Stresses in Steel and Concrete | Building Construction
In one of our previous articles, we discussed “Basic definitions and formulas”.
Now we will move on with our discussion on “Permissible stresses in concrete and steel” and “Understanding Modular ratio”.
Permissible Stresses in Concrete
Reinforced concrete designs make use of M15 grade concrete. The permissible stresses for different grades of concrete is different. They are given below:
|Sr. No.||Concrete Grade||M15||M20||M25||M30|
|1.||Stress in compression
|2.||Stress in bond (average) for plain bars||0.6||0.8||0.9||1.0|
|3.||Characteristics compressive strength||15||20||25||30|
Also refer for other values in IS:456-1978
Permissible Stresses in Steel
The permissible stresses for different grades of steel are given in the table above.
The different grades steel available in the market with their market names are as follows:
Grade I steel is known as mild steel. The abbreviation used for Mild steel is (m.s.)
High Tensile deformed steel has two types. They are as follows:
- Grade Fe415 (Tor-40 or Tistrong I)
- Grade Fe500 (Tor-50 or Tistrong II)
The names of the high tensile deformed steel have been derived from their manufacturers.
- Tor-Isteg Steel Corporation in Calcutta manufactures Tor-40 and Tor-50. Hence, the name.
- Tata Iron and Steel Co. Ltd, Calcutta manufactures Tistrong I and Tistrong II.
(Being aware of the names of the manufacturers is important for students especially those studying Civil and Structural Engineering.)
Understanding Modular Ratios
It is defined as the ratio of moduli of steel to the moduli of concrete. It is denoted by the letter “m”.
The modular ratio is not constant for all grades of concrete. It varies with the grade of concrete. Es/Ec is generally not used to calculate modular ratio for reinforced concrete designs.
As per IS: 456-1978;
m is calculated by the following formula:
m = 280/3σcbc
σcbc = permissible compressive stress in concrete in bending.
Calculation of Modular ratio values for different grades of concrete
|Grade of concrete||Modular ratio|
|M15||m = 280/3×5 = 18.66|
|M20||m = 280/3×7 = 13.33|
|M25||m = 280/3×8.5 = 10.98|
|M30||m = 280/3×10 = 9.33|
It should be remembered that rounding off the modular ratio values is not permitted by Indian Standard.
We shall discuss the following in our succeeding articles:
- Assumptions for singly reinforced sections
- Design procedure for Singly reinforced section – I
- Solved Numericals for Singly reinforced beam | Method I
- Design of Singly reinforced sections | Design Method 2
- Solved Numericals for Singly reinforced beam | Method 2
- Moment of Resistance for Singly reinforced sections
- Solved numerical example | Moment of resistance
- Solved numerical example 2 | Guide to singly reinforced sections