A completely rational approach to primary safety is not possible at present. But two closely linked semi-probabilistic or second-moment methods which take some account of the statistical nature of the loading and collapse (or limit state) strengths may now commend themselves to designers, owners, and certifying authorities. These are the partial safety factor approach (closest to the conventional safety factor) and the safety index. The paper outlines them both but concentrates on the second concept which is applied to five naval designs varying in length from 91-154 m and for which measured strain records exist. The safety indices are compared with those for pseudo-merchant ships designed by rule and having the same lengths, and then with those for larger merchant ships having a length range 158-328 m. If one accepts the traditional design approach, in which comparisons are made with the best "successful" past designs then the heterogeneous nature of the results suggests two closely linked findings. First, that there are some obvious shortcomings in the conventional use of the safety factor. Secondly, notwithstanding the possible fatigue implications, it does seem that there is appreciable scope for better weight and cost compromises in both naval and merchant designs, and especially for the larger merchant ships. It would, perhaps, be premature to accept by itself either of these hybrid approaches to design until further statistical data are available. Nevertheless, the authors advocate a design policy which adopts one or both of these approaches in parallel with current methods. This would assist in building up a data bank of relevant information for future use. But even more important,it could provide a stimulus to rational design thinking and to the necessary further research.

  • Supplemental Notes:
    • Paper presented at the meeting held April 12, 1978, London, England.
  • Corporate Authors:

    Royal Institution of Naval Architects, England

    10 Upper Belgrave Street
    London SW1X 8BQ,   England 
  • Authors:
    • Sadden, J A
  • Publication Date: 1978

Media Info

  • Pagination: 15 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00180030
  • Record Type: Publication
  • Source Agency: Royal Institution of Naval Architects, England
  • Report/Paper Numbers: Paper No. 3
  • Files: TRIS
  • Created Date: Sep 27 1978 12:00AM