prof. salah ce591compcol_f13

8/10/2019 Prof. Salah CE591compcol_F13

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CE591 Lecture 13: Composite ColumnsComposite Action, CompositeComponents, History

IntroductionEncased and FilledComposite Columns

Behavior of Composite Columns

AISC Limitations


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Benefits of Structural

Steel ConcreteHigh Strength

High Stiffness(Modulus ofElasticity)

High Ductility

Excellent FireResistance

Low Cost

Ability to Be Castinto Any Shape

+ speed of construction

Very good for floor framing Very good for floor slabs


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Composite Action

Developed when two load carryingstructural members are integrallyconnected and deflect as a single unit.


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Benefits (floor beam example)Reduced weight of steel

Increased stiffness for composite floor

beams/girders

Or shallower beams

for the same stiffness

increased floor-to-floor height


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Composite ElementsBeams

ColumnsFloor slabs

Shear Walls

Concrete

Metal

Deck

Beam-to-Column Connections (?)

Composite Columns considered to have a toughness; good

choice for designs where blast-loading is a concern


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HistoryEarly 1900s steelbeams encased inconcrete for

fireproofing1931Empire StateBuildings entire steelframe was encased in

concreteComposite sections were not considered in

capacity calculations, but lateral stiffness wasdoubled for drift calculations


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History

1988Bank of China

megatruss ofcomposite columns


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HistoryLate 1990sPacific First Center Supercolumns

(lateral system)

Gravity columns

Floorbeams


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Composite ColumnsEncased Composite Columns

SRCSteel Reinforced Concrete

Filled Composite Columns

CFTConcrete Filled Tubes


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Encased Composite ColumnsStructural shapessurrounded byconcrete

Vertical andhorizontalreinforcement tosustain encasement

Shear connectorscan be used to helptransfer forcesLongitudinal Bars

Lateral Ties/

Stirrups


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Encased Composite ColumnsConcrete provides stiffening,strengthening, fire protection

Steel carries construction loadMight use when exposedconcrete finish desired

Might use for transitions(concrete to steel columns)


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Encased Composite ColumnsDifficult toplace?

Might use U-ties instead


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Steel shell (pipe, tube, orhollow section built-up from

plate)Shell provides formwork forconcrete

Shell provides confinement toconcrete



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Concrete adds strength,stiffness

Might use when exposed steelis desired

Steel can buckle outwards

Shear connectors might be

needed near beam-to-columnconnections



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Shear bond between concrete & steelFriction

Coefficient of sliding friction ~0.5

Encased ColumnsPressure/friction only if concrete confinedlaterally to bear against steel shapelateral ties

Filled ColumnsPressure normal to interface exists


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Behavior of Encased ColumnsFlexural stiffness governed byconcrete encasement

Encasement prevents buckling ofsteel bars and steel shape

Concrete outside ties cracks andspalls, followed by rest of

encasementAfter spalling, post-yield buckling ofsteel, overall failure


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Behavior of Filled ColumnsFlexural stiffness governed by steelshell

Initial compressive strainsteelexpands more than concrete, causesmicrocracking

Expansion of concrete then

restrained by steelSteel reaches yield, inelasticoutward buckling may occur,concrete crushes


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Elephant-Foot Buckling


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ConfinementConfinement from steel shell canincrease effective strength of concrete

However, stiffness reduced bymicrocracking


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AISC LimitationsTo qualify as a composite column:

AISC I1.3,I2.1a and C-I1, I2

01.0

g

s

A

A

Concrete strength:

ksicfksi

ksicfksi

6'3

10'3

Normal weight

Lightweight

Supercolumns 12 ksi


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AISC Limitations, contd

Steel strength (used in calculations):

AISC I1.3 and C-I1

ksiFandF yry 75

Corresponds roughly to 0.003 strain limit for concrete


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AISC I2.1 and C-I2.1

Min. 1.5 db, 1-1/2 clear (betweensteel core and longitudinal reinf. bars)

axm"16@4.

max"12@3..

Noor

NoMin

g

srsr

AA Area of reinf. bars (in

2

)

Gross area of composite

member (in2)

004.0sr


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AISC Limitations, contd AISC I2.1and C-I2.1

d

dStr 5.0

Least column

dimension

provisions of ACI 318 shal l apply with exceptions and limitations

(as listed in AISC I 1.1); see ACI 318 Sections 7.10 and 10.9.3 for

additional tie reinforcement provisions


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Local Bucklinglpfor Axial Compression

AISC I1.4 and C-I1.4

b

t t

D

yFE

tb 26.2

yFE

tD 15.0


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Nominal Section Strength


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b

t

bis for longerside / dimension

AISC B4.1b= clear distance

between webs less inside

corner radius

Radius not known?

Use b = w3t

w

t = design wall thickness(0.93 x nominal wall thickness

(AISC B4.2))


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Load TransferAISC I6

Transfer of load to concrete by

direct bearing requires bearingcheck, etc.

Load applied to steel or

concrete onlyshearconnectors required

Good reference on Load Transfer is PowerPoint by

W Jacobs posted to CE591 website

prof. salah ce591compcol_f13

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