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

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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
Characteristics of BIGM
Typical Seismic excitation
Typical BIGM
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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)
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Impulse Shock Spectra

• If t1/T lt critical (0.4-0.5),
• - The maximum response of a linear structure
  depends on t1/T.

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Impulse-Response Relationship

• If t1/T lt critical (0.4-0.5),
• The maximum response factor is proportional to
  the total energy applied, regardless of the
  impulse shape.

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

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Typical Load

• Short duration impulse (lt 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

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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
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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 storys manage
  damage for this case characterized by higher
  column mode response?
• Paper also considers device on 2nd story and from
  ground to 2nd story

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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
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Semi-Active Customised Hysteresis
1
3
4
2
Only the 2 - 4 control law does not increase
base-shear
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The Very Basic Ideas
Independent two chamber design allows broader
range of control laws
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Specific Results

• Device on first floor and tendon versus
  uncontrolled
• First peak and free vibration reduced 40-50
• 1st story response

Displacement
Time
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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
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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