Composite Members (LRFD Approach)

48.13 Composite Members (LRFD Approach)

Composite members are structural members made from two or more materials. The majority of composite sections used for building constructions are made from steel and concrete, although in recent years the use of fiber-reinforced polymer (FRP) has been on the rise, especially in the area of structural rehabili- tation. Composite sections made from steel and concrete utilize the strength provided by steel and the rigidity provided by concrete. The combination of the two materials often results in efficient load-carrying members. Composite members may be concrete encased or concrete filled. For concrete-encased mem- bers ( Fig. 48.37a ), concrete is cast around steel shapes. In addition to enhancing strength and providing rigidity to the steel shapes, the concrete acts as a fireproofing material to the steel shapes. It also serves as a corrosion barrier, shielding the steel from corroding under adverse environmental conditions. For concrete-filled members ( Fig. 48.37b ), structural steel tubes are filled with concrete. In both concrete- encased and concrete-filled sections, the rigidity of the concrete often eliminates the problem of local buckling experienced by some slender elements of the steel sections.

Some disadvantages associated with composite sections are that concrete creeps and shrinks. Further- more, uncertainties with regard to the mechanical bond developed between the steel shape and the concrete often complicate the design of beam-column joints.

Composite Columns

According to the LRFD specification [AISC, 1999], a compression member is regarded as a composite column if:

(A) Concrete Encased Composite Section

(B) Concrete Filled Composite Sections

FIGURE 48.37 Composite columns.

1. the cross-sectional area of the steel section is at least 4% of the total composite area. If this condition is not satisfied, the member should be designed as a reinforced concrete column.

2. longitudinal reinforcements and lateral ties are provided for concrete-encased members. The cross- sectional area of the reinforcing bars shall be 0.007 in. 2 /in. (180 mm 2 /m) of bar spacing. To avoid spalling, lateral ties shall be placed at a spacing not greater than two thirds the least dimension of the composite cross section. For fire and corrosion resistance, a minimum clear cover of 1.5 in. (38 mm) shall be provided.

3. the compressive strength of concrete f c ¢ used for the composite section falls within the range of

3 ksi (21 MPa) to 8 ksi (55 MPa) for normal weight concrete and not less than 4 ksi (28 MPa) for lightweight concrete. These limits are set because they represent the range of test data available for the development of the design equations.

4. the specified minimum yield stress for the steel sections and reinforcing bars used in calculating the strength of the composite columns does not exceed 60 ksi (415 MPa). This limit is set because this stress corresponds to a strain below which the concrete remains unspalled and stable.

5. the minimum wall thickness of the steel sections for concrete-filled members is equal to b ÷(F y /3E) for rectangular sections of width b and D ÷(F y /8E) for circular sections of outside diameter D.

Design Compressive Strength

The design compressive strength, f c P n , shall exceed the factored compressive force, P u . The design compressive strength for f c £ 1.5 is given as

( ) ˚˙ l c

¢ c f A c (48.134)

1 yr

ËÁ ¯˜ 2 A c s ËÁ A s ¯˜

E Ê A m ˆ =+ EcE c 3 c (48.135)

ËÁ A s ¯˜

r m = the radius of gyration of steel section and shall not be less than 0.3 times the overall thickness of the composite cross section in the plane of buckling

A c = the area of concrete

A r = the area of longitudinal reinforcing bars

A s = the area of the steel shape

E = the modulus of elasticity of steel

E c = the modulus of elasticity of concrete

F y = the specified minimum yield stress of the steel shape

F yr = the specified minimum yield stress of longitudinal reinforcing bars

f c ¢ = the specified compressive strength of concrete

c 1 ,c 2 , and c 3 = the coefficients given in the table below.

Type of Composite Section

Concrete-encased shapes

Concrete-filled pipes and tubings

In addition to satisfying the condition f c P n ≥P u , shear connectors spaced no more than 16 in. (405 mm) apart on at least two faces of the steel section in a symmetric pattern about the axes of the steel section shall be provided for concrete-encased composite columns to transfer the interface shear force V u ¢ between steel and concrete. V u ¢ is given by

AF

Ô u - when the force is applied to the steel section

Ô V u when the force is applied to the concrete encasement Ó Ô ËÁ P n ¯˜

where V u = the axial force in the column

A s = the area of steel section

F y = the yield strength of the steel section P n = the nominal compressive strength of the composite column without consideration of slenderness effect

If the supporting concrete area in direct bearing is larger than the loaded area, the bearing condition for concrete must also be satisfied. Denoting f c P nc (= f c P n,composite section – f c P n,steel shape alone ) as the portion of compressive strength resisted by the concrete and A B as the loaded area, the condition that needs to be satisfied is

f c P nc £ . 0 65 1 7 [ . c ¢ B ] f A