Composite Columns

Summary:

• Composite columns may take the form of open sections partially or fully encased in concrete, or concrete-filled hollow steel sections.

• Both types require longitudinal reinforcing bars, and shear studs may be used to provide additional bond between steel and concrete.

• The confinement provided by a closed steel section allows higher strengths to be attained by the concrete. Circular concrete-filled tubes develop hoop-tension which further increases the overall load-carrying capacity of the concrete.

• Complete encasement of a steel section usually provides enough fire protection to satisfy the most stringent practical requirements.

• Eurocode 4 provides two methods for calculation of the resistance of composite columns; The General Method and the Simplified Method. The Simplified Method is covered in this article.

• Fully encased sections do not require any check on the thickness of the walls of the steel section. Other types of composite column are subject to a minimum thickness requirement.

• Loads from beams must be transmitted within a limited development length to both the steel and concrete parts of the column section. In some cases this may require the use of shear studs.

• In order to use the Simplified Method of design the column cross-section must be constant and doubly symmetric over its whole height.

• The bending stiffness of a column used in the calculation of its critical elastic resistance includes the steel section, reinforcement and concrete, which are assumed to interact perfectly. For short-term loading the concrete modulus is reduced by 20%, and for long-term loading it is further reduced to account for the effects of creep.

• The reduction of buckling resistance due to imperfections may be taken from the EC3 buckling curves. Equivalent imperfections are provided, which can be used directly as an eccentricity of the axial force in calculating the design moment.

• It is permissible to construct simply a linearised version of the interaction diagram

Structural Steelwork Eurocodes – Development of a Trans-National Approach Design of Composite Columns

reduction factor, provided that the design moment includes the effect of member imperfection and amplification by second-order effects.

• The resistance of a column under axial compression and biaxial bending is calculated using its uniaxial resistance about both axes, plus an interaction check between the moments at the specified axial force.

Structural Steelwork Eurocodes – Development of a Trans-National Approach Design of Composite Columns

Pre-requisites:

• Stress distributions in cross-sections composed of different materials and under combinations of axial force and bending moments about the principal axes.

• Euler buckling theory for columns in compression.

• The reduction of perfect elastic buckling resistance due to the presence of initial imperfections in columns.

• The difference between first-order and second-order bending moments in members which are subject to "P-δ" effects.

• Framing systems currently used in steel-framed construction, including composite systems.

Notes for Tutors:

• This material comprises one 60-minute lecture.

• The lecturer can break up the session with formative exercises at appropriate stages.

• The lecturer may apply a summative assessment at the end of the session requiring that students consider the consequences of adopting composite columns rather than steel H-section columns at the design stage for part of an example building.

Objectives:

The student should:

• be aware of the types of composite columns which may be used in building structures.

• appreciate the main characteristics of concrete-filled hollow sections and concrete-encased open sections when used as columns.

• understand how beam reaction forces must be transmitted through a

predetermined load path into both the steel and concrete parts of a column section close to a beam-column connection.

• understand the principles of the Eurocode 4 "Simplified Method" of design for composite columns.

• know that buckling resistance under axial load is given by the normal EC3 column buckling curves.

• know how to construct the simple linearised version of the interaction diagram for cross-sectional resistance to combinations of axial compression and uniaxial bending moment on a composite section.

• understand that, when designing for axial force and bending, the design moment must take account of “P-δ” effects, either by performing a second-order analysis

Structural Steelwork Eurocodes – Development of a Trans-National Approach Design of Composite Columns

References:

• ENV 1991-1: 1996 Eurocode 1: Basis of Design and Actions on Structures. Part 1: Basis of Design.

• ENV 1992-1-1: 1991 Eurocode 2: Design of Concrete Structures. Part 1.1:

General Rules and Rules for Buildings.

• ENV 1993-1-1: 1992 Eurocode 3: Design of Steel Structures. Part 1.1: General Rules and Rules for Buildings.

• Draft prEN 1994-1-1: 2001 Eurocode 4: Design of Composite Steel and

Concrete Structures. Part 1.1: General Rules and Rules for Buildings. (Draft 2)

• D J Oehlers and M A Bradford, Composite Steel and Concrete Structural Members – Fundamental Behaviour, Pergamon, Oxford, 1995.

Structural Steelwork Eurocodes – Development of a Trans-National Approach Design of Composite Columns

Contents:

1 Introduction

2 Calculation Methods

3 Local buckling of steel elements

4 Force transfer between steel and concrete at beam-column connections 5 Use of the simplified calculation method

6 Composite columns subject to axial compression 6.1 Resistance of the cross-section

6.2 Relative slenderness of a composite column 6.3 Member buckling resistance

7 Resistance to compression and bending

7.1 Cross-section resistance under moment and axial force 7.2 Second-order amplification of bending moments 7.3 The influence of shear force

7.4 Member resistance under axial compression and uniaxial bending 7.5 Member resistance under axial compression and biaxial bending

8 Conclusions

Structural Steelwork Eurocodes – Development of a Trans-National Approach Design of Composite Columns

Introduction

Composite columns may be classified into two principal types:

• Open sections partially or fully encased in concrete,

• Concrete-filled hollow steel sections.

Figure 1 shows different types of composite columns, and defines symbols used in this lecture.

Partially encased columns (Figs. 1b and 1c) are based on steel I- or H-sections, with the void between the flanges filled with concrete. In fully

encased columns (Figure 1a) the whole of the steel section is embedded within a minimum cover-depth of concrete.

Concrete-filled hollow sections (Figs. 1d to 1f) may be circular or rectangular.

The concrete fills the section, and its compressive strength is enhanced by its confinement. This is an additional advantage for the compression resistance of the column.

Figure 1 Typical cross-sections of composite columns Composite columns can provide considerable advantages compared to open steel columns. For example, a cross-section of slender exterior dimensions can resist high axial loads. Different cross-sections of the same exterior dimensions can carry very different loads, depending on the thickness of the steel section, the strength of the concrete and the area of reinforcement used. It is possible to keep the same column dimensions over several storeys of a building, which provides both functional and architectural advantages.

In the case of concrete-filled hollow sections, the steel provides a permanent formwork to the concrete core. This allows, for example, the steel frame to be erected and the hollow column sections subsequently to be filled with pumped concrete. This leads to appreciable savings in the time and cost of erection. In addition, the confinement provided by the closed steel section allows higher strengths to be attained by the concrete. In the case of circular concrete-filled tubes the steel, in providing confinement to the concrete, develops a hoop-tension which increases the overall load-carrying capacity, although this is often ignored in design. Creep and shrinkage of concrete, are also generally

Structural Steelwork Eurocodes – Development of a Trans-National Approach Design of Composite Columns

neglected in the design of concrete-filled tubes, which is not the case for concrete-encased sections. On the other hand, complete encasement of a steel section usually provides enough fire protection to satisfy the most stringent requirements without resorting to other protection systems. For partially encased sections, and for concrete-filled hollow sections, codes of practice on fire resistance require additional reinforcement.

Partially encased sections have the advantage of acting as permanent formwork; the concrete is placed in two stages with the section aligned horizontally, turning the member 24 hours after the first pour. In order to ensure adequate force transfer between the steel and concrete it is sometimes necessary to use stud connectors or reinforcement connected directly or indirectly to the metal profile. Another significant advantage of partially encased sections is the fact that, after concreting, some of the steel surfaces remain exposed and can be used for connection to other beams.

Structural Steelwork Eurocodes – Development of a Trans-National Approach Design of Composite Columns