What is a Column Base?
The column base transmits the column load to the concrete or masonry foundation blocks. In order to keep the intensity of bearing pressure on the foundation block within the bearing strength, the column base distributes the load over a larger surface.
Column Base Definiton
The column base is the interface between the columns and their foundation. Column base reduces the intensity of loading and distributing it over the foundation. The area of the column base or base plate is chosen so that the intensity of load distributed is less than the bearing capacity of concrete on which the plate resists. It also maintains the proper alignment of columns in a plan.
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Types of Column Base
Based on the stability of the foundation and on the bases in the steel column, column bases are classified on the basis of transmitting direct load and secondary carrying an appreciable bending moment in addition to the direct load. Column base types are:-
- Slab base
- Gusset base
- Grillage foundation
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Slab base is provided to the column subjected to only axial load. A steel plate is used to transfer the load to the concrete pedestal in the case of a slab base. In the slab base, the column is connected to a cleat angle. The critical moment will be at the edge of the column.
Design Criteria of Slab Base
- Depending upon the grade of concrete used, assume suitable allowable bearing pressure on the concrete.
- Area of a base plate (A) = Direct load(P)/Bearing stress of concrete
- The side of the base plate is calculated by,
L = B = √A(∵ In case of square base plate )
- The intensity of pressure is given by,
w = P/A
- σ= Bearing stress of concrete = 0.45 fck
- w = Intensity of pressure
- A = Area of the base plate (in mm2)
The thickness of the slab is calculated by WSM,
- t =Thickness of slab base
- a= Greater projection of plate beyond column
- b= Lesser projection of plate beyond column
- σbs= Permissible bending stress=165 MPa for flanged beam=185 MPa for solid beam
A gusseted base is provided when the heavy load on the column or axial load is accompanied by a bending moment. The column is connected to the base plate through the gusset angle and gusset plate. In this type of base, the load is transferred to the bearing and gusset simultaneously.
Gusset material ( two gusset plates and two gusset angles) is used in the base to increase the bearing area consequently it reduces the thickness of the base as compared to the slab base. The diagram shows how they are connected with each other.
Design Procedure for Gusset Base
The procedure is the same as slab base ( only the difference of thickness of slab base)
- Assume a suitable grade of concrete for calculating the bearing strength of concrete.
- Bearing strength =0.45 fck
(Assume pressure distribution uniformly distributed underneath)
Total thickness of plate, t = c√(2.75w/fy)(as per LSM, IS 800: 2007)
Total thickness of plate, t = c√(3w/σbs)(as per WSM, IS 800: 1984)
Where, Thickness of base plate, tb = t - tgusset angle(bolted connection)
The thickness of the base plate, tb = t (welded connection)
c= portion of base plate acting as a cantilever
w=Bearing pressure = P/A
When a column carries heavy axial loads and the bearing capacity of soil is very low. It consists of more tiers of steel beams placed one over another at a right angle to each other. It is embedded with a cover of 100 mm. The given figure shows how it is connected with each other. It is designed for critical bending moment section at the centre of the base plate, Shear force at the edge of the base plate and web crippling.
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