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Mr. A. M. Muir Wood (Sir William Halcrow B Partners): I think I am possibly the only one here who has been concerned with each stage of this project from its rebirth, as it were, in 1959 until the date of its suspended animation in 1975. While I have been concerned with it on and off-mostly off-nevertheless, I have seen a number of changes and constant evolution.
I find this paper particularly interesting because my firm were recently engaged by a company manufacturing precast prestressed concrete components to carry out a design study of the behaviour of their strip-type (66 mm deep section) lintels in composite action with brickwork, with the intention of preparing safe load tables and design charts. Our problem was essentially a special case of reinforced brickwork with the embedded bar reinforcement replaced by a prestressed concrete section able to sustain some tension at service loads. After studying the literature, we decided to make an allowance for the effect of the shear arm ratio which is vindicated by the authors' results.
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: