Kirkegaard, Poul Henning

Rytter, A.

English

In this paper the possibility of detecting and locating damages in a 20 m high steel lattice mast subjected to natural excitation has been investigated. For the damaged mast seven different damage states were considered. In these damage states a damage was assumed in one of the lower diagonals. These diagonals were cut and provided with a bolted joint implying that a damage could be simulated. Based on 20 periodical measurements during 6 months the sensitivity of the modal parameters, identified by an ARMA-model, to environmental conditions such as wind-direction, wind-speed and air-temperature have been investigated. These sensitivities have been compared with the changes of modal parameters due to a damage. It is found that the measured natural frequencies vary less than one per cent while the measured modal damping ratios vary more thai1 twenty per cent due to different environmental conditions. The measured bending natural frequencies and the measured rotational frequency approximately decrease few per cent and more than ten per cents, respectively, due to a damage corresponding to a removal of one of the lower diagonals. The results also show that a neural network trained with simulated data is capable for detecting location of a damage in the steel lattice mast when the network is subjected to the experimental data

    Aalborg UniversitetDamage Assessment of a Steel Lattice Mast under Natural ExcitationKirkegaard, Poul Henning; Rytter, A.Published in:Proceedings of "Dynamics of Structures"Publication date:1994Document VersionAccepted author manuscript, peer reviewed versionLink to publication from Aalborg UniversityCitation for published version (APA):Kirkegaard, P. H., & Rytter, A. (1994). Damage Assessment of a Steel Lattice Mast under Natural Excitation. InProceedings of "Dynamics of Structures": a workshop on dynamic loads and response of structures and soildynamics, September 14-15, 1994, Aalborg University, Denmark Dept. of Building Technology and StructuralEngineering, Aalborg University.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            ? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            ? You may not further distribute the material or use it for any profit-making activity or commercial gain            ? You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Downloaded from vbn.aau.dk on: November 29, 2020Damage Assessment of a Steel Lattice Mast under Natura1 Excitation P. H. Kirkegaard Department of Building Technology Aalborg University Sohngaardsholmsvej 57, 9000 Aalborg, Denmark e-mad: I6PHK@SVl.BUILD.A UC.DK A. Rytter Applied Mechanic~ Unit, Ispra, Italy Abstract: In this paper the possibility of detecting and locating damages in a 20 ni high steel lattice mast subjected to natural excitation I ias  been investigated. For the damaged mast ceven differeiit damage states were considered. In these damage states a damage was assumed in olie of the lower diagonals. These diagonals were cut and provided with a bolted joint implying tliat a damage could be simulated. Based on 20 periodical measuremeiits during G rnoiitlis tlie sensitivity of tbe niodal parameters, identified by an ARMA-model, to environmental conditions such as wind-directioii, %,ind- speed and air-temperature have been investigated. These sensitivities have been compared witli tlie clianges of modal parameters due to a damage. It is found that the measured natural frequencies vary less than one per cent while the measured modal damping ratios vary more thai1 turenty per cent due to different environmental conditions. The measured bending natural frequencies and the measured rotational frequency approxirnately decreuse few per cent and more than ten per cents, respectively, due to a damage corresponding to a removal of one of the lower diagonals. The results also show that a neural network trained with simulated data is capable for detechng location of a dainage in the steel lattice mast urhen the network is subjected to the experiinental dath. Keywords. Systein identitication, ARMA-model, damage detedion, civil engineering application, neural networks. 1. Introduction Structural diagnosis by rneasuriiig vibrational signals of civil eiigiiieeriiig structiirs i a su1,ject of researdl which has received increasing interest during the last decades. Tlie maiii iinpetus for doing vibrational based inspectioii (VBI) is caused by a wisli to est,ablisli aii alternative daiiiage asscssiiient. method to the more traditionally methods such as e.g. visiial inspection. Man? research projects liave concluded that it is possible to detect darnages in civil engineering structures by VBI, aiid some techniques to locate damages in civil engineeriiig structures have also beeii proposed. However, iiiiicli of the performed research has been based on numerical sirnulations and o11 laboratory iilodela. A throughoiit review of VRI techiiiques caii be found in Rytter [l]. In order to use VRI t,ecliiiiques it is necessary to be able to obtaiii reliable estiinates of tlie dynainic cliaracteristics, e.g. iiatiiral frequencies. Sucli quantities can be estiniat,ed froni tlie resiilting oiitpiit. caused by a kiiown well-defined input. liowever, tlie estirnates can also be estiniated Ib? using tlie so-called arnbieiit testiiig. i e .  tlie oiily escitat,io» oii tlie structure i.; Llie tiatural excitation. Tlie aiiii of tlie research presenI.ed in tliis papei was t,o answer the followiiig questions hy iisiiig fiill-scale ineasiirenieni.s I->ascd on iiatiiral escit.at.ioii: t ' 1) Is it possible to distinguisli between changes in modal psrameters due to effects produced by damages and those brought about as a result of changes in the ambient environmental coiiditions ? 2) How sensitive are measured modal parameters to a damage ? 3) Can the location of the damages be estimated 7 In order to answer these questions a 20 m high steel lattice mast subjected to wind excitation was experimentally investigated. The experimental arrangement are descnbed in section 2. In section 3 the experimental results are presented and discussed and at last in section 4 conclusions are given. 2. Experimental Arrangement An elevation of the 20 m high steel lattice test mast is shown in fig. 2.1. The four chords i<-frame test mast with a 0.9x0.9 m cross-section was bolted with twelve bolts, three for each chord, to a concrete foundation block founded on chalk and covered by sand. The mast was constructed with welded connections. At the top of tlie mast two plywood plates were placed in order to increase the wind-area. The eight lower diagonals were cut and provided with a bolted joint. Each bolted joint consists of.  4 slice plates giving tlie possibility of simulating a 1/4, 1/2, 3/4 and ful1 reductioii of the area of a diagonal. A damage wa5 simulated by reinoving olie or more splice plates in these bolted joiiits. Seveii different damage stat,es (1,2,5,6,9,10,11) were considered. The damage state 1.2,s and 6 correspond to a removal of diagonal AB101, BC101, AB102 and BC102, respectively. see fig. 2.2. Daiiiage states 9 and 11 correspond to fifty per cent reduction of tlie sectional area of diagoilal .4B101 and AB102, respectively. Damage state 10 corresponds to fifty per cent reduction of the sect.ional area of diagonal ABlOl, BCIO1, CDlO1 and DA101. Fig. 2.1 Elevatioii of Mast. Fig. 2.2 Diagonals of tlie lower tua  sectioiis The mast was instrumeiited with iiistrunients to rneasure tlie acceleratioiis, wiiid-directioii (wind-vanej and wind-speed (cup-anemometer). Rirtlier, the ambient air temperature was nieasiird. Tlie data acquisition and the analyse of the saiiipled data by an ARMA-model were perforriied \vit.li tlie MAT- LAB, see PC-MATLAB [2], based on program to Structural Time Domain Identification, STDI, see Kirkegaard et. al. [3]. A t.lirougliout descriptioii of tlie test arraiigeiiieiit caii he foiiiid i n  liirkegaai-d ct al. [.i]. 3. Experimental Results and Discussion In the period from December 92 to June 93 twenty measurements sessions were performed with the undamaged mast . The dates of the sessions were selected in sucli a wa)' that a data base containing measured responses due to different wind-directions and wind-speeds were created. A t  a measurement session 10 time series were recorded for each transducer, i.e. accelerometers as well as cupanemometer and wind-vane. In the same period 2 measurement sessions were performed whew dimages were simulated at the m g t .  In the period the lowest and the highest air temperature were -5'C and 20°c, respectively. 3.1 Modal Parameters of tlie Undamaged Mast I t  was the natura1 bending frequencies no. 1 and no. 4, the natural bending frequencies no. 2 and no. 5 and the natura1 frequency no. 3 corresponding to deflection parallel to the x-axis and deflection parallel to the y-axis and rotation, respectively, whicli wem estimated. The  estimates of tlie natural frequencies and the modal damping ratios are shown as function of the measurement number in fig. 3.1. Tlie 20 estimates in each figure have been obtained by rombining the measured estimates of natural frequeiicies and inodal dainping ratios, respectively, from eacli ineasure- ment session by weighting with the standard deviations. A t  each measurernent sessioii 10 times series were recorded, implying 10 estimat,es of tlie natural hequencies and modal damping ratios. respectively. The  solid lines in fig. 3.1 indicate a mean value of the 20 estimates rliile tlie daslied liiies give aii interval between the mean value plus olie per cent and the mean value ininus one per cent for tlie natural frequencies. In the same way an interval corresponding to the mean value plus ten per cent and the mean value minus ten per cent is shown with dashed lines for tlie modal dampiiig ratios. Fig. 3.1 shows that  the measured natural frequencies vary approximately oiily few per cent while the modal damping ratios vary more tlian twenty per cent. It is seen that the beiiding natural frequencies are more sensitive than the rotational frequeiicy. The standard deviation of the natural frequeiicies and modal damping ratios are approximately 0.003 Hz aiid 0.001, respecti\.ely. Tliis indicates tliat tlie variation of the ineasured modal paraineters is due to changes in tlie environinenral coiiditions aiid . .~ only not due to randomness. In order to investigate tlie sensitivity of iiat.ural frequencies ivitli respect to wind-direction and wind-speed the 200 estimates of tlie natural frequencies are sliown in fig. 3.2a as function of the wind-speed. The estimates have been divided into 4 groups. Each group coriespoiids to a wind-directioii interval of 90 degrees. Fig. 3.2a shows tliat tlie natural frequencies are sensitive to the wind-speed. However, it is most clear for the first and second natural frequency. Further, it is seen ttiat tlie natural frequencies have an increasefor a wind-speed corresponding to i - 8  tii/s wheii the wiiid-direction is changed. However, tliis cliange can also be a coiisequence of a cliaiige i n  reiiiperature. Iii fig. 3.2b the 200 estimates of tlie tiatural freq~iencies are shown as a f~inctioii of the \vind.speed. Tlie estiinates have beeii divided iiito Z groiips. correspondiiig t.o estimates obtained from iiieasiirenieiits wliere the air ternperattire was lower tliaii o O C  and Iiiglier than o°C. respectively. I t  is seeii tliat the iiicrease in natural frequeiicies for a wiiid-speed corresponding to i - 8  m/s can be due to an air temperature below o'C and iiot necessarily a chaiige in tlie wiiid-direction. However. niore data niost be obtained in order to investigate tliis problem. 3.2 Modal Parameters of tlie Damaged Mast In fig. 3.3a and 3.3b the measured natural frequeiicies froiii measureiiieiit. sessions 4 atid 6 are sliowii as a functioii of damage state, respecti\,ely. Tlie solid Lines in fig. 3.3a sliow tlie loii-ei houiid of the 95% confidence level oC tlie tintiiral frequeiicies froin iiieasiireiiieiit. sessioii :1. 'llir esriinales arc assumed Gaussian distributed. Ii i  tlic saiiie way i i i  lig. 3.3h tlie lowct. boiiii<i ol' Llir !15% coiilidenc<, level o€ the natural frequencies from measurement session 5 is shown. The measurement sessions 3 and t 5 (undamaged) correspond to measurement sessions 4 and 6 (damaged), respectively, with respect to environmental conditions, Le. approximately the same wind-speed, wind-direction and air-temperature. This means that a change in the measured natural frequencies caii be interpreted as a cliaiige due to a damage and not to a change in the environmental conditions. Fig. 3.3 shows that it is possible to detect a damage in the mast corresponding to a removal of one of the lower diagonals, damage states 1,2,5 and 6. Further, a damage, darnage states 9 and 11, corresponding to a fifty per cent reduction of the sectional area can also be detected. However, if such a damage should be detected it is important to compare modal parameters from the damaged and undamaged mast, respectively, obtained under the same environmental conditions. 3.3 Estimation of Damage Location by use of Neural Network The applicability of a neural based dainage assessrnent nietliod, see e.g Hertz et al. [5] and I<irkegaard e t  al. [6], is investigated by training a neural network with the relative changes of the natural frequendes of the 5 lowest modes. These clianges were estimated for a 20, 40, 60, 80 and 100 per cent reduction of tlie sectional area of diagonal ABlO1, BC101, AB102 and BC102, respectively. Further, the relative changes of the frequencies also were estiinat,ed for tlie undarnaged mast. By a trial-and-error approach i t  is found that  a 4 layers neural network with 5 input nodes, 5 nodes in eacli of the two Iiiddeii layers and 4 out,put iiodes gave the network with smallest output error. Each output node corresponds to a damage in one of the diagonals AB101, BC101, AB102 and BC102, respectively. The value for a single diagonal adopts the value 1 when not damaged, tlie value O when totally damaged and 0.2 corresponds to a 80 per cent reduction of a sectional area etc. The network was tested by subjecting the sirnulated input data corresponding to a 100 per cent reduc- tion of the sectional area of the four diagonals AB101, BC101. AB102 and BC102, respectively, to tlie network. It was found t,liat the neural network was capable of ieproducing tlie location aiid size of a damage used in traiiiing (Dainage state 1,2.5.6). Table 1 shows t.he outputs from the network subjected to experiineiital data. O"t,>"t I Node I Da)iiage Stale Table 1: Results froin network subjected to esperiinental data No. I (AB101)  ~ ( B C I O I )  3 ( A B 1 0 2 )  4(RC102) Tlie results in table 1 sliow tliat it is possiblc to detect a daiuage corresponding to a re~iioval of a diagonal (Dainage stat,e 1,2,5,G) Iby tlie iieural iietwork approach. It is also seeti tliat a. daniage correspondiiig to 50 per cent rediictioii of Llie sectioiial arra of a diagoiial AR102 <:an b<: detecrecl. biir iioi rluaniifed. 1 0.1 o.a 0.6 0.9 2 0.8 0.1 1.0 0.9 1 o O.& 0.4 U.?  1.1 5 0.9 0.8 0.1 0.9 G 0 .9  1.0 1.1 0.0 Y 0.5 0 .7  I I 2  O.g 1st Natura1 frequency ............... 204 E "2c 202 p i  O 1 O O Q- 7. .................................................. 2 Li 1.98- o 5 10 15 20 Measurement number 206 ,  2nd Natural frequency Measurement number 3rd Natural frcquency 8.25 .. ......................... 2 8.2 g 8.15 h 3 ~ o ~ o o ~ = v o o = ~ = o " - O " - ~ ~ ~  z" 8.1 r;; - x10-3 1st Modal damping ratio 8, 7 Measurernent number xlw 2nd Modal damping ratio 8, 1 Measurement number x l ~ - 3  3rd Modal damping ratio 1.5, I Measurement number Measurement number 4th Natural frequency N 11.5 V C O D O  0 0 O O  0 0  O C ..... .. .. .- .......... O 0  O .  a a .- & 11.4 ................. L m D O  O o o-. .................................................. u. a 0 0 11.3 o O 5 10 15 20 5 10 15 zn Measurement number Measurement number V C ........ 9 .......... a 5 10 15 2( Measurement number Measurement number Fig. 3.1.  Estimated natliral frequencies and modal dainpilig ratios as n f~inct ion of iiieasureineiit, niiniber. (Solid liiies slioiv tlie meaii value aiid daslied lines show tl-ie iiieaii value plus/miiiiis one per ceiii. a n d  plus/niinus t,eii per cent for t h e  iiatural freqiieiicies aiid the  modal dampiiig ratios, respectiveIy). Wind-speed (m/s) Wind-speed (m/s) 206 + + 20 1.98 Wind-speed (m/s) Windspeed (m/s) 3rd Natural bequency Wind-speed (m/s) Wind-speed (m/s) 4th Natural bequency 10 15 20 Wind-speed (m/s) Wind-speed (m/s) 5th Natural frequencj 5 10 15 20 Wind-speed (m/s) Wind-speed (m/s) Fig. 3 .2 .  Natiiral freqiieiicics as a fiiiictioii of wiiid-speed aiirl xviiicl-diirctioii ( a l  aiid as ;I. Ciiiictioi1 of wiiid-speed atid air teiiiperatizie (b).  a) 1st Natural frcqucncy b) 1st Natural frcqucncy 2 7 O 0 1.98 0 1.98 0 5 10 o 5 10 1.96 Damagc Statc Damage State 2nd Natural Ercquency 2nd Natural frcqucncy 2 L 5 10 O 5 10 1.96 Damage State Darnage State 3rd Natural frcquency 3rd Natural frcqucncy g 2z 8F?-=====j 7 i ; \ o "  6 6 2 O 0  2 o 0 6o 5 10 6o 5 10 Damage State 4th Natural frequency O æ m E L 10.5 o 5 10 Damage State 4th Naturai frequency Damage Statc Darnage State 5th Natural frequency 5th Natural frequency o Damage State Darnagr State Fig. 3.3  Estimated iiat,iira.l Creqiieiicies Croiii nieasureiiieiit session I (a )  an<l ri (h )  a s  a Iiiiictioii of daniagt st,ate. (Solid lines are the lower bound oF llie 95% coiifideiice level for rs(.iniated ~ial.iiral frrqueiicies from iiieasiiremeiit scssioiis 3 (a) aiicl 5 (b ) ,  respectively) 4. CONCLUSIONS In  this paper the natural frequencies and modal damping ratios of a 20 m high steel lattice mast subjected to natural excitatioii Iiave been experimentally investigated. The  conclusions of the paper can be stated as follows: Measured natura1 frequencies vary less than one per cent while the measured modal damping ratios vary more than twenty per cent due to different environmental conditions, such as- wind-speed and air-temperature The  measured bending natural frequencies and the rotational frequency approximately decrease few per cent and more than ten per cent, respectively, due to a damage corresponding to a removal of one of the lower diagonals. It is possible to  detect a damage corresponding to a removal of a diagonal using a system identifi- cation technique (ARMA) based on natura1 excitation. A fifty per cent reduction of the sectional area of a diagonal can be detected, if tlie ineasured modal parameter5 from the damaged mast and the undamaged mast, respectively, are obtained under the same environmental conditions. A neural network trained with simulated data is capable for detecting location of a damage, corresponding to  a removal of a diagonal wheii tlie network is subjected to experiinental data. 4. REFERENCES [l] Rytter. A.: Vibration Based Iiispection of Civil Engiiieering Structures. Ph.D-tliesis, Aalborg University, Denmark, 1993. [Z] PC-MATLAB for MS-DOS Personal Computers, The Math Works, Inc., 1989. [3] Kirkegaard, P.H. 8r. R. Briiicker: STDI: Program for Structural Time Domain Identification. Uiiiversity of Aalhorg, 1993. [4] Iiirkegaard, P.H. & A. Rytter: An Experimental Study of tlie Modal Pararneters of a Dainaged Steel Mast. Racture and Dyiiamic Paper No. 45, Uiiiversity of Aalborg, 1993. [S] Hertz, J . ,  A. I<rogh & R.G. Palmer: Introductioii to the Theory of Neural Computation. Addisoii- Wesley, Redwood City, CA, 1991. [6] Iiirkegaard, P.H. A. Rytter: Use of Neoral Networks for Dainage Detection and Location in a Steel Meiiiber. Proceedings of tlie t1iit.d Interiiatioiial Confereiice o11 Use of Artificial Intelligence iii Civil E~igiiieering, CIVIL-COMP93, 1993. 

Damage Assessment of a Steel Lattice Mast under Natural Excitation

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Damage Assessment of a Steel Lattice Mast under Natural Excitation

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