Hz of the U. S. wall of the Upper Tunnel Fig. (12) Mode shape at 1 Hz of D. S. wall of the Upper Tunnel

12 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt Table (6) Vibration level measured on the rock and clay part of the Upper Tunnel Measurement Rock Part Clay Part Locations (mm/sec 2 ) (mm) (mm/sec 2 ) (mm) Point Point Point Point Drainage Tunnel Modal parameters extracted from FRFs measurements for the Drainage tunnel is listed in Table (7). Dynamic characteristics of Drainage Tunnel are different from that of other tunnels. Natural frequency of the first mode was 0.4 Hz and damping ratio 2.7%, where other modes were 4.8 Hz, and 7 Hz. However, vibration level measured on the walls of the drainage tunnel was similar to that measured on other tunnels. Maximum acceleration measured was 33.9 mm/s 2, where maximum displacement was 0.74 mm, as shown in Table (8). However, vibration measured on the Drainage Tunnel was checked safe. Table (7) Modal parameters of the Drainage Tunnel Mode Frequency [Hz] Damping Ratio (%) Table (8) Vibration level measured on the Drainage Tunnel Locations (mm) (mm/sec 2 ) Point Point Point Point Point Structure of AHD at Two different Stations Operational Modal analysis and ambient vibration tests were done at two different stations: St above the core, and at St above Hydroelectric Power Station. Location of vibration measurements at St is shown in Fig. (13), where tests were done at 13 locations on the top surface of AHD and along D. S. slope at different elevations. Points 1, 2 & 3 were measured on the top surface at elev point 4 was measured at elev. 188, point 5 was measured at elev. 179, point 6 was measured at elevation 170, point 7 was measured at elev. 162, point 8 was measured at elev. 154, point 9 was measured at elev. 145, point 10 was measured at elev. 138, point 11 was measured at elev. 130, point 12 was measured at elev. 120, and point 13 at elev Vibration spectrum measured at St is shown in Fig. (14), where two dominant

13 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt exciting frequencies were recorded at 9 Hz and 18 Hz. However, the amplitude of vibration at such frequencies was very small in the order of 100 µm/s 2. On the other hand, average amplitude of vibration at different exciting frequencies was very small and in the order of 10 µm/s 2. Modal parameters for the AHD structure at St are shown in Table (9). First mode was of frequency 2.5 Hz and damping ration 5.8%, second mode is 9 Hz and damping ratio 3.2%, where third mode was 18 Hz and damping 0.55 %. Overall vibration levels measured on the 13 locations in terms of acceleration and displacement are summarized in Table (10). Acceleration levels ranged from 24 to 51 mm/s 2, where displacement levels ranged from 0.1 to 0.9 mm. Maximum acceleration and displacement were measured at point (9), elev Vibration tests were done at St above the hydroelectric power station at 6 locations. Two locations were at U. S. side at elev. 186 and elev Two locations were at the top surface at elev. 196 (one at the U.S. side and one at the D.S. side). Two locations were at D.S. side at elev. 180, and elev Modal parameters are derived and are shown in Table (9). First mode was 0.5 Hz frequency of 11.6 % damping, second mode was 1.25 Hz of damping 1.38%, and third mode was 2.75 Hz of damping 0.59%. Dynamic characteristics were different at the two tested stations. Natural frequencies and damping were completely different. Overall vibration levels measured in terms of acceleration and displacement are shown in Table (11). Acceleration level ranged from 25 to 48 mm/s 2, where displacement ranged from 0.16 to 0.96 mm. Maximum acceleration was measured at the surface, elev. 196 on the U.S. side, where maximum displacement was measured at elev. 191 on the U.S. side. Vibration level measured at the tunnels and AHD surface are similar. However, there is no consistency of vibration acceleration and displacement measured at each location. Sometimes, maximum acceleration was measured at a point of minimum displacement. This is due to sensitivity of vibration parameters to exciting frequencies. Acceleration is sensitive to high exciting frequencies, where displacement is sensitive to low exciting frequencies. The main exciting frequency for AHD structure was mainly from the hydroelectric power station of speed 100 rpm and exciting frequency 1.67 Hz as evaluated before (Abdel-Rahman [14]]). So, at this research both displacement and acceleration were measured as displacement evaluates vibration of low exciting frequency range produced from components of eclectic station and acceleration evaluates safety of the foundation and the structure. More results are in MERI [15].

14 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt Level 196 Level Level 162 Level 130 Level Measurement locations Level Fig. (13) Location of Vibration measurement at AHD surface at St [m/s²] Autospectrum(Signal 4) - Input OVERLOAD Working : Input : Input : FFT Analyzer 80u 60u 40u Table (9) Modal parameters for the AHD at two different stations St St Mode Freq. [Hz] Damp. (%) Freq. [Hz] Damp. (%) u [Hz] Fig. (14) Vibration spectrum measured on AHD structure at St

15 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt Table (10) Vibration level measured on AHD surface at St Locations (mm) (mm/sec 2 ) Point Point Point Point Point Point Point Point Point Point Point Point Point Table (11) Vibration level measured on AHD surface at St Level (mm) (mm/sec 2 ) 180 D.S D.S D.S U.S U.S U.S CONCLUSIONS Ambient vibration tests and OMA were done to determine dynamic characteristics and exciting forces affecting the AHD structure. Tests and analysis include Upper Inspection Tunnel, Lower Inspection Tunnel Upstream (U.S.), Lower Inspection Tunnel Downstream (D.S.), Drainage Tunnel, Surface of the AHD structure at St 22+50, and surface of the AHD structure at St Tests were done at the clay part and rock part of each tunnel. Modal parameters (modes of vibration) including natural frequencies, damping, and mode shapes are determined for each component of AHD structure. Amplitude of vibration in terms of acceleration and displacement were determined and evaluated according to standards. The structure is huge, composes of different materials with different damping ratios and no consistency of the boundary and operating conditions during performing OMA were thoroughly checked and assured. The present research proves that: Modal parameters were completely different for each component of AHD structure. Modes of vibration for rock parts of the tunnels were different than that for clay parts. Modes of vibration were different for different walls and sections for each separate tunnel. This was due to non linearity of various elements of different materials of the structure. Dynamic forces affecting the AHD structure is small and safe. Vibration level measured at the tunnels and AHD surface were similar. Maximum amplitudes of vibration recorded along the different components tested of AHD structure do not exceed 65 mm/sec 2 peak acceleration and 1 mm peak to peak displacement. Natural frequencies extracted are in the range of 0.2 Hz up to 18.0 Hz with very small and safe amplitudes. Damping reached to 12 % for small modes.

16 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt No coincidence of one of the natural frequencies with the exciting frequencies of the hydroelectric power station (1.67 Hz) implying the structure strength. The research helps to predict any local damage and modify structure weakness. The present results indicate consistency and durability of the different components of AHD structure and are dynamically safe. REFERENCES 1. Tayeb, M.M., Modal testing to detect and locate damages in large composite structures, Proceedings of the 16 th International Modal Analysis Conference (IMAC), pp , Santa Barbara, CA, USA, Afolabi, D., An anti-resonance technique for detecting structural damage, IMAC, Vol. 1, pp , London, England, Yang, J.C.S., et al., Application of the induced cracks on an off-shore platform model, Comp. Methods for Offshore Structures, ASME Pub., AMD-37, Vandier, J.K., Detection of structure failure on fixed platforms by measurements of dynamic response, Jr. of Petroleum Tech. (JPT), p. 305, Juang, J., et al., An eigensystem realization algorithm for modal parameter identification and modal reduction, Journal of Guidance (AIAA) 8 (5), James, G.H., et al., The natural excitation technique (NExT) for modal parameter extraction from operating structures, Journal of Analytical and Experimental Modal Analysis 10(2), pp , Brownjohn, J.M.W., et al., Assessment of highway bridge upgrading by dynamic testing and finite element model updating, J. Bridge Eng., Volume 8 (3), p. 162, Mohanty, C.H., et al., Testing a dynamic mechanical analyzer-influence of the measuring column dynamics, IMAC XXI, Kissimmee, Fl., USA, Doebling, S., et al., Computational of structural flexibility for bridge health monitoring using ambient modal data, Proc. 11 th ASCE, pp , Parboo, E., et al., Damage assessment using mode shape sensitivities, Mechanical systems and signal Processing 17(3), pp , Mohanty, C.H., Operational modal analysis in the presence of harmonics excitations, Ph.D. Thesis, Tech. Univ., Delft, The Netherlands, Richart, F.E., et al., "Vibrations of Soils and Foundations", Prentice-Hall, INC., RGBS (Reservoir and Grand Barrage Sector, Zefta Rehabilitation/reconstruction Reversibility Study, Summary Report, RGBS Tech. Workshop, Cairo, April Abdel-Rahman, S.M., et al., Dynamic loads generated due to hydro-electric station of the Aswan High Dam, Proceedings of 3 rd Arab Water Regional Conference, Cairo, Egypt, Mech. & Elect. Research Institute (MERI), Dynamic characteristics and ambient vibration of AHD, Tech. Report, Improvement of Monitoring and Control System for AHD, Authority of High Dam and Aswan Reservoir, 2006.