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.
Functions of Concrete Pedestal
The main functions of pedestal provision are as follows.
To avoid contact between soil and metal elements or wood elements.
To offer support for elements at some elevation
To allow thinner foundation footings.
1. To Avoid Contact Between Soil and Metal Elements or Wood Elements
Metal columns or wood columns may easily be effected by the moisture conditions of soil. Corrosion or weathering may occur. To resist corrosion and weathering pedestal is provided just above the footing on which these metal or wood elements are constructed above the ground surface. Hence the column durability increases.
2. To Offer Support for Elements at Some Elevation
In some cases, the depth of designed footing may be very less. Then we have to provide long supporting element above the footing, to avoid this we can provide pedestal in between them to offer some elevation. Because of this the supported elements are provided in normal designed height and they support the structure without any buckling.
3. To Allow Thinner Foundation Footings
When the allowable bearing pressure of soil is very low, wide footing is to be provided with required bearing area. But, the depth of footing is to be reduced. If pedestal is provided on the footing it is not a problem with thinner footing. It permits the way to provide thinner footings in such type of soils.
Construction of Concrete Pedestal
As discussed above, pedestals are provided above the footings. For small load carrying columns, the pedestals are designed without any reinforcement and with height less than three times its width. Generally hollow concrete blocks are used and the hollow place is filled with concrete. But this is only limited to small load ranges.
But in the case of large load columns, construction and design of pedestals is same as reinforced columns for major loads. Reinforcement is also provided for the pedestals but the amount of reinforcement is 0.15% of its cross sectional area. Similarly if the pedestal height is more than three times of its width then also, vertical reinforcement, ties and dowels should be provided. The mortar used for pedestal is same as mortar used for column.
Theoretically, it is possible to provide the reinforcement in the same manner as footing but it is not practical. The area of contact between pedestal and supported column should be more and pedestal should have equal height as its width to avoid bending and shear movements.
The allowable bearing stress depends upon the ratio of column contact are to pedestal gross area. The ratios of contact area to pedestal area in different conditions are shown in below graph.
If the pedestal height is more than two times of its thickness then we can provide the vertical reinforcement in the corners of pedestal. By the provision of vertical reinforcement, the pedestal can withstand against temperature changes, shrinkage etc. It also protects pedestal during the construction of column above it.