CE591 Lecture 13: Composite Columns

Composite Action, Composite Components, History

Introduction – Encased and Filled Composite Columns

Behavior of Composite Columns

AISC Limitations

Benefits of Structural… …Steel …Concrete

High Strength

High Stiffness (Modulus of Elasticity)

High Ductility

Excellent Fire Resistance

Low Cost

Ability to Be Cast into Any Shape

+ speed of construction

Very good for floor framing Very good for floor slabs

Composite Action

Developed when two load carrying structural members are integrally connected and deflect as a single unit.

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

Composite Elements

Beams

Columns

Floor 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

History Early 1900’s – steel beams encased in concrete for fireproofing

1931 – Empire State Building’s entire steel frame was encased in concrete

Composite sections were not considered in capacity calculations, but lateral stiffness was “doubled” for drift calculations

History

1988 – Bank of China

“megatruss” of composite columns

History Late 1990s – Pacific First Center

Supercolumns

(lateral system)

Gravity columns

Floor

beams

Composite Columns

Encased Composite Columns

SRC – Steel Reinforced Concrete

Filled Composite Columns

CFT – Concrete Filled Tubes

Encased Composite Columns Structural shapes surrounded by concrete

Vertical and horizontal reinforcement to sustain encasement

Shear connectors can be used to help transfer forces Longitudinal Bars

Lateral Ties/

Stirrups

Encased Composite Columns Concrete provides stiffening, strengthening, fire protection

Steel carries construction load

Might use when exposed concrete finish desired

Might use for transitions (concrete to steel columns)

Encased Composite Columns Difficult to

place?

Might use U-ties instead

Steel shell (pipe, tube, or hollow section built-up from plate)

Shell provides formwork for concrete

Shell provides confinement to concrete

Filled Composite Columns

Concrete adds strength, stiffness

Might use when exposed steel is desired

Steel can buckle outwards

Shear connectors might be needed near beam-to-column connections

Filled Composite Columns

Shear bond between concrete & steel

Friction Coefficient of sliding friction ~0.5

Encased Columns Pressure/friction only if concrete confined laterally to bear against steel shape lateral ties

Filled Columns Pressure normal to interface exists

Behavior of Encased Columns Flexural stiffness governed by concrete encasement

Encasement prevents buckling of steel bars and steel shape

Concrete outside ties cracks and spalls, followed by rest of encasement

After spalling, post-yield buckling of steel, overall failure

Behavior of Filled Columns Flexural stiffness governed by steel shell

Initial compressive strain – steel expands more than concrete, causes microcracking

Expansion of concrete then restrained by steel

Steel reaches yield, inelastic outward buckling may occur, concrete crushes

“Elephant-Foot Buckling”

Confinement

Confinement from steel shell can increase effective strength of concrete

However, stiffness reduced by microcracking

AISC Limitations

To qualify as a composite column:

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

01.0g

s

AA

Concrete strength:

ksicfksi

ksicfksi

6'3

10'3

Normal weight

Lightweight

Supercolumns 12 ksi

AISC Limitations, cont’d

Steel strength (used in calculations):

AISC I1.3 and C-I1

ksiFandF yry 75

Corresponds roughly to 0.003 strain limit for concrete

AISC Limitations, cont’d

AISC I2.1 and C-I2.1

Min. 1.5 db, 1-1/2” clear (between

steel core and longitudinal reinf. bars)

axm"16@4.

max"12@3..

Noor

NoMin

g

srsr

A

A

Area of reinf. bars (in2)

Gross area of composite

member (in2)

004.0sr

AISC Limitations, cont’d AISC I2.1and C-I2.1

d

dStr 5.0

Least column

dimension

provisions of ACI 318 shall apply with exceptions and limitations

(as listed in AISC I1.1); see ACI 318 Sections 7.10 and 10.9.3 for

additional tie reinforcement provisions

Local Buckling lp for Axial Compression

AISC I1.4 and C-I1.4

b

t t

D

yF

E

t

b26.2

yF

E

t

D15.0

Nominal Section Strength

AISC Limitations, cont’d

b

t

b is for longer

side / dimension

AISC B4.1

b = clear distance

between webs less inside

corner radius

Radius not known?

Use b = w – 3t

w

t = design wall thickness (0.93 x nominal wall thickness

(AISC B4.2))

Load Transfer

AISC I6

Transfer of load to concrete by direct bearing requires bearing check, etc.

Load applied to steel or concrete only – shear connectors required

Good reference on Load Transfer is PowerPoint by

W. Jacobs – posted to CE591 website.