IN SITU VIBRATION EXPERIMENTS ON INTACT AND MODIFIED BUILDINGS INTEREST FOR VULNERABILITY ANALYSIS...
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Transcript of IN SITU VIBRATION EXPERIMENTS ON INTACT AND MODIFIED BUILDINGS INTEREST FOR VULNERABILITY ANALYSIS...
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