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Grillage Foundation – Design, Installation & Types

What Grillage Foundation?
Grillage foundations are made of one, two, or more levels of beams (usual steel) superimposed on a layer of concrete to distribute load across a large area. It is found at the bottom of the columns. These layers are concrete-encased and at right angles to one another. This foundation form commonly supports heavy building columns, piers, and scaffolds.

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:

Design of Grillage Foundation:

The loads and moments from the superstructure calculate for the grillage foundation design. Based on this, the critical foundation area for a sufficient allowed ground bearing pressure must be determined. We’ll divide this space to determine the number and size of each grillage layer.

The layer must then be designed to cantilevers from the edge of the layer above it. It will determine how big the beams need to withstand the bending moments and shear forces. The grillage cannot be encased in concrete and the order since this would demoralize the composite action of the beam and concrete. The construction and loading methods must be compliant with the design requirements.

Features of Grillage Foundation:

Installation of the Grillage Foundation:

Types of Grillage Foundation:

1. Timber Grillage Foundation:

A timber-type foundation is supplied for strongly heavy masonry walls of timber columns. This foundation is particularly effective in wet areas where the soil’s bearing ability is low, and the weight on the soil is limited to 50-60 KN/m2. Steel joists are replaced with timber planks and beams.

Fig 1 Timber Grillage Foundation
Fig 1: Timber Grillage Foundation
Courtesy: researchgate.net

Between the timber joints, no concrete is present. However, the steel grillage’s bottom concrete is replaced by a timber platform made of wood boards. The base excavation has been leveled. The bottom layer comprises 20-30cm wide, 5-7.5cm thick timber boards set side by side with no gaps.

A timber beam of the same section as the wooden post is installed at right angles on this layer. Then, at right angles to the path of the beams, another layer of boards is laid. The top layer of planks could be 7.5-10 cm thick. Thick and spanning the entire width of the wall base, over which a masonry wall is built.

1. Steel Grillage Foundation:

Steel grillage foundation consists of steel beams, also known as grillage beams, offered in single or double levels. The upper tier of a double-layer grillage foundation is built at right angles to the lower tier. 20 mm spacer bars with 25 mm diameter pipe separators hold the grillage beams of each tier in place. The beams are spaced enough apart to allow for the placement and compacting of concrete between them. A minimum clearance of 8 cm is deemed most optimal. The space between the beams’ flanges should be one and a half to two times the flange width, with a maximum of 30 cm.

Fig 2 Steel Grillage Foundation
Fig 2: Steel Grillage Foundation
Courtesy: civicconcept.com

If the beams are placed further apart, there is a risk that the concrete filling will not operate monolithically with the beams, resulting in the foundation failing. A minimum cover of 10 cm is preserved on the outside edges of the external beam. Above the top tier’s upper flange to protect the beams from corrosion. Under the lower beam, the concrete cover should not be less than 15 cm thick.

Advantages of Grillage Foundation:

Disadvantages of Grillage Foundation:

Conclusion:

For huge expanses, grillage foundations are commonly used instead of poured concrete. Because of the straightforward installation process, the grillage foundation saves time. The grillage foundation constructs to meet a variety of construction needs. As a result, grillage foundations are suited for various construction applications.

References:

1. Mahajan, B. (2021, June 17). Grillage Foundation | Types Of Grillage Foundation | Steel Grillage Foundation | Grillage Footing. Civiconcepts; civiconcepts.com. https://civiconcepts.com/blog/grillage-foundation
2. Lead, C. (2020, November 12). Grillage Foundation – Types, Advantages & Disadvantages – Civil Lead. Civil Lead; www.civillead.com. https://www.civillead.com/grillage-foundation/
3. Rajput, K. (2021, August 21). Grillage Foundation | Types of Grillage Foundation | Features of Grillage Foundation. CivilJungle; civiljungle.com. https://civiljungle.com/grillage-foundation/
4. Satheesh. (2020, May 11). Grillage Foundation – Types & Installation [Civil Planets]. Civil Planets; civilplanets.com. https://civilplanets.com/grillage-foundation/
5. Grillage Foundation: Types, Construction, and Advantages – The Constructor. (2022, January 13). The Constructor; theconstructor.org. https://theconstructor.org/building/grillage-foundation-types-construction/560853/
6. What is Grillage Foundation? Construction and Advantages. (n.d.). PRODYOGI; www.prodyogi.com. Retrieved March 18, 2022, from https://www.prodyogi.com/2021/03/what-is-grillage-foundation.html

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Kanwarjot Singh

Kanwarjot Singh is the founder of Civil Engineering Portal, a leading civil engineering website which has been awarded as the best online publication by CIDC. He did his BE civil from Thapar University, Patiala and has been working on this website with his team of Civil Engineers.

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