De la competencia comunicativa a la competencia sociocultural Ante todo, con el fin de entender el concepto sociocultural, es necesario volver a los fundamentos, o

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150mm (6in.) centres around the perimeter should be regarded as a minimum amount for even the thinnest section. For crack control the spacing of the reinforcement is as important as the quantity provided, so that large bars spaced well apart are unlikely to be effective.

Where applicable, UK design regulations relating to bridges call for the calculations to be based on a modular ratio of 15. Other national codes call for different values, some as low a ratio as eight. The discrepancy stems from the fact that the elastic modulus for high-strength concretes is, in fact, quite high but shrinkage and creep have the effect of adjusting the relative stresses within the concrete and the reinforcement. British design regulations have taken the view that the calculations should represent the long-term effect—i.e. after shrinkage and creep have taken place—and stresses/which are calculated using a modular ratio of 15 are representative of this. Lowering of the modular ratio has the effect of increasing the calculated stresses in the concrete of a reinforced concrete section, thereby tending to require slightly deeper members with marginally less reinforcement.

Concern about fatigue in reinforced concrete sections has recently led to the introduction of limitations on the total range of stress to which any given reinforcement

may be subjected in the loading history of a member, as well as the limiting tensile and compressive stresses which the reinforcement may be designed to resist.

Detailing

The objective when detailing is to achieve a simple arrangement of reinforcement which provides bars where they are needed with the minimum of superfluous steel and without imposing restrictions on the formwork. A simple arrangement of reinforcement means using straight bars where possible to produce a layout of reinforcement which creates a readily identifiable pattern that a steel fixer can easily memorize, thus enabling him to work without needing to make constant reference to the details. These objectives almost inevitably mean the use of a small number of different bars within an arrangement.

Putting reinforcement where it is needed sounds obvious, but it is all too easy to detail arrangements of reinforcement which take no sensible account of tolerances that may reasonably be expected, either in bar bending or fixing. If a link passes round the full perimeter of a column section and its dimensions are calculated from

the column size and cover required, without making any allowance for tolerances, difficulties are likely to arise in maintaining suitable cover to the reinforcement.

There is often a conflict between the desire to adopt simple arrangements and the need to minimize superfluous reinforcement. Any attempt to restrict the reinforcement provided to the amounts theoretically required at closely spaced sections within a member would lead to an arrangement of reinforcement in which each reinforcing bar had its own individual dimensions and shape. There are, however, points within a structure where there is a tendency to duplicate reinforcement. In a cellular deck, for instance, the junction between the web and slab members suggests the adoption of a detail in which the vertical reinforcement is bent into the plane of the slab reinforcement to provide anchorage. If this is done, then that reinforcement should be arranged to contribute to the strength of the slab section, rather than simply to include it in addition to the reinforcement already provided in the horizontal plane.

For an arrangement of reinforcement to be convenient for construction it is essential that the constructional sequence and the location of construction joints are taken into account when deciding on the arrangement of steel. For example, where the head of a wall is to provide support for a slab, a simple reinforcement layout would provide an L-bar in the corner linking the wall to the slab. However, during the process of constructing the wall, the bar is projecting through the plane of the face of the wall, which restricts the height of the wall formwork, and this can prove costly. Where the members are sufficiently thick it is often possible to adopt alternative reinforcement layouts which allow the use of wall forms of any height.

Where long lengths of slab or wall are constructed without joints, cracks will inevitably develop. If it is important to control the positions of these cracks this can be attempted by creating lines of weakness. However,

to be effective such lines need to coincide with the construction joints, since cracks within monolithic concrete sections usually express their own opinion as to where a plane of weakness is, regardless of what the designer does in terms of reducing the reinforcement locally or providing rebates in the face of the section.

Design data

Typical reinforced concrete details for bridgeworks are illustrated on Data Sheet 40. Some information concerning the design of diaphragms is presented on Data Sheet 41, and Data Sheet 42 deals similarly with the design of slabs linking spine beams.

Design may be undertaken either by employing elastic-strain (so-called modular ratio) theory or a limit- state analysis based on load-factor methods. The principal formulae and data necessary for the application of these alternative theories are summarized on Data Sheets 44 and 45 respectively. Modular-ratio design can be facilitated by the use of various design factors corresponding to the permissible working stresses in the concrete and the reinforcement and, for a modular ratio of 15, these factors may be read from the chart forming Data Sheet 46: brief examples of the use of the chart are given on Data Sheet 47, which also includes a chart that relates the stress in the reinforcement to the bar spacing in order to achieve adequate crack control.

The areas of various numbers and spacings of reinforcing bars of different sizes are tabulated on Data Sheet 49, while Data Sheets 50 and 51 present details of the requirements of BS 4466 for reinforcing bars of preferred shapes and other permitted shapes, respectively.

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Reinforced concrete details