What is Grade Beam?
It is the part of the foundation of the building that is constructed from RCC. It has a continuous reinforcement beyond the supported column anchored within the foundation cap or footing As per ACI 318-14.
Generally, it is constructed on the ground or grade level spans between foundations to serve as horizontal ties between foundations. It helps to prevent differential settlement.
The grade beam footing helps transfer the building’s load to the ground or its supporting caps, whereas its bases spread the load to the ground.
What is Grade Beam Foundation?
It is a concrete foundation with a rectangular cross-section beam running around the house’s perimeter below the exterior walls known as a grade beam foundation or perimeter beam foundation.
We say it grade beam because it has a beam that rests on grade or ground.
The main objective of providing the grade beam foundation is to transfer the building’s load to the ground.
Based on the load intensity, types of load, and the bearing capacity of the soil, the size of the grade beam is determined.
Usually, its width should not be less than eight inches, and depth is equal to the span between supports.
Grade beam foundation needs less concrete compared to a traditional foundation; hence it is economical.
But we can’t use a grade beam foundation for the basement, which is the disadvantage of this foundation.
The minimum reinforcement needed for the grade beam is 6 bars of 10 mm, 3 bars at the top, and 3 bars at the bottom.
Grade Beam Construction Process
Construction of grade beam comprises of four steps that are:
- Excavation and preparation
- Installation of formwork
- Reinforcement Placement
- Concrete Pouring
Excavation and preparation
Trenches are excavated at the desired level for the construction of grade beams. If we construct it directly on the ground, we must first level and prepare the ground surface.
And if it is constructed between the foundation, then preparation and excavation will occur after the construction of piles.
But, trenches excavation will occur along with footing where the grade beam is provided with footings cap.
The ground’s surface should be levelled and prepared with PCC (plain cement concrete) to reinforce the grade beam.
Installation of formwork
After trench excavation and site, preparation formwork is installed according to the dimensions as specified in the drawing.
If the grade beam’s construction is performed on the ground, then the bottom shutter will be placed earlier than the sides for which soling bricks are used instead of the bottom.
After that, the cage of reinforcement may be tied on the flat soiling brick surface before placing side shuttering, or one side of shuttering may be installed first, and then reinforcement works may be carried out.
If the grade beam’s construction is carried out above the ground level, then the formwork installation will be the same as that of the conventional beam.
These construction steps are decided based on convenience and ease of construction.
After completing the formwork installation, check the dimensions for the specified grade beam with sufficient reinforcement cover.
After completing the formwork, it’s time to place the reinforcement for the beam. The placement of the bars should be as per the structural drawing.
Essential details about bars such as diameter and number of longitudinal reinforcements, and stirrups bar diameter and its spacing, extra bars details may be seen in structural drawings.
After finishing the reinforcement, placing work check for lap length, reinforcement cover, hooks, etc., as specified in the drawing.
The casting of the grade beam may be done with ready mix concrete or concrete mixed at the machine on site. If the volume of concrete is large, RMC is preferable.
If casting is done with ready mix concrete, the concrete supplier requires only concrete strength. While in the case of machine-mixed concrete, the ratio of concrete ingredients should be decided to ensure the desired strength of the concrete.
While pouring the concrete, it should be adequately compacted using a vibrator and then finish the top surface.
Sides of the formwork for the beam may be removed after 24 hours of concrete casting, while its bottom should not be released until concrete gains its strength. The time for bottom form removal depends upon the length of the clear span.
Advantages of Grade Beam
- Grade beams are flexible that means it can be built in any size and shape.
- It helps to prevent differential settlement.
- The construction of grade beams requires less excavation and less concrete.
- The grade beam’s construction process is simple that needs less labour and time to provide the overall economy.
- Grade beam foundation is less expensive as compared to conventional foundation.
Disadvantages of Grade Beam
- We can’t provide a grade beam foundation for the basement.
- It is not suitable for hilly or slope areas where its construction needs a large amount of excavation.
What is the purpose of a grade beam?
The main purpose of providing a grade beam is to transfer the load from the bearing wall into the foundation or to the ground and control differential settlement.
What is the difference between grade beam and plinth beam?
The grade beam is constructed at the grade level or earth level, while the plinth beam is provided at the plinth level. Also, the grade beam is stronger than the plinth beam. It creates better bonding between pile caps.
What is the difference between grade beam footing and wall footing?
The grade beam is provided to control bending and usually spans between the footing cap while a wall footing stands on soil and transmits the wall’s weight directly into the ground.
What is the difference between grade beam footing and strap footing?
A grade beam is used to distribute the wall’s weight to supporting foundations, while a strap beam is used to redistribute the column’s load between footings.