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

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 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

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impact Instantaneous impact Momentum balance and coefficient of restitution to modify velocities Inconvenient: no longer valid if the impact duration is large Models for impact zone Position of impact elements

6 Observation from the analyses: An elastic model cannot predict correctly the behaviour Structure pounded on the rightStructure pounded on the left Elastic model Non- elastic model

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 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

12 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. Pounding mitigation example - Adjacent buildings of equal height, with aligned floor levels and similar structural types, in particular their stiffness

13 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

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 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