Learn : Characteristic Strength of Materials :Characteristic Strength of Concrete, Characteristic Strength of Steel. and Characteristic Load or ultimate load

CHARACTERISTIC VALUES  (REFER CLAUSE 36, IS CODE)

Characteristic Strength of Materials

The characteristic strength is based on the statistical analysis of the test results because there are variations in the strength of the material used. In order to simplify the analysis, it may be assumed that the variation in strength follows a normal distribution curve which is symmetric about the mean value as shown below in Fig. 3.1.

Therefore, characteristic strength = Mean strength – k S

where S is the standard deviation, k=1.64, corresponding to 5% probability

\[\therefore                f_{ck}=f_{m} – 1.64 S\]

Characteristic Strength of Concrete

The term characteristic strength means that value of strength of material below which not more than 5% of the test results are expected to fall. It is denoted by fck is N/mm2. The value of fck for different grades of concrete are specified by IS code 456:2000 and given in Table 1.1

TABLE 1.1. Grades of Concrete, Characteristic Strength of Concrete

 Group Designation Characteristic Compressive Strength, fck (N/mm2) Ordinary Concrete M10 M15 M20 10 15 20 Standard Concrete M25 M30 M35 M40 M45 M50 M55 25 30 35 40 45 50 55 High Strength Concrete M60 M65 M70 M75 M80 60 65 70 75 80

Characteristic Strength of Steel

The characteristic strength of steel is taken as the minimum yield stress or 0.2 percent proof stress specified by various Indian standards. In the case of mild steel it is taken as equal to minimum yield strength and is the case of HYSD bars it is taken as equal to 0.2 percent proof stress the values of characteristic strength of various types of steel are given in Table 1.9

TABLE 1.9.

 S.No. Type of Reinforcement Sub-Type Relevant Indian Standard 1. Mild steel plain bar (a) Cold worked mild steel bars (b) Hot rolled mild steel bars IS 432-1996 (Part-I) IS 1139-1966 (Part-II) 2. High yield strength deformed bars (Tor steel) (a) Cold worked deformed bars (i) Grade Fe 415 (ii) Grade Fe 500 (b) Hot rolled deformed bars IS 1786-1979     IS 1139-1966 (Part-II) 3. Hard drawn steel wire fabric Wire mesh IS 1566-1967 4. Rolled steel members Angles, T-sections Joists, Channels etc. IS 226:1975 5. (i)     Thermo-mechanically treated bars (TMT) (ii)    Corrosion resistant steel (CRS) bars Latest in use deformed bars

\[W_{u}=W_{m} + 1.64 S\]

Characteristic load is that value of load which has a 95 percent probability of not being exceeded during the life time of the structure. In absence of any data, loads given in various standards shall be assumed as the characteristic loads. The following standards may be used for this purpose.

IS 875 (Part 2) – for imposed loads

IS 875 (Part 3) – for wind loads

IS 875 (Part 4) – for snow loads

IS 1893 (Part 1) – for earthquake loads