5/29/2013 1 Behavior and Design of Concrete‐Filled Composite Columns Roberto T. Leon Virginia Tech, Blacksburg, VA Jerome F. Hajjar Northeastern University, Boston, MA Larry Griffis Walter P. Moore, Austin, TX Scope • Brief introduction to composite columns (LG) • Research motivation and experimental results (RL) • Analytical modeling and system studies (JH) • Conclusions and design recommendations (LG) Work is based on the dissertations of: Tiziano Perea, UAM, Mexico City (MX) – Georgia Tech Mark Denavit, SDL, Atlanta (GA) – UIUC In‐Kind:
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Behavior and Design of Concrete‐Filled Composite
Columns
Roberto T. LeonVirginia Tech, Blacksburg, VA
Jerome F. HajjarNortheastern University, Boston, MA
Larry GriffisWalter P. Moore, Austin, TX
Scope• Brief introduction to composite columns (LG)
• Research motivation and experimental results (RL)
• Analytical modeling and system studies (JH)
• Conclusions and design recommendations (LG)
Work is based on the dissertations of:Tiziano Perea, UAM, Mexico City (MX) – Georgia TechMark Denavit, SDL, Atlanta (GA) – UIUC
In‐Kind:
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Composite or hybrid system (concrete & steel)System which combines the advantages of concrete and structural steel
Concrete* Rigid * Economic* Fire resistant * Durable
Structural steel* High strength * Ductile* Easy to assembly * Fast to erect
Frames with CFT columns• Steel tube confines concrete• Concrete restricts the buckling of the steel tube• Increase in strength & deformation of the concrete • Delay in the buckling of the steel tube
Frames with SRC columns• Steel element supports the construction loads• The concrete gives final stiffness and fire resistant• Shear connections become FR once concrete is cast• System fast to erect & build (redundancy)
Uses for Composite Columns
• Extra capacity in concrete column for no increase in dimension
• Large unbraced lengths in tall open spaces– Lower story in high rise buildings– Airport terminals, convention centers
• Corrosion, fireproof protection in steel buildings• Composite frame – high rise construction• Transition column between steel, concrete systems• Toughness, redundancy as for blast, impact
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Composite Systems• Perimeter moment frames for stiffness in hurricane zones.
• Extension to seismic based on Japanese experience.
• Distributed systems vs. supercolumns
Buildings with SRC Columns (Martinez‐Romero, 1999 & 2003)
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Composite Braced Frame
Bank of China Hong Kong
Composite Column
Bank of ChinaHong Kong
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Composite Moment Frame“Tube” Design
3 Houston CenterHouston, Texas
Composite Column Forming
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“Tree Columns”Composite Columns
3 Houston CenterHouston, Texas
Composite “Erection Columns”
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Composite ColumnsReinforcement Cage
Composite Shear Walls
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Composite Braced Frame
2 Union SquareSeattle, Washington
Composite Frame Construction
Dallas, Texas
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Composite Frame Construction
Possible configurations in composite columns
a) SRC b) Circular and Rectangular CFT
c) Combinations between SRC and CFT
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FlexibilitySizes and Shapes
Filled Composite Column(Covered in this Webinar)
Round HSS Square or Rectangular HSS
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Encased Composite Column
Motivation for Research
• Lack of design information for the stiffness of columns to be used for buckling and lateral rigidity calculations
• Lack of knowledge on the interaction between axial load and bending at ultimate (2D and 3D)
• Lack of knowledge on system factors (force reduction and deflection amplification for seismic design)
• Gaps in data for slender columns (local and overall buckling)
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(1) Flexural rigidity for lateral forces
• Advanced computational analysis:
eff s s s c c cEI EI EI
HSSSection
t
D
Fiber element analysis
Finite element analysis
• Semi‐empirical :
• Concrete‐only or Steel‐onlyfor calculating column capacity, not for lateral analysis
Selected Systems R CdS‐SMF (Steel Special Moment Frames): 8.0 3.0 5.5
C‐SMF (Composite Special Moment Frames): 8.0 3.0 5.5
ConstitutiveRelations• Constitutive formulations, calibration, and validation developed for five
separate steel and steel‐concrete composite cross sections plus connections– CCFT, RCFT, and SRC beam‐columns– WF beams– WF and Rect. HSS braces– Moment frame and braced frame connections
• “Proposed for Behavior” constitutive model– Aims to capture the behavior as accurately as possible
• “Proposed for Design” constitutive model– Follows typical assumptions common in the development of design
recommendations (e.g., no steel strain hardening, no concrete tension)
• Calibrated and validated against detailed results of over 100 monotonically‐ and cyclically‐loaded experiments of composite beam‐columns, connections, and frames
• For the “Proposed for Behavior” model, based on the bounding‐surface plasticity model of Shen et al. (1995).
• Modifications for the analysis of composite members– Local buckling– Residual stress defined with
initial plastic strain
• For the “Proposed for Design” model, either elastic‐perfectly plastic (SRC WFs; rebar) or based on the model of Abdel‐Rahman & Sivakumaran 1997 (CFTs)
0 2 4 6 8 100
0.2
0.4
0.6
0.8
1
1.2
1.4
Normalized Strain (/y,flat)
Nor
mal
ized
Str
ess
( /F
y,fla
t)
Et1 = Es/2
Et2 = Es/10
Et3 = Es/200
Et1
Et2
Et3
Flat
Corner
Elastic Unloading
Es
Fp = 0.75 Fy
Fym = 0.875 Fy
Et3
Et1
Et2
Fp
Fym
Fy
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SRCBeam‐ColumnValidationRiclesandPaboojian 1994
-150 -100 -50 0 50 100 150-400
-300
-200
-100
0
100
200
300
400
Lateral Displacement (mm)Test #4: 4 (Ricles and Paboojian 1994)
Late
ral L
oad
(kN
)
Expt.
PfB
-150 -100 -50 0 50 100 150-500
-400
-300
-200
-100
0
100
200
300
400
500
Lateral Displacement (mm)Test #8: 8 (Ricles and Paboojian 1994)
where,Rn = nominal bond strength, kipsCin = 2 if the CFT extends to one side of the point of force transfer
= 4 if the CFT extends to both sides of the point of force transferFin = nominal bond stress = 60 psiB = overall width of rectangular steel section along face transferring load, in.D = outside diameter of the round steel section, in.
= 0.45 = 3.33
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Experimental Setups for Assessing Bond Strength
(a) Push-off test(b) Push-out test
without shear tabs(c) Push-out test with shear tabs
(d) Typical CFT connection
Air Gap
Air Gap
Proposed Design Provisions
For CCFT:
Rn = πDLbondFin
Lbond = CinD
Fin = 30.9(t/D2) ≤ 0.2
For RCFT:
Rn = 2(B+H)LbondFin
Lbond = CinH
Fin = 12.8(t/H2) ≤ 0.1
where,Rn = nominal bond strength, kipsFin = nominal bond stress, ksit = design wall thickness of steel section, in.B = overall width of rectangular steel section (B ≤ H), in.H = overall height of rectangular steel section (H ≥ B), in.D = outside diameter of round steel section, in.Lbond = length of the bond region (the bond region of adjacent connections shall not overlap), in.Cin = 4 if load is applied to the steel tube and the CFT extends to both sides of the point of force transfer
= 2 otherwise
For RCFT: Both Lbond and Fin are based on the larger lateral dimension of the tube (H ≥ B)
Zero Length Spring Representing the Panel Zone Shear
Behavior
Nonlinear Column Element
Nonlinear Beam
Element
Elastic Beam
Element
Nonlinear stress‐resultant‐space multi‐surface kinematic hardening model used for rotational spring formulation (after Muhummud 2003)
Rigid Links
Nonlinear Column Element
Nonlinear Beam
Element
Nonlinear Brace
Element
Moment Release
Modeling assumptions established by Hsiao et al. (2012)
EvaluationofSeismicPerformanceFactors
Archetype frames are categorized into performance groups based on basic structural characteristics
Group Number
DesignGravity Load
Level
DesignSeismic Load
Level
Period Domain
Number of C‐SMFs
Number of C‐SCBFs
PG‐1 High Dmax Short 6 4
PG‐2 High Dmax Long 2 2
PG‐3 High Dmin Short 6 4
PG‐4 High Dmin Long 2 2
PG‐5 Low Dmax Short 6 4
PG‐6 Low Dmax Long 2 2
PG‐7 Low Dmin Short 6 4
PG‐8 Low Dmin Long 2 2
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TypicalStaticPushoverAnalysis
0 10 20 30 40 50 600
100
200
300
400
500
600
700
800
900
1000
Roof Displacement (in)
Bas
e S
hear
(ki
ps)
Vmax
= 879.3 kips
V80
= 703.4 kips
V = 153.9 kips
u =
50.
8 in
SFRS: C-SMF, Frame: RCFT-3-1
SystemOverstrengthFactor,Ωo
• By the FEMA P695 methodology, Ωo should be taken as the largest average value of Ω from any performance group– Rounded to nearest 0.5– Upper limits of 1.5R and 3.0
• High overstrength for C‐SMFs– Displacement controlled design– Current value (Ωo = 3.0) is upper limit
and is acceptable
• Overstrength for C‐SCBFs near current value (Ωo = 2.0)– Higher for PG‐3 and PG‐4 (High gravity
ResponseModificationFactor,R• ACMR10% = Acceptable value of the Adjusted
Collapse Margin Ratio for 10% collapse probability
• ACMR10% = 1.96 for both C‐SMF and C‐SCBF and are less than the ACMR shown for each performance group in the table
• Similarly positive results for ACMR20% per frame
• ACMR values show correlation with the overstrength
• C‐SMFs
– Current value (R = 8.0) is acceptable
• C‐SCBFs
– Current value (R = 5.0) is acceptable
Group Number
ACMR
C‐SMF C‐SCBF
PG‐1 4.8 3.3
PG‐2 3.7 2.3
PG‐3 7.5 5.1
PG‐4 8.5 5.4
PG‐5 4.9 2.6
PG‐6 3.9 2.9
PG‐7 7.1 3.8
PG‐8 6.9 3.7
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DeflectionAmplificationFactor,Cd• By the FEMA P695 methodology, Cd = R for these systems
• Would represent a minor change for C‐SCBF – Current values: Cd = 4.5, R = 5.0– Typically strength controlled design
• Would represent a significant change for C‐SMF– Current values: Cd = 5.5, R = 8.0– Typically already displacement controlled design
• Four C‐SMF archetype frames designed with the current Cd value – Lower overstrength with current Cd (average 4.9 vs. 6.4 with Cd = R)
– Acceptable performance with current Cd
Key Conclusions from the Research
Experimental Research• A comprehensive and unique data set for axial strength and beam‐column
strength has been generated for slender CCFTs and RCFTs.
• CFTs demonstrated great toughness under complex cyclic loadings.
• Local buckling did not lead to substantial strength or stiffness losses.
Computational Research
• New mixed element analysis formulation developed for composite beam‐columns
• Composite beam‐columns exhibit robust performance under severe cyclic loading
• Analysis formulation enables benchmark studies of stability and strength of composite frames (non‐seismic and seismic)
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Proposals for AISC 360‐16 (2016)Specification for Structural Steel Buildings
• New commentary on addressing wet weight of concrete during concrete pour for CFTs
• New EIeff value for calculating column strength of SRCs to better reflect computational data
• New recommendations for EIelastic value to use for calculating elastic stiffness of CFTs and SRCs for use in elastic analysis and use in Direct Analysis
• New interaction equation that addresses possible unconservative errors for very slender composite members
• New CFT bond provisions that more accurately reflect the change in bond strength with CFT diameter and that clarify how to compute bond strength in load transfer regions
• Validation of current seismic performance factors in ASCE 7‐10 and recommendation to consider increasing the deflection criteria for C‐SMFs if Cd = R
Future Work
• Finalize recommendations for AISC 360‐16
• Prequalified composite connections
• Incorporate creep and shrinkage effects into design of composite systems
• Effects of elevated temperature in composite systems, and effects of internal reinforcement
• Innovative composite framing systems:
– Prefabricated composite construction systems
– Integration of new materials, including higher strength materials
440 – System Behavior Factors for Composite and Mixed Structural System
Roberto T. Leon, Jerome F. Hajjar, Nakin Suksawang
References and a list of papers and publications for this work are available at the NEES site for this webinar: https://nees.org/events/details/190
The work described here is part of a NEESR project supported by the National Science Foundation under Grant No. CMMI‐0619047, the American Institute of Steel Construction, the Georgia Institute of Technology, and the University of Illinois at Urbana‐Champaign. These experiments were conducted at the Multi‐axial Subassemablage Testing System (MAST) at the University of Minnesota.