413 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 has been rising especially in the area of structural rehabilitation. 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 members (Figure 4.37a), concrete is cast around steel shapes. In addition to enhancing strength and providing rigidity to the steel shapes, the concrete acts as a fire-proofing 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 (Figure 4.37b), 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. Furthermore, uncertainties with regard to the mechanical bond developed between the steel shape and the concrete often complicate the design of beam-column joints.

4.13.1 Composite Columns

According to the LRFD Specification (AISC 1999), a compression member is regarded as a composite column if

• 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.

• 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 mm2/m) of bar spacing. To avoid spalling, lateral ties shall be placed at a spacing not greater than 3 the least dimension of

FIGURE 4.37 Composite columns: (a) concrete encased composite section and (b) concrete filled composite sections.

the composite cross-section. For fire and corrosion resistance, a minimum clear cover of 1.5 in. (38 mm) shall be provided.

• The compressive strength of concrete /.' used for the composite section falls within the range 3 ksi (21 MPa) to 8 ksi (55 MPa) for normal weight concrete and not less than 4 ksi (28 MPa) for light weight concrete. These limits are set because they represent the range of test data available for the development of the design equations.

• 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.

• The minimum wall thickness of the steel sections for concrete filled members is equal to bsj(Fy/3E) for rectangular sections of width b and D^J(Fy/8E) for circular sections of outside diameter D. Design Compressive Strength

The design compressive strength, f cPn, shall exceed the factored compressive force, Pu. The design compressive strength is given as follows:



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