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Transcript of 1 LESSLOSS Sub Project 7 Techniques and Methods for Vulnerability Reduction Analyses of hammering...
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LESSLOSS Sub Project 7 Techniques and Methods for Vulnerability Reduction
Analyses of hammering and joints problems between buildings
Lisbon 24th May 07 LESSLOSS Dissemination Meeting
Viviane Warnotte
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Problem description
Building collision - ‘pounding’:•during an earthquake •different dynamic characteristics•adjacent buildings vibrate out of phase •at-rest separation is insufficient
Pounding: an instance of rapid strong pulsationSometimes repeated heavy blows: ‘Hammering’
Building separations often insufficient => need for safe and economical retrofitting methods
Damage to the façades (Mexico 1985)
Total collapse (Mexico 1985)
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• An extended review of the state of the art on pounding and mitigation
Some comments: SDOF cannot provide realistic evaluation of:
- Required plastic rotations- Local shear or bending failure- Sequence and amplitude of relative displacements- Distribution of impacts
=> Need to assess: – Complete structure– Non-elastic response– Various typical situation of adjacent buildings– Good models of impact
• Numerical modelling in various typical situations
• Conclusions
• Guidance for mitigation
Work developed in LessLoss
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Pounding Situations analysed
pounding
Case A Case B Case C
Case E Case F Case G
Case D
Hypothesis of the numerical models in LessLoss:• 2-D analyses• 3 accelerograms and 3 PGA: 0.4g, 0.25g & 0.10g• No spatial variations of the ground motion• No soil-structure interaction• Buildings design Eurocode 8 DCM• Non-linear time history analyses, point plastic hinge models & impact element
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Contact element method (piece-wise impact)
• Linear solid
• Kelvin solid classic
• Hertz contact law
Stereomechanical impact• Instantaneous impact • Momentum balance and coefficient of restitution to modify velocities• Inconvenient: no longer valid if the impact duration is large
Models for impact zone
gapm1
kc
m2
gap
m1
kc
cc m2
Position of impact elements Contact element
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Observation from the analyses:An elastic model cannot predict correctly the behaviour
Structure pounded on the right Structure pounded on the left
Elastic model
Non-elastic model
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time (sec)
Dis
pla
cem
en
t (m
)
MRF 1 Elastic WO Pounding
MRF 1 Elastic e=0.004m -0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time (sec)
Dis
pla
cem
en
t (m
)
W 1 Elastic WO Pounding
W 1 Elastic e=0.004m
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time (sec)
Dis
pla
cem
en
t (m
)
MRF 1 Nonelastic WO Pounding
MRF 1 Nonelastic e=0.004m
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time (sec)
Dis
pla
cem
en
t (m
)
W 1 Nonelastic WO Pounding
W 1 Nonelastic e=0.004m
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Example of results in the analyses of pounding – The case of
Adjacent buildings of equal height, with aligned floor levels
Case A1
Observation:
• Pounding amplifies the displacements of both structures
=> Danger: P-Δ effects and damage to secondary element
• Amplification of the shear action effect => Brittle failure
• Peaks of accelerations => Damage to the contents of the buildings
Case A2
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Example of results in the analyses of pounding – The case of Adjacent buildings of unequal height, with aligned floor levels
Observation:• Lower building massive and strong
=> sway of the taller building abruptly restricted: Whiplash
• Amplification of the shear action effect
=> Brittle failure
• Pounding amplifies the displacements of tall structure
=> Danger: P-Δ effects damage to secondary element
• Peaks of accelerations => Damage to the contents of the buildings
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Pounding mitigation methods
Methods to avoid or limit pounding problems: Seismic gaps (prescribed in codes) Increasing the stiffness of one or both buildings
Methods to strengthen structures : Supplemental energy dissipation in buildings (add X brace…) Strengthening: concrete or steel jacketing local or fibre reinforced
polymers)
Alternative load paths Other techniques
Primary structure away from property limits “crash box interface” Devices between structures PRD’s = Pounding Reduction Devices
=> Techniques alone or combined
Local strengthening
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Link Element
Possible mechanical behaviour of PRD’s
Elastic spring => short/long rod as link
Elasto plastic spring => short/long rod as link
Dampers in link => - fluid damper
- friction damper
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• ability to sustain large force levels and dissipate large
quantities of energy over short displacements;• ability to sustain high strain rate;• ability to sustain many cycles of loading without degradation
of mechanical properties;• predictable and stable mechanical properties over the range of
possible loading amplitudes, displacements and frequencies;• the possibility to test the device to check its properties;• resistance to weather (if not protected);• initial and maintenance cost: links may require strengthening
of their connection zone. Dampers have high initial cost.
Possible criteria in the definition of a PRD
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Recommended type: Hinged barsMain advantages: • prevent from oscillating out of unison. • forces through the connections are small (due to similar dynamic properties).
Effects: • change the dynamic behaviour • could enhance undesirable torsional response.
Links properties: • Stiffness of links kc sufficiently high to preclude pounding; • Not too high, not to create too high restraint forces. • A starting point in design: stiffness of the building, K, evaluated by applying a concentred force at the top storey kc=K.• Maintain elastic response in the linkage.
Pounding mitigation example - Adjacent buildings of equal height, with aligned floor levels and similar structural types, in particular their stiffness
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• Number of links: regular distribution in elevation
• Devices at only few floors
cost
disruption in functionality
• Too few floors: possibly too high forces
• Less effective at the bottom of the buildings
Number and location of the links
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Conclusions
Pounding can cause significant damage
Simplified methods can provide wrong estimates
(elastic non – elastic, SDOF MDOF)
There is a high sensitivity of the system response to data: accelerogram used, relative stiffness, relative mass…of adjacent buildings
There exist various ways of mitigation : seismic gap, links between structures