Engineering Concepts

House Plans, Excels, E-books & pdfs

Calculation of Foundation Bearing Capacity as per IS 6403 – 1981

Posted by:

RC

|

On:

|

,
, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,

The IS 6403:1981 standard provides guidelines for calculating the bearing capacity of shallow foundations, ensuring safety and stability of structures. The standard defines methods to determine ultimate bearing capacity (qᵤ), safe bearing capacity (qₛ), and allowable bearing pressure based on Terzaghi’s and Meyerhof’s theories.

1. Types of Bearing Capacity

  • Ultimate Bearing Capacity (qᵤ): Maximum pressure soil can withstand before failure.
  • Safe Bearing Capacity (qₛ): The ultimate bearing capacity divided by a factor of safety (FOS = 2.5 – 3.0).
  • Allowable Bearing Pressure (qₐ): Safe bearing capacity considering settlement criteria.
  • Net Bearing Capacity (qₙ): Ultimate bearing capacity minus overburden pressure.

2. Bearing Capacity Formula as per IS 6403:1981

The ultimate bearing capacity (qᵤ) for strip footings is given by:

qu= cNc + γDfNq +0.5γBNγ ​

For square or circular footings:

qu= 1.3cNc + γDfNq + 0.4γBNγ

Where:

  • qᵤ = Ultimate bearing capacity (kN/m²)
  • c = Cohesion of soil (kN/m²)
  • γ = Unit weight of soil (kN/m³)
  • D_f = Depth of foundation (m)
  • B = Width of foundation (m)
  • N_c, N_q, N_γ = Bearing capacity factors (depend on soil’s angle of internal friction, φ)

Factors from IS 6403 for Different Soil Types

φ (°)N_cN_qN_γ
05.141.000.00
108.341.780.59
2017.76.405.38
3030.118.422.4
4092.081.381.8

For cohesion less soils (sand, gravel), c = 0.

3. Calculation Steps

Example: Find Safe Bearing Capacity for a Foundation

Given Data:

  • Soil Type: Dense Sand
  • γ = 18 kN/m³
  • c = 0 (for sand)
  • φ = 30°
  • D_f = 1.5m
  • B = 2.0m
  • Factor of Safety (FOS) = 3.0

Step 1: Find Bearing Capacity Factors (From IS 6403 Table)

For φ = 30°:

  • N_c = 30.1, N_q = 18.4, N_γ = 22.4

Step 2: Calculate Ultimate Bearing Capacity (qᵤ)

qu= (0×30.1) + (18×1.5×18.4) + (0.5×18×2.0×22.4)

qu​=0+496.8+403.2=900 kN/m²

Step 3: Calculate Safe Bearing Capacity (qₛ)

qs= qu / FOS

= 900 / 3

= 300 kN/m²

Final Answer:

Safe Bearing Capacity (qₛ) = 300 kN/m²

4. Adjustments Based on Soil and Conditions

For Cohesive Soils (Clay, Silt)

  • Use undrained shear strength to determine cohesion (c).
  • Typically, N_q = 1 and N_γ = 0.

For Seismic Conditions

  • Apply seismic correction factors to adjust N_c, N_q, N_γ.
  • Follow IS 1893:2016 guidelines.

For Deep Foundations

  • Consider pile foundations, use Meyerhof’s or Vesic’s method.

5. Alternative Methods for Verification

  1. Plate Load Test (IS 1888:1982) – Field test to directly measure soil bearing capacity.
  2. Standard Penetration Test (SPT – IS 2131:1981) – Uses N-values to estimate bearing capacity.
  3. Geotechnical Reports – Provides precise soil strength data.

6. Conclusion

  • IS 6403:1981 provides empirical methods for calculating bearing capacity of shallow foundations.
  • The Terzaghi equation is widely used but requires adjustments for soil type and loading conditions.
  • Field tests (SPT, Plate Load Test) are recommended for accurate site-specific data.
Click Here To Download Calculation of Soil Bearing Capacity as per IS 6403:1981 Excel Spreadsheet
BEARING CAPACITY IS 6403Download
Related Posts:

Fencing with Angle Post and Pipe Post Details & Arrangements

Church Design Drawing

Effective Length for RCC Columns

BEHAVIOUR OF STEEL CHIMNEY UNDER DYNAMIC LOADINGS

Deep Excavations

Laboratory Building Plan and Architecture Details

Apartment House Plan | West Facing 60 x 60 |

Moody Chart | Moment Reactions for Rectangular Plates |

Guard House Layout and Details

SHEAR FORCE AND BENDING MOMENT DIAGRAMS WITH FORMULA

Posted by

RC

in

,
Almost there!
We are confirming your order with
Taking you back to the merchant website shortly.