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
Filled Composite Columns
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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
Filled Composite Columns
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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 Limitations, contd
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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AISC Limitations, contd
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
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