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Foundations near trees
The discussion on construction near trees, generated by the paper in the issue of 16 August 1994 (Building near trees - the development of NHBC standards for the foundations of low-rise dwellings) has generated divergent views from members. Should we design to prevent all visible signs of ground movement in our buildings or should the cost-effectiveness of accepting minor cracking be a more prominent consideration? To what extent would the public go along with us, bearing in mind that they are the ultimate paymasters even if insurance cover spreads the costs? With the current arguments regarding the excessive cost of construction, should we do more to influence public opinion?
Over recent years there has been discussion of the possibility of reducing the partial safety factor for reinforcement. This paper puts the case for a reduction from the present value of 1.15 to 1.05. The case is made on three grounds:
- 30 years experience with the current safety levels
- comparison with structural steel design where lower factors are used
- comparison of current sufety level with the recommendations of Eurocode 1.
Professor A.W. Beeby
The problem of occupant-induced vibrations in buildings is one of growing importance. Until recently this problem was thought to be confined to floors of timber or composite construction, but in this paper it is shown that some recently constructed concrete floors are sufficiently light and flexible to give rise to disturbing levels of vibration. Methods of designing concrete floors against occupant-induced vibrations are evaluated against an extensive programme of field tests and are found to be deficient in two respects: prediction of the free vibration characteristics of 2-way
spanning floors and estimation of structural response to excitations such as walking and heel drop.
Martin S. Williams and Peter Waldron