The grillage foundation consists of 1, 2, or more tiers of beams (usually metal), and these beams are mounted on a layer of concrete to distribute the load over a large area, grillage foundations are used at the base of columns, scaffolding, and heavy-structure columns.
When it comes to transferring heavy loads from column to soil with low load-bearing capacity, this foundation is essentially the most economical foundation.
Here we will learn about grillage foundation, types of grillage foundation, merits & demerits of grillage foundation.
Introduction to grillage foundation:
This foundation normally consists of various layers of beams placed at right angles to one another; steel, precast concrete, or wooden grills are most typically used in grillage beams.
The grillage connects all the foundation and contributes to the uniform distribution load of all piles and structure.
In a monolithic reinforced concrete construction, this foundation is most often arranged as it provides strength by a metallic frame called a metal grill foundation.
Need of grillage foundation:
- This type of foundation is appropriate to construct for buildings of light construction materials on weak soils.
- This foundation is needed to save up to 30%.
- Don’t permit to build the other type of foundation in areas where soil characteristics and conditions of their occurrence weak.
- The load must be anticipated to be relatively small for the construction.
Types of Grillage Foundation:
The grillage foundation is of two types, according to the material used in the structure as follows:
1.Steel Grillage Foundation:
This foundation consists of beams or steel joints that can be set in single or double-tier as-rolled steel joists made of steel beams.
On the outer sides of the exterior beams, a minimal cover of 10 cm is kept above the higher flanges of the top tier and 15 cm must be the depth of concrete.
We must always examine that if the compaction is finished correctly and shaped an impervious layer of a thickness not lower than 15 cm after levelling the base and pouring the concrete.
Then with the help of pipe separators, lay the primary layer of beams over the concrete bed at a distance of 100mm to 300mm across the beams of the primary tier pour the concrete.
Then place the second tier of the beam at the right angle to the primary tiers and again pour the concrete with the help of a base plate, facet angles, and a gusset plate connect the steel stanchions to the upper tier.
2.Timber Grillage Foundation:
For heavily loaded masonry walls of timber columns, timber foundation is provided and it is very helpful in waterlogged areas where the bearing capacity of the soil could be very low.
Between the joints of the timbers, no concrete is surrounded and the width of the bottom layer of the timber planks is about 20 cm to 30 cm in width and 5 cm to 7.5 cm, with the gap between them laid side by side.
A timber beam of the identical section of the wooden post is positioned at right angles to the direction of beams again another layer of planks is laid and perhaps 7.5cm to 10cm thick upper layer of planks extending over the total width of the wall base.
Design of Grillage Foundations:
It is required to calculate the loads and moments from the superstructure for the design of the grillage foundation and suitable allowable floor bearing pressure should determine for the required base area.
We are going to discover the numbers and dimension of every grillage layer by dividing this area then we have to design the layer that it cantilevers from the edge of the layer above.
To withstand the bending moments and shear forces, it would determine the beam sizes required with the design requirements the method of building and loading should be compatible.
Installation of Grillage FoundationS:
- Firstly, we have to provide and install the framework for stable monolithic grillage.
- From the edge boards within the type of rectangular troughs, the formwork is made and its width is equal to the minimum wall thickness of the house and its height is 1 foot, and a gap of about 6 to 8 inches between each grill.
- We have to determine an outline of the reinforcement connection using the binding wire contained in the formwork.
- The least distance is required from the body to the side of the formwork after which with the identical binding wire, the reinforcement is linked.
- Subsequently, the cement mixer must be prepared in a continuous cycle using concrete, it is poured within the formwork.
- At a height of about 25mm to 30mm fittings have to be positioned and to keep away from undesirable cavities concrete should be carefully poured.
- The floor must be leveled and left to dry after filling the surface and the formwork will be eliminated when it is dried.
- Then the foundation is prepared.
Advantages of Grillage Foundation:
There are the following benefits of the grillage foundation as follows:
- This foundation reduces the heat-evoke of the home.
- If the home is constructed near highways it reduces the level of the home vibration.
- Contractors can save time using a grillage foundation as it has the speed of Installation.
- To keep away from poured concrete and saves time with easy installation this foundation is used.
- With the minimal disruption of transport infrastructure, this technology is cost-effective.
Disadvantages of Grillage Foundation:
- At sufficiently great depth, there is a requirement for the construction of piles on this foundation.
- In the grillage foundation house underneath the grillage must be warmed and crammed.
Also read: Pier and Beam Foundation, Caisson Foundation & Mat Foundation
This foundation is mostly used in places where the weight of the structure is heavy and the bearing capacity of the soil is comparatively poor.