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IN SITU VIBRATION EXPERIMENTS ON INTACT AND MODIFIED BUILDINGS
INTEREST FOR VULNERABILITY ANALYSIS
C. BOUTIN, S. HANS
1. Experiment : structural identification
2. Integrity threshold : first structural damage
3. Interest for vulnerability analysis
Experimental program on 7 buildings (1960-80) before demolition in Lyon suburbs
IN SITU METHODS
0 20 40 60 80 100 120-1
-0.5
0
0.5
1
secondes
mm
/s²
0 20 40 60 80 100 120-30
-20
-10
0
10
20
30
secondes
mm
/s²
10 12 14 16 18 20-60
-40
-20
0
20
40
60
secondes
mm
/s²
Ambient noise Harmonic Shock
~10-5 g ~10-3 g ~10-2 g
MODAL IDENTIFICATIONFREQUENCY – SHAPE – DAMPING
Autocor.
S3
SBF
Ambient noise Harmonic Shock
mm
/s²
Time (s)
Fre
qu
ency
(H
z)
Time (s)
3
21
12
3
Frequency (Hz)
HANS S.&al. , Journal of Sound and Vibration, 2000
BUILDING C (~1975)
MODAL CHARACTERISTICS OF BUILDING C
Ex : Mode LMode 2 L Mode 3 L
0
1
2
3
4
5
6
7
8
0 0,5 1
Eta
ges
BdfHarmoniqueOsc. LibresChoc
0
1
2
3
4
5
6
7
8
-1 -0,5 0 0,5 10
1
2
3
4
5
6
7
8
-1 -0,5 0 0,5 1
S3
First modal frequency evolution
3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5 4.60
0.2
0.4
0.6
0.8
1
PRECAST FACADE PANELS
• Measurable decrease of frequency
• Shear beam model 20 % of story rigidity
Progressive modification
BOUTIN C., HANS S. & IBRAIM E , Revue Française de Génie Civil, 2000
BUILDINGS D-E-F (~1973)
Stories plan
DE F
STRUCTURE-STRUCTURE INTERACTION
kinematic interactions soil impedance
SUPPRESSION OF MASONRY WALLS
Suppressed walls
before
after
Longitudinal direction
Transversal direction
TORSION
FIRST CONCLUSION
• STRUCTURAL INFORMATION
– quasi-elastic behaviour 10-2 g
– identification with ambient noise 10-5 g
– modal characteristics including participating elements frequency < > empirical formula (statistic specific)
• FIRST LEVEL OF USE
– retrofitting
– recalculation (reliable data for fitting complex numerical modelling)
MORE DETAILLED ANALYSIS ?
INTERPRETATION OF MEASUREMENTS
• FACT– Measurements not sufficient– Need of model as simple as possible
• BEAM MODEL (SHEAR, TIMOSHENKO …) ?– Plan + simple assumptions on structural behaviour
(distribution of rigidity …)
• FIT – 1 parameter Econcrete
– Fit of the firt frequency : ‘Ereal’
• CHECK – comparison with higher frequencies
MODELLING OF DYNAMIC BEHAVIOUR
BOUTIN C., HANS S., Computer & Geotechnics, 2003
Modelling by homogenisation
BUILDING C ~ SHEAR BEAM MODEL
• E= 20 GPa => f1 = 3,6 Hz
• Fit of the 1st frequency
Econcrete ~ 31 GPa
– {4,45 Hz, 13,3 Hz, 21,8 Hz} model
– {4,45 Hz, 14,1 Hz, 23,5 Hz} experiment
Comparison of the Shapes
Model Experimental
BUILDING G (~1975)
Story plan
• Fit of the 1st longitudinal frequency Econcrete ~ 16,5 GPa
– longitudinal frequencies (L) : {2,15 ; 6,6 ; 11,8 ; 16,6 } model
{2,15 ; 7,24 ; 14 ; 20,5} experiment – transversal frequencies (T)
{1,86 ; 8,7 ; 19,1} model
{1,56 ; 6,64 ; 14,4} experiment
• Fit of the 1st et 2nd frequencies :L {2,15 ; 7,24 ; 11,8 ; 20,1} model
T {1,56 ; 6,64 ; 14,4} model
BUILDING G ~ TIMOSHENKO BEAM MODEL
Comparison of the Shapes
LINK WITH VULNERABILITY
LIMIT OF ELASTIC DOMAIN UNDER SEISMIC EXCITATION (FRENCH NORMS PS 92)
• CALCULUS
– 1st mode of vibration
– Damage criteria : maximal concrete extension ( = 10-4)
• INTEGRITY THRESHOLD
INTEGRITY THRESHOLD
• Extension criteria max ~10-4 Umax
• Elastic response spectra (norm) U(Asol)
• U(Asol) = Umax Smax : integrity threshold
(S1 , Ia ) Asol = 1 m/s² C8 : Smax = 0,45 m/s² C4 : Smax = 1,07 m/s²
Umax (mm)0,38 0,42 1,8
SECOND CONCLUSION
• INTEGRITY THRESHOLD– Quantified available value based on structural characteristics and
seismic motions
• INTEREST FOR VULNERABILITY ANALYSIS ?– First indicator on safety– Check for strategic buildings and facilities :
• stay in service ?• First structural damage
• LIMITATION : first damage vulnerability
• BEYOND INTEGRITY ?
PLAUSIBLE COLLAPSE SCENARIO
S = 0,45 m/s²
Brittle failure of panel (1st-2nd storey)
Kst 1, 2 = 0,6 Kst
no change in 1st mode shape and frequency
S = 0,52 m/s²
Brittle failure of lift walls (1st-2nd storey)
Kst 1, 2 = 0,2 Kst
Strong change in 1st mode shape and frequency
f =1
2 p H2 n+1L &Kst
mst= 4, 45 Hz f =
1
2 p &0, 2 Kst
n mst= 3, 8 Hz
S = 0,41 m/s²
Failure of last walls (1st-2nd storey)
In this real case:
Integrity Collapse
Other situations
CONCLUSIONS
• INTEREST OF IN SITU EXPERIMENTS
– Structural informations
– Reliable data to fit sophisticated numerical modelling
• INTEGRITY THRESHOLD
– Discrimination of buildings – Presumption of safety
Brittle materials (unreinforced
concrete, masonry)
Wrong design Transparency
(even with ductile materials)
Good design Ductile materials
•Building brittle failure•Vulnerability indicator
•Estimated need of ductility
•Real mode Push-over analysis
?
Used carefully, interesting informations can be drived from in-situ low level experiments, complementary to other methods