design of beam-column joints: lessons from the 2010-2012...
TRANSCRIPT
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Design of Beam-Column Joints:Lessons from the 2010-2012
Christchurch Earthquake Swarm
Roberto T. León
The Via Department of Civil and Environmental EngineeringVirginia Tech, Blacksburg, VA
The Fifth Kwang-Hua ForumTongji University, Shanghai, PRC
December 2012
Design of Beam-Column Joints:Lessons from the 2010-2012
Christchurch Earthquake Swarm
Roberto T. León
The Via Department of Civil and Environmental EngineeringVirginia Tech, Blacksburg, VA
The Fifth Kwang-Hua ForumTongji University, Shanghai, PRC
December 2012
BackgroundBackground• Objective: Discuss performance of structures
built to the most modern standards and compare to older standards → How well are we doing?
• Motivation: • The Christchurch earthquake swarm can be
construed, locally, as the MCE (or multiple ones?).
• Historically there has been a substantial philosophical difference between USA, NZ, and Japan on design approaches to beam-column joints.
• Some interesting and potentially very important conclusions for joint modeling, cumulative damage, and post-earthquake condition assessment.
• Some bright Ph.D.s needs to look at these!
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Kaikuro (New Zealand)
Canterbury Swarm (10/10 –01/12)
Kelvin Berryman GNS –New Zealand
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ChristchurchCentral Business District (CBD)
ChristchurchCentral Business District (CBD)
0.9 miles
1.2
mile
s
Area of highest damage
1. Four stations within a mile or so or one another
2. Large variation on records due to local soil conditions
3. No instrumented structures
CCCC
REHS
CBGS
CCHC
Googlemaps
CENTRAL CITY AND NZS1170 SPECTRACLASS 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
NZS 1170.5 Spectra and Largest Horizontal Direction Recorded from CBD Strong Motion Records• Dotted line = ULS design spectrum for normal importance buildings for CBD soil type• Solid red line = the average from the 5 recording stations• Solid black line = the MCE spectrum for normal importance buildings for CBD soil type
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The Earthquake Sequence: Impact on Christchurch CBDThe Earthquake Sequence: Impact on Christchurch CBD
• Damaging 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 ≈ 0.6 x design*
• 22 Feb 2011: M 6.3, MM 9 to 10, ≈ 2.0 x design*
• 6 June 2011: M 5.3?, MM 7 to 8 ≈ 0.6 x design
• 13 June, 2011: M 5.4?, MM 7 to 8 ≈ 0.6 x design
• 13 June 2011: M 6.3, MM 8 to 9, ≈ 0.9 x design*+
• 23 December 2011: M 5.5, MM 6 to 7, ≈ 0.6 x design*
• Series is still on-going ( over 10,000 felt earthquakes)
• Cumulative damage and residual strength(>6 x design!)
CCCC: Christchurch Cathedral College
Comparison of Accelerations
Very short duration of strong ground motion ( 6 seconds)
Strong high frequency vertical motion
Studies of cumulative damage needed!!
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Christchurch (New Zealand)
Saturday February 24, 2011 (4 days after)Saturday February 24, 2011 (4 days after)
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Human Losses – Two Main Reasons Human Losses – Two Main Reasons • About 2/3 of the casualties came from the complete
collapse of two older RC buildings (CCTV & PGC) • About 1/3 came from falling debris from URM structures
(facades, awnings and parapets)
Total: 182 casualties
NZ vs. USA RC Construction Pre-1970: Little or no awareness of seismic design issues NZ similar to Eastern USA practice
1974-1984: Minimum seismic design adopted Discussion between USA, NZ, and Japan on
beam-column joint design NZ begins to utilize precast systems
Post 1984: ChCh introduced more seismic design, roughly
equivalent to USA UBC Zone 2 Beam-column joint design diverges
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Pyne Gold Corp. BuildingPyne Gold Corp. Building
• Built 1964• Five bays by five bays• Walls carried lateral loads
but had significant penetrations on the ground floor
Pyne Gold Building• Small, weak gravity columns• Poorly detailed boundary elements• Lightly reinforced walls• No beam-column joint reinforcement• Inadequate splice lengths• Inadequate diaphragm connections
Weaknesses well known for this building
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CCTV BuildingCCTV Building
Hyland, C. and Smith, A.,CTV Building Collapse Investigation,Report to the Building Housing Authority,January 2012
• Built in 1987 as office building• Drag struts added in upper floors to North core wall
CCTV Building
Hyland, C. and Smith, A., CTV Building Collapse Investigation,Report to the Building Housing Authority, January 2012
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Poor Detailing: Joints for Gravity
System
Poor Detailing: Joints for Gravity
System
• Little or no shear reinforcement• No confinement• Numerous discontinuous planes• Displacement to yield and failure very similar
Building 2: 7-story (1970s)Building 2: 7-story (1970s)
Figure 1: An 8-storey building with two-way RC frames system had a mix of column and beam-column joint shear
failures.
8” thick C-shapedcore wall
North
North-elevation frame
Short columns failure
South-elevation frame
• Figure from Kam, Pampanin & Elwood, 2011 NZSEE Bulletin
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Building 2: Joint / Column ShearBuilding 2: Joint / Column Shear
Beam-Column Joint DesignBeam-Column Joint Design
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• Allowable joint shear (US = 2NZ)• Required bar anchorage lengh (US =⅔NZt• Column-to-beamMoment ratio (US = ¾NZ)
© R. Leon, 1981
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NZ vs. USA Design
NZ connection expected to have far better performance under MCE – little stiffness deterioration in joint area
New Zealand Design
• Moderate joint shear stress and large anchorage lengths; hinging in beams with large cracks at joint interface due to extensive, concentrated yielding.
• Force transferred by smaller struts sustained by large amount of transverse reinforcement
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Typical NZ Performance
• Distributed shear cracking in the joint• Appreciable plastic hinge formation in the beams• Large cracks at interface
USA Design
• High joint shear stress and poor anchorage = large cracks, loss of stiffness and bar slip
• Relies on large concrete strut sustained by framing members
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Typical USA Performance
• No clear plastic hinges in the beams;• Shear failure in the joint
Precast Seismic Systems
Price Waterhouse Coopers Building
Great economy and excellent quality control
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Clarendon Towers
Clarendon Towers
• Mid-1990s• Precast system with
complex elements• Moment frame in
two directions• Apparently
undamaged (no visual cues to actual structural damage)
Clarendon Towers (precast units) Clarendon Towers (precast units)
Precast moment frame system
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Clarendon Towers Clarendon Towers Floor cracking from beam elongation after the February 22, 2011 earthquake. Cracks ranged from 20 mm to 40 mm; unseating of precast floor evident.
Kam, Pampanin and Elwood, BNZNSEE, 44(4), 2011
Clarendon Towers Clarendon Towers Floor cracking from beam elongation after the February 22, 2011 earthquake. Cracks ranged from 20 mm to 40 mm; unseating of precast floor evident.
Epoxy injection repairs underway after the September 2010 earthquake
Kam, Pampanin and Elwood, BNZNSEE, 44(4), 2011
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Clarendon Towers Clarendon Towers
Façade gaps?
Beam Growth Beam
Growth
Peng, B.H.H., Dhakal, R.P., Fenwick, R.C., Carr, A.J., and Bull, D.K., Elongation of Plastic Hinges in Ductile RC Members: Model Development, J. of Advanced Concrete Technology, Vol. 9, No. 3, 2011
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Experimental EvidenceExperimental Evidence
Bidirectional loading and slabsBidirectional loading and slabs
This behavior has been flagged as a potential issue in all beam-column joint testing (Leon and Deierlein, EERI, 1996)
Horizontal beam end displacement
Hor
izon
tal b
eam
dis
plac
emen
t
This problem is not newLeon, 1984 & Fenwick et al. 1985
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Price Waterhouse CoopersPrice Waterhouse Coopers
Price Waterhouse CoopersPrice Waterhouse Coopers
• 22 storey reinforced concrete building from mid-late 1980s (Restrepo, 1993; Park, 2002 )
• “text-book” in terms of ductile seismic response according to a beam-sway mechanism.
© J. Restrepo
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Price Waterhouse CoopersPrice Waterhouse Coopers
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• Corner joints – bidirectional loading• Notice proportions of beam to column sizes
© J. Restrepo
Lack of Clear Plastic Hinge Lack of Clear Plastic Hinge
Note single large crack = low reinforcement %
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Damage to Gravity SystemsDamage to Gravity Systems
Lack of deformation compatibility
Damage to Lateral SystemsDamage to Lateral SystemsDamage to exterior and roof joints in modern moment frames
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Deconstruction
Less than 4% of major 500 buildings will survive
Grand Chancellor
Deconstruction
Less than 4% of major 500 buildings will survive
Grand Chancellor
www.stuff.co.nz/the-press
SummarySummary• Modern beam-column joints designed to NZ standards
preformed extremely well for the ultimate limit state.• Most NZ frames are not repairable; situation is likely
worse for USA buildings.• Precast system emulating conventional RC
construction are a viable option for high seismic areas.• Cumulative damage, decosntructiuon, beam
elongation and lack of plastic hinge formation are two important issues that need further analytical study.
• Poor beam-column joint detailing in older construction remains a big problem; it is not easy to detect level of damage from smaller events.