Expert: Jeanette A - 9/28/2009

Question
Hi,i would like to know the definition of pad foundation,its importance and typse if any,reasons for adoptind pad foundation and its design

Answer
Hello Neville,

Here is some info which will answer your questions:

4. Pad Foundations

Pad foundations are generally used to transfer the load from a column, pier or heavy machinery to the ground. Pad foundations are usually square or rectangular in plan; they are formed of either unreinforced concrete or reinforced concrete. Pad foundations are generally designed to support higher loads over a limited area.

4.1 Unreinforced Concrete Pad Foundation

The main feature of unreinforced concrete pad foundations is that the concrete does not carry any reinforcement.

4.1.1 Mass Concrete Pad Foundation

This kind of foundation usually carries very light loads. The foundation is designed to be of such a thickness that no tensile stresses will develop on the underside. This can be achieved by making the length of the projection equal to the depth of the foundation, on the normal practice of a 45° distribution of loading.


4.1.2 Mass Concrete Pad Foundations with Grillage Base

These foundations carry heavily loaded structural steel columns (stanchions). They usually have a very large area and the steel grillage is designed to transfer the load to the ground. The function of the concrete in this case is to protect the steel against corrosion.

The grillage may consist of two or three tiers of universal steel beams placed across each other. These beams are spaced such that the concrete is able to flow between and underneath them and, at the same time, be fully compacted.


4.2 Reinforced Concrete Pad Foundations

These foundations carry reinforcement and placed at the bottom of the foundation to resist tensile stresses. Compression and shear reinforcement are normally not provided.

4.2.1 Reinforced Concrete Plain Pad Foundations

They are usually square in plan. The plan area of the base is determined by dividing the total in-service loads of the column and the base by the allowable load-bearing pressure on the soil.

The thickness of the pad, which is designed to resist punching shear, should be checked for beam shear stress at critical sections. This is to ensure that no shear reinforcement is required. The amount of tensile reinforcement in the base is designed to resist tensile stresses caused by bending moments in the base.


4.2.2 Reinforced Concrete Pad Foundations with Sloping Top Face

They are similar to plain pad foundations. They are based on the same design principle as plain pad foundations, and are square in plan. The only difference between this foundation and the plain pad is that the top face of this foundation has a sloping surface. The greatest depth of a sloping face foundation is at the column face, and the least depth is at its perimeter. The reason for having a sloping face is to reduce the quantity of concrete used.


4.2.3 Stepped Reinforced Concrete Pad Foundation

This foundation has a stepped top surface. This reduction in depth along the edges of the foundation reduces the quantity of concrete used. However, although less concrete is used, the cost of formwork may increase.

4.2.4 Eccentrically Loaded Reinforced Concrete Pad Foundations

They are required for peripheral columns which have to be built close to adjacent buildings. They are rectangular in plan and may have a plain or sloping top face.

They are designed on the same principles as the other types of pad foundations except that the columns are eccentric (that is, the column is closer to one edge of the foundation than to the other).


Here is a video to pad foundations: http://www.itct.hku.hk/ctglossary/foundations/media/a_pad_foundation.avi

Here is some extra info:
www.decodingeurocode7.com/.../10.%20Design%20of%20footings%20(sample).pdf
http://environment.uwe.ac.uk/geocal/foundations/gifs/pad.gif


Hope this has helped. Have a good one.


Warm Regards,
Jeanette