seismic performance of dissipative devices martin williams university of oxford japan-europe...

Seismic Performance ofDissipative Devices

Martin WilliamsUniversity of Oxford

Japan-Europe Workshop on Seismic RiskBristol, July 2004

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 Clément, Neil Woodward


Introduction to knee braced frames

Cross brace

Knee element

Seismic energy dissipated through yielding/hysteresis of knee elements


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:


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


Test set-up

Load cell

Actuators

Kneeelement

Universal load cell

Test frame

Cable extensiondisplacementtransducers


Loading regimes

Slow cyclic: Real-time loading:

u

t

Apply measured forces tonumerical substructure

External loads(eg. earthquake)

PHYSICAL SUBSTRUCTURE

Measure forces and actualdisplacements of test specimen

NUMERICAL SUBSTRUCTURE

Calculatedisplacementsat interfacebetweenphysical andnumericalsubstructures

Command actuators to applyinterface displacements to

physical substructure


Under-stiffened element

Displacement [mm]

Mom

ent [

kNm

]

-70

-35

0

35

70

-25 -20 -15 -10 -5 0 5 10 15 20 25

Failure mode

Hysteresis:


Well-stiffened section

Displacement [mm]

For

ce [k

N]

-700

-350

0

350

700

-30 -20 -10 0 10 20 30

Displacement [mm]

Mom

ent [

kNm

]

-80

-40

0

40

80

-25 -20 -15 -10 -5 0 5 10 15 20 25

Failure mode:

Hysteresis


Perforated web

Displacement [mm]

For

ce [k

N]

-500

-250

0

250

500

-28 -21 -14 -7 0 7 14 21 28

Displacement [mm]

For

ce [k

N]

-400

-200

0

200

400

-20 -15 -10 -5 0 5 10 15 20

Failure mode: Hysteresis:


Thermal monitoring system

Plastic strain distributions during tests could be deduced from measurements of the knee element temperature

Thermal imaging system:

Typical images:


Thermal analysis results

Amplitude = 20 mm 30 mm

Energy:

Plastic strain:

Von Mises stress:


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


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:


Buckling analysis

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


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


Design of a knee braced frame

Facade 1 Facade 2

Brace

Kneeelement

Pin

5-storey building designed to EC8, for earthquake with peak ground acceleration 0.35g


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

0

500

1000

1500

0 50 100 150d* (mm)

F*

(kN

)

(b) Uniform pattern

0

500

1000

1500

0 50 100 150

d* (mm)

F*

(kN

)

Pushover

EC8

FEMA356


Comparison with time-history analysis

(a) EC 8

0

1

2

3

4

5

0 0.5 1

Storey drift (%)

Sto

rey

(b) FEMA 356

0

1

2

3

4

5

0 0.5 1

Storey drift (%)

Sto

rey

(c) ATC 40

0

1

2

3

4

5

0 0.5 1

Storey drift (%)

Sto

rey

Mean Mean +/- st. dev.

Uniform Modal

Time history analysis:

Pushover analysis:


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


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


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

seismic performance of dissipative devices martin williams university of oxford japan-europe...

Documents

seismic risk bristol

seismic design

hysteresis slide

knee element requirements

braced frames seismic

test setup slide

spread of yielding slide

knee elements finite