Bearing Pressure Limit Explained

When a base plate is placed on a support, such as a concrete pedestal or foundation, it transfers the load from the column or structure above to the support. The pressure exerted by the base plate on the support is called bearing pressure. It is crucial that this bearing pressure does not exceed the strength of the support material to prevent crushing or failure.

Characteristic Strength ($f_{ck}$)

The characteristic cube strength of concrete or bedding material, denoted as $f_{ck}$, is a standard measure of its compressive strength. It is defined as the strength below which not more than 5% of the test results are expected to fall.

Maximum Allowable Bearing Pressure

Structural design codes and standards specify limits for the maximum bearing pressure allowed between the base plate and the supporting material. This limit is based on the strength of the weaker material, which is typically either the concrete or the bedding material used below the base plate.

According to standard practice in structural design, the maximum bearing pressure should not exceed a certain fraction of the characteristic strength of the supporting material. This fraction is applied to the smaller of the characteristic cube strength of the concrete pedestal/foundation or any bedding material used.

The specified limit for the maximum bearing pressure is commonly taken as 0.6 times the smaller of the characteristic cube strength of concrete or bedding material ($f_{ck}$).

Therefore, the maximum bearing pressure allowed is:

Maximum Bearing Pressure $\le 0.6 \times \text{smaller of } (f_{ck} \text{ of concrete, } f_{ck} \text{ of bedding material})$

This ensures that the support material has adequate capacity to resist the load transferred by the base plate without exceeding its safe bearing strength.