The basic requirements of a structure are to sustain all likely actions and influences, to remain fit for purpose, and to have adequate structural resistance, durability, and serviceability. These requirements must be met for the structure's entire design working life, including construction.

The structure must not suffer disproportionate damage owing to adverse events, such as explosions, impact, or human error. The events to be taken into account are those agreed with client and relevant authorities.

In addition, the design must avoid or limit potential damage by reducing, avoiding, or eliminating hazards. This can be achieved by tying structural members together, avoiding collapse without warning (e.g. by employing structural redundancy and providing ductility), and designing for the accidental removal of a structural member.

The design working life is the assumed period for which a structure or part of it is to be used for its intended purpose with anticipated maintenance but without major repair being necessary. [en 1990 ยง1.5.2.8]

Figure 2.3 compares the design working life of various structures according to EN 1990 (dark lines) with the modifications made to these time periods by the UK National Annex to EN 1990 (lighter lines). The most significant change is the extension of Category 5 to 120 years - although this only really affects fatigue calculations.

Figure 2.3. Design working life of various structures

deemed to meet, if assumptions satisfied

Requirements adequately maintained used in conjunction with design assumptions adequately maintained used in conjunction with design assumptions as specified -in EN 1990-9

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