11
th
Lecture: Basis for the Design of Structures
Aim of the present lecture
The present lecture outlines the philosophy underlying the development of codes for the
design of structures. First a short introduction is given explaining the development of design
codes over time. Thereafter it is explained how reliability is ensured in code based design by
use of safety formats, and it is shown how the Load and Resistance Factor Design (LRFD)
formats can be related to the results of First Order Reliability Methods (FORM). Following
this, a general optimization problem concerning cost optimal design of structures is
formulated, which should be understood as the first principle for the optimal development of
design basis for structures. Subsequently target reliability indexes are provided which may be
applied as criteria for the minimum acceptable reliability for new structures. Then a simpler
and more practically applicable procedure recommended for the calibration of design codes
by the Joint Committee of Structural Safety (JCSS) is introduced and finally an example is
given, which illustrates how calibration of load and resistance based design codes can be
performed using the JCSS software CodeCal (manual included in ANNEX A).
Based on the introduced material, that follows Faber and Sørensen (2003), in this lecture it is
aimed for that the students should acquire knowledge and skills in regard to:
What is the principle of the LRFD safety format?
What is the meaning and purpose of the different factors entering into the LRFD safety
format?
How may the LRFD safety format be related to FORM results?
What is the principle for cost based optimization of design formats?
How should target reliability indexes for the design of structures be understood?
Which are the main steps of the JCSS procedure for calibration of design codes?