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.

Even though foundation and grillage appear to be the same, they are not. Grillage disperses huge loads across broad areas, similar to how foundations distribute the load from the structure to the ground.

Uses of Grillage Foundation:

  • Grillage foundation is utilized when hard rock soil is unavailable, and mat foundation is not an option.
  • Deep excavations are not required for grillage foundations. As a result, it is cost-effective.
  • A grillage foundation allows for the safe transfer of heavy building loads.
  • The grillage foundation is employed in the main project for an electrical transmission line and the foundation for a telephone tower.

Features of Grillage Foundation

  • The strength of the grillage foundation is higher than other foundations. A minimum of approximately 800 mm (8 cm) is allowed between the grillage beams to obtain the appropriate strength in the grillage foundation. The concrete can be poured easily and compacted properly.
  • The heavyweight of the house safely contributes to an even distribution on the ground through the Grillage foundation.
  • In this type of foundation, the depth of concrete is kept at least 150mm (15cm) to protect the lower level steel beam against corrosion.
  • No more space is required to transfer the heavy load of the building to the ground.
  • This type of Grillage foundation can be used very effectively in poor soil and waterlogged area.

Installation of the Grillage Foundation:

  • The framework for solid monolithic grillage must be built and installed.
  • The formwork comprises rectangular troughs made of edging boards, with a width equal to the house’s minimum wall thickness and 1 ft.
  • The minimum distance between the frame and the side of the formwork must be the same, and the reinforcement must be linked with the same binding wire.
  • A 6 to 8-inch space between each grillage must be established, and a framework of reinforcement connections must be established inside the formwork using binding wire.
  • Next, concrete must be made with the cement mixer and put into the formwork in a continuous cycle. Grillage foundation fitting should be installed at around 25mm to 30mm, and concrete should be carefully poured to eliminate voids.
  • After filling the surface, it must be leveled and allowed to cure, after which the formwork can be removed. The foundation has been laid.

Types of Grillage Foundation:

  1. Steel Grillage foundation
  2. Timber Grillage Foundation

Steel Grillage Foundation

Steel Grillage foundation consists of steel joints or beams which are provided in single or double tiers. Its name defines its function and structure as it is made up of steel beams, structurally known as rolled steel joist. A minimum cover of 10 cm is kept on the outer sides of the external beams as well as above the upper flanges of the top tier. The depth of concrete should be at least 15 cm. After we level the base and pour the concrete, we should check that if the compaction is done properly and formed an impervious layer of a thickness not less than 15 cm. It shields steel joist from groundwater, which can lead to corrosion. Then we lay the first layer of beams over the concrete bed at a distance of 100mm to 300mm, with the help of pipe separators. Next, we pour concrete between and around the beams of the first tier. After that, we place the second tier of the beam at the right angle to the first tiers with the help of the separator. Then again, we pour concrete between and around the steel beams. By doing so, we connect the steel stanchions to the upper tier with the help of a base plate, side angles, and a gusset plate. These connecting elements are also embedded in the concrete to make the joint rigid.


Timber Grillage Foundation

Timber grillage foundation is used where the soil is soft and is permanently water-logged. A suitably designed timber grillage foundation can economically support the building walls. 

By restricting the soil weight to 5.5 tonne/sq.mt, a wood platform replaces the concrete block that is generally given below the wall footing in this construction style.

The wood platform comprises boards that are typically 8 to 10 cm thick and are stacked in two layers, one longitudinal and the other across the wall, reaching 45 to 60 cm beyond the footing foundation on each side.

The boards are 5 cm to 10 cm thick at the bottom levels, depending on the loads and site conditions. Rectangular portions of timber spaced at no more than 38 cm center to center, with a depth of 0.75 times the width, divide the two layers of planks.

Advantages of Grillage Foundation:

  • It takes less time to install and uses fewer resources.
  • It lowers heat transfer from the home via the foundation because it is not sprayed on frozen earth.
  • It reduces the amount of vibration in the house (it is very real if the house is built close to highways).
  • Installation of Grillage Beams takes less time and requires fewer resources.
  • This type of foundation may sustain significant structures like column piers and scaffolds.
  • It can spread the weight across a larger area.

Disadvantages of Grillage Foundation:

  • This foundation required the insertion of large-diameter piles.
  • It is necessary to heat and fill the empty area between steel beams. As a result, skilled personnel are needed.
  • The excavation of a bigger area is required to construct a grillage foundation, which increases the construction cost.
  • This type of foundation is susceptible to uplift pressure.
  • The structural arrangement of the column bed is subject to modification in grillage foundation construction at times.

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