Reinforced Concrete Beam Design : Failure Mode

In reinforced concrete beam design, failure is designed for safety. If the structure fails by sudden crushing, huge damage and loss of life may take place. So, it is designed so that structure would give a warning before collapse. Reinforced concrete beam failure may take place in 3 ways.

1. Concrete failure: Crushing of concrete
2. Steel failure: Yielding by steel
3. Concrete & steel combined failure

Concrete Failure

In concrete failure, concrete reaches to its ultimate strength before steel reaching to its yield point. As a result concrete crushes down before steel yields. This type of beam failure is unwanted as the concrete crushes without giving any warning. Concrete failure occurs due to the following reasons.
1.       If large amount of reinforcement is used.
2.       Compressive strength of concrete is less than tensile strength of steel.
3.       Sudden destruction occurs without giving any warning.

Steel Failure

In steel failure, steel reaches to its yield point before concrete reaches to its ultimate strength. So, steel yields before crushing of concrete which gives ample warning to the inhabitants. So, steel failure is preferable during structure design. Some common notes on steel failure of rcc beams are given below.
1.       Relatively moderate amount of reinforcement are employed.
2.       Tensile strength of steel is less than compressive strength of steel.
3.       Steel yields and stretches and tension crack propagates concrete cracks.
4.       Secondary compression failure.
5.       Steel failure of rcc beams gives ample warning to the inhabitants. So, damage can be avoided.
6.       For engineering, steel failure is preferable.

Combined Concrete Beam Failure

In this type of failure, concrete and steel fails at the same time. That means concrete should reach to its ultimate strength just at the same time when steel fails so that both fails together. Combined failure is practically almost impossible because it is not possible to tell the exact behavior of concrete & steel so that they fail at the same time.

Basic Reinforced Concrete Beam Design Question

Question: Which type of failure is preferable in design of reinforced concrete beam?

Answer:  Steel failure. It gives ample warning to the inhabitants reducing the damage.