Development of a Real-Time Test Method for Structures

Development of a Real-Time Test Method for Structures

Seismic Performance of Dissipative Devices Martin Williams University of Oxford Japan-Europe Workshop on Seismic Risk Bristol, July 2004 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 Clment, Neil Woodward Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 Introduction to knee braced frames Seismic energy dissipated through yielding/hysteresis of knee elements Cross brace Knee element Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 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: Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 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 Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 Test set-up

Test frame Cable extension displacement transducers Universal load cell Knee element Load cell Actuators Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 Loading regimes Slow cyclic: Real-time loading: External loads (eg. earthquake) Apply measured forces to numerical substructure

u PHYSICAL SUBSTRUCTURE NUMERICAL SUBSTRUCTURE Calculate displacements at interface between physical and numerical substructures t Measure forces and actual displacements of test specimen Command actuators to apply interface displacements to physical substructure Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 Under-stiffened element Failure mode Hysteresis: 70

35 0 -25 -20 -15 -10 -5 0 5 10 15 20 -35 -70 Displacement [mm]

Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 25 Well-stiffened section 700 Hysteresis Failure mode: 350 0 -30 -20 -10 0 10 20 30 -350

-700 Displacement [mm] 80 40 0 -25 -20 -15 -10 -5 0 5 10 15 20

25 -40 -80 Displacement [mm] Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 Perforated web Failure mode: Hysteresis: 400 200 0 -20 -15 -10 -5

0 5 10 15 20 -200 -400 Displacement [mm] 500 250 0 -28 -21 -14 -7

0 7 14 21 -250 -500 Displacement [mm] Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 28 Thermal monitoring system Plastic strain distributions during tests could be deduced from measurements of the knee element temperature Typical images: Thermal imaging system:

Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 Thermal analysis results Amplitude = 20 mm 30 mm Energy: Plastic strain: Von Mises stress: Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 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 Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 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: Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 Buckling analysis Over-predicted buckling load of unstiffened web by 20% Unable to model buckling of stiffened web Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 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 Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 Design of a knee braced frame 5-storey building designed to EC8, for earthquake with peak ground acceleration 0.35g Knee element Brace Pin Facade 1 Facade 2 Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 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: (a) Modal pattern (b) Uniform pattern 1500 F* (kN) 1500 F* (kN) 1000 500 0 1000 Pushover EC8 FEMA356 500 0

0 50 100 d* (mm) 150 0 50 100 150 d* (mm) Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 Comparison with time-history analysis (a) EC 8 (b) FEMA 356 (c) ATC 40

5 5 5 4 4 4 3 3 3 2 2 2 1 1

1 0 0 0 0.5 1 Storey drift (%) 0 0 0.5 Storey drift (%) 1 0 0.5 1

Storey drift (%) Time history analysis: Mean Mean +/- st. dev. Pushover analysis: Uniform Modal Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 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 Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 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 Japan-Europe Workshop on Seismic Risk, Bristol, July 2004 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 Japan-Europe Workshop on Seismic Risk, Bristol, July 2004

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