35.1 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 2. Until the relevant Indian Standard Specifications for reinforcing steel are modified to include the concept of characteristic strength, the characteristic value shall be assumed as the minimum yield stress/0.2 percent proof stress specified in the relevant Indian Standard Specifications.
35.2 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 standard, dead loads, imposed loads, wind loads, snow load and seismic forces in accordance with Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code shall be assumed as the characteristic loads.
PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5A PLAIN AND REINFORCED CONCRETE
35.3 Design Values
35.3.1 Materials
The design strength of the materials, is given by:
where
characteristic strength of the material (see 35.1), and
partial safety factor appropriate to the material and the limit state being considered.
35.3.2 Loads
The design load, is given by:
where
characteristic load (see 35.2), and
= partial safety factor appropriate to the nature of loading and the limit state being considered.
35.3.3 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 35.4.1 and 35.4.2 may be applied.
35.4 Partial Safety Factors
35.4.1 Partial Safety Factor for Loads
The values given in Table 18 shall normally be used.
Table 18 Values of Partial Safety Factor for Loads
(Clauses 17.2.3.1, 35.4.1 and B-4.3)
| Sl No. | Load Combination | Limit State of Collapse | Limit States of Serviceability | ||||
| DL(3) | IL(4) | WL(5) | DL(6) | IL(7) | WL(8) | ||
| i) | 1.5 | 1.5 | - | 1.0 | 1.0 | - | |
| ii) | - | 1.5 | 1.0 | - | 1.0 | ||
| iii) | 1.2 | 1.2 | 1.2 | 1.0 | 0.8 | 0.8 | |
1 While considering earthquake effects, substitute for . 2 For the limit states of serviceability, the values of given in this table are applicable for short term effects. While assessing the long term effects due to creep the dead load and that part of the imposed load likely to be permanent may only be considered.
This value is to be considered when stability against overturning or stress reversal is critical.
35.4.2 Partial Safety Factor for Material Strength
35.4.2.1 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.
NOTE — values are already incorporated in the equations and tables given in this standard for limit state design.
35.4.2.2 When assessing the deflection, the material properties such as modulus of elasticity should be taken as those associated with the characteristic strength of the material.