Column Base - Definition, Types, Design

By Aina Parasher|Updated : August 26th, 2022

Column base is generally placed over concrete blocks When the load supported by these columns is large and the bearing pressure of concrete from below is not sufficient to resist the load. So, in order to place the column load on a concrete block, distribute the column load on a steel base plate to avoid crashing on the base of the column.

The column base is subjected to axial compression loading and Shear force only. Column base is generally used to transfer the load from column to substructures. The column base acts as an intermediate medium between sub-structures and superstructures. In this article, many types of Column bases and their uses are described in a detailed manner.

Table of Content

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.

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

Slab Base

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.

slab base

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

Where,

  • 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,

equation

Where,

  • 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

Gusset Base

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.

gusset base

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

Grillage Foundation

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.

grillage

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FAQs on Column Base

  • Column bases are used as a steel-concrete interface between steel columns and their foundations. Their main function is to transmit the structural stresses to the concrete footing surface and to link the column's bottom end to the footing.

  • Depending upon the load condition and bearing capacity of the soil. The column base is divided into 3 types:

    • Slab base
    • Gusset base
    • Grillage foundation
  • Gusseted base plates are used for columns carrying heavy loads and loads accompanied by moments. In this case, fastenings are used to connect the base plate and the column in the form of vertical gusset plates and gusset angles.

  • A foundation made of two or more two-tier beams superimposed in a concrete layer to distribute the load over a large area refers to as the Grillage Foundation. It is a suitable column base when the load transmitted by a column or wall is heavy and the bearing capacity of the soil is low.

  • Slab bases are used where the columns have independent concrete pedestals and when the column is subjected to only direct loads of less intensity and no bending moment. Along with a thick steel base plate, there are also two cleat angles, which connect the flanges of the column to the base plate.

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