Types of Failures in RCC beams
1. Flexure Failure
Flexure failure occurs when the imposed loads exceeds the flexural capacity of the structural member. Flexural strength is defined as stress in material just before the yielding starts in flexure test; it represents the highest stress value experienced at moment of rupture.
Depending on yielding of steel and crushing of concrete, flexure failure is further divided into following types:
a. Flexural Tension Failure
In this type of failure, the steel reinforcement starts yielding before crushing of concrete. It is also known as ductile failure because it gives enough warning of failure in form of deflection.
This failure is occurs in case of under-reinforced sections i.e., the reinforcement is less than balances section. At the beginning, the cracks are developed at tension side and then the cracks further extend towards compression side.
The cracks so formed are vertical in orientation and lies perpendicular to most tensile fibre of beam. From designer’s point of view, it is the most favourable failure.
- Choosing appropriate dimensions for beam, as per IS 456:2000 specifications.
b. Flexural Compression Failure
In this type of failure, the crushing of concrete occurs prior to yielding of reinforcement steel at tension side of beam.
This is also called as brittle failure of structural member as there is no warning in form of deflection, before the structural failure of beam.
This failure predominantly occurs in over-reinforced sections, i.e. reinforcement provided is greater than required in balanced section.
- Flexure compression failure could be avoided by designing under-reinforced sections, rather than over-reinforced.
- Introducing compression side steel reinforcement to increase compression strength of beam.
2. Shear Failure
Shear failure occurs when the acting shear force exceeds the shear capacity of the corresponding section of the structural member. A shear force acts along a plane parallel to the direction of force and hence, producing a sliding failure on beam.
Shear failure is considered as brittle failure as it does not provide any warning before complete failure. It is further classified in following forms:
A. Diagonal Tension Failure
This type of failure begins with development of flexural cracks at the tension of beam due to flexural tensile stress.
Due to further increase in applied load on beam, growth of crack amplifies and cracks grow further wider and lengthy with a slight bend in diagonal direction, towards the load application point.
- Web reinforcements should be mandatorily provided to avoid diagonal tension failure
- The ratio of shear span to depth must be kept greater 2
B. Shear Compression Failure
This failure occurs due to initiation and further development of cracks to the compression zone in cross-section of beam and compressive strength is compromised.
Shear compression failure results into crushing of concrete at the tip of diagonal crack around the point of load application.
- Shear reinforcement must be provided as per codal provisions provided by IS 456:2000.
- The ratio of span to depth must be kept less than 4, as it will decrease the probability of crack to reach to compression zone.
C. Splitting Shear Failure
It is a common case of shear failure in case of deep beams, as the loads are directly transferred to supports so higher shear stresses are generated.
- The ratio of shear span to depth must be preferably kept greater than 2, in any case should not be kept less than 1.
D. Anchorage Failure
When the main reinforcement are not adequately anchored, splitting of concrete occurs along the longitudinal reinforcement, due to small diagonal cracks.
- High grade concrete, deformed bars must be used for proper anchorage.
- Large number of small diameter bars should be used to form proper bond.
Failure In Columns
The modes of failure for a column is based on ratio of effective length (further depends on support conditions) of column to the radius of gyration about the axis under consideration i.e. slenderness ratio of the column.
Assuming the columns are concentrically loaded, based on slenderness ratio, modes of failure of column are classifies as:
1. Failure Due to Pure Compression
When a column is subjected to high compressive load, the concrete and steel experiences high stresses, as a result the column fails without undergoing any lateral deformation. The concrete is crushed and steel yields, hence, column fails due to material failure.
It is a common failure case for pedestals, as pedestals have slenderness ratio value less than 3, due to which they do not undergo any lateral deformation (bending) due to concentric loads.
- To keep the generated stresses under permissible limits, the concrete column must be designed with sufficient cross-section area.
2. Failure Due to Combined Compression and Bending
All long columns and most short columns are susceptible to this type of failure. Long columns undergoes lateral deflections even under influence of axial loads only, due to both material and member abnormality.
Short columns are mostly subjected to lateral loads, moments and axial loads. Due to action of moments and lateral loads, short columns undergo bending and lateral deflection. Due to lateral deflection and bending, the steel acquires its yield stress and eventually column fails.
- Columns must be constructed keeping centre line straight; any abnormality might cause excessive lateral deflection, under influence of lateral or axial loads.
- As long as not specifically required, slenderness ratio must be kept small. This will reduce the unsupported length of column.
3. Failure due to Inelastic Inability
Long columns with slenderness ratio greater than 12 have larger unsupported length, due to which their load carrying capacity is majorly affected.
These columns are highly susceptible to buckling and become unstable even under influence of much smaller loads than their carrying capacity. This failure is highly unacceptable as per designer’s point of view.
- Columns with slenderness ratio greater than 30 are totally neglected as per IS 456:2000 guidelines. Moreover, columns with slenderness ratio greater than 12 must be designed with respect to Rankine’s and Euler’s theories, according to requirement to calculate critical load.
– Tushar Meena