Design of Shallow Foundation (Isolated Footings)

Design of Shallow Foundations (isolated footing)

AIM:
To design a square footing for a column size of 400x400 for given constraints and do the following check:
• If the gross bearing pressure under the footing is within safe bearing capacity Provide sufficient depth and longitudinal steel for resisting bending and shear
• Sufficient anchorage for column steel into the footing
• If the bearing pressure at the column-footing joint is within 0.45 times the maximum concrete compressive strength.
Also, to provide a sketch showing the details of geometric and structural features of footing.

INTRODUCTION:
• Footing is a part of the foundation which is constructed with concrete or brickwork masonry and acts as a base for the floor columns and floor walls.
• The main function of footing is to transfer the vertical loads directly to the soil.
• The four main types of footing are strip/wall footing, spread footing, combined footing, and raft footing.
• A wall footing or strip footing is a continuous strip of concrete that serves to spread the weight of a load-bearing wall across an area of soil.
• Spread footing is a pad that distributes the column load in 2 directions to an area of soil around the column. It can be stepped, tapered, etc.
• Combined footing is provided when two columns are closely placed or when the column is at the property line.
• Raft footing is provided for very weak soil or when the area of the overall column to the area of the site footprint exceeds 50%.
• Isolated footing is an individual footing that transfers the load to the underground soil. It is provided when a single column is to be provided.

TASK -

Design a square footing for a column size of 400x400. The compression axial load for the load combination of (1.5 DL + 1.5 LL) is 2000 KN. The safe soil bearing capacity is 150 KN/m2 at a depth of 2 meters below E.G.L. Participants are free to go for either a tapered or stepped footing. Besides the total axial load, also account for the self-weight of the footing and soil above it. Assume M25 concrete grade. Do the following checks:

• If the gross bearing pressure under the footing is within the safe bearing capacity
• Provide sufficient depth and longitudinal steel for resisting bending and shear
• Sufficient anchorage for column steel into the footing
• If the bearing pressure at the column-footing joint is within 0.45 times the maximum concrete compressive strength

Provide a simple sketch showing the size of the footing, maximum and minimum depth of the section, flexural reinforcement in both directions, and anchorage of column steel.

PROCEDURE:

• Data are given:

• Column size 400mmx400mm

• Compression axial load for the load combination of (1.5 DL + 1.5 LL) is 2000 KN

• Safe soil bearing capacity at a depth of 2 meters below E.G.L is 150 kN/m²

• Assuming the self-weight of the footing and soil above it as a 10% load

• Grade of concrete=25 kN/m²

• Assuming fy=415 N/mm²

Here a sloped footing is designed.

• Gross bearing capacity

• Assuming the unit weight of the soil to be 18kN/m³

• Load on the footing= 1333.3kN

• Weight of the footing,g = 3.5 X 3.5 X 25 X 0.8 = 245KN

• weight of the soil= 3.5 X 3.5 X (2 - 0.8) X 18 = 261.6kN

• Total load= 1852kN
  
  
• Gross bearing capacity= 1852/3.5 X 3.5 =257KN/m2
  
  
• As 257 kN/m² >150kN/m², hence we need to increase the size of the footing to 3.8m X 3.8m

• Now total load=1934KN

• gross bearing capacity=1934/3.8*3.8=133.9KN/mm2

• 133.9 kN/m² <150kN/m², hence safe

• Corrected net soil pressure = 2000/3.8*3.8= 138.5kn
  
  

Reinforcement Bars Placement Plan

Section of the Footing

RESULT:

Sloped isolated footing for the square column is designed for the given conditions and also checked.