Seismic Performance of Dissipative Devices Martin Williams University of Oxford

1. Seismic Performance ofDissipative DevicesMartin WilliamsUniversity of Oxford Japan-Europe Workshop on Seismic Risk Bristol, July 2004

2. 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

3. Introduction to knee braced frames Seismic energy dissipated through yielding/hysteresis of knee elements

4. 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:

5. 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

6. Test set-up

7. Loading regimes Slow cyclic: Real-time loading:

8. Under-stiffened element Failure mode Hysteresis:

9. Well-stiffened section Hysteresis Failure mode:

10. Perforated web Failure mode: Hysteresis:

11. Thermal monitoring system Typical images: Plastic strain distributions during tests could be deduced from measurements of the knee element temperature Thermal imaging system:

12. Thermal analysis results Amplitude = 20 mm 30 mm Energy: Plastic strain: Von Mises stress:

13. 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

14. FE analysis of knee elements using ABAQUS Cyclic + thermal analysis – comparison of temperature rise in one half-cycle with test: Cyclic analysis with three different hardening laws:

15. Buckling analysis • Over-predicted buckling load of unstiffened web by 20% • Unable to model buckling of stiffened web

16. 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

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

18. 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:

19. Comparison with time-history analysis

20. 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

21. 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

22. 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