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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impact• Instantaneous impact • Momentum balance and coefficient of restitution to modify velocities• Inconvenient: no longer valid if the impact duration is large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

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Guide for designers is provided

in Lessloss Deliverables N°46 Rev (LessLoss Website)

Guidance:

- indicates mitigation methods which can be successful and why.

- does not give simple quantitative design method because analyses have shown that simplicity is not possible.

Conclusions - continued