LESSLOSS Sub Project 7 Techniques and Methods for Vulnerability Reduction

1. LESSLOSS Sub Project 7 Techniques and Methods for Vulnerability Reduction Analyses of hammering and joints problems between buildings Viviane Warnotte Lisbon 24th May 07 LESSLOSS Dissemination Meeting

2. 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 pulsation • Sometimes 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)

3. Work developed in LessLoss • 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

4. Pounding Situations analysed • 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

5. Models for impact zone • 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impact • Instantaneous impact • Momentum balance and coefficient of restitution to modify velocities • Inconvenient: no longer valid if the impact duration is large Position of impact elements

6. Observation from the analyses: An elastic model cannot predict correctly the behaviour

7. Example of results in the analyses of pounding – The case of Adjacent buildings of equal height, with aligned floor levels • 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

8. 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

9. 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

10. 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

11. Possible criteria in the definition of a PRD • 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.

12. Pounding mitigation example - Adjacent buildings of equal height, with aligned floor levels and similar structural types, in particular their stiffness • Recommended type:Hinged bars • Main 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.

13. Number and location of the links • 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

14. 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

15. Conclusions - continued • 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.