Learn : Characteristic Strength of Materials :Characteristic Strength of Concrete, Characteristic Strength of Steel. and Characteristic Load or ultimate load
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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 f_{ck} is N/mm^{2}. The value of f_{ck} 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, f_{ck} (N/mm^{2}) 
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  SubType  Relevant Indian Standard 
1.  Mild steel plain bar  (a) Cold worked mild steel bars
(b) Hot rolled mild steel bars 
IS 4321996 (PartI)
IS 11391966 (PartII) 
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 17861979
IS 11391966 (PartII) 
3.  Hard drawn steel wire fabric  Wire mesh  IS 15661967 
4.  Rolled steel members  Angles, Tsections Joists,
Channels etc. 
IS 226:1975 
5.  (i) Thermomechanically treated bars (TMT)
(ii) Corrosion resistant steel (CRS) bars 
Latest in use deformed bars 
Characteristic Load or ultimate load
The characteristic load is the ultimate load that is liable to come on structure during its lifetime. For simplicity, the variation in the loading is also assumed to follow a normal distribution cure, the characteristic load or ultimate load is given as
Characteristic load or ultimate load = Mean load + 1.64 S
\[W_{u}=W_{m} + 1.64 S\]
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 1) – for dead loads
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

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