Characteristic Strength of Materials Characteristic Load or ultimate load

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 f_ck is N/mm². 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, 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

---

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