Continuous composite beams I

OBJECTIVE/SCOPE

To describe the behaviour of continuous composite beams. To explain the use of rigid-plastic analysis to determine internal moments and forces and to derive plastic resistance moments.

SUMMARY

The advantages of continuous beams are summarised and failure modes which result from continuity in composite beams are identified. Plastic methods may be used to determine internal moments and forces, provided that rotation capacity is sufficient and lateral-torsional buckling does not occur. The scope for plastic methods is related to classification of cross-sections in terms of limiting breadth/thickness ratios for structural steel elements in compression. Other measures needed to ensure adequate rotation capacity are also described. Simple values for effective width of the concrete flange are presented and expressions for the negative resistance moment of Class 1 and Class 2 sections are given. The application of rigid-plastic analysis to determine the distribution of bending moments is demonstrated.

1. INTRODUCTION

Continuous beams offer the following advantages over simple construction:

  1. greater load resistance
  2. greater stiffness

These result in a smaller steel section being required to withstand specified loading.

In this lecture, members are assumed to be continuous over simple supports or to be rigidly connected to columns in braced frames. Additional cost will be incurred if special methods, such as more complicated jointing, have to be provided to achieve continuity. However, continuity of structural steel can be achieved economically by running a single length of section across two or more spans. The concrete is cast continuously over the supports and, to control shrinkage and other cracking, the concrete is reinforced. A typical cross-section of a composite beam, in the region of an internal support, is shown in Figure 1.

Figure 1 Cross-section of composite beam at an internal support

The disadvantages associated with continuous construction are:

  1. increased complexity in design
  2. susceptibility to buckling in the negative moment region over internal supports (see Figure 2a): Indeed, this negative moment region may extend over the whole of one span during construction (see Figure 2b). Two forms of buckling may also be involved: local buckling of the web and/or bottom flange and lateral-torsional buckling. Only the former is treated here; lateral-torsional buckling is discussed in the following lecture: Lecture 10.4.2.

 

After conceptual structural design has been done, which might possibly include initial sizing of members based on experience or rough calculations, the designer will wish to proceed to detailed calculations for the structure. The next stage is, therefore, the determination of internal moments and forces in critical regions for the various loading cases and limit states. This is known as 'global analysis' and procedures for this, at the ultimate limit state, are discussed here and in the following lecture.

Internal moments in continuous composite beams may be conveniently determined by elastic analysis or, subject to certain conditions, by rigid-plastic analysis. Whether plastic analysis is appropriate depends on the ductility of the reinforcement and on the susceptibility of the steel section to local buckling, as explained below. Elastic analysis is treated in the following lecture.

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Prerequisites

Cross-section classification

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Local buckling

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Behaviour of beams

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Single span composite beams

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Related lectures

Continuous composite beams II

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Composite beams - Design for serviceability part 1

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