Behaviour of beams


To describe the basic behaviour of composite beams including a geometric description of a typical beam, its construction, and the stress strain relationships that develop under load.


Composite beams are described in terms of the steel section, concrete slab, and connectors used in a typical building floor. The material behaviour of each of the components is briefly reviewed and reference is made to the slender nature of the steel section and the anisotropic nature of the concrete slab. The structural behaviour of a typical composite beam is described in three stages by reference to the strain and stress in each component part. Firstly, at low loads when full interaction and a linear elastic response occurs; secondly, as slip takes place with increasing load; and finally, as the materials reach failure stresses. Propped and unpropped construction gives rise to different beam behaviour, which is described. Partial interaction is also explained in qualitative terms. The lecture concludes with a summary of the constraints that the engineer must take into account when designing composite beams.


This lecture outlines, in general terms, the behaviour of the most common form of composite element - the composite beam. In doing this it will be possible to explain many of the problems associated with the analysis and design of other elements such as columns and slabs. The lecture, therefore, forms a basis on which to build an understanding of composite behaviour.

A general description of a composite beam is followed by a more detailed discussion on the component parts and their individual structural behaviour. The structural action is described by reference to the strain and resulting stress and the history of a typical composite beam as it deforms under increasing load to failure.

The way in which composite beams are constructed may alter their resistance to applied loads. Consequently, it is essential to design composite beams for both the construction and in-service condition. It is also possible to design a beam for 'partial connection' so that each condition is equally critical. A definition of partial connection and brief reasons for the two-stage design requirement are described. Simple single spans are a common form of beam and their behaviour is explained. The behaviour of continuous spans is also introduced. Finally, a summary of the design criteria for composite beams is given. These criteria are covered in more detail in subsequent lectures.

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