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5/29/2013

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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



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



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



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


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


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’ Dmin 4 ksi 3






Low 20’ Dmax 4 ksi 9


Low 20’ Dmin 4 ksi 11






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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:

eeri nees final print - datacenterhub.org

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