Seismic Performance of Dissipative Devices Martin Williams University of Oxford

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Seismic Performance ofDissipative
DevicesMartin WilliamsUniversity of Oxford

• Japan-Europe Workshop on Seismic Risk
• Bristol, July 2004

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Outline

• Introduction to knee bracing
• Optimisation of the knee element design
• Full-scale experiments on knee elements
• Finite element modelling
• Seismic design and analysis of knee braced frames
  
• Conclusions and future work
• Acknowledgements Tony Blakeborough, Denis
  Clément, Neil Woodward

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Introduction to knee braced frames
Seismic energy dissipated through
yielding/hysteresis of knee elements
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Knee bracing

• Knee element requirements
• Early yield
• Large energy dissipation shear vs flexure
• Stable under large non-linear excursions web
  buckling
• Easily replaceable no damage to ends
• Pursued via testing and FE analysis
• Focus on standard section types

Flexural hinge
Shear yield in web
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Knee element designs

• Column sections provide high lateral stability
• Different stiffener patterns explored to prevent
  plastic web buckling
• Perforation of webs explored as a way of giving a
  designer greater flexibility over choice of shear
  yield load

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Test set-up
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Loading regimes

• Slow cyclic Real-time loading

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Under-stiffened element
Failure mode
Hysteresis
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Well-stiffened section
Hysteresis
Failure mode
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Perforated web
Failure mode
Hysteresis
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Thermal monitoring system

• Plastic strain distributions during tests could
  be deduced from measurements of the knee element
  temperature
• Thermal imaging system

Typical images
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Thermal analysis results
Amplitude
20 mm
30 mm
Energy
Plastic strain
Von Mises stress
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Summary of experimental findings

• Full scale cyclic loading gives responses
  representative of a real earthquake
• Yielding in shear is optimal
• UC sections are are less prone to lateral
  instabilities
• To prevent buckling, web stiffeners are required
  at a spacing approximately equal to the section
  depth
• At a realistic design deflection the load on a
  knee element is approximately 1.7 times the yield
  load
• Perforating the web was unsuccessful
• Thermal imaging is an effective method for
  identifying the energy dissipation areas and
  tracking the spread of yielding

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FE analysis of knee elements using ABAQUS

• Cyclic analysis with three different hardening
  laws

Cyclic thermal analysis comparison of
temperature rise in one half-cycle with test
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Buckling analysis

• Over-predicted buckling load of unstiffened web
  by 20
• Unable to model buckling of stiffened web

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Summary of FE results

• An accurate hardening law is essential for
  realistic cyclic analysis
• Thermal analysis showed reasonable agreement with
  thermal imaging results
• It was not possible to build a model that agreed
  with all aspects of behaviour - shear forces,
  axial forces, moments and thermal dissipations
• Buckling analysis overestimated the critical load
  by 20 for an unstiffened knee element and was
  unable to predict the failure mode for knee
  elements with stiffeners

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Design of a knee braced frame
5-storey building designed to EC8, for earthquake
with peak ground acceleration 0.35g
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Design using pushover analysis

• Designed using EC8 pushover approach
• Also FEMA 356 approach, ATC 40 capacity spectrum
  method
• Key difference is idealisation of pushover curve

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Comparison with time-history analysis
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Summary of results

• Pushover analysis shows that frames possess high
  ductility and post-yield stiffness
• Knee elements begin to yield at just 0.08g but
  remain stable up to 0.56g
• EC8 approach appears highly conservative for this
  type of structure, ATC40 approach unsafe

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Conclusions

• Stable dissipative behaviour can be achieved
  using standard sections, appropriately reinforced
• Large increases in knee element load occur after
  initial yield
• Yielding and energy dissipation in experiments
  can be tracked using thermal imaging
• Accurate FE modelling of all aspects of knee
  element behaviour did not prove possible web
  buckling was particularly problematic
• Design methods based on pushover analysis may be
  suitable for frames incorporating dissipative
  elements, but some further development of these
  approaches is desirable

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Current/future work

• Testing of other dissipators, e.g. Jarret, Hyde
  devices
• Real-time substructure testing
• Further design and analysis studies using
  ten-storey frames, different dissipators