Identification of the Swiss Z24 Highway Bridge by Frequency Domain Decomposition Brincker, Rune; Andersen, P.

Transcription

1 Aalborg Uiversitet Idetificatio of the Swiss Z24 Highway Bridge by Frequecy Domai Decompositio Bricker, Rue; Aderse, P. Published i: Proceedigs of IMAC 2 Publicatio date: 22 Documet Versio Publisher's PDF, also kow as Versio of record Lik to publicatio from Aalborg Uiversity Citatio for published versio (APA): Bricker, R., & Aderse, P. (22). Idetificatio of the Swiss Z24 Highway Bridge by Frequecy Domai Decompositio. I Proceedigs of IMAC 2: A Coferece o Structural Dyamics : februar 4-7, 22, The Westi Los Ageles Airport, Los Ageles, Califoria (pp ). Society for Experimetal Mechaics. Geeral rights Copyright ad moral rights for the publicatios made accessible i the public portal are retaied by the authors ad/or other copyright owers ad it is a coditio of accessig publicatios that users recogise ad abide by the legal requiremets associated with these rights.? Users may dowload ad prit oe copy of ay publicatio from the public portal for the purpose of private study or research.? You may ot further distribute the material or use it for ay profit-makig activity or commercial gai? You may freely distribute the URL idetifyig the publicatio i the public portal? Take dow policy If you believe that this documet breaches copyright please cotact us at vb@aub.aau.dk providig details, ad we will remove access to the work immediately ad ivestigate your claim. Dowloaded from vb.aau.dk o: jauar 1, 218

2 Idetificatio of the Swiss Z24 Highway Bridge by Frequecy Domai Decompositio Rue Bricker Departmet of Buildig Techology ad Structural Egieerig Aalborg Uiversity Sohgaardsholmsvej 57, DK 9, Aalborg Palle Aderse Structural Vibratio Solutios ApS NOVI Sciece Park, Niels Jeres Vej 1, DK 922 Aalborg East, Demark Abstract This paper presets the result of the modal idetificatio of the Swiss highway bridge Z24. A series of 15 progressive damage tests were performed o the bridge before it was demolished i autum 1998, ad the ambiet respose of the bridge was recorded for each damage case. I this paper the modal properties are idetified from the ambiet resposes by frequecy domai decompositio. 6 modes were idetified for all 15 damage cases. The idetificatio was carried out for the full 3D data case i.e. icludig all measuremets, a total of 291 chaels, a reduced data case i 2D icludig 153 chaels, ad fially, a 1D case icludig 2 chaels. The modal properties for the differet damage cases are compared with the modal properties of the udamaged bridge. Deviatios for frequecies, dampig ratios ad MAC values are used as moitorig variables. From these results it ca be cocluded, that frequecies ad model shapes for the structure chaged sigificatly durig damage. Further, it ca be cocluded, that the spatial iformatio gaied by a large umber of chaels does ot seem to result i sigificat better estimates of frequecy ad mode shape deviatios. Itroductio to data ad idetificatio This paper describes the modal idetificatio of the old Swiss highway bridge Z24 at highway A1 betwee Ber ad Zurich. The bridge was demolished i the autum of 1998, but before demolitio, a series of 15 progressive damage tests (PDT s) were carried out. The descriptio of the PDT s ca be foud i Kramer et al. [1]. The damage cases ca shortly be summarized as follows: PDT2: 2 d referece measuremet PDT3: Settlemet of pier, 2 mm PDT4: Settlemet of pier, 4 mm PDT5: Settlemet of pier, 8 mm PDT6: Settlemet of pier, 95 mm PDT7: Tilt of foudatio PDT8: 3 rd referece measuremet 2 PDT9: Spallig of cocrete, 12 m 2 PDT1: Spallig of cocrete, 24 m PDT11: Ladslide PDT12: Cocrete higes PDT13: Failure of achor heads PDT14: Achor heads #2 PDT15: Rupture of tedos #1 PDT16: Rupture of tedos #2 PDT17: Rupture of tedos #3 The data was measured i 9 data sets, 8 data sets with 33 chaels ad oe with 27 chaels (data set 5, the data from the middle of the bridge). Three referece sesors were used, oe uidirectioal, ad oe 3D sesor. All sesors measured acceleratios with a sesitivity of 5 Volts/g. The bridge was loaded by atural loads ad by the traffic passig uder the bridge. Each data set cosists of 1.9 miutes log time series sampled simultaeously at 1 Hz. Before aalysis the time series were decimated by a factor three correspodig to a effective samplig rate of 33.3 Hz. All data were aalyzed usig frequecy domai decompositio (FDD) as described i Bricker et al. [2], however all data preseted i this paper were estimated usig the ehaced versio of the FDD (EFDD), Bricker et al. [3]. All idetificatios were performed usig the ARTeMIS Extractor software. The full 3D data case The first two data sets are show i Figure 1. As it appears three sesors are keep as referece sesors at the same positio i all data sets while the remaiig sesors are moved alog the bridge, oe row at each side of the bridge, 442

3 oe row i the middle. There were 12 measuremet poits o the bridge deck, 3 i each row, ad 16 o the piers, 8 o each of the two piers. Thus, the total umber of measuremet poits was 136. It is assumed that the trasverse ad logitudial horizotal movemet of the bridge deck is the same over the cross sectio of the bridge deck. Usig this assumptio, the 3D movemets ca be estimated i all the 136 measuremet poits resultig i a modal model with 48 degrees of freedom. Figure 1. Referece sesors ad movig sesors i the first two out of the ie data sets for the Z24 bridge. The 6 modes estimated i all tests are idicated i Figure 2. The results of the idetified frequecies ad dampig ratios for all PDT s are listed i Table 1 ad Table 2 respectively. Typical mode shapes are show i Figures 3-8. The details of the bridge deformatios are determied rather well, deote the clamped behavior betwee pier ad bridge deck as show i Figure 9. Figure 2. Modes idetified i all tests of the Z24 bridge Figure 3. Mode 1, Vertical bedig. Figure 4. Mode 2, Trasversal rockig, horizotal bedig. Table 1. Idetified atural frequecies i Hz PDT Mode 1 Mode 2 Mode 3 Mode 4 Mode 5 Mode Table 2. Idetified dampig ratios i % PDT Mode 1 Mode 2 Mode 3 Mode 4 Mode 5 Mode

4 The 2D data case Removig a large part of the measuremet poits ad discardig all logitudial sesors creates the 2 dimesioal case. I this case oly the vertical ad the trasverse horizotal movemets of the bridge are determied. Figure 5. Mode 3, Vertical bedig ad torsio. This leads to a case with a total 153 chaels distributed i the 9 data sets, 36 measuremet poits o the bridge deck, oe row at each side, ad all measuremet poits kept o the piers. A total of 52 measuremet poits each with 2 degrees of freedom, thus, a case with a total of 14 degrees of freedom. The placemet of sesors for the first two data sets are show i figure 1, ad typical mode shapes for the first two modes are show i Figure 11 ad 12 respectively. Figure 6. Mode 4, Vertical bedig ad torsio. All atural frequecies ad dampig ratios determied i the 2D case were rather close to the estimated values for the full 3D case. The stadard deviatio of the differeces betwee the atural frequecies for the two cases was determied to.4 Hz, ad the stadard deviatio of the differece betwee dampig ratios for the two cases was determied to.2 %. The 1D data case By removig all horizotal sesors ad keepig oly 2 measuremet poits i the side spas ad 4 i the mid spa creates the 1D case. I this case, the vertical displacemets of all supports are assumed to be zero. Figure 7. Mode 5, Vertical bedig. Still all 3 referece sesors was kept, but oly the data sets 2, 4, 6 ad 8 were applied for this aalysis. This leads to a case with a total of 2 chaels distributed i 4 data sets, 8 measuremet poits each with oe degree of freedom, thus a total of oly 8 degrees of freedom. All atural frequecies ad dampig ratios determied i the 1D case were rather close to the estimated values for the full 3D case. The stadard deviatio of the differeces betwee the atural frequecies for the two cases was determied to.6 Hz, ad the stadard deviatio of the differece betwee dampig ratios for the two cases was determied to.33 %. Damage Moitorig Figure 8. Mode 5, Vertical bedig. Before damage ca be detected ad idetified, the modal estimates must be combied with other sources of iformatio to determie whether or ot the structure has bee subject to ay sigificat physical chages. 444

5 Figure 9. Close-up of mode shape for mode 3 showig the stiff coectio betwee pier ad bridge e vi cyd e qu re e f v t R el a i Mode 1 Mode 2 Mode 3 Mode 4 Mode 5 Mode 6 Average g p i D a m Figure 1. The two first data sets for the reduced 2D case. M AC Figure 13. Moitorig variables for the 3D case For that purpose three simple moitorig variables were estimated: frequecy deviatio, dampig deviatio, ad fially mode shape deviatio. Figure 11. Mode 1 for the 2D case. Figure 12. Mode 2 for the 2D case The frequecy deviatio was calculated as the relative frequecy drop, i.e. the drop of atural frequecy betwee the specific PDT ad the referece test divided by the atural frequecy of the referece test. The mode shape deviatio was calculated as the deviatio of MAC value from uity. The moitorig variables were calculated for the 3D, the 2D ad the 1D cases, ad the results are show i Figures 13, 14 ad

6 Mode 1 Mode 2 Mode 3 Mode 4 Mode 5 Mode 6 Average Mode 1 Mode 2 Mode 3 Mode 4 Mode 5 Mode 6 Average e vi d cy e qu re e f v t R el a i e vi d cy e qu e r e f v t R el a i g p i D a m g p i D a m M AC.5 M AC.1 Figure 13. Moitorig variables for the 2D case Figure 14. Moitorig variables for the 1D case As it appears from the results of the moitorig ivestigatio, a clear frequecy drop for all 6 modes ca be detected for damage states 4-6. For later damage states a somewhat smaller but sigificat drop ca be detected. Regardig frequecy deviatio o sigificat differece ca be see betwee the 3D, the 2D ad the 1D case. For damage states ad later, a sigificat icrease i the dampig ratio ca be detected for the 3D ad the 2D case, however, a similar icreased caot be see for the 1D case. Some icreased i mode shape deviatio ca be see for all damage states. However, sice the deviatio is oe-sided, some of the deviatio is due to radom errors, ad ot ecessarily due to physical chages. Later states show a clear icrease i the deviatio idicatig that damage has bee itroduced. The later mode shape deviatio is stroger i the 2D ad 1D case tha it is for the 3D case. Coclusios With the frequecy domai decompositio techique it was possible to detect all 6 bedig/torsio modes i the frequecy rage from -16 Hz for the full 3D case icludig 48 degrees of freedom, for the 2D case icludig

7 degrees of freedom, ad the 1D case icludig 8 degrees of freedom. Natural frequecies ad dampig ratios was practically the same for the 3D, 2D ad the 1D case. Moitorig of the data was performed usig frequecy, dampig ad mode shape deviatios. All three deviatios clearly idicated that damage had bee itroduced, however, the clearest idicatio is believed to be based o frequecy deviatio. Moitorig based o the 3D, 2D ad the 1D case showed that there is oly a very little differece betwee the moitorig of the three cases. Thus, i practice whe a structure is to be moitored, the results of this ivestigatio suggests that a good moitorig ca as well be based o a limited umber of sesors. Ackowledgemets The mai part of the elaborate work of performig all the idetificatios were doe i Oslo i the kitche of Igu Mari Eide. Her compay ad support is gratefully ackowledged. Refereces [1] Kramer, C., C.A.M. de Smet ad G. de Roeck: Z24 Bridge Damage Tests, Proc. Of the 17 th Iteratioal Modal Aalysis Coferece (IMAC), Kissimee, Florida, [2] Bricker, R., L. Zhag ad P. Aderse: Modal Idetificatio from Ambiet resposes usig Frequecy Domai Decompositio, i Proc. of the 18 th Iteratioal Modal Aalysis Coferece (IMAC), Sa Atoio, Texas, 2. [3] Bricker, R., C. Vetura ad P. Aderse: Dampig Estimatio by Frequecy Domai Decompositio, i Proc. of the 19 th Iteratioal Modal Aalysis Coferece (IMAC), Kissimee, Florida,