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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
S‐IMF (Steel Intermediate Moment Frames; SDC B, C, D): 4.5 3.0 4.0
C‐IMF (Composite Intermediate Moment Frames; SDC B, C): 5.0 3.0 4.5
S‐OMF (Steel Ordinary Moment Frames; SDC B, C, D): 3.5 3.0 3.0
C‐OMF (Composite Ordinary Moment Frames; SDC B!!): 3.0 3.0 2.5
SCBF (Steel Concentrically Braced Frames): 6.0 2.0 5.0
C‐SBF (Composite Special Braced Frames): 5.0 2.0 4.5
OCBF (Composite Ordinary Conc. Braced Frames; SDC B‐F): 3.25 2.0 3.25
C‐OBF (Composite Ordinary Braced Frames; SDC B, C!!): 3.0 2.0 3.0
(2) Behavior factors for seismic design?ASCE/SEI 7‐10, Table12‐2‐1
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0.0
0.5
1.0
1.5
2.0
2.5
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Pexp/Po
Pn/Po
AISC
P/P o
CCFT columns database
(3) Lack of Slender Experimental Tests
Databases compiled by León et al., 2005 and Goode et al., 2007
1375 Circular CFT• 912 columns• 463 beam‐columns
798 Rectangular CFT• 524 columns• 274 beam‐columns
267 Encased SRC• 119 columns• 148 beam‐column
(4) Interaction Equations
How do we get a simplified expression that is close to the design strength?
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(5) Biaxial Interaction SurfaceAnalytical vs. Experimental Data
(6) Local Buckling
Theoretical difference of 1.73 between two cases not reflected in code provisions
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Project Objectives• Obtain and evaluate experimental response:
– Critical load (Pcr)
– P‐M interaction diagram (uniaxial and biaxial bending)
– Cyclic lateral force (uniaxial and biaxial bending)
– Torsion (torsional strength and rigidity)
– Wet concrete pressure due to the pouring
– Flexural rigidity (EIeff)
– Steel local buckling and concrete confinement
• Develop new computational formulations for complete frame analysis of composite systems
• Provide recommendations on construction, analysis, and design of CFTs.
NEES – UMN MAST Lab
MAST capabilities:• 6 DOFs• Pz = 1320 kip • Px, Py = 880 kips• Ux=Uy=+/‐16” • 14’ < L < 28’
Databases gaps: • L = 18 ft. and 26 ft.• , < 2.7• D/t 86 (CCFT) • B/t 67 (RCFT)• fc’ = 5 ksi and 12 ksi
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Specimen L Steel section Fy fc’ D/t
name (ft) HSS D x t (ksi) (ksi)
1-C5-18-5 18 HSS5.563x0.134 42 5 45
2-C12-18-5 18 HSS12.75X0.25 42 5 55
3-C20-18-5 18 HSS20x0.25 42 5 86
4-Rw-18-5 18 HSS20x12x0.25 46 5 67
5-Rs-18-5 18 HSS20x12x0.25 46 5 67
6-C12-18-12 18 HSS12.75X0.25 42 12 55
7-C20-18-12 18 HSS20x0.25 42 12 86
8-Rw-18-12 18 HSS20x12x0.25 46 12 67
9-Rs-18-12 18 HSS20x12x0.25 46 12 67
CFT Test Matrix (18 specimens)
Similar for specimens 10‐18 but at 26 ft.
CCFT10352 (S)
RCFT5634 (S)
Setup and Instrumentation
• Video and Still ImagesFour towers for images of whole specimen as well as base
• Krypton Coordinate Measurement Machine
• String PotsDistributed along height
• LVDTsSets of three for biaxial curvature measurement
• Strain GagesUniaxial and rosettes distributed along heightMeasurements during concrete pouring and testing
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Hydrostatic Pressures on Slender RCFT
FE Analysis:max ≈ max ≈ 36.1 ksimax ≈ ¼ in
≈2’
Stiffeners to reduce expansion in the RCFTs during the concrete pouring
Surveyed Initial Imperfections Length (ft) Length (ft)
Initial imperfection Initial imperfection CCFTs, L=26ft RCFTs, L=26ft
0 0.5 1 1.5 20
5
10
15
20
25 10
11
14 1518
o=
L/50
0=0.
63
0 0.5 1 1.50
5
10
15
20
2512 13
16
17
o=
L/50
0=0.
63
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LC1
Load protocol
Pcr
0, P
A
ME, PE
MB, 0
MB, PC
MD, P
C/2
0, PAPA, PA
LC1
Stability Effects
LC 1 – Axial load only
Load protocol
0, P
A
ME, PE
MB, 0
MB, PC
MD, P
C/2
0, PAPA, PA
LC1
MLC2a, 2PALC2aunidirectional
MLC2b, PALC2bunidirectional
Fmax
P
LC2
Stability Effects
LC 2 – Axial load plus lateral displacement along Xat two different axial load levels
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LC3
y
x
Load protocol
0, P
A
ME, PE
MB, 0
MB, PC
MD, P
C/2
0, PAPA, PA
LC1
MLC2a, 2PALC2aunidirectional
MLC2b, PALC2bunidirectional
LC3abidirectional
LC3bbidirectional
LC3cbidirectional
Fmax
P Stability Effects
LC 3A – Axial load at three levels plus lateral displacement along both X and y in a diamond‐spike configuration
LC3
Load protocol
0, P
A
ME, PE
MB, 0
MB, PC
MD, P
C/2
0, PAPA, PA
LC1
MLC2a, 2PALC2aunidirectional
MLC2b, PALC2bunidirectional
LC3abidirectional
LC3bbidirectional
LC3cbidirectional
Fmax
P
-10 -5 0 5 10-30
-20
-10
0
10
20
30
Lateral Displacement (in)
Late
ral F
orce
(kip)
-6 -4 -2 0 2 4 6
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Lateral Drift (%)
Cracking of concrete
Steel yielding in compression
Steel yielding in tension
Crushing of concrete
Steel local buckling
y
x
LC 3B – Axial load at three levels plus lateral displacement along both X and y in a “figure eight” configuration
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Load protocolLC4
T
Pcr
0, P
A
ME, PE
MB, 0
MB, PC
MD, P
C/2
0, PAPA, PA
LC1
MLC2a, 2PALC2aunidirectional
MLC2b, PALC2bunidirectional
LC3abidirectional
LC3bbidirectional
LC3cbidirectional
T
-10 -5 0 5 10-30
-20
-10
0
10
20
30
Lateral Displacement (in)
Late
ral F
orce
(kip)
-6 -4 -2 0 2 4 6
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Lateral Drift (%)
Cracking of concrete
Steel yielding in compression
Steel yielding in tension
Crushing of concrete
Steel local buckling
-600
-400
-200
0
200
400
600
-10 -5 0 5 10
P=0
P=0.2Po
Angle of twist (deg)
Torsional M
oment (kip‐ft)
CCFT20x0.25‐18ft‐5ksi
LC 4 – Torsion at two levels of axial load
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Load protocol: LC1 – Pure compression
0 200 400 600 800 10000
500
1000
1500
2000
2500
3000
Cross-section
Beam-column
Experimental
P (kip)
M (kip‐ft)
Stab
ility Effects
Specimen 17‐Rs‐26‐12P
M
Load protocol: LC2 – Uniaxial bending
Specimen 3‐C20‐18‐5
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Probe
-5000
500 -5000
5000
500
1000
1500
Y Moment (k-ft)X Moment (k-ft)
Z F
orce
(k)
AISC Beam Column Strength (K=2)All Load CasesExperimental Interaction Points
Load protocol: LC3 – Biaxial bendingCCFT Specimen
20x0.25
Fy = 42 ksif’c = 5 ksi
L = 18 feetKL = 36 feet
Corrected Column Strengths (LC1)
MAST capacity reached: 3, 5, 7, 9
Large imperfection: 1, 8, 11, 17
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Local Buckling ‐ 2010
Composite Members Subject to Axial Compression
Description ofElement
Width-Thickness
Ratio
p
Compact/Noncompact
r
Noncompact/Slender
Max.Permitted
Sides of rectangular box and hollow structural sections of uniform thickness
b/t 2.26 3.00 5.00
Round filled sectionsD/t 0.15 E/Fy 0.19 E/Fy 0.35 E/Fy
yF
E
yF
E
yF
E
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Extraction of EI from the experimental M‐ curves
M (kip-ft)
(10-4/in)Specimen 4-Rw-18-5
0 1 2 3 4 50
100
200
300
400
500
600EI
eff=21081046 kip-in2
EIexpL
=21865004 kip-in2
EIexpL
/EIeff
=1.0372
EIexpU
=21868261 kip-in2
EIexpU
/EIeff
=1.0373
Specimen 13Rs‐26‐5, LC2
M (kip‐ft)
(1/in)
Load protocol: LC4 –Torsion
PT
Specimen 3‐C20‐18‐5
-600
-400
-200
0
200
400
600
-10 -5 0 5 10
P=0
P=0.2Po
T (
kip
-ft)
z (deg)
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-10 -5 0 5 10-500
-400
-300
-200
-100
0
100
200
300
400
500
-10 -5 0 5 10-500
-400
-300
-200
-100
0
100
200
300
400
500GJ
exp=17430909 kip-in2
GsJ
s=11003678 kip-in2
GcJ
c=31370173 kip-in2
T=0.2049
RCFTs, P=0 to 0.2Po 4‐Rw‐18‐5, P=0 kipT (kip‐ft) T (kip‐ft)
z (deg) z (deg)
eff s s T c cGJ G J G J
Load protocol: LC4 –Torsion
Summary of Experimental Results
A comprehensive and unique data for:
• Slender CCFTs and RCFTs
• Axial strength and beam‐column strength for CFTs
• Complex cyclic loadings
• Initial imperfections
• Construction stresses/deformations
• Local buckling
• Ductility
Current AISC equations predict strength well for these specimens
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AnalysisofCompositeFrames:MixedBeam‐ColumnElement
• Mixed beam finite element formulation was developed using both displacement and force shape functions
• Distributed plasticity fiber formulation: stress and strain modeled explicitly at each fiber of cross section
• Perfect composite action assumed (i.e., slip neglected)
• Total‐Lagrangian corotationalformulation
• Implemented in the OpenSees framework
0 L
0
1Shape Functions
Tra
nsve
rse
Dis
plac
emen
t
0 L0
1
Ben
ding
Mom
ent
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
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UniaxialCyclicConcreteConstitutiveRelationsforCFTsandSRCs
• “Proposed for Behavior” constitutive relation:– Based on the rule‐based model of Chang and Mander (1994)
– Backbone stress‐strain curve for the concrete is based on Tsai’s Equation, which is defined by:
• Initial stiffness Ec• Peak coordinate (´cc, f´cc)• r, which acts as a shape factor for Tsai’s equation and enables calibration for
confinement in CFTs, between the flanges in SRCs, etc.
• “Proposed for Design” constitutive relation: simplified version of PB
-10000 -9000 -8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0 1000-5
-4
-3
-2
-1
0
1
Strain (strain)
Str
ess
(ks
i)
-10000 -9000 -8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0 1000-5
-4
-3
-2
-1
0
1
Strain (strain)
Str
ess
(ks
i)
UniaxialCyclicSteelConstitutiveRelationsforCFTs,SRCs,WFs,Rebar
• 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)
Late
ral L
oad
(kN
)
Expt.
PfB
H = 406 mm; B = 406 mmW8x40
Fy = 372 MPa4 #9; Fyr = 448 MPa
f′c = 31 MPaP/Pno = 0.19L/H = 4.8
H = 406 mm; B = 406 mmW8x40
Fy = 372 MPa12 #7; Fyr = 434 MPa
f′c = 63 MPaP/Pno = 0.11 L/H = 4.8
RCFTBeam‐ColumnValidationVarma2000
-100 -80 -60 -40 -20 0 20 40 60 80 100-500
-400
-300
-200
-100
0
100
200
300
400
500
Lateral Displacement (mm)Test #5: CBC-32-46-10 (Varma 2000)
Late
ral L
oad
(kN
)
Expt.
PfB
-80 -60 -40 -20 0 20 40 60 80-500
-400
-300
-200
-100
0
100
200
300
400
500
Lateral Displacement (mm)Test #8: CBC-48-46-20 (Varma 2000)
Late
ral L
oad
(kN
)
Expt.
PfB
H/t = B/t = 35Fy = 269 MPaf′c = 110 MPaP/Pno = 0.11L/H = 4.9
H/t = B/t = 53 Fy = 471 MPaf′c = 110 MPaP/Pno = 0.18L/H = 4.9
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CCFTBeam‐ColumnValidationSpecimen11– LoadCase3a
L = 7.9 m; D = 508 mm.; t = 5.9 mm.; D/t = 85.8; Fy = 305 MPa; f′c = 55.9 MPa
BenchmarkFrameStudiesforCompositeFrames:Schematic
L = oe1g EIgrossPno,gross
ktop = 6 EIgrossGg,top L
kbot = 6 EIgrossGg,bot L
P P P
HM
M
EIelasticEIelastic
x
EIgross = EsIs + EsIsr + EcIcPno,gross = AsFy + AsrFysr + Acf′c
Initial Imperfections:
Out-of-plumbness o = L/500Out-of-straightness o = L/1000 (sinusoidal)
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SelectedSections
Index D t sA 7 0.500 24.82%
B 10 0.500 17.70%
C 12.75 0.375 10.65%
D 16 0.250 5.72%
E 24 0.125 1.93%
Index H B t sA 6 6 1/2 27.63%
B 9 9 1/2 19.06%
C 8 8 1/4 11.13%
D 9 9 1/8 5.05%
E 14 14 1/8 3.27%
CCFT RCFT
Index Steel Shape sA W14x311 11.66%
B W14x233 8.74%
C W12x120 4.49%
D W8x31 1.16%
Index Rebar srA 20 #11 3.98%
B 12 #10 1.94%
C 4 #8 0.40%
SRC
Gross dimensions of all SRC sections = 28″ x 28″Fy = 50 ksi; Fyr = 60 ksi; ; f′c = 4, 8, 16 ksi
Fy = 42 ksi; f′c = 4, 8, 16 ksi Fy = 46 ksi; f′c = 4, 8, 16 ksi
ElasticFlexuralRigidityinCompositeBeam‐Columns
• EIeff – used to determine the axial compressive strength of columns in AISC 360‐10
• EIelastic – used in a 1st or 2nd order static, dynamic, or
eigenvalue analysis– in conjunction with Direct Analyses stiffness reductions to perform strength checks
– to compute story drifts used in interstory drift checks– to compute fundamental periods and mode shapes (including for response spectrum analysis)
– as the elastic component of a concentrated plasticity beam‐column element
• EIDA – used in the Direct Analysis method
For Structural Steel: EIeff = EIelastic = EsIs
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AISC360‐10SectionI2:CalculationofAxialCompressiveStrength:EIeff
10.5 (SRC)eff s s s sr c cEI E I E I C E I
1 0.1 2 0.3s
c s
AC
A A
3 (CFT)eff s s s sr c cEI E I E I C E I
3 0.6 2 0.9s
c s
AC
A A
/ 2
0/
CompositeAxialCompressiveStrengthfromBenchmarkStudy
CCFT RCFT
SRC (strong axis)
SRC (weak axis)
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ProposedFormulaforAxialCompressiveStrengthofSRCs
, 1, (SRC)eff proposed s s s sr proposed c cEI E I E I C E I
1,
20.60 0.75s
proposedg
AC
A
SRC (strong axis)
SRC (weak axis)
AxialCompressiveStrengthofSRCColumns:ExperimentalValidation
, 1, (SRC)eff proposed s s s sr proposed c cEI E I E I C E I
1,
20.60 0.75s
proposedg
AC
A
0 0.5 1 1.50
0.5
1
1.5
oe,proposed
P exp/P
no,p
ropo
sed
Column Curve
Anslijn & Janss 1974
Chen, Astaneh-Asl, & Moehle 1992
Han & Kim 1995
Han, Kim, & Kim 1992
Roderick & Loke 1975
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BenchmarkStudyResults:SecantValuesofEIelastic forElasticAnalysis
0 0.5 1 1.50
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Normalized Bending Moment (M/Mn)
Section 13: RCFT-E-4, Frame 37: UA-67-g1
Nor
mal
ized
Axi
al C
ompr
essi
on (
P/P no
)
0.4
0.6
0.8
1
elastic
s s c c
EI
E I E I
“Serviceability” Level Strength/1.6
0 0.5 1 1.50
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Normalized Bending Moment (M/Mn)
Section 4: RCFT-B-4, Frame 37: UA-67-g1
Nor
mal
ized
Axi
al C
ompr
essi
on (
P/P no
)
0.4
0.5
0.6
0.7
0.8
0.9
1First-Order Applied Load
Interaction
elastic
s s c c
EI
E I E I
EIelastic valueprovidescomparabledeflectiontofullynonlinearanalysisforforcesshown
Calculation of Required Strengths• Analysis Requirements
Second‐Order Elastic Analysis• Consideration of Initial Imperfections
• Adjustments to Stiffness
Calculation of Available Strengths• Chapters D though K without further consideration of overall structure stability
0.8
0.8DA b elastic
DA elastic
EI EI
EA EA
0.002i iN Y
AISC360‐10DirectAnalysisMethodChapterC
1K
Mr
Pr
cPn,K=1
cPn,K=K
Effective Length Factor Method
Direct Analysis Method
Distributed Plasticity Analysis
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DirectAnalysis
• From a practical standpoint it is best to maintain a stiffness reduction of 0.8b
• Thus, differences between composite and steel may be embodied in proposed EIelastic
:
0.8DA b elasticEI EI
1.0 for 0.5
4 1 for 0.5r no
br no r no r no
P P
P P P P P P
10.75 (SRC)elastic s s s sr c cEI E I E I C E I
30.75 (CFT)elastic s s c cEI E I C E I
CompositeInteractionStrength
P
M
(PA,0)
(PA,0)
(PC,MC)
(PC,MC)
(0,MB)
Nominal Section Strength
Nominal Beam-Column
Strength
= Pn/Pno
(PA,0)
(PA,0)
(PC,MC)
(CPA,0.9BMB)
(0, BMB) (0,MB)
NominalBeam-Column
Strength
P
M
= Pn/PnoNominal Section Strength
for 0.5
0.2 0.5 for 0.5 1.5
0.2 for 1.5
C A oe
C C A C A oe oe
oe
P P
P P P P
1 for 1
1 0.2 1 for 1 2
0.8 for 2
oe
B oe oe
oe
AISC 2010 Proposed
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VariationoftheCompositeInteractionDiagramwithSlenderness
01
230 0.5 1 1.5
0
0.2
0.4
0.6
0.8
1
1.2
Normalized Bending Moment (M/Mn)
Norm
alized Axial Load
(P/Pno)
CFT Bond Provisions in AISC 360‐10
For CCFT:
Rn = 0.25πD2CinFin
For RCFT:
Rn = B2CinFin
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)
= 0.50, = 3.00
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SeismicPerformanceFactors:FEMAP695ArchetypeFrameStudy:SelectionandDesignofArchetypeFrames
= Location of Braced Frame= Fully Restrained Connections
= Shear Connections
Moment Frames Braced Frames
SelectedCompositeArchetypeFramesDesign Gravity Load
Bay Width
Design Seismic Load
Conc.Strength
(f′c)Index
Moment Frames Braced Frames
RCFT RCFT SRC RCFT‐Cd CCFT CCFT
3 Stories 9 Stories 3 Stories 3 Stories 3 Stories 9 Stories
High 20’ Dmax 4 ksi 1
High 20’ Dmax 12 ksi 2
High 20’ Dmin 4 ksi 3
High 20’ Dmin 12 ksi 4
High 30’ Dmax 4 ksi 5
High 30’ Dmax 12 ksi 6
High 30’ Dmin 4 ksi 7
High 30’ Dmin 12 ksi 8
Low 20’ Dmax 4 ksi 9
Low 20’ Dmax 12 ksi 10
Low 20’ Dmin 4 ksi 11
Low 20’ Dmin 12 ksi 12
Low 30’ Dmax 4 ksi 13
Low 30’ Dmax 12 ksi 14
Low 30’ Dmin 4 ksi 15
Low 30’ Dmin 12 ksi 16
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TypicalCompositeConnectionRegionModeling:ValidatedAgainstTests
Rigid Links
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
load, SDC Dmin)
Group Number
Average Ω
C‐SMF C‐SCBF
PG‐1 5.9 2.1
PG‐2 5.3 1.9
PG‐3 7.6 2.8
PG‐4 9.9 2.7
PG‐5 6.2 1.8
PG‐6 5.5 1.7
PG‐7 7.5 2.3
PG‐8 6.5 2.2
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TypicalDynamicTimeHistoryAnalyses:IncrementalDynamicAnalysis
0% 5% 10% 15%0
2
4
6
8
10
12
14
16
18
Maximum Story Drift
ST =
SM
TS
F 2 (g)
SFRS: C-SMF, Frame: RCFT-3-1
ˆ 5.72CTS g
1.50MTS g
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
– Etc.
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Thank YouNEES Project Warehouse: https://nees.org/warehouse/project/440
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.
In‐Kind: