Plinth beam and tie beam are often interchanged and confused. Even civil engineer get confused with plinth beam and tie beam. In this post we will look in detail about plinth and tie beam.
Plinth beam is a beam that is provided at the plinth level. Usually plinth beam is the beam that is constructed above the ground level after foundation.
What is Plinth Level
Plinth level is the top most level of ground level that is where sub structure ends and super structure starts.
Tie beam does not transfer load from beam to columns but they are used to connect two or more columns or rafters.
As the Tie beam name itself suggest that it ties column, thus tie beam is a beam that ties column in a building with high floor levels.
Tie beams are provided in roof truss and at plinth level. Thus tie beam don’t transfer load but act as length breaker in columns.
Purpose of Plinth Beam
- Load of walls are transferred to foundation through plinth beams.
- Plinth beam prevent cracks from foundation to walls due to settlement or any other reasons.
- If due to any reason there is differential settlement then plinth beam helps in avoiding settlement.
- Plinth beams connect all columns.
Purpose of Tie Beam
- Tie beam carry axial compression.
- Column buckling can be prevented by using tie beams.
- Rafter loads are transferred to the column.
What is Plinth Level
Plinth level of a building is that raised level of ground floor of a building with respect to the ground level.
Generally speaking ground floor level is kept at a higher level as compared to the locality or natural ground level to prevent rain water and other problem.
Plinth level is decided based on the regional mean sea level and future trends of the road level. So that building ground floor is above the road and drainage is not a problem in it.
What is Difference Between Plinth Beam and Tie Beam
Truly speaking there is not much difference between plinth beam and tie beam. Every plinth beam is a tie beam. But every tie beam is not a plinth beam.
Tie beam can be a plinth beam if it is provided at the plinth level. The only difference between plinth beam and tie beam is the level of height at which they are provided.
Thus plinth beam is provided at the plinth level and tie beam is provided at any level above plinth level and floor level.
Plinth Beam Design
Plinth beam is actually a beam provided at the plinth level above the ground. Plinth beam span throughout the structure periphery.
Plinth beam consist of minimum 2 numbers of 12mm dia bars provided at bottom and 2 nos. at top of the beam. Depth of plinth beam should not be less then 200 mm. Clear cover of plinth beam should be 25mm from all side.
Plinth beam design:
- 12mm dia bar at bottom
- 10mm dia bar at top
- depth of beam more then 20cm
- Clear cover = 25mm
In order to avoid shear failure 3mm dia bars stirrups are provided at 150mm center to center.
How Plinth Height is Decided
Deciding the height of plinth needs various factors to be taken care. Most important facts for deciding plinth beam height are as follows:
- Water drainage level
- Road Level
- Ground Level
- Frequency of road maintenance and recarpeting of pavement
In most areas road level and highways are provided at a greater height then the natural surface level. This is done to provide easy drainage of rain water and road surface run off level.
Roads are generally laid on the previously used road. Thus the level of road goes on increasing in future due to re-carpeting of road. To avoid the building ground floor level being below the road level future trend should be kept in mind before deciding the plinth beam height.