The foundation is a vital part of any structure which transfers all types of loads to the ground.
What is the grillage Foundation?
A column or pier foundation formed by two or more cross-layer beams placed at right angles to each other covered by concrete is called the grillage foundation.
When & Where It Is Used?
- The grillage foundation is suitable when the foundation of a structure is constructed as a shallow type foundation over the wet or loose soil.
- Mostly deep foundation method is used for loose soil. However, the grillage foundation will be recommended where the deep foundation is not possible for some reason.
- Grillage foundation is considered when the foundation depth is restricted to 1 to 1.5 m for reasons such as rocky earth.
- Moreover, the grillage foundation can sustain & transfer heavy loads from the structure to large areas.
Components of Grillage foundation
- The depth of the foundation will be in the range of 1m to 1.5m, but should not be less than 900mm.
- The steel beam or timber planks should be placed over the concrete bed, and the thickness of the concrete bed was not less than 150mm.
- Two or more steel beam or timber layers have been placed over the concrete bed, and those layers are at right angles to each of them.
- The number of bottom layers beams higher than the top layer. For example, if the top layer has 3 beams, then the bottom layer should have 5 or 6 beams based on the load design criteria.
- After fixing the steel column, the entire framework should be covered by concrete.
Types of Grillage foundation
The grillage foundation has been classified as two by their make.
- Steel grillage foundation
- Timber Grillage foundation
Steel Grillage foundation & Installation
The steel grillage foundation consists of two or more layers of steel beams.
- Firstly, a concrete bed formation should be made with a minimum thickness of 150mm on the soil.
- The steel framework will be placed appropriately over the concrete bed.
- The top layer of beams should be laid at the right angle to the bottom layer.
- The beams will be separated by a spacer bar, which is 20mm in diameter for concrete penetration in each layer.
- The distance between the beam joist should not be greater than 300mm. If the space between the beams exceeds more than 300mm, then the concrete will not act as monolithically.
- Now the concrete has to be poured properly in and around the steel section with proper compaction.
- A base plate has to be fixed over the top layer with cement grout and ensure the plate has not oscillated.
- Then, fix the steel stanchion on the base plate along with the gusset plate and tighten the nuts & bolts.
Finally, the concrete has to be poured in & around the foundation with a minimum clear cover of 15mm to prevent steel corrosion from the water.
Timber Grillage foundation & Installation
The timber grillage foundation is suitable for a lightweight structure where the soil bearing capacity is not more than 6 Ton/Sqm. The brick or block masonry wall will be constructed over this foundation.
- The timber planks which are 80 mm to 100 mm thickness used to make the foundation.
- The timber planks consist of two or more cross layers. There is no spacer provided between the planks & the clear cover distance of the planks must be 450 mm to 600 mm for each side of the footing.
- In this foundation, minimal concrete has been poured in & around the foundation.
- Then the brick or block masonry wall can be constructed over the foundation.
Advantages And Disadvantages
|Economical & cost savings||It may be affected by the uplift force.|
|Deep excavation not required for loose soil foundation.||To excavate a larger area.|
|Less manpower is enough to complete the foundation.||The column alignment may slightly change.|
Uses Of Grillage Foundation
The main use of a grillage foundation is to transfer a heavy load to the loose soil where the deep foundation is not possible for some reason.
Hope you enjoyed the article. Happy Learning 🙂