What is Grillage Foundation | Where it’s Used?

What Is Grillage Foundation?

A grillage foundation is a special type of isolated footing, generally provided for heavily loaded steel stanchions, especially in those locations where the bearing capacity of soil is poor. The depth of foundation is limited from 1000 mm to 1500 mm. The load of the column or stanchion is distributed or spread to a very large area by means of layers or tiers of joists, each tier being placed at right angles to the next tier.

Grillage Foundation

Types of Grillage Foundation 

Grillage foundations are of two types:

  1. Steel grillage foundation.
  2. Timber grillage foundation.

STEEL GRILLAGE FOUNDATION

Steel grillage foundation is constructed of heavy steel beams which are structurally known as rolled steel joists (R.S.J.), provided in two or more tiers. In the case of double tier grillage (which is commonly provided), the top most tier of grillage beams is laid at right angles to the bottom tier.

grillage foundation drawing

The joists or beams of each tire are held in position by 20 mm diameter spacer bars with 25 mm diameter pipe separators. The grillage beams are embedded in concrete below it. Generally, a minimum clearance of 80 mm is kept between the grillage beams so that concrete can be easily poured and properly vibrated to achieve the desired compaction.

However, the distance between the flanges of the beams should not exceed 300 mm or 1½ times of the flange width (whichever is small) so that the filled up concrete acts monolithically with the beams, it should be noted that the filled-up concrete does not carry any load, it simply provided to keep the beams in position and to prevent their corrosion.

grillage foundation drawing

A minimum concrete cover of 100 mm is kept on the outer sides of the external beams, as well as upper flanges of the top tier. The depth of concrete below the lower tier should be at least be150 mm.

Step by Step Method of Steel Grillage Construction

  • At first, the foundation is excavated for steel grillage construction to the required depth. In general, the depth of foundation is shallow, it should be sufficient enough to accommodate the two tiers of grillage beams and the gusset plates angle cleats, etc. connecting the stanchion to the base (*some allowance in the excavation shall be provided for working space).
  • However, this depth should not be less than 900 mm in any case. Then the foundation base should be properly leveled and thereafter rich mix of concrete is poured over the foundation soil and compacted, the formed thickness is not less than 150 mm. Compaction of concrete should be done properly so that the layer becomes impervious to protect steel joists against subsoil water.
  • After leveling and compacting the concrete bed, the first layer of grillage beams of designed sizes is laid over it, at required distances, with the help of separators. The upper surface of all the beams is then leveled and lied in one horizontal plane.
  • A rich cement or chemical grout is then poured all around the lower flanges of the beams so that they are fixed and secured to the concrete bed. Then the cement concrete is poured between and around the beams of the first tier.
  • The second tier of beams is then placed at right angles to the first tier and over the top flanges of the beams of the first tier. They are properly spaced with the help of separators. Then the second layer of concrete is poured between and around the steel beams.
  • The steel stanchion is then connected to the upper tier with the help of a base plate, anchor bolts, side angles, and gusset plate. These connecting elements are also embedded in the concrete so that the whole joint becomes homogenous and rigid.
  • Steel grillage foundation is also be provided for a masonry wall on soils of low bearing capacity. The grillage foundation for such a case consists of a single tier, though in some circumstances when the wall is wider and it carries heavy loads, two tiers may also be provided.

TIMBER GRILLAGE FOUNDATION.

Timber grillage foundation is provided in case of heavily loaded timber column or heavy load-bearing masonry wall. In this foundation timber planks and timber beams are used in the place of steel joists.

Timber grillage foundation drawing

This foundation is especially useful in waterlogged areas where the bearing capacity of the soil is very less, and where the steel beams may get corroded due to undersoil water caused by any kind of seepage.

The load on the soil is limited to 50 to 60 kN/m where no concrete is poured between the timber joists, However, the bottom level concrete is provided in the case of steel grillage foundation which is replaced by a timber platform constructed of timber-planks,

Step by Step Method of Timber grillage Construction

  • Firstly, excavation for the foundation up to the desired depth is done and leveling it properly, then the bottom layer of planks 50 to 75 mm thick and 200 to 300 mm wide is laid.
  • The planks are arranged side by side, without any gap between them. Over this platform, a wooden beam tire which is sized about 150 mm x 100 mm, spaced 300 to 500 mm apart, is laid at right angles to the direction of the planks.
  • Over the top of this layer, a timber beam of the same section as that of the wood post is placed at right angles. The timber post is then fixed at right angles to this timber beam.
  • The foundation consists of two layers of wood planks, separated by rectangular sections (beams) of timber placed at right angles to the direction of the wall.
  • The upper layer of the planks, placed side by side maybe 75 to 100 mm thick, extending over the full width of the wall base, and running longitudinally along the wall.
  • The lowermost part or layer of planks is 50 to 75 mm in thickness, which is placed along the wall longitudinally.
  • However, the lower layer of timber planks, and also the middle tier of timber beams, should extend to at least 450 to 600 mm on either side of the wall footing base.

Grillage Foundation Advantages & Disadvantages 

ADVANTAGESDISADVANTAGES 
Time of construction of grillage foundation is lessErection of column or stanchion is difficult in order to keep it aligned.
Grillage foundations provide better performance against seismic actions and vibrationsThe underneath base of the foundation must be filled and compacted properly and the soil must be heated up to make it hard
This foundation performs better than any other foundation.The piles below the foundation must be higher in-depth
Grillage foundations are capable of transferring loads to a larger area thus, it is a safe option.The cost of excavation is more in this foundation as it needs a larger area to cover
Grillage foundations save a lot of concrete pouring, thus it is eco-friendly and cost-effective.The heavy materials like beams and columns must be hoisted safely using a crane, which is again a cost involving matter.
This foundation reduces the heat evoke from the houseThis type of foundation can be encountered by a large uplift of the soil.

Grillage Foundation Application 

The major areas where grillage foundation system is adopted are;

  • Suitable where bedrock is not available and raft or mat foundation is not feasible.
  • The foundation is lighter and cost-effective does not require deep excavation unlike the pile foundation
  • This type of foundation is lighter on the site constraints

The major projects where grillage foundation is used.

  • Electrical Transmission line project.
  • Telephone Or TV tower foundation.
  • Any other tower-like structure relied on steel stanchions or columns.

Features Of Grillage Foundation 

The main few features of grillage foundations are;

  • It is successful and durable to choose as a foundation if you got plenty of steel and durable timber.
  • It is beneficial to reuse the rolled steel sections after get dismantled.
  • If the bearing capacity of the soil is low it is a great choice as it is low cost and lighter.
  • This foundation is the best choice for large area pressure distribution.

Design of Grillage Foundation 

Designing of grillage foundation is a critical task it shall always be approved by a licensed structural engineer. Thus in this section, we will discuss the typical steps involved in the designing process.

  • The very first step involved in the process of designing the grillage foundation is the calculation of the loads and the moments of the superstructure coming into it.
  • From the above process, engineers will get be able to analyze the base area required for the foundation. Considering the present allowable bearing capacity of the soil.
  • Then by dividing the area, the number and the size of the grid will be obtained.
  • The slope of the layers of timber or rolled steel joists must be upward from the edge to above.
  • The considered amount of the slope given will determine the size of the beam to withstand all the moments and forces coming into it.
  • The concrete is poured between grillage beams in order the keep the beams in position and to protect them from corrosion.
  • Minimum clearance of 800 mm must be given around the grillage beams so that the concrete can seamlessly be poured.
  • The depth of the base concrete must minimum of 15 cm in order to protect from atmospheric effects and to provide a firm base.

 

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