In the construction of reinforced concrete (RC) structures, beams are the main horizontal components that transfer loads from slabs and walls to columns, which then pass them to the foundation and ground. Among the various types of beams, plinth beams and tie beams play a special role. Plinth beams distribute wall/column loads to the foundation, and tie beams connect columns to reduce their effective length and improve stability, thereby maintaining structural integrity. Although these two share a common trait as horizontal RC members associated with columns, dissimilarities in the aspects of location, functions, and design characterize them further.
Overview of Plinth Beam
The plinth beam is basically a beam that is built at the plinth level (normally 450-600 mm above the ground level). It tops foundation walls or isolated footings, thus forming a continuous ring around the building’s perimeter.
Key Functions:
- Load Distribution: It takes the vertical load from the walls or columns and hands it over to the foundation. Hence, the load distribution is carried out in a proper way.
- Prevention of Differential Settlement: The plinth beam, through its even load distribution, prevents differential settlement of the foundation.
- Structural Stability: It also gives structural stability to the building, which is the connecting element that links different parts of the building.
Design Considerations:
- Dimensions: The width of the plinth beam should be the same as the width of the wall or column it supports. Depth (height) generally ranges from 200 mm to 450 mm, depending on design requirements.
- Reinforcement: As per IS 456:2000, the minimum steel percentage is about 0.85% of the cross-sectional area for HYSD bars or 0.24% for mild steel. A common practice is to provide 2 bars of 12 mm dia at the bottom and 2 bars of 10 mm dia at the top, with adequate stirrups, subject to design.
- Concrete Grade: M20 grade of concrete or above is recommended.
Overview of Tie Beam
A tie beam is a horizontal structural member that connects two or more columns at a height above the plinth level, such as at the lintel or roof level.
Key Functions:
- Column Stability: Stops lateral movement or changes in the shape of columns caused by the application of external forces by inserting a mechanical stop in the form of a brace or a tie at the place where the unsupported length is minimum. Hence, the columns get their effective length reduced and become less vulnerable to lateral forces.
- Structural Rigidity: Gives the structure more stability, for instance, in the case of tall buildings or when the structure gets hit by lateral forces.
Design Considerations:
- Dimensions: The depth of the tie beam is typically less than that of the plinth beam, ranging from 200 mm to 400 mm, depending on the span and load.
- Reinforcement: Minimum reinforcement should be 0.85% of the cross-sectional area for HYSD bars, as per IS 456:2000. The specific reinforcement details depend on the structural design and load requirements.
- Concrete Grade: M20 grade of concrete is commonly used, based on the structural requirements.
Plinth Beam vs Tie Beam
| Aspect | Plinth Beam | Tie Beam |
| Location | At plinth level (450–600 mm above ground level) | Above plinth level (lintel or roof level) |
| Primary Function | Distributes loads from walls/columns to the foundation | Connects columns to prevent lateral movement |
| Load Transfer | Transfers vertical loads from walls/columns to the foundation. | Provides horizontal support and shares forces between columns under settlement or lateral action. |
| Reinforcement | Higher reinforcement (0.2% of cross-sectional area) | Lower reinforcement (0.15% of cross-sectional area) |
| Concrete Grade | M20 or higher | M20 or higher |
| Dimensions | Depth: 300–450 mm; Width: Equal to wall thickness | Depth: 200–400 mm; Width: Varies |
Function and Location
- Plinth Beam: At the plinth level, it is the main element that combines the foundation walls or footings, hence ensuring load distribution that is both uniform and preventing differential settlement.
- Tie Beam: Situated at higher levels (lintel, roof, or between floors), tie beams are those that connect columns at a higher level, thus cutting down their effective length and avoiding the occurrence of buckling which leads to overall stability of the structure.
Design Considerations
- Plinth Beam: The design must focus on load distribution and ensuring structural stability.
- Tie Beam: The design of the beam aims to reduce the effective length of the column, thereby stopping lateral movement. Not only should the beam be able to withstand axial loads, but also be able to provide structural support.
Load Distribution
- Plinth Beam: The main function of the plinth beam is to distribute the vertical loads of the walls or columns to the foundation.
- Tie Beam: Provides horizontal support and shares forces between columns under settlement or lateral action, thereby improving the overall stability of the structure.
Construction Materials
- Plinth Beam: The main material used in its production is reinforced cement concrete (RCC) of a minimum grade M20. The reinforcement normally consists of two 12 mm diameter bars at the bottom and two 10 mm diameter bars at the top.
- Tie Beam: Also made of RCC with a minimum of M20. Depending on the structural design and load requirements, the reinforcement details might differ.
Applications
- Plinth Beam: Used in buildings with load-bearing walls is the most common application, structures in areas with a high water table or buildings susceptible to differential settlement.
- Tie Beam: Found in tall buildings, the most critical areas are structures that experience lateral forces and buildings with widely spaced columns.
Structural Benefits
- Plinth Beam: The beam acts as a source of the level base for the walls to be placed on, it avoids differential settlement and also helps in the protection against moisture ingress.
- Tie Beam: Keeps the stability of the columns at a high level; the columns cannot be twisted because bracing is provided, and the overall structure becomes more rigid.
Conclusion
It is true that both plinth beams and tie beams contribute significantly to securing the stability of the building. Plinth beams, placed at the base level of a structure, help in distributing loads uniformly to the foundation. On the other hand, tie beams, located at higher levels, connect columns and provide lateral stability to the building. Understanding their roles, considering proper design, and selecting suitable materials are the first steps toward stronger and more durable construction.
FAQs
The distinguishing feature, to a large extent, is in the functions of the beams respectively. Plinth beams are used to transfer loads to the foundation, while tie beams are those that are connected to the columns and provide stability.
They are essential in load-bearing structures, buildings constructed in areas with a high water table, and those prone to differential settlement.
Tie beams are very important in tall buildings as well as in structures that are exposed to side forces. Nevertheless, in low buildings with closely spaced columns, they may be removed after checking if the structure can support the load requirement.
These two beams are both made of reinforced cement concrete (RCC). Concrete grades and reinforcements are designed based on the load and structural requirements.
Apoorva H P Achar