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Christchurch Earthquake Series: The Case for Structural Steel Systems
Presentation by G Charles Clifton
AP of Civil EngineeringSpecialist in Structural Steel and Composite Construction
to the:Canterbury Earthquakes Royal Commission
ENG.UOA.0006.1
The Earthquake Sequence: Impact on Christchurch CBD
Magnitude and Intensity of events to date:
4 Sept 2010: M 7.1, MM 7, ≈
0.7 x design*
26 Dec 2010: M 5.5?, MM 7 to 8
22 Feb 2011: M 6.3, MM 9 to 10, ≈
1.8 x design*
6 June 2011: M 5.3?, MM 7 to 8
13 June, 2011: M 5.4?, MM 7 to 8
13 June 2011: M 6.3, MM 8 to 9, ≈
0.9 x design*
design* = design for ultimate limit state to current seismic loading standard
Cumulative effect ≡
close to maximum considered event
ENG.UOA.0006.2
22 February Earthquake – Intensity of Shaking and Duration
•
Above the MCE level (data courtesy of GNS)CENTRAL CITY AND NZS1170 SPECTRA
CLASS D DEEP OR SOFT SOILLarger Horizontal Components
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Period T(s)
SA(T
) (g)
NZS1170 2500-yr Class D
NZS1170 500-yr Class D Deep orSoft Soil
CHHC_MaxH_FEB
CCCC_MaxH_FEB
CBGS_MaxH_FEB
REHS_MaxH_FEB
GM_Larger_FEB
ENG.UOA.0006.3
Performance requirements of modern buildings in this level of event (> DLE)
For normal importance buildings to conventional ductile design,
they are:•Shall remain standing under DLE,
should under MCE•Structural and non structural
damage will occur•Building will probably require
replacement•Actual performance needs to be
viewed in this light
ENG.UOA.0006.4
EBF Systems•
V braced (K braced) and D
braced both present; V braced shown opposite
•
Capacity design procedure to force
inelastic demand into the active link
•
This was first earthquake worldwide severe enough to push the frames into
the inelastic range•
Minor yielding only; no
link replacement required except in one instance
•
Repair cracking stairwells and light‐weight walls
•
Minimum floor slab damage
•
Buildings typically self‐ centred
ENG.UOA.0006.5
Case Study: Pacific Tower 23 storey mixed EBF and MRF, composite floors, transfer diaphragm level 6
•
Building has effectively self centred:–
60 mm out of plumb midheight
–
30 mm out of plumb at top–
under 0.1% residual deflection
•
Minimum damage compared with other buildings
–
Minimal structural or non structural repair or replacement needed
–
Requires only realignment of lift guide rails
–
Could re‐occupy but is in red zone so no public access
–
All other buildings of same height severely damaged; replacement likely
ENG.UOA.0006.6
Case Study: Club Tower 12 storey mixed EBF and MRF, composite floors, torsionally irregular
•
Building has effectively self centred:–
45;35 mm out of plumb top; within
construction tolerances–
0.14% maximum residual deflection
•
Minimum damage–
Lift guide rail realignment required:
this will cost approx $250k–
No other structural or non structural
repair or replacement needed–
Building now fully occupied including
CERA and CCC–
The only normal importance high‐
rise building in Christchurch now reoccupied (other is Police Station)
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Examples of Low/Medium Rise EBF and CBFs
2 to 5 storey
•
No structural or non structural damage to frame or suspended
floors•
Some instances ground
has slumped from under building
•
Two cases brace system failure due to bad
detailing
ENG.UOA.0006.16
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Connections
•
Performed as intended
•
No damage to splices and secondary element connections
•
Gusset plate connections out of plane movement in endplates to column as
intended
•
Sliding connections in stairs etc worked as intended
ENG.UOA.0006.20
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Steel Moment Frames
•
No visible damage to frames
themselves•
No visible
structural or non‐
structural damage
ENG.UOA.0006.22
Summary of Steel Frame, Composite Floor System Building Behaviour
•
Steel frame EBF or MRF
•
Composite floor system, concrete on steel deck on steel beams
•
All existing buildings have delivered low damage performance
in MCE earthquake
•
Minimal floor slab cracking, no frame elongation or unexpected behaviour
•
Capacity design procedure has been robust
Why? –
still finding out, but key points appear to be:•
Capacity design procedure ensures only primary member
undergoes inelastic demand
•
Elastic columns assist in building self centering
•
Composite floor slab assists in building self centering, including pushing the EBF collector beams back to level
ENG.UOA.0006.23
Recommended Low Damage Solutions for New Construction (or Retrofitted Construction): 1 of 2
•
EBF system with bolted in active links; or
•
Rotational bolted active links; or•
CBFs with buckling restrained
braces•
Dual frames with MRF semi‐rigid
connections•
Designed for limited ductile or
lower levels of ductility demand•
Proven performance in
Christchurch earthquakes (only well performing multi‐storey
system in that earthquake)
or, a REBF
ENG.UOA.0006.24
sacrificialposition bolts
bottom flange bolts
beamcolumn
Sliding Hinge Joint general viewSliding Hinge Joint general view
ENG.UOA.0006.25
Structure sways to rightStructure sways to right
beamcolumn
Action of the Sliding Hinge Joint in Severe Earthquakes
ENG.UOA.0006.26
Structure sways to leftStructure sways to left
Action of the Sliding Hinge Joint in Severe Earthquakes
beamcolumn
ENG.UOA.0006.27
Recommended Low Damage Solutions for New Construction (or Retrofitted Construction): 2 of 2
•
Columns with self centering, no residual
rotation bases, eg as in Te Puni building, in conjunction with SHJs
•
Sliding hinge joints for rotation with no replacement
•
Composite floor system with slab, enhancing resistance
and self centering capability
ENG.UOA.0006.28
Long Span Composite Floor Solutions
•
Use composite slab on steel deck on welded,
cellular secondary beams•
Easy to pass services
through•
Clear spans of 20 m readily
achieved•
Slab Panel Design for fire
means only approx 30% of beams need passive fire
protection
Britomart East Building –
12m
x 12m column grid; every 7th
secondary beam passive fire
protected
ENG.UOA.0006.29
Steel in Strengthening of Existing Buildings
•
When added with care is
very effective
•
Tying together masonry
•
Stiff frames in concrete
buildings
ENG.UOA.0006.30
Innovative Uses For Steel Containers
•
Road barriers
•
Rock fall stoppers
•
Shop windows
•
Temporary pubs
ENG.UOA.0006.31
Light Steel Framed Housing
•
Not that many houses in
affected region•
Excellent
performance; no internal cracking and minor
loosening of bricks only issues
ENG.UOA.0006.32
Has a Building’s Earthquake Life Been Used Up? : 1 of 2
•
Depends on design and construction of building and nature of
earthquake(s)
•
Older buildings: often decision is clear but this
depends on importance of building
Decision = demolish
Decision ? rebuild or demolish
ENG.UOA.0006.33
Has a Building’s Earthquake Life Been Used Up? : 2 of 2
•
Modern buildings:–
Designed for controlled damage
in earthquake for life safety(Like crumple zone in car)
–
What reserve of strength and ductility is left depends on:
•
Strains in steel (structural or rebar)•
Number of cycles
•
Extent of concrete crushing•
Integrity of overall structural system
•
Difficult decision to make: need understanding of failure mechanisms and
representative test data•
Easier with steel framed buildings as can
see main structural components
RC beams: damage in lab
(top) and in actual buildings
(right)
EBF active links: leave
in place (top left and
right ) and replace
(bottom right)
ENG.UOA.0006.34