bridge-design of shallow foundations (1)
TRANSCRIPT
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
1/35
LRFD Design of
Shallow Foundations
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
2/35
Nominal Geotechnical
Resistances
ASD Failure Modes
Overal l Stabil i ty Bearing Capacity
Sett lement
Sl id ing
Overturning
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
3/35
Nominal Geotechnical
Resistances LRFD Service Limit State
Overal l Stabil i ty
Vertical (Sett lement) and Horizontal
Movements
LRFD Strength Limit State
Bearing Resistance Sl id ing
Eccentr ic i ty Lim i ts (Overturning)
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
4/35
Stabilize Destabilize
Service Limit State
Global Stability
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
5/35
Global Stability Factor of Safety
Method of Slices
+
WT
WT
WTWT
NN
TT
T T
cc
N tan fN tan f
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
6/35
ASD Factors of Safety
Soil/Rock Parameters and
Ground Water Conditions
Based On:
Slope Supports
Abutment or
Other
Structure?Yes No
In-situ or Laboratory Tests and
Measurements1.5 1.3
No Site-specific Tests 1.8 1.5
Resistance Factors
LRFD
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
7/35
Stability Wrap-Up
Unfactored loads Service Lim it State
Applied stress must be limited Foot ings supported in a slope
0.65 (FS 1.5) Stress criteria for stability can control
footing design
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
8/35
Service Limit State Design
Settlement
Cohesive Soils Evaluate Using Conso l idat ion Theory
Cohesionless Soils Evaluate Using Empir ical or Other Conventional
Methods
Hough Method
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
9/35
Impact on Structures
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
10/35
Settlement of Granular vs.
Cohesive Soils
Relative importance of settlement
components for different soil types Elast ic
Primary Consol idat ion
Secondary Settlement (Creep)
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
11/35
Settlement of Granular vs.
Cohesive Soils
Structural effects of settlement
components
Include Transient Loads if Drained
Loading is Expected and for Computing
Initial Elastic Settlement
Transient Loads May Be Omitted When
Computing Consolidation Settlement of
Cohesive Soils
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
12/35
Hough MethodSettlement of Cohesionless Soils
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
13/35
Stress
BelowFooting
BoussinesqPressure
Isobars
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
14/35
Nominal Bearing Resistance at
Service Limit State
Rn
Bf
For a constant valueof settlement
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
15/35
ML M
B
LB
P
LB
P
LB
eB eL
BL
P
Eccentricity of Footings on Soil
eB= MB/ Pe
L= M
L/ P
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
16/35
ML M
B
LB
eB eL
BL
P
Effective Dimensions for
Footings on Soil
B= B 2eB
L= L 2eL
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
17/35
ML M
B
LB
eB eL
BL
P
q
Applied Stress Beneath Effective
Footing Area
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
18/35
Stress Applied to SoilStrip Footing
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
19/35
Footings on RockTrapezoidal Distribution
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
20/35
Footings on RockTriangular Distribution
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
21/35
Use of Eccentricity and Effective
Footing Dimensions
Service Limit State Nom inal Bearing Resistance Lim ited b y
Sett lement
Strength Limit State Nom inal Bear ing Resistance Lim ited by Bear ing
Resistance
Prevent Overturning Al l App l icable Lim it States
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
22/35
Strength Limit State
Bearing Resistance
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
23/35
Strength Limit State Design
Bearing Resistance
Footings on Soil Evaluate Using Conventional Bearing Theory
Footings on Rock Evaluate Using CSIR Rock Mass Rating Procedu re
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
24/35
1
2 233
dade= C + s tan fSoil Shear Strength
Df
B>Df
B
GroundSurface sv= gDf
PpPp
ccb
aIb
bb
Bearing Resistance Mechanism
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
25/35
Table 10.5.5.2.1-1 Resistance Factors for Geotechnical Resistance of Shallow
Foundations at the Strength Limit State
METHOD/SOIL/CONDITION RESISTANCE FACTOR
Bearing
Resistanceb
Theoretical method (Munfakh, et al. (2001), in clay 0.50
Theoretical method (Munfakh, et al. (2001), in sand,
using CPT 0.50
Theoretical method (Munfakh, et al. (2001), in sand,
using SPT0.45
Semi-empirical methods (Meyerhof), all soils 0.45
Footings on rock 0.45
Plate Load Test 0.55
Sliding
Precast concrete placed on sand 0.90
Cast-in-Place Concrete on sand 0.80
Cast-in-Place or precast Concrete on Clay 0.85
Soil on soil 0.90
epPassive earth pressure component of slidingresistance
0.50
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
26/35
Footings on Rock
Service Limit State use published
presumptive bearing
Published values are al lowable
therefore settlement-limited
Procedures for computing settlement
are available
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
27/35
Very little guidance available for
bearing resistance of rock
Proposed Specification revisionsprovide for evaluating the cohesion and
friction angle of rock using the CSIR
Rock Mass Rating System
Footings on RockStrength Limit State
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
28/35
CSIR Rock Mass Rating System
CSIR Rock Mass Rating developed for
tunnel design
Includes life safety considerations and
therefore, margin of safety
Use of cohesion and friction angle
therefore may be conservative
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
29/35
LRFD vs. ASD
All modes are expressly checked at a
limit state in LRFD
Eccentricity limits replace the
overturning Factor of Safety
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
30/35
Width vs. Resistance - ASD
SettlementcontrolsShear Failurecontrols
Footing width, B (m)
0.0 1.0 2.0 3.0 4.0 5.0
800
Bear
ingPressu
re(kPa)
Allowable Bearing Capacity, FS = 3.0
Bearing Pressure for 25-mm (1in) settlement
600
400
0
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
31/35
Settlement vs. Bearing
Resistance
00
1212
N=30N=30
B, ftB, ft
qqaa,
ksf
,ksf N=25N=25
N=5N=5
N=20N=20
N=15N=15
N=10N=10
22 44 66 1414101088 1212
22
00
44
66
88
1010
00
1212
N=30N=30
B, ftB, ft
qqaa,
ksf
,ksf N=25N=25
N=5N=5
N=20N=20
N=15N=15
N=10N=10
22 44 66 1414101088 1212
22
00
44
66
88
1010
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
32/35
Width vs. Resistance - LRFD
Effective Footing width, B (m)
0 4 8 12 16 20
NominalB
earing
Resistanc
e(ksf)
Strength Limit State
Service Limit State
5
15
25
35
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
33/35
Recommended Practice
For LRFD design of footings on soil
and rock; Size footin gs at the Service Lim it State
Check footing at al l other appl icable Lim it States
Settlement typically controls!
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
34/35
Summary Comparison of ASD
and LRFD for Spread Footings
Same geotechnical theory used to
compute resistances, however
As per Limit State concepts,
presentation of design
recommendations needs to be modified
-
8/13/2019 Bridge-Design of Shallow Foundations (1)
35/35
METHOD/SOIL/CONDITION
RESISTANCE
FACTOR
Bearing
Resistance
f All methods, soil and rock 0.45
Plate Load Test 0.55
Sliding f Precast concrete placedon sand
0.90
Cast-in-Place Concrete on
sand0.80
Clay 0.85Soil on soil 0.90
fep Passive earth pressure
component of sliding
resistance
0.50
Strength Limit State Resistance Factors