The opening section of the clause on design in the Code of Practice for concrete reads: 'The purpose of design is the achievement of acceptable probabilities that the structure being designed will not become unfit for the use for which it is required.. . ' The clause goes on to refer to variations in loading and in the properties of materials used; it mentions the need for statistical data on these variations and introduces the partial safety factors necessary to achieve this design objective. It does not, however, define the acceptable probability of a structure becoming unfit for use which is basic to the design concept. At present this is not possible in quantitative terms and may never become so; it can however be stated qualitatively after appraisal whether individual structural failures or cases of unserviceability are acceptable or not. Experience shows that structural inadequacy is seldom due to a single cause; it is often the result of a combination of effects, which may include overloading, fire, impact or explosion, settlement, design errors and faults in construction. Of these, faults in construction have played a major contributory part in causing failure, as illustrated by a few examples drawn from the building field: