Semi-active Management of Structures Subjected to High Frequency Ground Excitation

1. Semi-active Management of Structures Subjected to High Frequency Ground Excitation C.M. Ewing, R.P. Dhakal, J.G. Chase and J.B. Mander 19th ACMSM, Christchurch, New Zealand, 2006

2. The Scene • Structures can be highly vulnerable to a variety of environmental loads • These days, man-made events can also have significant impact on the life, serviceability and safety of structures, and must be accounted for in new designs • i.e. blast loads • However, what do you do about already existing and potentially vulnerable structures? • In particular, how do you manage to protect the structure without overloading shear or other demands? • Particularly true for relatively older structures • Semi-active methods offer the adaptability to reduce response energy without increasing demands on the structure, but add complexity • Passive methods offer simplicity and ease of design, but are not adaptable or as effective.

3. Horizontal 50 m May cause sudden collapse. Impulsive nature. Post-BIGM response is also important. Large amplitude (~100 g) Short duration (<0.05 sec) Characteristics of BIGM Typical Seismic excitation Typical BIGM

4. Characteristics of BIGM Typical Seismic excitation Typical BIGM Horizontal 50 m May excite high frequency vibration Modes during major shock duration. High frequency (~200 Hz)

5. Impulse Shock Spectra • If t1/T < critical (0.4-0.5), - The maximum response of a linear structure depends on t1/T.

6. T = 1 sec Impulse-Response Relationship • If t1/T < critical (0.4-0.5), • The maximum response factor is proportional to the total energy applied, regardless of the impulse shape.

7. A Simple Structure & Damage 450kN live • Loads are impulsive • Excite higher order modes • Plastic first peak response is not unusual • Plastic deformation on return or second peak response may also occur • After initial pulse the response is transient free response from a large initial value • Main forms of damage: • Residual deformation • Low cycle fatigue Blast load based on pressure wave and face area 630kN live 1000kg/story E = 27GPa

8. General Dynamic Response Fundamental local modes Fundamental global mode Higher order global mode Frequency increases Acceleration increases Displacement decreases

9. P More Detailed Model Basic Elements: • Multiple elements per column to capture higher order responses [Lu et al, 2001] • Mass discretised over all elements in column • Blast load discretised to each storey based on pressure wave and face area • Simple frame used to characterise basic solutions available for something more complex than a SDOF analysis • Non-linear finite elements (elastic-plastic with 3% post yield stiffness) • Fundamental Period = 1 sec • Main structure model captures all fundamental dynamics required for this scenario

10. Typical Load • Short duration impulse (< T1/5) • Any shape will give the same result, as the basic input is an applied momentum • Provides an initial displacement • Pblast = 350kPa pressure wave • Triangular shaped pulse of duration Dt = 0.05 seconds or 5% of fundamental structural period

11. Typical Uncontrolled Response • A first large peak that is plastic • Second and third peaks may also have permanent deformation • Free vibration response after initial pulse (not linear) • Residual deformation Permanent deflection may be larger or even negative depending on size of the load

12. Tendon in shape of moment diagram Resetable device 1st floor Possible Solutions • Passive = Tendons • Restrict first peak motion = initial damage • Add slightly to base shear demand on foundation • Match overturning moment diagram [Pekcan et al, 2000] • Tendon yields by design during initial peak • Semi-Active = Resetable devices using 2-4 control law • Do not increase base shear • Reduce free vibration response = subsequent damage • Therefore, in combination these devices are designed to reduce different occurrences of damage in the response • However, can devices hooked to story’s manage damage for this case characterized by higher column mode response? • Paper also considers device on 2nd story and from ground to 2nd story

13. End Cap Seal Cylinder Piston Becoming A Proven Technology More later in conference from Mulligan et al, Rodgers et al and Anaya et al on resetable devices and semi-active applications/experiments

14. Viscous Damper Resist all velocity Resist all motion Reset at peaks 1-4 Resetable Resist motion away from 0 From 0Peak 1-3 Resetable Resist motion toward 0 From Peak0 2-4 Resetable Semi-Active Customised Hysteresis 1 3 4 2 Only the 2-4 control law does not increase base-shear

15. a) Valve b) Valves Cylinder Piston Cylinder Piston The Very Basic Ideas Independent two chamber design allows broader range of control laws

16. Specific Results • Device on first floor and tendon versus uncontrolled • First peak and free vibration reduced ~40-50% • 1st story response Displacement Time

17. Device Stiffness is Critical • Results normalised to uncontrolled response • Device stiffness in terms of column stiffness k • 50-100% of column stiffness = good result in free vibration per [Rodgers et al, 2006] Response Energy 2-norm 1st Peak 2nd Peak

18. Conclusions • Blast can be completely represented by the applied momentum rather than shape, pressure or other typically unknown values • Simple robust system shows potential in this proof of concept study on an emerging problem of importance for structural designers • Complexity added is minimal • Results show that significant improvements that could be critical to safety and survivability can be obtained • Minimal extra demand on foundations makes it particularly suitable for retrofit of existing (relatively older) structures