What is plinth Beam ?
A reinforced concrete Beam which is provided either below, above, or at ground level is known as plinth beam. It differentiates superstructure and substructure levels. It connects all the columns of the structure that helps to resist the earthquake forces.
Why Plinth Beam is Provided / Structural Benefits of Providing Plinth Beam
The Structural benefits of providing Plinth Beam are as follows:
1. It Prevents differential settlement of Buildings
All the loads of superstructure act on the plinth beam which is uniformly distributed to the soil that prevents the building from the settlement. If the Plinth beam is not provided in structure, then superstructure loads unequally distributed to the foundation that causes the differential settlement in buildings.
2.To reduce the slenderness of Column
I would like to show an example of slenderness of column.
- Floor to Floor height of structure = 3.2 m
- Height of Column from top of the foundation up to Ground level= 2.5 m.
- Total height of column (L) = 3.2 + 2.5 = 5.7 m.
- Column size = 300 mm x 300 mm.
As per IS 456-2000, Clause 25.1.2 If the Slenderness ratio is greater than 12 is known as long columns. The slenderness ratio is the ration of its effective length to its lateral dimension. Slenderness is the geometrical property of a compression member.
As per Table 28 of IS 456-200, for effectively held in position and restrained against rotation in both ends,
Effective length of compression member (Leff) = 0.6 x L = 0.65 x 5.7 = 3.705 m.
Slenderness Ratio = Effective length (Leff) / least lateral dimension
= 3.705/0.3= 12.35 which is greater than 12 that indicates a long column.
Hence if we do not provide a plinth beam then the column will behave as a long column. The long column fails due to buckling. Buckling develops due to gravity and lateral load.
3. To carry load of masonry wall
It carries the load of masonry wall which is at the Ground floor level. Let’s find how much masonry wall load acts on plinth Beam in KN/m.
- Floor to Floor height = 3.2 m
- Size of First floor Beam = 230 mm x 450 mm.
- Unit weight of Bricks = 19.2 KN/m3
- Unit weight of plaster = 20.4 KN/m3
- Thickness of Mortar in Wall = 25 mm
- Width of wall = 9″ =229 mm.
Wall height = Floor to Floor height – Depth of Beam
= 3.2-0.45 =2.75 m
Full wall Plaster = Wall height x thickness of plaster x unit weight of Mortar
= 2.75 x 0.025 x 20.4 =1.4025 KN/m
Wall load = Wall height x thickness of wall x unit weight of Bricks
= 2.75 x 0.229 x 19.2 =12.0912 KN/m
Total Load acts on Plinth Beam = Wall load + Full wall plaster = 12.0912 + 1.4025 =13.5 KN/m
Hence, this much amount of wall carried by the plinth Beam and transfer to the Soil indirectly.
4. Plinth Beam Resist lateral forces
As we can see in the above image, the plinth beam tie all the columns and behaves as a single unit. Whenever an earthquake hits the building, the Plinth beam doesn’t allow the building from lateral movement or sway. Hence it resists the lateral forces and imparts strength.
What is the ideal height of the plinth for Residential Building?
If the plot is located in a society where the local municipalities road is not there and will construct near future, in that situation plinth level may at the height of 4 to 5 feet from road level.
If the road is already constructed, it requires repairing every year that removes old surfaces and adds new strata. In every addition of layer, the thickness of the road level increases so in that circumstances plinth level shall above from Road level. It is because the construction of roads in the future reduces the height of the plinth level.
The advantage of providing the plinth level above Road level is to prevent the rainwater from entering the house. The height of the plinth should be 1 to 2 feet from the adjacent structure.