Geometrical data

Geometrical data are also introduced into design as characteristic values (ak), which may be taken as the nominal values (anom) given on the design drawings. This has the considerable virtue of not over-complicating design calculations. [EN 1990 §4.3(1)P]&[EN 1990 §4.3(2)]

Nominal dimensions ^ characteristic dimensions

Although geometrical data are random variables, their level of variability is generally small compared with those of actions and material properties. It is therefore usual to treat geometrical data as known values, taking the characteristic value as the nominal value provided on the design drawings. Examples include the size of structural members; the height and inclination of slopes; and footing formation depths.

For structures, nominal dimensions are used for member sizes (and their variation is catered for by partial material factors) but imperfections are considered in the structure's geometry, e.g. the lean of piers. These imperfections are based on tolerances given in the construction specification.

When selecting nominal dimensions, it is usual to choose conservative values relevant to the limit state being analysed taking into account imperfections in setting out, workmanship, and other construction issues. Where a significant variation in dimension is likely, either initially (due to loading, production, setting-out, or erection) or over time (due to loading or various chemical and physical causes),9 the design dimension should reflect this likely variation (see Section 2.13.3).

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