ARCH BRIDGES IN PROVINCIA DI TERAMO: TESTS, IDENTIFICATION AND NUMERICAL MODELS

F. Benedettini, DISAT University of L’Aquila, Italy

R. Alaggio, DISAT University of L’Aquila, Italy

P. Manetta, Professional Engineer, Teramo Italy

ben@ing.univaq.it

Abstract Between 1917 and 1969 six reinforced concrete twin-arch bridges were built in the territory Provincia di Teramo, in Italy. The paper is concerned with the analysis of the structural conditions of such bridges and, among other aspects, with their modal identification in operational conditions. The aim is to calibrate (in the sense of the model updating) their FE models by using the experimental data to constitute reference modal models for a future identification of possible damages should occur to the bridges controlling the evolution of the modal properties as the years go by. The combined use of an unknown excitation (ambient) and a measurable known external excitation will be discussed as well. The veering phenomenon of two subsequent structural modes has been evidenced in two cases and the theoretical aspects have been enlightened in the framework of the linear oscillations theory. Eventually, aspects linked to the systematic use of repeated visual inspection will be addressed.

1 Introduction In the mountainous district of “Provincia di Teramo”, in Italy, more than 300 bridges are operating to overpass rivers and deep valley and, among them, six bridges constituted by two or three r.c. parallel arches (twin arches) surmounted by different kind of frames sustaining the bridge-deck. In the recent past, before the large development of pre-stressed r.c. technology, the structural typology of twin-arches was largely used for bridges ranging in the field of medium spans. The paper is concerned with a reliable condition assessment of such bridges pursued through the methodology of structural identification and combined visual inspections useful to obtain different kind of information and, after combining them, to classify the structural conditions of the bridges by means of synthetic indexes. The six bridges are localized, respectively in Aprati (span 68.5 m), Frattoli (span 31.8 m), Morrice-S.Giovanni (span 28.4 m), Montefino (span 47.3 m), Valle Castellana (span 27.2 m) and Villa Passo (span 58.2 m) in the territory Provincia di Teramo. On this respect, recent

Figure 1 The district Provincia di Teramo

research studies have followed the idea to define a complete framework to asses the health condition through structural identification methodology that contains: experimental and analytical analyses, information technology, decision making arts and non-technical issues [1-5], even integrated with structural control strategy [6].

The paper tackles with the analysis of such bridges revealing surprising maintenance conditions even after several decades from their construction. Moreover, the clear and simple structural scheme permits to obtain, without particular effort, a clean dynamical identification useful, among other possibilities, to monitor the structural conditions of the bridges as the years go by and to be used to identify and fit FE models of the bridges able, in principle, to furnish with good precision the prevision of the structural behaviour under non-frequent load conditions (seismic events and so on). Concerning the model updating procedures, even knowing the need to use multi-parameters updating models, in this work only a simple tuning on few global parameters (Young modulus, mass density and boundary conditions) has been adopted. The updating procedure was based on an error function accounting for the difference of the first natural frequencies and relevant modal shapes between the experimental data and the numerical prediction. Even with the mentioned limit on the number of adopted parameters the reached agreement after the updating process is quite good in all the six cases.

After performing the dynamical tests and the modal identification, a detailed condition analysis based on visual inspections has been conducted on the same bridges with the aim of linking the results of the dynamic analysis and the results of the visual inspections to the general level of degradation and an ad hoc software named PONTILIA [7] was developed to create an integrated data base permitting to easily manage all the information relevant to each structure (text, images, and graphics).

For the analyzed bridges a test and identification procedure has been pursued to obtain reliable results by generally using output only measures and ambient excitation (regular traffic) with the minimum number of transducers able to furnish the first natural frequencies and relevant modal shapes. The bridges have been tested by using 10 accelerometers on 10 to 28 spatial points.

The ambient excitation guarantees the low cost of the testing procedure conducted without closing the bridges. Two different techniques have been used to identify the modal properties; both of them are based on output-only measures. The first method is frequency-domain based and makes use of the decomposition of the power spectral density matrix. The second method is a time-domain based method using a stochastic subspace identification technique to fit a parametric model directly from the data returned by the transducers. Both of them were conducted by using software available in the literature.

Even if output only techniques have been used to identify the dynamical properties of the bridges, in a sample case, a combined use of an unknown and known (measurable) excitation has been considered to verify the level of accuracy reachable, in principle, with the use of output only measures and to understand the level of the possible improvement in the identification by using a more complex technique making use of the knowledge of both input and output.

Among the six cases the dynamical analysis permitted to put into evidence in two cases a seemingly anomalous behaviour interpreted by unfolding the observed phenomenon with the help of the FE companion model; in particular:

• On the Valle Castellana bridge, the splitting of the two first modal shapes (first symmetric and first anti-symmetric) into two hybrid flexural modes has been observed and the phenomenon has been interpreted as a veering of subsequent structural modes, characteristic phenomenon, among other cases, of curved structures [8]; the phenomenon has been reproduced on the

identified FE model unfolding the variations of frequencies and eigenvectors as the shallowness of the arch is varied around its initial value (geometrical parameter).

• On the Frattoli bridge, the splitting of the first anti-symmetric and first torsional modal shape into two hybrid flexural-torsional modes has been observed. In this second case, differently from the preceding one, the veering parameter has not been evidenced yet, and the relevant unfolding is still under study.

The merging of information coming from the detailed and repeated dynamical tests and visual inspections permitted to classify the maintenance level of each bridge forecasting, on the same time, a coherent even approximate repairing cost by means of the dedicated section built inside the software PONTILIA.

2 The analyzed bridges

Villa Passo Aprati Montefino

Morrice Frattoli Valle Castellana

Figure 2 The analyzed bridges

The analyzed bridges have, in all the cases, the deck sustained by the arches by means of variable shape, interconnecting, spatial frames. The main structure is constituted by two or three twin arches having a variable section (constant base and decreasing height between the foundation and the arch top). The arches are, in general, connected in the transversal direction by means of secondary structural elements having different geometries. In all the cases the bridge-deck is constituted by a variable number of r.c. principal and secondary beams forming a planar grid and a surmounting r.c. slab. In the case of the Villa Passo bridge (top-left picture in Fig.2) the main structure incorporates also two caisson Vierendeel beams also visible in the figure. In all the cases the arches have direct foundations. The present research tackles the problem of the structural identification of bridges and tries to develop a more general framework useful for the assessment of bridge health-conditions. The synthesis of the information is also useful for the management of inspections and maintenance works. To this end the use of modal testing techniques will be discussed together with the construction of FE models updated by using the available experimental information concerning the

modal properties. The relevant discussion is based on the results obtained during an extensive campaign of on-site testing conducted in the last three years on the six bridges.

3 The identification procedure The adopted identification techniques (frequency and time domain) are both based on ambient excitation and consequently are classified as Natural Input Multiple Output (NIMO) being the ambient input not measurable. The first tests have been conducted during the summer 2002 and had the twofold objective to constitute the reference term for the subsequent tests coherently to a monitoring program of the structural health conditions and to permit a first updating process on the FE models of the structures. In the examined cases, the cited ambient excitation has been the regular traffic on the bridge (in operational condition tests). On the different bridges an instrumental lay-out oriented to obtain a sufficiently detailed identification (within the above mentioned limits) by using the minimum number of sensors has been always adopted. The number of measured points has been always 10, into 2 parallel lines of 5 sensors (Fig. 3a), except for the Villa Passo bridge on which 28 points of measures into 2 different instrumental set-up has been used (Fig. 3b and 3c). In fact, on this bridge, apart the standard tests generally conducted, more detailed tests oriented to discuss the problem of the possible use of combined unknown and measurable excitations has been adopted [10]. The demand to use a small number of sensors has been essentially due to the quite big number of independent tests conducted in the relatively small period of three years. In Fig.3 the used instrumental lay-out for the bridges is shown.

Figure 3 The standard setup (a), the Villa Passo setup 1 (b) and setup 2 (c)

The length of the acquired time series correspond to a minimum of 3000 1st natural period of each structure and the adopted sampling rate has been of 400 Hz. The modal properties have been determined by using two different software [10-11]. Two different identification techniques have been used:

• The Enhanced Frequency Domain Decomposition (EFDD), technique working in the frequency domain and based on the singular value decomposition of the spectral density matrix. By means of this technique the structural response is decoupled in a series of independent 1 d.o.f. response each of one representing a natural mode of the examined structure. The technique permitted to have very quickly a first draft identification directly during the tests and permitting, in such a way, to be sure about the possibility to identify the modal properties of the structure and to know the good response of the measure chain.

• The Stochastic Subspace Identification (SSI), technique working in the time domain. In this case a parametric model is fitted directly to the raw times series data returned by the accelerometers. The technique, computationally more time consuming than the first one, has been adopted with the twofold objective of validating the results obtained with the first quicker technique and to have a more accurate identification mainly in the cases of nearly coincident modes.

The results obtained for each bridge and relevant to the first two identified modes is reported in the following Fig.4.

dB | 1.0 / Hz

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Frequency Domain Decomposition - Peak PickingAverage of the Normalized Singular Values of

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ARTeMIS Extractor, f 29-4dba-a144-5766, ARTX-0320A-090103PRO, Academic License

Villa Passo: 5 identified modes: 1st anti-sym. planar mode f=3.67 Hz 1st symm. planar mode f=6.64 Hz dB | 1.0 / Hz

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Frequency Domain Decomposition - Peak PickingAverage of the Normalized Singular Values of

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ARTeMIS Extractor, f 29-4dba-a144-5766, ARTX-0320A-090103PRO, Academic License

Aprati: 5 identified modes: 1st symm. planar mode f=2.97 Hz 1st anti-sym. planar mode f=4.03 Hz dB | 1.0 / Hz

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Frequency Domain Decomposition - Peak PickingAverage of the Normalized Singular Values of

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ARTeMIS Extractor, f 29-4dba-a144-5766, ARTX-0320A-090103PRO, Academic License

Montefino: 5 identified modes: 1st anti-sym. planar mode f=4.40 Hz 1st symm. planar mode f=6.76 Hz dB | 1.0 / Hz

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Frequency Domain Decomposition - Peak PickingAverage of the Normalized Singular Values of

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ARTeMIS Extractor, f 29-4dba-a144-5766, ARTX-0320A-090103PRO, Academic License

Morrice: 5 identified modes: 1st anti-sym. planar mode f=7.64 Hz 1st symm. planar mode f=12.81 Hz dB | 1.0 / Hz

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ARTeMIS Extractor, f 29-4dba-a144-5766, ARTX-0320A-090103PRO, Academic License

Valle Castellana: 4 identified modes: 1st hybrid planar mode f=8.30 Hz 2nd hybrid planar mode f=8.75 Hz dB | 1.0 / Hz

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Frattoli: 3 identified modes: 1st hybrid flex-tors mode f=5.27 Hz 2nd hybrid flex-tors mode f=5.99 Hz

Figure 4 The identification results (frequency domain analysis)

4 The Villa Passo bridge and OMA vs. OMAX techniques In the present paragraph the case study relevant to the Villa Passo bridge is presented: the principal steps of the identification procedure in the frequency domain, in the case of output only measures identification, is reported in Fig.5 where the spectral density functions for the two considered setups are reported in the meaningful frequency ranges.

Frequency [Hz] 0 4 8 12 16 20-80 -60 -40 -20

0 20 p y

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40p y

a) anti-s. sym tors b) anti-s. sym

Figure 5 The peak peaking: a) in plane modes, b) out of plane modes.

Three and two spectral lines are easily recognizable respectively and correspond (left) to the first anti-symmetric, first symmetric and first torsional planar modes and (right) to the first two out of plane modes (anti-symmetric and symmetric). For each identified frequency the corresponding modal shape has been evaluated and the relevant results are reported in the Fig. 6.

Figure 6 OMA identification results

After identifying each mode in the frequency domain, going back in the time domain it is possible to estimate the relevant modal damping by means of the logarithmic decrement method. In the civil engineering field, several aspects suggest the choice of operating in the field of OMA (operational modal analysis) and output only measures more than operating in the more classical environment of EMA (experimental modal analysis):

• the interest to obtain the dynamical properties of the structures during their operational conditions including the modelling of the interaction between the structure and its environment,

• the objective difficulty to apply external forces able to furnish a sufficient level of energy and able to excite the whole interesting frequency range,

• the difficulty to reduce or eliminate the interaction between the structure and the ambient, • the impossibility to measure the ambient forces Nevertheless, to asses the capability of OMA in identifying (at a reasonable level) a modal model of a civil engineering structure, a subsequent test in which an extra exogenous and measurable input was used together the ambient excitation. The adding of an exogenous input produces the situation reported in Fig.7:

Figure 7 Operational Modal Analysis with eXogenous inputs (OMAX)

In a EMA the only interest is in finding the deterministic relation between the measured input (Fi) and the outputs (Yi) and the contribution to response of the unmeasurable forces is considered undesirable and generally treated as disturbing measurement noise. On the contrary by using the OMA approach, the focus is considering the output Yi to a stochastic process as the primary font to estimate the modal parameters. The combined EMA-OMA approach usually called OMAX (Operational Modal Analysis with eXogenous inputs) considers the response Yi as constituted by a combination of a deterministic contribution depending on the Fi and a stochastic contribution from the unmeasurable forces; using such a technique results in a maximal data exploitation. A mode weakly excited by the Fi (usually low frequency modes) could be strongly excited by the unmeasurable forces and could be not accurately identified in a purely EMA framework. The case of the bridge excited by both the regular traffic and an artificially measurable excitation like a drop-mass or an impact hammer represents a good example to show the possible identification improvement obtainable by using such a combined approach. By using output only measures the difficulty to identify modes having frequencies higher than about 10 Hz, suggests adding to the ambient excitation an impact hammer able to excite the bridge in the medium-high frequency-range. While the low frequency modes are also easily identified by using the OMA approach, two more modes located all above the threshold of 10 Hz are identified with the combined excitation (see the following Fig.8 extracted from a deeper treatment of the same case reported in [10]). This circumstance suggests the combined excitation and OMAX analysis when higher modes are needed as is usually the case of identifying localized damages not easily observable on the base of few

lower modes. On the contrary, being the gain not so high and being the identification procedure considerably more difficult, in the standard cases in which a small number of modes is needed, the OMA appears as the easiest and faster technique to obtain, with a reasonable effort, quite good results.

Figure 8 Mode shapes of the Villa Passo bridge (OMAX technique):

a) 1.6 Hz, b) 2.6 Hz, c) 3.6 Hz, d) 5.7 Hz, e) 9.3 Hz, f) 14.9 Hz, g) 17.5 Hz

5 The Valle Castellana bridge and the veering phenomenon Eigensolutions depend on system parameters: the mutual approaching, the possible intersection (crossover), the subsequent avoidance (veering) of the root-loci of two neighbours modes when plotted versus a system parameter is generally of interest in analyzing the spectral properties of a structure. The phenomenon is interesting since affects eigenvectors shapes and the forced response of the system. In the literature the crossover or veering phenomena are often related to initially curved structures were some classes of eigenvalues and eigenvectors show a strong dependence on an elastic-geometric parameter. If the eigenvalues loci of such structures are plotted versus a significant parameter the relevant curves can cross each other (crossover phenomenon, see Fig.9-left representing the case of a symmetric suspended cable). If the initial configuration is non-symmetric the loci of neighbouring eigenvalues, after approaching, veer instead to cross each other (see fig Fig.9-right [8]).

Figure 9 Crossover on a symmetric (left) and veering on a non symmetric (right)

suspended cables

Differently from the crossover an important characteristics of the veering of loci is that the modal shapes associated with each locus are interchanged during the veering in a continuous way giving rise to the so called hybrid modes strongly evident in the approaching zones (see Fig.9 right).

Fig. 10a 1st hybrid mode f = 8,295 Hz Fig. 10b 2nd hybrid mode f = 8,875 Hz

Even if the case of nearly-symmetric structures is one of the most relevant concerning this phenomenon, other situation (e.g. a system sub-matrix with low out diagonal coupling terms) can produce the same behaviour even with symmetric initial configurations. As reported in the introduction, two bridges enlightened an anomalous dynamical behaviour classified as examples of veering of two neighbouring vibration modes. In the case of the Valle Castellana bridge, the identification of two neighbours hybrid modes (see Fig.10) has been unfolded putting into evidence with the help of the FE model the veering parameter and showing the sequence of the modes-hybridization of the first two flexural vibration modes (1st symmetric and 1st anti-symmetric, see Fig.11).

_

8,6800

8,6900

8,7000

8,7100

8,7200

8,7300

8,7400

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0,85700 0,85750 0,85800 0,85850 0,85900 0,85950 0,86000 0,86050 0,86100 0,86150 0,86200 0,86250 0,86300

Z_estese

Freq

uenz

e (H

z)

emisymm symm Fig. 11 The veering phenomenon on Valle Castellana bridge FEM

In the case of the Frattoli bridge the phenomenon appears similar even if the hybridization sequence pertains to the 1st flexural (anti-symmetric) and 1st torsional modes. The relevant unfolding structure is still under study.

6 The Aprati bridge and the FE modelling The identification of a FE model able to correctly reproduce the actual behaviour of the studied structure is always a complicated task. Many questions have to be addressed and, among these the main aspects can be grouped in choice of starting parameters, convergence (nodes and elements numbers, mesh size ..) and sensitivity analysis to system parameters. In all the studied cases, the identification results have been used to update the FE companion models of the bridges to obtain updated models able to reproduce the real dynamical behaviour of each structure [1-3]. The case study presented is relevant to the FE model of the Aprati bridge whose sensitivity analysis showed a quite strong dependence on the material properties and boundary condition.

a

b Fig. 12 The Aprati bridge (a), the FE model of the structure (b)

During the tuning operation [2, 3] the target was minimizing a suitable objective function of frequencies and relevant MAC indexes of corresponding numerical and experimental response.

c d

e

Fig. 13 Instruments lay-out (c), the first experimental (d) and analytical (e) modal shapes.

Nat. frequency Experimental Numerical (base)

Numerical (updated)

Modified parameters

1st symmetric 2.97 Hz 2.601 Hz 2.976 Hz 1st anti-symm. 4.03 Hz 4.454 Hz 4.022 Hz

E +25.78% g -3.60%

Tab. 1 Numerical-experimental comparison

In Tab.1 the comparison between the numerical and experimental frequencies obtained before and after the model updating is showed as well as the values of the variation of the updating parameters on the respect of their base values at the end of the minimization procedure.

7 The health monitoring program and the software PONTILIA The applied research project herein described establishes an extensive modal identification procedure based on ambient excitation and in operational modal analysis to be applied to the bridges of the given cluster to evaluate the modal properties and to use them to control the structural performances as the years go by. Moreover the modal identification has the purpose to constitute the reference basis of the dynamical properties used to update companion FE models of the bridges. Nevertheless, for each analyzed bridge the health condition assessment is also performed by repeating, with a given frequency, detailed visual inspections able to classify, according to well established rating procedures [4], the maintenance condition of every analyzed bridge. An ad hoc software named PONTILIA (Fig. 13) was developed to manage all the information pertaining every catalogued element. The software is organized in the following sections: i) a GIS able to identify each catalogued structure, ii) a section containing every detailed information on the bridge (historical and ambient data, structural information, technical drawings, general pictures and so on), iii) a “damage recognition” section in which every observed damage is recognized in a built in damage catalogue (Fig. 14) and then is rated according to objective classification criteria, iv) a “damage index evaluation” section, v) a “computation of repair costs” section, vi) a “final report” section in which a detailed technical document concerning the maintenance conditions of every bridge is automatically extracted.

Fig. 13 The Software PONTILIA Fig. 14 Rating of different damages

8 Conclusions The research project reported in the paper uses the merging of all the information coming from a detailed dynamic analysis and combined visual inspections to classify the level of maintenance of every analyzed bridge. Sample cases pertaining to the class of r.c. twin arch-bridges present in the territory “Provincia di Teramo” in Italy are presented. Several aspects concerned with the research program have been discussed by means of case studies. The research is still in progress and constitutes the basis for a correct management program to be applied to the analyzed bridges.

References

[1] Aktan A. E., Farhey D. N., Helmicki A. J., Brown D.L., Hunt V. J., Lee K-L., Levi A.: Structural identification for condition assessment: experimental arts, ASCE Journal of Structural Engineering, vol.123(12) pp. 1674-1684, 1997.

[2] Aktan A. E., Catbas N., Turer A., Zhang Z.: Structural identification: analytical aspects, ASCE Journal of Structural Engineering, vol.124(7) pp. 817-829, 1997.

[3] Alaggio R., Gattulli V., Benedettini F.: Modal testing of a retrofitted concrete arch bridge for structural identification and condition assessment, Proceedings of the XVI Italian Congress of Theoretical and Applied Mechanics, Ferrara, Italy, 2003.

[4] Gattulli V., and Chiaromonte L.: Condition assessment by visual inspection for a bridge management system, Computer-Aided Civil and Infrastructural Engineering, vol. 20(2), pp. 95-107, 2005.

[5] Benedettini F., Alaggio R., Fusco F.: Repeated Modal Testing on the Bridges of a Local Public Territorial Authority for Structural Identification and Condition Assessment, on Quality, Reliability and Maintenance-QRM 2004 (Ed. G.J. McNulty), Professional Engineering Publishing, Bury St Edmund and London, UK, pp. 193-197, 2004.

[6] Gattulli V., Romeo F.: Integrated procedure for identification and control of MDOF structures, ASCE Journal of Engineering Mechanics, vol. 126(7), pp. 730-737, 2000.

[7] PONTILIA: An integrated software to manage health monitoring information on a cluster of bridges, issued by DISAT-University of L’Aquila, 67040 Monteluco di Roio, L’Aquila, Italy.

[8] Srinil, N., Rega, G., Chucheepsakul, S.: Large amplitude three-dimensional free vibrations of inclined sagged elastic cables, Nonlinear Dynamics 33, pp. 129-154, 2003.

[9] Caubergue, B.: Applied frequency domain system identification in the field of experimental and operational modal analysis. Ph. D. thesys. Vrije Universiteit, Brussel, 2004.

[10] ARTeMIS Extractor software ver.3.2, 2002, issued by Structural Vibration Solution ApS, NOVI Science Park, Niels Jernes Vej 10, DK 9220, Aalgorg East, Denmark.

[11] SPICE Modal Analysis on Civil Engineering Constructions v.2.0, 1999, issued by Katholieke Universiteit Leuven, Dept. Of Civil Engineering, Belgium.

The authors want to thank Dr. Eli Parloo and Dr. Bart Caubergue for the discussions and the help during the tests of some of the bridges conducted during the summer 2003.