Question

In steel structures, the thickness of the base plate in a column base is determined from the 
1. Flexural strength of plate
2. Shear strength of plate
3. Bearing strength of concrete pedestal
4. Punching criteria

Answer 1

Correct Answer - Option 1 : Flexural strength of plate

Concept-

The thickness of the slab base is dependent upon the flexural strength of the plate.

A column base consists of a column, a base plate, and an anchoring assembly. In general, they are designed with unstiffened base plates, but stiffened base plates may be used where the connection is required to transfer high bending moments.

Size of Base plate:

(1) Find the bearing strength of concrete which is given by = 0.45 f_ck

(2) Therefore, area of base plate required \(= \frac{{{P_u}}}{{0.45\;{f_{ck}}}},\) where P_u is factored load.

(3) Select the size of base plate. For economy, as far as possible keep the projection ‘a’ and ‘b’ equal.

Thickness of Base Plate:

(1) Find the intensity of pressure

\(w = \frac{{{P_u}}}{{Area\;of\;base\;plate}}\)

(2) Minimum thickness required is given by

\({t_s} = {\left[ {\frac{{2.5w\left( {{a^2} - {{0.36}^2}} \right){y_{mo}}}}{{{f_y}}}} \right]^{0.5}} > {t_f}\)

Where

t_s = thickness of the base plate

and t_f = thickness of the flange.

The above formula may be derived by taking μ = 0.3 and using plate theory for finding the bending moment

From the above formula it is clear that the thickness of slab base is dependent upon the flexural strength of the plate.