What is A Grillage Foundation, Definition, Construction, Uses

What is A Grillage Foundation, Definition, Construction, Uses

A Grillage Foundation is a type of foundation often used at the base of a column. It consists of one, two or more tiers of steel beams superimposed on a layer of concrete, adjacent tiers being placed at right angles to each other, while all tiers are encased in concrete.

Before going for details; let’s see what you’re going to learn today in this post: 

  1. Definition
  2. Design
  3. Types
  4. Construction

It is the most economical foundation in case of transferring heavy loads from columns to soil of low bearing capacity.Please Note that the word tier is used for such an arrangement in which items are placed in layers one above the other like the seats that are placed in a cinema or auditorium.

Grillage Foundation Diagram
Grillage Foundation Diagram
Definition of Grillage Foundation
Definition of Grillage Foundation

Now because in electrical transmission towers, the footing experience extensive corrosion which is a big challenge. Now because a grillage foundation is underground and is not easily accessible for inspection, it can suddenly fail. That’s why Galvanized steel material is preferred for grillage foundation in electrical transmission line projects.

Design of Grillage Beams

Now that you’ve covered the basics of grillage footing, let’s explore its design. You have to calculate the load and moments of the superstructure. Use these applied forces to calculate the required base area. For that, use suitable value of allowable ground bearing pressure. Don’t forget to use the condition of underneath soil.

 
Uses of Grillage Foundation
Uses of Grillage Foundation

From this area, you can decide the number and size of each grillage layer. The layers are then designed to cantilever from the edge of the layer above. This will further help to determine the beam sizes. These beam sizes are required to resist the applied bending moments and shear forces.

Stuck with some design problem? Don’t worry; Here is a practical example where geotechnical engineer has designed grillage footing for an auditorium. Find it here

Design of Grillage Foundation
Design of Grillage Foundation
The design has to consider composite action where you encase grillage in concrete. The sequence of loading and method of construction has to be compatible with design requirements.
 

Types

Mostly there are two types based on type of material used;

  • Steel grillage foundation
  • Timber grillage foundation

Mostly out of these two types the decision is made on the basis of availability of material and overall cost is the deciding factor.

If you want to become a good geotechnical engineer in United States, I’d rather say you learn about main characteristics of grillage types.

Mostly out of these two types the decision is made on the basis of availability of material and overall cost is the deciding factor.

If you’re still unsure whether this type of footing is suitable for soil type of your area. You can perfield full-scale field uplift tests on steel grillage foundations. First soil characterization of the sites is studied with variety of in-situ tests including standard penetration, cone penetration, pressuremeter, borehole shear, drive cone, and dilatometer tests.

Steel Grillage Foundation

This type of grillage foundation is one of the most common. Steel Grillage foundation includes steel grillage beams. These are called rolled steel joists (RSJ). They are provided in layers one above the other and is usually referred as tiers of steel beams. At the end of this article, I’ve explained the structural construction details of steel grillage foundation.

But if you’re opting this, make sure you adhere to the minimum cover requirements. So, to protect steel grillage base from ground water, you have to provide concrete coverage of 10cm around the outer beams and the upper flanges. This will prevent corrosion and will make the joints rigid.

Timber Grillage Foundation

If you’re up with a timber frame structure where timber columns are heavily loaded, timber grillage foundation has a lot to offer. You just have to use timber planks and timber grillage beams instead of the steel joists. Similar to steel grillage foundation, here you’ll place timber beams one above the other. Timber grillage foundation is very useful for water logged areas where the soil bearing capacity is low and where steel beams can get corroded due to underground water.

Unlike steel type, here we don’t need to embed wooden beams in concrete. Instead, you can provided a timber base at the bottom.  

Construction of Grillage Foundation

This method of construction is for steel grillage embedded in concrete. The procedure for construction is as follows:

  • First we have to excavate the soil to a desired depth of foundation. The depth is generally shallow. Just enough to embed the two tiers of grillage beams and connections. However, the minimum depth is 90 cm.
  • After excavation, we have to tam the surface to make it flat. Now, pour the concrete and compact it so that we have a concrete pad of 15 cm thickness.
  • Now we will place the first layer of grillage beam on top. The number, size, and spacing of beams is as per design. The beams have to be leveled.
  • Now pour rich cement grout all around the lower flanges of the grillage beams. It will make them monolithic with the concrete pad.
  • Now place the second tier of beams at right angles. We have to place and space them with the help of separators. Now pour the concrete between and around the steel beams.
  • At the end, we will place steel stanchion by connecting with base plate. We will use gusset plate and side angles for secure connection. These connections are also embedded in the concrete to make them monolithic.
Grilage foundation details
Grillage Foundation Details

Disadvantages of Grillage Foundation

Grillage foundation is not a perfect foundation as it is only useful in some specific scenarios and ground conditions. The biggest disadvantage is: you need to protect the steel joists from corrosion by covering it with concrete. You can’t expect this type of foundation to be useful for multi-story tower, for such you have to go with pile foundation. You just can’t rest the building load on fragile soil.

But if you have a light structure like timber, concrete blocks or steel structure, you can make a grillage foundation to support on soil of low bearing capacity. It will save up to 30% on construction of foundation.  

Is Grillage Foundation Shallow or Deep?

Well to answer this question, first we need to know what is deep foundation or shallow foundation.

If you already know you can skip this paragraph and move on. A shallow foundation is the one when the depth of foundation is equal or less than the width of the foundation. With such, it is placed immediately beneath the lowest part of the structure. Whereas, a deep foundation goes many meters beneath the soil and the most common type of deep foundation is pile foundation.

Well, coming back to grillage. From definition, this type of foundation should come under the heading of shallow foundation. But we can use it in place of deep foundation i.e. when the underground water condition and soil condition doesn’t allow other type of foundation, you can use grillage foundation.

So, grillage foundation is a shallow foundation that can be used as alternative to deep foundation.   

Want to know More?

If you want to know more about foundation definition in civil engineering and explore many other types of foundation. Enter this article here about different types of foundation.

Bored of reading? Don’t worry. Sit back and watch this amazing explainer video with your cup of coffee.

Saad Iqbal
Hi there, I am Saad Iqbal from Pakistan. I am an enthusiastic blogger, passionate content creator, construction geek, and a creative graphic designer. Connect with me at my social channels.