Limit state design and partial safety factors


To explain the philosophy of limit state design in the context of Eurocode 3: Design of Steel Structures. To provide information on partial safety factors for loads and resistance and to consider how the particular values can be justified.


The need for structural idealisations is explained in the context of developing quantitative analysis and design procedures. Alternative ways of introducing safety margins are discussed and the role of design regulations is introduced. The philosophy of limit state design is explained and appropriate values for partial safety factors for loads and strength are discussed. A glossary of terms is included.


The fundamental objectives of structural design are to provide a structure which is safe and serviceable to use, economical to build and maintain, and which satisfactorily performs its intended function. All design rules, whatever the philosophy, aim to assist the designer to fulfil these basic requirements. Early design was highly empirical. It was initially based largely upon previous experience and inevitably involved a considerable number of failures. Physical testing approaches were subsequently developed as a means of proving innovative designs. The first approaches to design based upon calculation methods used elastic theory. They have been used almost exclusively as the basis for quantitative structural design until quite recently. Limit state design is now superseding the previous elastic permissible stress approaches and forms the basis for Eurocode 3 [1] which is concerned with the design of steel structures. In the following sections, the principles of limit state design are explained and their implementation within design codes, in particular Eurocode 3, is described.

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