Grillage Foundation
A foundation consisting of one, two or more layers of beams (typically steel) superimposed on a concrete layer to disperse the load over a large area is a foundation for grilling.
It is used at the base of the columns. These layers are wrapped in concrete and are at right angles to each other. This type of foundation is generally used for pillars and column scaffolds with a heavy structure.
Although the foundation and the grid look the same, they are different. Where the foundation transfers the load from the structure to the ground, the grid disperses heavy loads over large areas.
What is a grillage foundation?
A Grillage foundation is a type of shallow foundation used to distribute heavy structural loads over a large area. It is commonly employed in constructing bridges, towers, or other heavy structures where the soil has a low bearing capacity.
The loads must be spread out to avoid excessive settlement or failure. The foundation consists of one or more layers of steel beams, timber, or reinforced concrete beams, arranged in a grid-like pattern, which transfers the load from the structure above to the underlying soil.
With the use of a grillage foundation, deep excavations can be avoided, and the depth of the foundation can be limited from 1 m to 1.5 m.
This type of foundation is beneficial in situations where traditional spread footings or piers may not be suitable due to the soil conditions, size of the load, or other factors.
Types of Grillage Foundation
Here, two different types of grillage foundations are as follows.
- Steel Grillage Foundation.
- Timber grill foundation.
1. Steel Grillage Foundation
The Steel Grillage foundation consists of joints or steel beams supplied in single or double layers. Its name defines its function and structure, as it is composed of steel beams, structurally known as rolled steel beams.
A minimum coverage of 10 cm is maintained on the outer sides of the outer beams, as well as above the upper flanges of the upper layer.
The depth of the concrete must be at least 15 cm. After leveling the base and pouring the concrete, we must check that the compaction is done properly and that an impermeable layer of a thickness of not less than 15 cm is formed.
It protects steel beams from groundwater, which can lead to corrosion. Then, we place the first layer of beams on the concrete bed at a distance of 100 to 300 mm, with the help of tube separators.
Then, we pour concrete between and around the beams of the first layer. After that, we place the second beam level at right angles to the first levels with the help of the separator.
Then again, we pour concrete between and around the steel beams. In doing so, we connect the steel supports to the top layer with the help of a base plate, side angles, and a reinforcement plate. These connecting elements are also embedded in the concrete to make the joint rigid.
2. Timber Grillage Foundation
The timber-type foundation is provided for masonry walls heavily loaded with Timber columns. This foundation is especially useful in flooded areas, where the carrying capacity of the soil is very low and where the load on the soil is limited to 50-60 KN/M2.
Timber planks and Timber beams are used instead of steel beams. there is no concrete enclosed between the Timber joints. However, the lower concrete supplied in the steel grid is replaced by a Timber platform built with Timber planks.
The excavation of the base is level. The bottom layer of Timber boards of size 20 to 30 cm and width of 5 to 7.5 cm thick is placed side by side, with no space between them. Above this layer, a Timber beam of the same section as that of the Timber post is placed at a right angle.
Then, another layer of planks is placed again at right angles to the direction of the beams. The top layer of boards maybe 7.5-10 cm. Thick, extending the entire width of the base of the wall, on which the masonry wall is built.
Grillage Foundation Project
For the grating foundation project, it is necessary to calculate the loads and moments of the superstructure.
From this, we need to determine the base area required for an adequate allowable ground bearing pressure of the condition.
Dividing this area, we will find the numbers and the size of each grid layer. Next, we have to design the layer so that it slopes from the edge of the layer above. It will determine the beam sizes needed to withstand bending moments and shear forces.
The grid cannot be enclosed in concrete and in order, as the composite action of the beam and concrete will be demoralized. The method of construction and loading must be compatible with the needs of the project
Features of Grillage Foundation
- The grill connects the entire foundation to the single structure and thus contributes to an even distribution of the weight of the house and all the piles.
- Most of the time, this foundation is organized as a monolithic reinforced concrete structure. The resistance is provided by the metallic structure (steel foundation for grilling).
- Minimum clearance of around 800 mm (8cm) is provided between the grillage beams so the concrete will be easily poured and properly compacted.
- As the concrete filling doesn’t carry a load, it functions to keep the beams in position and prevents them from getting corroded.
- The depth of concrete below the lower tier is kept at a minimum of 150mm (15cm).
Installation of the Grillage Foundation
The steps for installing the foundation for grilling are shown below:
- First, for the solid monolithic grid, we need to produce and install the structure. We prefer this barbecue because it is more reliable.
- The formwork is made of edge boards in the form of rectangular gutters. Its height is 1 foot and width is equal to the minimum thickness of the house wall. A space of about 15 to 30 cm should be defined between each grid.
- Within the formwork, we have to define a reinforcement connection structure using the connecting wire. The shortest distance from the structure next to the formwork must be the same.
- Then, the reinforcement is connected to the same connecting wire used previously.
- Then, the concrete must be prepared with the cement mixer. It is poured into the formwork in a continuous cycle. The connections must be placed at a height of about 25 to 30 mm so that they are completely immersed in the concrete. Concrete must be poured with care to avoid unwanted cavities. 6. After filling the surface, it should be leveled and left to dry. When dry, the formwork can be removed. The foundation is ready now.
Advantages of Grillage Foundation
- It requires less time and materials for installation.
- It reduces the evocation of heat from the house with the help of the foundation, since it is not applied to frozen soil.
- It reduces the vibration level of the house (it is very real if the house is built close to highways).
- The installation process of Grillage Beam requires less time and materials.
- This type of foundation can be used for significant structures such as column piers, and scaffolds.
- It is capable of transforming load over a larger area.
Disadvantages of Grillage Foundation
- It requires the need to build piles at a sufficiently large depth.
- Space under the grid must be filled and heated.
- Excavation of a larger area is needed, which increases the excavation cost.
- It gets affected by a larger uplift force.
- Column alignment in this type of foundation might slightly change.
FAQ: Grillage Foundation
What is a grillage foundation?
A grillage foundation is a type of shallow foundation used to distribute heavy structural loads over a large area. It consists of one or more layers of steel beams, timber, or reinforced concrete beams arranged in a grid-like pattern.
Where is a grillage foundation typically used?
Grillage foundations are commonly used in constructing bridges, towers, or other heavy structures where the soil has a low bearing capacity.
How does a grillage foundation differ from a traditional foundation?
While traditional foundations transfer loads directly to the ground, a grillage foundation disperses heavy loads over a larger area to avoid excessive settlement or failure.
What are the main types of grillage foundations?
The main types are:
- Steel Grillage Foundation
- Timber Grillage Foundation
What materials are used in a steel grillage foundation?
A steel grillage foundation uses steel beams arranged in single or double layers, encased in concrete to protect against corrosion.
What is the typical depth for a grillage foundation?
The depth of a grillage foundation is typically limited to 1 m to 1.5 m, avoiding the need for deep excavations.
What are the steps involved in installing a grillage foundation?
The installation involves:
- Leveling the base and pouring a concrete bed
- Placing the first layer of beams and encasing them in concrete
- Placing subsequent layers at right angles and encasing them in concrete
- Connecting the steel supports with base plates and reinforcement plates
What are the advantages of a grillage foundation?
Advantages include:
- Requires less time and materials for installation
- Reduces heat loss from the house
- Lowers the vibration level of the structure
- Can transform load over a larger area
- Suitable for significant structures such as column piers and scaffolds
What are the disadvantages of a grillage foundation?
Disadvantages include:
- Requires piles at a large depth
- Space under the grid must be filled and heated
- Larger excavation area increases cost
- Affected by larger uplift forces
- Possible slight misalignment of columns
Why would a timber grillage foundation be used?
A timber grillage foundation is used for masonry walls with heavy loads, especially in flooded areas with low soil bearing capacity, where the load on the soil is limited to 50-60 KN/m².
How is the load distribution calculated for a grillage foundation project?
Loads and moments of the superstructure are calculated to determine the required base area. This area is divided to find the size and number of grid layers, and beam sizes are determined to withstand bending moments and shear forces.
