Characteristic strength and characteristic load

Characteristic strength

The characteristic strength of a material is the value of strength below which not more than 5 percent of the test results are expected to fall (95 percent reliability). It is the basis of design in limit state method (IS 456:2000, cl. 36.1).

• Concrete: characteristic compressive strength $f_{ck}$ is the 28-day strength of 150 mm cubes. For M20, $f_{ck} = 20$ N/mm$^2$.
• Steel: characteristic strength $f_y$ is the yield stress (or 0.2% proof stress). For Fe415, $f_y = 415$ N/mm$^2$.
• Statistically, $f_{ck} = f_m - 1.65\,s$, where $f_m$ is the mean strength and $s$ the standard deviation.

Characteristic load

The characteristic load is the value of a load which has a 95 percent probability of not being exceeded during the life of the structure (IS 456:2000, cl. 36.2). Since statistical data are rarely available, the loads given in the codes are taken as characteristic loads: dead load from IS 875 (Part 1), live load from IS 875 (Part 2), wind load from IS 875 (Part 3), and earthquake load from IS 1893.

Use in design

Design values are obtained with partial safety factors:

• Design strength $= \dfrac{f_k}{\gamma_m}$, with $\gamma_m = 1.5$ for concrete and $1.15$ for steel.
• Design load $= \gamma_f \times$ characteristic load, with $\gamma_f = 1.5$ for DL+LL, for example.