What is Plinth Beam in Building Construction?

Plinth beam is a horizontal structural element that is provided at the plinth level of the structure, connecting the columns as shown in the figure below.  It is constructed above the top of the plinth level in load bearing walls to help in uniform load distribution and even out the settlement of the buildings. 

Figure.1. Plinth Beam Constructed for Building

Features of Plinth Beam

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Plinth beam as mentioned are constructed at the foundation or the ground level. They act as tie beams at the plinth level, which finally will reduce the effective length of the columns. This arrangement tend to carry the wall load at the ground floor level to the columns, thus acting as as settlement controller. 

Figure.2. Plinth Beam Arrangement

In situations, when there are different settlements, stresses cannot be taken completely by foundations. This would result in cracking in rubble foundation, and slowly creates cracking in the brick wall. Hence, the introduction of beam below these walls (plinth beams) helps to bear the additional tension, bending and shear forces. This helps to minimize the cracking in the brick wall.

The introduction of plinth beams carry the wall loads on the ground floor. The plinth beam size and reinforcement is designed based on the wall loading. 

In the case of low rise buildings, the loads from the floor beams are not transferred to the plinth beams. Beams can be designed as plinth beam in these situation, by designing it as a normal beam taking into account the loads from the slabs.

Plinth beams are a compulsory structural element in building wherever there are load bearing walls. It is mainly constructed using reinforced cement concrete. In view, it act as a base for the block works or brickworks of the structure.

Objectives of Plinth Beam

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The important objectives of plinth beam in building construction are:

1. Plinth beam helps to even out settlement in buildings.
2. They provide damp proofing course (DPC) to the building.
3. Plinth beam constructed as continuous band at the plinth level in addition to other beams at lintels and roof level to improve building strength in seismic regions.
4. It prevent the development and propagation of cracks due to differential settlement.
5. It transfers the incoming load coming from the superstructure to the foundation below.
6. Plinth beam construction reduces the slenderness ratio of columns by reducing the effective length.
7. Plinth beams act as ties that holds the columns in position with the walls of the structure.

Structural Dimensions of Plinth Beam

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The minimum reccomended depth of plinth beams  are 10 cm to 15 cm. It is provided at full depth of the wall above the plinth beam. Generally, minimum of 3 numbers of reinforcement bars of minimum diameter MS 6mm ( Generally, 2 nos: of 8mm dia bars Fe415) at top and bottom as main or longitudinal reinforcement with 6mm diameter stirrups at 230 mm spacing is provided.

The span to overall depth of plinth beam must range between 15 to 18 cm.