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