Bearing Capacity of Shallow Foundations - PowerPoint PPT Presentation

1 / 22
About This Presentation
Title:

Bearing Capacity of Shallow Foundations

Description:

We design for the general shear case (for shallow foundations) ... Foundation shape (cost and labor) Moment loads and eccentricity. Weight of the foundations ... – PowerPoint PPT presentation

Number of Views:3990
Avg rating:4.0/5.0
Slides: 23
Provided by: Val88
Category:

less

Transcript and Presenter's Notes

Title: Bearing Capacity of Shallow Foundations


1
Bearing Capacity of Shallow Foundations
  • Ch. 6.

2
B.C. Failures
3
B.C. Failures
Sand Circular foundations
(Vesic, 1963 and 1973)
4
We design for the general shear case (for shallow
foundations)
5
Bearing Capacity Theory LIMIT EQUILIBRIUM
  • Define the shape of a failure surface
  • Evaluate stresses vs. strengths along this surface

6
Bearing Capacity Theory LIMIT EQUILIBRIUM
Ultimate bearing capacity qult ? (Bearing
press. required to cause a BC failure)
Moments about point A
7
Terzaghis Bearing Capacity Theory
  • Assumptions
  • D lt or B
  • Homogenous and isotropic s c stan(f)
  • level ground
  • rigid foundation
  • full adhesion between soil and base of footing
  • general shear failure develops

8
Terzaghis Bearing Capacity Theory
9
Terzaghis Bearing Capacity Theory
  • Terzaghi developed the theory for continuous
    foundations (simplest, 2D problem).

From model tests, he expanded the theory to
10
Terzaghis Bearing Capacity Theory
Nc cohesion factor Nq surcharge factor N?
self wt factor
fn (f) See table 6.1 for values
11
Groundwater level effects
groundwater
by
  • Reduction in apparent cohesion - cap (sat. soil
    for lab tests)
  • Decrease in s

12
Groundwater level effects
D
13
Groundwater level effects
Case I
14
Groundwater level effects
Case II
15
Groundwater level effects
Case III
16
Groundwater level effects
For total stress analysis
regardless of the case (gw effects are implicit
in cT and fT)
17
FS for BC
Allowable BC qa
FS function of
soil type
structure type
soil variability
extent of site characterization
18
BC of shallow foundations in practice (per Mayne
97)
Undrained
Nc 5.14 for strip footing 6.14 for square
or circular footing
The value of su is taken as the ave. within a
depth to 1B to 1.5B beneath the foundation base
19
BC of shallow foundations in practice (per Mayne
97)
Drained
Ng fn (foundation shape and f)
Consider gw cases (I, II, or III to determine g)
20
BC of shallow foundations in practice (per Mayne
97)
Sands
Perform drained analysis
Clays
Perform both
21
Problem formulation BC design
1. Find B so that FS 3
Get q Get q ult (by BC analysis) Set FS ratio and
solve for B
22
Problem formulation BC design
2. Find B and D so that FS 3
Get q Get q ult (by BC analysis) Set FS ratio and
solve for B
Determine this for various D values
Write a Comment
User Comments (0)
About PowerShow.com