What Is Pedestal?
The term “Pedestal”, so far in the Civil Engineering field is concerned, is a compressive structural member which is provided in between the footing and column, in order to transfer the load uniformly into the footing.
In early times, the pedestals were not much in use. It is later found out, when the loads are (dead load & live load) are subjected to the column, the column transfers the loads to the footing.
Without a compression member, the loads act in the footing, and not being uniformly spread in the total footing. To reduce this tendency of loads, the pedestal is adopted for the construction of footing & column.
Functions of Pedestal
There are many functions of a pedestal, some of which have been included in the following—
- A concrete pedestal is a compression element provided to carry the loads from supported elements like columns, statues etc. to footing below the ground.
- It is generally provided below the metal columns. In general pedestal, width is greater than its height.
- In Wood or Metal (Steel columns, “I-section” steel column) columns, there are chances that the column section may contact the subsoil, below it.
- If the soil is of inferior quality, soils which are near the water table, sulphate infested soils, there is a chance that the corrosion or decay may occur in the column section.
- If the pedestal is provided, then there are very little chances of contact between the column section and the subsoil earth.
- Sometimes, the thickness of the footing may become very less, after an appropriate design of the footing.
- If a long and thick supporting element is placed on the thin footing, then there are chances of structural failure of the footing (followed by cracks), and column buckling.
- If a pedestal is provided in between the footing and the column, then the height of the column may become short due to the increased elevation of the column, which substantially reduces the tendency of column buckling.
- If the allowable bearing capacity of the soil is low, then a wide footing may be provided with required bearing capacity.
- In such a case, the thickness of the fitting is to be reduced. A pedestal is extremely helpful in such situations, which support the supporting elements over the thin footing, and suitable for construction of thin footing in such soils.
Methods of Construction: Pedestals
The method of construction of pedestal is similar to that of the column, which includes:
- In classical architecture, a pedestal is used as a base to support columns, statues or other ornaments. A classical pedestal may be square, octagonal or circular and is usually made up of three elements:
- Plinth: This is the lowest part of the base of a column or pedestal.
- Die: This is a rectangular block that separates the base from the cap.
- Cap: This is the uppermost element in a pedestal.
- At first, the length, breadth and height of the pedestal is determined.
- Which is then followed by an amount of reinforcement which is to be provided in a pedestal, to safely transfer the loads from column to the footing (design of the pedestal is to be conducted in the first stage)
- After the theoretical determinations, the actual fieldwork begins. As the footing, pedestal, and column at plinth level are cast monolithically.
- After placing the reinforcement cage, the shuttering are fixed (timber or steel shuttering, based on the quality of work required, atmospheric condition & financial implications) adding the dimension of reinforcement placed with adequate cover (40mm most of the case).
- The shuttering of footing, pedestal and column are to be fixed consecutively and precautions are to be taken so that there is a gap between, resulting into leakage of concrete.
- After the shuttering is fixed, the final phase begins which is concreting.
- The concrete is manufactured from the rotating drum mixer (for a larger quantity of work, ready mix concrete, conveyed to the site through transit mixer, and placed at the site through a placing boom), which is then placed at the site.
- After placing the concrete, the concrete is vibrated by vibrating equipment for a certain time, in order to fully compact the concrete and eliminating the air voids.
- The concreting operation is conducted at the footing, then pedestal, then the column, simultaneously in order to result in a monolithic structure.
- After the concreting finishes, it is then left for 24-48 hours for the concrete to complete set.
- After that time, the shuttering are unfixed, and it is cured for 7-14 days (sometimes it is covered with jute mat cloth or damp cloth for curing).
- This follows the completion of the construction of pedestal and it then ready to apply loads.
Types of Pedestal: Advantages and Disadvantages
There are different types of a pedestal, based on the elements it is placed over or below, and the materials of construction. Some of those are—
1. Column Pedestal
A concrete pedestal is a compression element provided to carry the loads from supported elements like columns, statues etc. to footing below the ground.
Column pedestal is placed under column. In a pedestal, the length of the member is less than three times it’s least lateral dimension.
The difference between column and pedestal is that, when the length of the structural member is higher than three times than it’s a least lateral dimension, it is known, column, and when it’s less than three times than it’s a least lateral dimension, it is known, pedestal.
Advantges of Column Pedstal
- Column pedestals are placed under the column, which reduces the height of the column, and reduces the tendency of buckling of the column.
- The pedestal increases the elevation, thus making the column short, which also reduces the amount of reinforcement the column requires to maintain its structural stability.
2. Footing Pedestal
A concrete pedestal is a compression element provided to carry the loads from supported elements like columns, statues etc. to footing below the ground. It is generally provided below the metal columns. In general pedestal, width is greater than its height.
In Footing Pedestal, the pedestal is placed over the footing. The footing and the pedestals are cast monolithically so that there are uniform transfer loads from pedestal to the footing, and then the subsoil.
The length of the footing pedestal is less than three times than it’s least lateral dimensions. Footing pedestals are being adopted widely in modern building construction.
Advantages of Footing Pedestal
- In places where the soil bearing capacity is considerably low, wider footings are required with estimated bearing capacity.
- In such cases, the depth of the footing becomes less, resisting into the thin footing. As long supporting elements are placed over the footing, the footing is subjected to excessive deflection & subsequent cracking. In order to prevent that, pedestals are provided.
- Pedestals prevent the application of heavy concentrated loads directly into the footing, by distributing it evenly over the footing surface.
3. Concrete Footing
The purpose of footings is to support the foundation and prevent settling. A footing is placed below the frost line and then the walls are added on top.
Almost all of the footings are constructed using concrete, the construction procedure of concrete has been mentioned above.
Advantages of Concrete Footing
- Concrete is very strong as a material to be used in the construction of the pedestal. As the main function of the pedestal is to distribute the compressive forces acting on the element over it, to a large base area in the footing, the concrete is the best-suited material for this purpose, being very strong at resisting compressive forces.
- Concrete is very durable, resulting in long-lasting of the pedestal constructed.
Advantages of Pedestal
The upward soil pressure (like you bending eraser) on footing induces moments which is usually maximum at the junction between column and footing.
Providing a pedestal increases the moment-resisting capacity of the footing base slab and also reduces the deflection of the base slab.
Disadvantages of Pedestal
- Since the footing is below ground, it is not a very safe option to continue the steel of the column below the ground into the footing. Steel bars may come in contact with water below and corrode.
- The only disadvantage of the pedestal is that it necessitates additional reinforcement to be fixed and an additional amount of concrete for the construction of the pedestal.
- Thus it increases the cost of the project, which is not very suitable in places where financial implications prevail.
- As the number of advantages of pedestal far exceeds than its disadvantages, it is a very suitable choice in building construction, adopted nowadays.