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8/20/2019 Use of Limit State Design in Foundation Engineering
http://slidepdf.com/reader/full/use-of-limit-state-design-in-foundation-engineering 1/60
Engineers Australia
The Civil and Structural Engineering Panel
Use of Limit State Design in Foundation
Engineering
(Patrick Wong 17 April 2012)
8/20/2019 Use of Limit State Design in Foundation Engineering
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Presentation Outline
1. Working Stress Design Method
2. Limit State Design Method
3. Comparison between the two methods
.
5. Conclusions
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Working Stress Design Method
• “allowable” bearing pressure for footings
“ ” “ ”
Structural Engineers often ask the Geotechnical
Engineers to provide:
bearing pressure for piles
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But what does “Allowable” mean? Is it:
• Ultimate Capacity / Factor of Safety (FOS) ?
• FOS = Ultimate Capacity / Working Load
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But what is the appropriate FOS (2, 2.5, 3)?
Rug
PileFoundation
Load
ShallowFootingLoad
Pw
Deflection
ug
Pw
Deflection
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Choice of FOS is also affected by uncertainty inmaterial properties
u e n c y
Concrete
Legend
F r e
Material Strength
Soil
Mean strength
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Working Stress Design Summary:
• “Allowable” load = Ultimate Capacity / FOS
• FOS should be dependent on:
– tolerable deformation
– foundation stiffness (linear or non-linear?)
– uncertainties in material properties/behaviour
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But these factors are rarely considered adequately in
Working Stress Design
RockRock Strength
Allowable End
Bearing
Allowable
Shaft
Example of “Allowable” Design Parameters given in a Recent SiteInvestigation Report (settlement <1% footing width)
(kPa) (kPa)
IV Shale Extremely low to verylow 800 80
III Shale Medium strength 3000 300
II Shale High strength 8000 800
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Why Limit State Design?
1960’s identified two specific classes of problem notadequately addressed by working stress designs:
• Failures under extreme low probability events
• -
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Failures under low probability events
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Failures under low probability events
DESIGN WIND LOAD
Self weight less overtuning
= net compression
frictional resistance
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Failures under low probability events
DESIGN WIND LOAD EXTREME WIND LOAD BUCKLE AND COLLAPSE
Self weight less overtuning Self weight less overtuning
= net compression = net uplift SLIDING
frictional resistance
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Redundancy and indeterminate structures
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Foundation Engineering
Definition of the Ultimate Limit State (ULS)
Definition in Austroads 1992 for foundations:a. a limit state at which a mechanism is formed in the
ground
. rupture of a critical section of the structure due to
movements in the ground
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Definition of the Serviceability Limit State (SLS)
• Similar to working stress loads BUT strength of thestructure is not directly considered.
• Acceptance criteria relate to the structure being able to,
crack widths, appearance etc.
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The Ultimate Limit State Inequality Formula
ULS Loads≤
Design Geotech ResistanceS* =ΨS ≤ ΦgRug = Rug*
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The Ultimate Limit State Inequality Formula
S* =ΨS ≤ ΦgRug = Rug*
Load factortypically >1
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The Ultimate Limit State Inequality Formula
S* =ΨS ≤ ΦgRug = Rug*
(Resistance)Strength factor
typically <1
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The Ultimate Limit State Inequality Formula
S* =ΨS ≤ ΦgRug = Rug*
• Assumes probability approximates to a constant factor
• Assumes loads and resistance are independent
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Idealised Probability Distribution
Idalised Probability Distribution using MEAN Values
0.6
0.8
1.0
1.2
1.4
b i l i t y D e n s i t y
load resistanceFactor of Safety ~ 3.0
Stren th Factor ~ 0.6
Load
Factor ~ 1.8
0.0
0.2
0.4
0 1 2 3 4 5
Action/Resistance
P r o b
probability
of failure
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More Realistic Probability Distribution"Realistic" Probability distributions of a simple loading including 5% characteristic values
0.8
1.0
1.2
1.4
1.6
1.8
b i l i t y D e n s i t y
load positive resistance negative resistance
load
factor
(LF)
~ 1.8
LFSF
5%strength factor
(SF) ~ 0.6
5%
5% 5%
0.0
0.2
0.4
0.6
-3 -2 -1 0 1 2 3 4
Action/Resistance
P r o
b a
~ 1.8~ 0.6
Higher probability forsmaller loads
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Confusions In Terminology
Structural Engineer:
“ULTIMATE LOAD”
(= ULS Load = S*)
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Confusions In Terminology
Structural Engineer:
“ULTIMATE LOAD”
(= ULS Load = S*)
Geotechnical Engineer:
“ULTIMATE CAPACITY”
(= Rug)
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Confusions In Terminology
Structural Engineer:
“ULTIMATE LOAD”
(= ULS Load = S*)
Geotechnical Engineer:
“ULTIMATE CAPACITY”
(= Rug)
S* ≤ Rug* = Φg.Rug
=> Out by a factor of
Φ = 0.4 to 0.8?
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Confusions In Terminology
Geotechnical Engineer:
“ALLOWABLE LOAD”
(= Rug /FOS)
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Confusions In Terminology
Geotechnical Engineer:
“ALLOWABLE LOAD”
(= Rug /FOS)
Structural Engineer:
“???”
(S or S* ?)
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Confusions In Terminology
Geotechnical Engineer:
“ALLOWABLE LOAD”
(= Rug /FOS)
Structural Engineer:
“???”
(S or S* ?)
Ψ.S = S* ≤ Rug* = Φg.Rug
⇒Out by a factor of
Ψ = 1.2 to 1.5?
8/20/2019 Use of Limit State Design in Foundation Engineering
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Quick Recap:
Limit State Design Requirements
Part 1 - Strength Limit State
R * ≥ S*
ΦgRug ≥ ΨS
Failure mechanism does notform due to deflections
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The Limit State Design Requirements
Part 2 - Serviceability Limit State
n r h rvi ili l in h r l in l i n
does not exceed the tolerable limit.
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• Shallow Square Pad Footing Problem on a Stiff Clay
S and S*
Comparison with Working Stress
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Comparison with Working Stress
Working Stress:Rug / FoS ≥ S
e.g. Rug / 3.0 ≥ S for a shallow footing
Limit State:
* *ug
ΦgRug ≥ ΨSRug / (Ψ/Φg) ≥ S
For typical Ψ = 1.3 and Φg = 0.7, (Ψ/Φg) = 1.86
Rug / 1.86 ≥ SBut the design is not quite finished !
For the this example:
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Serviceability Limit State
Rug
Shallow Square
Pad FootingLoad
For the this example:• S = 1200kN• S* = 1.3 x 1200 = 1560kN
• Rug = 600kPa
Working stress:• Rug/3 = 200kPa
• B = 2.45m• E at working stress = 20MPa• Settlement = 22mm
Ss
Deflection
Limit State:
• Rug* = Rug x 0.7 = 420kPa• B = 1.95m• Serviceability stress = 316kPa• E at this stress level = 16MPa
• Settlement = 35mm (may needto increase footing width ifsettlement is unacceptable)
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In both methods, likely settlement shouldbe assessed but rarely done in Working
Stress Design
Deformation is a Key Design Issue
Why?
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Case Study – Barangaroo South (the site)
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Case Study – Barangaroo South (site plan)
~ 4 ha
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Case Study – Barangaroo South (geotech conditions)
C S d B S h (R k L l )
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Case Study – Barangaroo South (Rock Levels)(top of rock RL0m to -28mAHD)
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1. Geotechnical investigation commissioned on a
competitive basis
2. Standard brief requested the provision of “designparameters”
Case Study – Barangaroo South (design parameters)
3. Report provided geotechnical design parametersbased on well known guide used in Sydney (Pells et
al, 1998)
4. Both “Limit State” and “Serviceability” design valuesprovided in geotechnical investigation report
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Case Study – Barangaroo South (design parameters)
Sand-stone
RockClass
IV
III
II
Note FOS ranges
from 3 to 10
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Case Study – Barangaroo South (Pile Design Loads)
Building
C4
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Case Study – Barangaroo South (Pile Design Loads)
Building C4 – Typical Loads & Settlement Limits
Pile Diameter
(m)ULS (MN) SLS (MN)
Pile ToeSettlement
Limit at SLS(mm)
1 9.4 to 17.5 7.2 to 14.4 3
1.18 20.5 to 28 16.7 to 23 3.5
1.5 26.9 to 32 24.2 to 25.9 4.5
2.4 90.5 to101.4 72.6 to 81.4 7.2
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1. Preliminary design using “allowable” design valuesresulted in long socket lengths (e.g.10m in Class II for2.4m diameter piles)
2. Detailed design using Limit State Design method, withrock socket settlement performance analysed using
numerical methods:
Case Study – Barangaroo South (Design)
• Base alone would achieve adequate “Strength” for theULS load
• Design was governed by “serviceability” limits
• Rock socket required to provide sufficient stiffness
• Rock socket lengths reduced to 50% to 60% of thepreliminary design lengths
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Case Study – Barangaroo South (Design)
Why is there adifference?
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Case Study – Barangaroo South (Design Example)
C S d B S h (D i E l )
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Case Study – Barangaroo South (Design Example)
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1. Rough side walls and clean pile bore baseconsidered to be vital to achieve design stiffness andcapacity
Case Study – Barangaroo South (Construction)
3. Independent site monitoring by geotechnical staff
4. Pile load testing to validate design assumptions
C St d B S th (L d T ti )
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Case Study – Barangaroo South (Load Testing)
Prototype Pile Load
Testing using O-Cells
2 x 0.75m dia pilesdesigned to be loadedto 17.4MN
bi-directionally (i.e.total 34.8MN)
Case Study Barangaroo South (test pile BAR 19)
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Case Study – Barangaroo South (test pile BAR-19)
Test piledesigned tomobilise ~75%
of estimatedultimate shaft
,
~50% ofestimatedultimate baseresistance
Case Study Barangaroo South (great project for big toys)
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Case Study – Barangaroo South (great project for big toys)
Case Study Barangaroo South (rock auger tool)
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Case Study – Barangaroo South (rock auger tool)
Case Study Barangaroo South (boring bucket)
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Case Study – Barangaroo South (boring bucket)
Case Study – Barangaroo South (boring bucket)
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Case Study – Barangaroo South (boring bucket)
Case Study – Barangaroo South (cleaning bucket)
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Case Study – Barangaroo South (cleaning bucket)
Case Study Barangaroo South (Happy snaps with O Cell)
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Case Study – Barangaroo South (Happy snaps with O-Cell)
Case Study – Barangaroo South (O-Cell being lowered)
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Case Study Barangaroo South (O Cell being lowered)
Case Study – Barangaroo South (O-Cell installed)
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Case Study Barangaroo South (O Cell installed)
Case Study – Barangaroo South (O-Cell Test Results)
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Final load was 26MN
(i.e. 52MN total) at65mm shaft deflectionbut O-Cell not calibratedbeyond 17.4MN
Mobilised values in
• End bearingpressure > 59MPa
• Shaft friction >2,000kPa, but non-
linear beyond1,200kPa
Conclusions
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1. The design principles for both Working Stress andLimit State methods are similar, if done properly ….
2. Limit State Method has the following advantages:
• Partial factors for different levels of uncertainties
• Designers are “reminded”, or “forced” to consider“ ”
“Strength”, and hopefully also “Uncertainties”• Ability to optimise design based on both strength
and serviceability criteria rather than lumping
these together using an arbitrary FOS
Conclusions (Con’t)
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3. Standard geotechnical investigation and reporting
commissioned under competitive biddingarrangement is more likely to result in “conventional”or “ordinary” results
4. Better results can be achieved by collaborativeapproach between the structural and geotechnical
foundation assessment, validation testing, andconstruction monitoring.
5. “I am not advocating reducing safety factors - I am
advocating replacing arbitrary safety factors ”
Prof. Malcolm Bolton, Cambridge University - 52nd Rankine Lecture 2012
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Clear communication and closeworking relationship betweengeotechnical and structural
engineers is essential
Acknowledgement