Reinforced Concrete Beam Design : Failure Mode

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

 

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.

 

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.

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