What is Grillage Foundation? Construction and Advantages

Grillage foundation is a special type of shallow foundation that is constructed to take heavy structural loads from columns, piers or beams and also transfer to the soil below. It is employed when the bearing capacity of the soil is poor and need to avoid the use of deep foundation. It essentially consist of one or more layers of steel beams placed at right angles to each other as shown in Figure-1 below. They  resemble a grill arrangement and hence called grillage foundation.

What is Grillage Foundation? Construction and Advantages
Image Credits: www.fondex.com.au


Also Read: Top 7 Types of Shallow Foundation

Grillage foundation have an amazing feature of distributing the heavy loads coming over it to a wider area of the subsoil. These foundation are constructed for a depth in between 1 to 1.5m. 

Construction of Grillage Foundation

As shown in figure-2 below, the whole grillage foundation system lies over a level bed of concrete and the whole is cased in concrete to prevent rusting or corrosion. The system itself consist of steel beams that are placed in right angled directions in layers or tiers. The grillage beam used can be made out of steel, timber or precast concrete. 

Fig.2. Construction of Grillage Foundation

The beams are placed one over the other by maintaining a proper spacing. This designed spacing is achieved by using circular pipe separators with bolts with considerable length that would run through the entire length and breadth of the beams. These circular pipe separators, not only help to maintain the spacing but also helps to provide lateral stability to the foundation when subjected to imposed loads.

Adequate spacing must be there between the beams to ensure proper pouring of concrete and to facilitate compaction.

Design Specifications for Grillage Foundation

Some important guidelines to be followed for quality construction of grillage foundation are:

Fig.3. Design Details of Grillage Foundation
  1. The grillage beams provided must maintain a clear cover not less than 100mm.
  2. The clear distance between the edges of the adjacent beam flanges should be not less than 75mm to allow proper placement and concrete compaction.
  3. Near the ends of the grillage beams, provide adequate pipe spacers or diaphragms and also under the points where intermediate concentrated loads are placed.
  4. The allowable bending stress in grillage beams must be increased by 1/3rd above the normal values for dead and live loads.
  5. The allowable bending stress in grillage beams is increased by 50 percent above their normal values for load combination with wind or seismic loads.

Advantages of Grillage Foundation

  1. Easy Installation
  2. Reduce building vibration
  3. Reduce heat evoke of the house
  4. Minimum disruption of transport infrastructure  makes this cost-effective 

Disadvantages of Grillage Foundation

  1. The base and space where the grillage  foundation is placed must be warmed and filled.
  2. It is not suitable for sufficiently higher depth
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