A grillage foundation made of two or more than two-tier beams placed at the right angle to disperse the load evenly over a large area.
A foundation made of two or more than two-tier beams superimposed in a concrete layer to distribute the load over a large area refer to Grillage Foundation.
It is suitable when the load transmitted by a column or wall is hefty, and bearing capacity of the soil is deficient.
It also helps to eliminate the deep excavation required for the foundation. It is provided at the column’s base.
The grillage beam of one tier is at the perpendicular of the grillage beams of the second tier, and these tiers of beams are encased in concrete.
Generally, this type of foundation is the right for columns, piers and scaffolds of massive structures.
Foundation and grillage look similar, but their function is different. Foundation transmits the load from to structure to the ground while grillage distributes the heavy load over the broad area of ground.
Types of Grillage Foundation
Based on the material, there are two types of grillage foundation.
- Steel Grillage
- Timber Grillage
Steel Grillage Foundation
This foundation consists of one or two-tier of RSJ ( Rolled steel joints) embedded in cement concrete. RSJs used in this foundation are also known as grillage beams.
In this foundation, depth is limited to 1 m to 1.5 m, and the width is increased considerably to pressure the soil within the permissible limit.
Typically this foundation has limits constructed by providing RSJ or grillage beams in two-tiers. The direction of grillage beams of one tier is perpendicular to the grillage beams of the second tier.
Grillage foundation may consist of only one tier when the wall is not carrying a hefty load.
The load carried by the steel stanchion or wall is first transmitted to the upper tier of the beams, then to the second tier lying below the upper one, and finally to the soil below the foundation.
A spacer bar of 30 mm diameter or pipe separator of 25 mm is used to hold each tier’s grillage beams in the position.
The minimum clearance between the adjacent beam’s flanges should not be less than 8 cm to facilitate significant placing and consolidation of the concrete.
The maximum clearance between the adjacent beams’ flange should be restricted to 300 mm or 1.5 to 2 times the flange’s width, whichever is smaller.
If beams are spaced at more than the maximum specified spacing, it is likely that the concrete between beams may not act monolithically with the rays, and such may become the cause of failure of the foundation.
A minimum concrete cover of 100 mm is maintained throughout the beams’ exposed surface and ends to protect the girder beams from corrosion. Concrete cover under the lower tier of beams should not be less than 150 mm.
Also, Read - Density of Cement, Sand and Aggregate, Bulk Density of Aggregate
Timber Grillage Foundation
Timber grillage foundation consists of timber planks, and timber beams can also be used to support heavy loads on weak soils. This foundation is suitable for the ground that always remains waterlogged.
In this foundation, no concrete block is used, but instead, the timber platform is used that consists of 50 mm to 75 mm thick wood planks installed touching each other.
Above this platform lower-tier of scantlings, nearly 80 mm × 120 mm size is placed covering the full length and breadth of the rectangular platform.
Lastly, a massive log is used at the centre of the scantlings of lower-tier and timber columns is centrally placed over it as shown in the image.
The load from the timber column is transferred to the lower tier of the scantling through massive logs placed directly below the column.
Lastly, lower-tier scantling transfers the entire load to the foundation soil through timber platform.
Sometimes timber grillage foundations may also be used below permanent walls. But in such case, timber must be preserved from possible harmful effects.
Also, Read - Difference Between Plinth Level, Sill Level and Lintel Level
Features of Grillage Foundation
The grillage foundation helps connect the entire foundation into a unity structure and uniformly distribute the house’s load.
Mostly, it is designed as a monolithic RCC structure, and the metal frame gives its strength.
Design of Grillage Foundation
- For design purposes, it is essential to calculate the loads and moments of the superstructure.
- Now calculate the required base area with a suitable allowable ground bearing pressure for the structure load.
- From this value, we can determine the numbers and sizes of beams for each grillage layer.
- It will help to determine the dimensions of beams required to resist the shear forces and bending moments.
- The loading and building methods should be agreeable with the design requirements.
Practical Example of grillage foundation design for the auditorium
Construction Procedure of Grillage Foundation
Grillage foundation made of steel or wooden joints fitted in a stepped manner. It disperses the load over the broad area of ground.
- First, a trench is excavated of depth 90 to 150 cm and levelled. A layer of concrete ratio 1:2:4 or 1:1.5:3 of thickness 23 to 30 cm is spread and compacted.
- Over this concrete base, steel I-beams are laid and placed at suitable intervals 45 to 90 cm. This I-beams length is equal to the foundation’s width.
- Then concrete is poured to fill the space between the I beams. Above these, the next layer of I-beams is installed perpendicular to the primary layer.
- Again concrete is poured into the internal space to fill it.
- Now fix steel stanchions by connecting with the base plate. A gusset plate and side angles are used for a secure connection. To make them monolithic, these connections are also secured in the concrete.
Generally, the grillage foundation is employed for heavy buildings such as town halls, towers, and factories.
Also, Read - What is Plinth Beam? Plinth Protection, Difference Between Plinth Beam and Tie Beam
Advantages of Grillage Foundation
- Its installation process is easy and fast.
- The steel used in steel grillage foundations can be reused several times, making it environment friendly.
- The technology and material used in installation and manufacturing are simple and readily available, making it affordable.
- You can customize grillage as per the project requirement, making it suitable for different applications.
- It assists in reducing the vibration of the house near the highway or railway line.
- It also serves to reduce the heat evocation of the house.
- Suitable for low bearing capacity soil. You can save up to 30% on foundation construction.
Disadvantages of Grillage Foundation
- Its steel joints require protection from corrosion by concrete.
- For greater depth, the construction of piles is needed in this foundation.
- Space below the grillage needs to be filled and warmed.
- Not suitable for multi-story buildings.
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