20.1 Characteristic Strength of Materials
The term ‘characteristic strength’ means that value of the strength of the material below which not more than 5 percent of the test results are expected to fall. The
characteristic strength for concrete shall be in accordance with Table 1. The characteristic strength shall be assumed for prestressing steel and the characteristic strength for reinforcing steel shall be assumed as minimum yield/0.2 percent proof stress.
20.2 Characteristic Loads
The term ‘characteristic load’ means that value of load which has a 95 percent probability of not being exceeded during the life of the structure. Since data are not available to express loads in statistical terms, for the purpose of this Section, the dead load, imposed load, wind load, seismic forces and snow load as given in 2, 3, 4, 5 and 6 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code shall be assumed as the characteristic loads.
20.3 Design Values
20.3.1 Materials
The design strength of the materials, is given by:
where
= characteristic strength of the material (see 20.1), and
partial safety factor appropriate to the material and the limit state being considered (see 20.4).
20.3.2 Loads
The design load, is given by:
where
characteristic load (see 20.2), and
partial safety factor appropriate to the nature of loading and the limit state being considered (see 20.4).
20.3.3 Consequences of Attaining Limit State
Where the consequences of a structure attaining a limit state are of a serious nature such as huge loss of life and disruption of the economy, higher values for and than those given under 20.4.1 and 20.4.2 may be applied.
20.4 Partial Safety Factors
20.4.1 Partial Safety Factor for Material Strength
20.4.1.1 When assessing the strength of a structure or structural member for the limit state of collapse, the values of partial safety factor should be taken as 1.5 for concrete and 1.15 for steel.
PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5B PRESTRESSED CONCRETE
NOTE — values are already incorporated in the equations and tables given in this Section.
20.4.1.2 When assessing the deflection, the material properties such as modulus of elasticity of concrete should be taken as those associated with the characteristic strength of the material and safety factor shall not be applied.
20.4.2 Partial Safety Factor for Loads
The value of given in Table 7 shall normally be used.