34 convegno gngts 17-19 novembre 2015, trieste€¦ · introduction structural identification of...

Chiara Bedon1*, Antonino Morassi2

1) University of Trieste, Italy, [email protected]) University of Udine, Italy

34° Convegno GNGTS

17-19 Novembre 2015, Trieste

1

Introduction

Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing

C. Bedon, A. Morassi

Among the tools currently available for structural investigations, dynamic techniques play an important role for several motivations, and are particularly suitable for flexible systems like suspension of cable-stayed bridges

The paper, in this context, focuses on the experimental and numerical assessment of the dynamic behavior of the cable-stayed bridge in Pietratagliata (UD)

Based on FE-model updating and refinement, the sensitivity of the bridge dynamic response to damage in the cables is investigated

It is expected, based on the current outcomes, that this study could represent a solid background for diagnostic investigations and monitoring programs on the Pietratagliata bridge

2


C. Bedon, A. Morassi 3

Pietratagliata (Pontebba, UD)568m

Val Canale / Canal del Ferro23/08/ 2003


C. Bedon, A. Morassi 4

28/12/2006

The bridge – General description

Cable-stayed bridge located in Pietratagliata (Udine, Italy)

Steel-concrete composite deck

23.3m high steel tower

3 groups of cables

5Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing


Deck support on the pier (NR n.13 side): 2 unidirectional supports

Stays-RC abutment connection (Pietratagliata side)

Stays-tower connection

Stays-deck connection

The bridge - Details



The bridge - Details



2010

2012

https://www.youtube.com/watch?v=QqfxLRAUEoQ

Experimental measurements

Dynamic testing carried out to identify the lowest vibration modes of the bridge

Pure ambient vibration testing

16-channel data acquisition system

Two separates setups for the 16 instruments

Time acquisition of 45’ (≈1600 times the fundamental period of the bridge)

Sampling rate of 400Hz

Setup A

Setup B



Experimental results

Enhanced Frequency Domain Decomposition (EFDD) technique (ARTeMIS)

Six vibration modes identified

Several repeated identifications using different baseband and sets of data

Damping ratios derived from

the inverse Fourier transform

of the fully or SDOF

auto-spectral density function

EMA

Mode

orderMode type

Frequency Damping

[Hz] [%]

1 1st B 1.665 ± 0.001 1.2 ± 0.5

2 1st T 2.669 ± 0.014 0.6 ± 0.1

3 2nd B 3.411 ± 0.012 0.7 ± 0.2

4 2nd T 4.750 ± 0.007 0.4 ± 0.0

5 3rd B 5.261 ± 0.009 0.7 ± 0.2

6 3rd T 7.336 ± 0.002 0.9 ± 0.2



Preliminary FE investigations (M01-A)

Preliminary 3D FE model (SAP2000) created to define the experimental layout

Nominal dimensions of the bridge components

Concrete and steel described as linear elastic materials (Es=206GPa, rs= 78.5kN/m3, ns= 0.2; Ec=42GPa, rc= 25kN/m3, nc= 0.3)

RC deck: 4-node shell elements

Girders and tower: 3D frame elements

Cables: 3D truss elements + lumped masses

RC pier fully neglected

≈50,000 DOFs , ≈6,900 elements

Boundaries Deck: simply supports (NR n.13 side)

spherical hinges (Pietratagliata side) Tower: spherical hinges



EMA and FEA (M01-A) modal correlation

EMA FEA (M01-A)

nf

nf D MAC

[Hz] [Hz] [%] [%]

1 1.665 1 1.452 12.8 99.6

2 2.669 2 2.243 16.0 89.3

3 3.411 3 2.958 13.3 97.3

4 4.750 7 5.160 -8.6 97.3

5 5.261 6 4.561 13.3 93.4

6 7.336 9 7.483 -2.0 91.7

FEA correlation was obtained for all the six EMA vibration modes

Modal correlation:

High MAC values (> 97.3% for bending modes and > 89.3% for torsional modes)

FEA frequencies with discrepancies up to 13% and 16% for bending / torsional modes



FE updating (ii)Refined FE model (M02)

Refined FE model of the bridge (ABAQUS/Standard)

Nominal dimensions of the bridge components

RC deck and girders: 4-node shell elements

Tower: 4-node shell elements

RC pier: 8-node solid elements

Cables: 3D truss elements with lumped masses

Metal bracings: 3d beam elements

Lumped masses for the footbridges

≈700,000 DOFs, ≈160,000 elements

(≈ 50,000) (≈ 6,900)



FE updating (ii) - Refined FE model (M02)

Detail AStays-tower connection

Detail BStays-deck connection

Detail DDeck end restraint(Pietratagliata side)

Detail CTower base restraint

Detail EDeck end (NR n.13 side):unidirectional connector




14

FE-model refinement:

Improved dynamic estimations

BUT high modeling and computational cost

iterative solution of numerical instabilities and uncertainties

OkNo




15

FE-model refinement:

Improved dynamic estimations

BUT high modeling and computational cost

iterative solution of numerical instabilities and uncertainties



Ok

EMA and FEA modal correlation

EMA FEA (M01-A) FEA (M02)

nf

nf D MAC

nf D MAC

[Hz] [Hz] [%] [%] [Hz] [%] [%]

0 1.619 1 1.599 1.2 98.5

1 1.665 1 1.452 12.8 99.6 2 1.619 2.8 99.5

2 2.669 2 2.243 16.0 89.3 3 2.691 -0.8 97.3

3 3.411 3 2.958 13.3 97.3 5 3.234 5.2 96.0

4 4.75 7 5.160 -8.6 97.3 7 4.717 0.7 76.3

5 5.261 6 4.561 13.3 93.4 8 5.295 -0.6 48.4

6 7.336 9 7.483 -2.0 91.7 13 7.371 -0.5 78.4

16

Step I: Application of dead loads (nonlinear procedure)

Step II: Linear modal analysis



Dynamic estimation of the axial force on the cables Ambient vibration measurements carried out on the cables

Measurement of transverse acceleration

time-histories on the vertical plane

of each cable

Estimation of the natural frequencies

Calculation of axial forces on the cables

1 2 3

17

Analytical model:

pinned straight elastic beam subjected to unknown axial force T, in undamped free-vibrations



Dynamic estimation of the axial force on the cables Almost uniform distribution of axial forces between groups

of stays belonging to the same transversal cross-section

High discrepancies (up to 16%) in the axial forces of cables belonging to a same group

Possible symptom of structural anomalies?

Tgroup D

[kN] [%]

Group Cable 1 Cable 2 Cable 3 Cable 4

1U 386.5 1.9 3.2 -2.2 -2.9

1D 380.1 -1.9 1.6 2.8 -2.5

2U 529.6 0.0 2.7 4.4 -7.1

2D 545.2 -10.3 -13.4 15.9 7.8

3U 460.6 5.8 -2.3 6.8 -10.3

3D 452.5 -5.7 1.6 -4.4 8.5

1 2 3

18

UD



Sensitivity analyses (FE-M02 M02-DAM)

Six damage scenarios obtained by removing

1 or 2 cables from the upstream cable groups

1U, 2U, 3U

Major effects of localized damage:

Redistribution of axial forces on the stays

Up to 15% and 60% in the damaged groups

Up to 12% in the undamaged groups, on the upstream side

Almost negligible (3-4%) in the undamaged groups on the downstream side

Modification of the FEA mode order

Small average variation of the predicted natural frequencies (≈0.5-1%, up to ≈5% for few cases only)

High sensitivity of some modal shapes to damage MAC decrease or

anomalous variations

1 2 3

x

x




Redistribution of axial forces on the stays

20

1 2 3

x

x




Variation of the predicted FEA natural frequencies, compared to the undamaged bridge (M02-FULL)

21

1 2 3

x

x




High sensitivity of the modal shapes to damage

22

Downstream Upstream




High sensitivity of the normalized modal shapes to damage

23

Downstream Upstream





24

Downstream Upstream





25

Downstream Upstream




MAC decrease or anomalous variations



‘EMA 0’‘EMA 1’

Conclusions The dynamic characterization of the cable-stayed bridge of Pietratagliata was

carried out using ambient vibration tests and FE analyses

A refined 3D FE model of the bridge was calibrated (FE-OPT), based on natural frequencies and modal shapes extracted from FRFs, as well as dynamic measurements of the axial forces on the cables

The optimized 3D model highlighted the importance of geometrical refinement and computationally expensive but accurate modelling assumptions

The sensitivity of the bridge dynamic parameters to damage in the cables was assessed (FE-DAM), emphasizing the main effects of damage on natural frequencies, mode shapes and axial forces on the cables

It is expected, based on the current outcomes, that the present study could be used as a baseline for further monitoring programs



Acknowledgements

This research was made possible thanks to the interest and the support of the “Dipartimento della Protezione Civile”, Friuli Venezia Giulia. The authors would like to

gratefully acknowledge the cooperation of Drs. G. Berlasso and C. Garlatti.

The authors would like to commemorate the dear friend and colleague Prof. Francesco Benedettini (University of L'Aquila), a great scholar of Structural Dynamics, ambient

vibration testing and operational modal analysis methods on bridges.

The collaboration of Prof. Rocco Alaggio and Dr. Daniele Zulli (University of L'Aquila) during dynamic testing is gratefully appreciated.



34 convegno gngts 17-19 novembre 2015, trieste€¦ · introduction structural identification of...

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