Prof. Salah CE591compcol_F13

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 Concrete Beams Columns Floor slabs Metal Deck Shear Walls 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)

Floor beams

Gravity columns

Composite Columns Encased Composite Columns SRC – Steel Reinforced Concrete

Filled Composite Columns CFT – Concrete Filled Tubes

Encased Composite Columns Lateral Ties/ Stirrups

Longitudinal Bars

Structural shapes surrounded by concrete Vertical and horizontal reinforcement to sustain encasement Shear connectors can be used to help transfer forces

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

Filled Composite Columns 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

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

AISC Limitations To qualify as a composite column:

As  0.01 Ag Concrete strength:

Supercolumns 12 ksi

3ksi  f ' c  10ksi Normal weight 3ksi  f ' c  6ksi Lightweight

AISC I1.3 and C-I1

AISC Limitations, cont’d Steel strength (used in calculations):

Fy and Fyr  75ksi Corresponds roughly to 0.003 strain limit for concrete

AISC I2.1 and C-I2.1

AISC Limitations, cont’d Min. 1.5 db, 1-1/2” clear (between steel core and longitudinal reinf. bars)

Min. No.3 @ 12" max or No.4 @ 16" max

Asr sr  Ag

sr  0.004

Area of reinf. bars (in2) Gross area of composite member (in2)

AISC Limitations, cont’d

AISC I2.1and C-I2.1

Str  0.5d Least column dimension

d 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

b

AISC I1.4 and C-I1.4

D t

b E  2.26 t Fy

t

D E  0.15 t Fy

Nominal Section Strength

AISC Limitations, cont’d

b is for longer side / dimension

w b AISC B4.1 b = clear distance

t t = design wall thickness (0.93 x nominal wall thickness (AISC B4.2))

between webs less inside corner radius Radius not known? Use b = w – 3t

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.