Steel concrete composite column is compression member by design with a cross-section made up of a hot rolled steel profile encased in the concrete or concrete infilled inside them. In case when steel shape is encased inside the concrete, longitudinal and transverse reinforcement is also used in the concrete.
The design of a steel composite steel concrete column under compression takes into account the limit state of flexural buckling. In the case of the encased section, local buckling is also taken into account during the design.
Chapter I of the AISI 360-16 specification for Structural steel buildings covers the design of composite columns. The structure of the design guidelines is quite similar to the guidelines for the design of steel columns.
In this article, we will see in detail how to design a composite steel concrete column?
What are the common cross-sectional shapes of sections in steel concrete composite columns?
- In the case of encased sections, usually, a W shape section is infilled inside a rectangular or circular reinforced concrete section.
- In the case of infilled sections, usually a rectangular or circular Hollow Steel Section (HSS) is infilled with the concrete. No additional reinforcement is required in this case.
Advantages of using steel concrete composite columns.
Protect from fire.
Steel is weak in fire as compared to concrete. Encasing a steel section within the concrete will make the section stay longer against the fire.
Permit use of smaller shapes
With concrete combined with steel shape, it will increase the area and inertia of the section, which in turn will result in a stronger section. This can permit the designer to use smaller shapes for steel sections, which in turn will be a financial benefit.
Offer more lateral resistance
With increased inertia, the column would have more flexure stiffness (EI), which in turn will lead to increased lateral resistance against lateral loads like wind and earthquake actions
No formwork is required
Infilling the concrete inside a steel shape will obviously eliminate the need for formwork. The steel itself can serve the purpose of formwork.
Fewer construction times
With no formwork required, construction times will obviously reduce. Columns can be casted independently of the casting of beams and slabs.
How to design a composite steel concrete column?
Before beginning the design process, let’s have a look into some general requirements for the design of steel concrete composite columns given in the AISI specification.
Requirements for encased columns
- ACI 318 provisions specifically intended for composite columns shall be excluded in their entirety.
- Transverse reinforcement limitations shall be as specified in Section I2.1a(b) and I2.2a(c) of the specification, in addition to those specified in ACI 318.
- Concrete shall have a compressive strength, f ′c, of not less than 3 ksi (21 MPa) nor more than 10 ksi (69 MPa) for normal weight concrete
- Concrete shall have a compressive strength, f ′c, of not less than 3 ksi (21 MPa) nor more than 6 ksi (41 MPa) for lightweight concrete.
- The specified minimum yield stress of structural steel used in calculating the strength of composite members shall not exceed 75 ksi (525 MPa). This limit avoids the earlier appearance of local buckling in steel reinforcement.
- The specified minimum yield stress of reinforcing bars used in calculating the strength of composite members shall not exceed 80 ksi (550 MPa).
- The cross-sectional area of the steel core shall comprise at least 1% of the total composite cross-section.
- Continuous longitudinal bars and lateral ties or spirals should be used for concrete encasement.
- For lateral ties, a minimum of either a No. 3 (10 mm) bar spaced at a maximum of 12 in (300 mm) on center, or a No. 4 (13 mm) bar or larger spaced at a maximum of 16 in (400 mm) on center shall be used. Deformed wire or welded wire reinforcement of equivalent area is permitted. Maximum spacing of lateral ties shall not exceed 0.5 times the least column dimension.
- The minimum reinforcement ratio for continuous longitudinal reinforcing, ρsr, shall be 0.004.
- If there is more than one shape encased, they should be laced together
- A concrete cover of 1.5 in should be provided
Requirements for infilled columns
- The cross-sectional area of the steel core shall comprise at least 1% of the total composite cross-section.
- For compression, filled composite sections are classified as compact, non-compact, or slender.
- Table I1.1a of the specification gives the limiting values for slenderness ratio to qualify section as compact, non-compact, or slender.
Following figure outlines the entire design process of composite steel concrete columns under compression.
Design of composite steel concrete column using Table 4-13 to Table 4-20 of AISC Manual
Alternatively, infilled columns with 4 and 5 ksi concretes can also be designed using the tables 4-13 to 4-20 given in chapter 4 of the AISC manual.
Design Shear Strength of composite member (AISC 360 Section I4)
The design shear strength, φvVn, and allowable shear strength, Vn /Ωv, can be determined based on one of the following:
- The available shear strength of the steel section alone as specified in Chapter G.
- The available shear strength of the reinforced concrete portion (concrete plus steel reinforcement) alone as defined by ACI 318 with φv = 0.75 (LRFD) Ωv = 2.00 (ASD)
- The nominal shear strength of the steel section, as defined in Chapter G, plus the nominal strength of the reinforcing steel, as defined by ACI 318, with a combined resistance or safety factor of φv = 0.75 (LRFD) Ωv = 2.00 (ASD)
Any of the above-prescribed methods can be used to obtain the design shear resistance of a steel concrete composite column.
Design Tensile Strength of composite member (AISC 360 Section I4)
The available tensile strength of the axially loaded encased composite column is determined by the limit state of tensile yielding. Partial safety factors for LRFD is 0.9 while 1.67 for ASD.
Pn=FyAs + FysrAsr