Grillage Foundation
Table of Content: 1. What is grillage foundation.
2. Installation of grillage foundation.
3. Types of grillage foundation.
4. Construction of grillage foundation.
5. Advantages and disadvantages of grillage foundation.
A foundation which consists of one, two or more tiers of beams (typically steel) superimposed on a layer of concrete to disperse load over an extensive area is grillage foundation. Grillage foundation is used when heavy structural loads from columns, piers or stanchions are required to be transferred to a soil of low bearing capacity. Grillage foundation is often found to be lighter and more economical. It is used at the base of columns. These tiers are encased in concrete and are at right angles to each other. This type of foundation is generally used for heavy structure columns piers and scaffolds
Installation of Grillage Foundation:
First, for solid monolithic grillage, we have to produce and install the framework. We prefer this grillage because it is more dependable.
Formwork is made from edging boards in the form of rectangular troughs. Its height is 1 ft and width are equal to the minimum wall thickness of the house. It must be set about 6-8 inches gap in between each grillage.
Inside the formwork, we have to set a framework of reinforcements connections using binding wire. The least distance from the frame to the side of the formwork is required to be same.
Next, the reinforcement is linked with the same binding wire used before.
Then, Concrete has to be prepared using the cement mixer. It is poured in the formwork in a continuous cycle. Fittings must be placed on a height about 25-30mm so that it completely immerses in concrete. Concrete should be carefully poured to avoid unwanted cavities. 6. After filling the surface, the surface has to be leveled and left to dry. When it is dried, the formwork can be removed. The foundation is ready now.
Types of Grillage Foundation:
1. Steel Grillage foundation
2. Timber Grillage 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.
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.
Construction of Grillage Foundation:
The steps followed for the construction of the grillage foundation are listed below:
The framework must hold the grillage in place for a solid monolithic grillage foundation.
The formwork is prepared from edging boards in the form of rectangular troughs.
The height of the formwork ranges from 1-1.5 ft, and the width shall be equal to the minimum wall thickness of the building
Concrete is poured into the formwork to form a concrete pad of 450 mm to 600 mm.
The grillage sections (timber or steel) are placed above the concrete pad with sufficient cover on all the sides.
Concrete is poured into the foundation up to the desired depth.
A minimum cover of 10 mm shall be maintained at the top grillage to avoid corrosion of steel beams.
Advantages of Grillage Foundation:
1. The time required for the installation and casting of grillage foundation is comparatively less than that of conventional foundation.
2. Grillage foundation is capable of successfully transforming heavy loads over a larger area.
3. These foundations are used to construct significant structures such as column piers and scaffolds.
4. Grillage foundation avoids deep excavations in soft soils, and the depth of the foundation can be limited to 1-1.5 m.
Disadvantages of Grillage Foundation:
1. Its steel joints require protection from corrosion by concrete.
2. For greater depth the construction of piles is needed in this foundation.
3. Space below the grillage needs to be filled and warmed.
4. Not suitable for multi-story buildings.
