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
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi 3
Pietratagliata (Pontebba, UD)568m
Val Canale / Canal del Ferro23/08/ 2003
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
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
C. Bedon, A. Morassi
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
6Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
The bridge - Details
7Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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
8Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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
9Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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
10Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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
11Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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)
12Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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
13Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
FE updating (ii) - Refined FE model (M02)
14
FE-model refinement:
Improved dynamic estimations
BUT high modeling and computational cost
iterative solution of numerical instabilities and uncertainties
OkNo
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
FE updating (ii) - Refined FE model (M02)
15
FE-model refinement:
Improved dynamic estimations
BUT high modeling and computational cost
iterative solution of numerical instabilities and uncertainties
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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
19Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
Sensitivity analyses (FE-M02 M02-DAM)
Redistribution of axial forces on the stays
20
1 2 3
x
x
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
Sensitivity analyses (FE-M02 M02-DAM)
Variation of the predicted FEA natural frequencies, compared to the undamaged bridge (M02-FULL)
21
1 2 3
x
x
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
Sensitivity analyses (FE-M02 M02-DAM)
High sensitivity of the modal shapes to damage
22
Downstream Upstream
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
Sensitivity analyses (FE-M02 M02-DAM)
High sensitivity of the normalized modal shapes to damage
23
Downstream Upstream
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
Sensitivity analyses (FE-M02 M02-DAM)
High sensitivity of the normalized modal shapes to damage
24
Downstream Upstream
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
Sensitivity analyses (FE-M02 M02-DAM)
High sensitivity of the normalized modal shapes to damage
25
Downstream Upstream
Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
Sensitivity analyses (FE-M02 M02-DAM)
MAC decrease or anomalous variations
26Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
‘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
27Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi
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.
28Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing
C. Bedon, A. Morassi