lrfd lecture for the egyptian code (handouts)

Housing & Building National Research Centre (HBRC)May 18, 2011

By

Dr. Sherif S. AbdelSalamLecturer – British University in Egypt

Members of Committee # 4

Prof. Dr. Amr Darrag

Eng. Ashraf Wahby

Prof. Dr. Gehan El-Sayed

Dr. Sherif AbdelSalam

Dr. Tarek Thabet

Prof. Dr. Fatma Baligh

Prof. Dr. Fathalla El-Nahhas

Dr. Mohamed El-Nabarawi

Prof. Dr. Nadia Shenouda

Prof. Dr. Nagwa El-Sakhawi

Prof. Dr. Yasser El-Mossallamy

2

LRFD Philosophy – Current Practices –Factors Development – Construction Control –Design Example – Conclusions & Recommendations

Outline

LRFD Philosophy and Advantages

Current International Practices

Resistance Factors Development

Construction Control Aspects

Pile Design Example

Conclusions and Recommendations

Pile Design Approaches

Definition of LRFD

Advantages of LRFD

Typical R.F. Values

WSD vs. LRFD

3

Pile Design ApproachesWorking Stress Design (WSD or ASD)

FS based on experience and subjective judgment

Ignores various sources and levels of uncertainties

Variation of soil properties, behavior, and GWT

Capacity and performance of deep foundations

Therefore, highly conservative

No consistent degree of reliability

In some cases leads to unsafe designs

𝐿 =𝑅

𝐹𝑆

L: LoadsR: ResistancesFS: Factor of Safety

4LRFD Philosophy – Current Practices –Factors Development – Construction Control –Design Example – Conclusions & Recommendations

Load and Resistance Factors Design (LRFD)

Uncertainties are quantified using probability-theory

Overlap area between loads and resistances is failure

Depends on: 1)mean; 2) st. dev.; and 3) best-curve-fit

Pile Design Approaches cont’d

𝛾𝑖 𝑥 𝑄𝑖 ≤ 𝑅 𝑥 𝜑


Qi : Loadsγi : Load Factors

R: Resistanceϕ: Resistance Factor

Definition of LRFD

LRFD quantifies various uncertainties using statistics Achieves designs with a chosen level of reliability Loads x L.F. > 1.0 and Capacities x R.F. < 1.0 Failure is when factored loads exceed factored capacities To avoid failure, LRFD control the overlap area The overlap area is limited to an acceptable level


7

Constant and controllable degree of reliability

Consistent design for the entire structure

Improves the construction control process

Reliabilities higher than WSD approach

Higher efficiency and cost effectiveness

No assumptions are needed

No experience and engineering judgment

Easy to use for design engineers

Advantages of LRFD


Allows for Regional Calibration to be more cost effective

Soil Type Static Analysis Methodβ=2.33 β=3.00

φ φ/λ φ φ/λ

SandSPT-Meyerhof 0.42 0.25 0.27 0.16

β-Method 0.32 0.37 0.23 0.27

Nordlund 0.31 0.34 0.21 0.24

ClaySPT-Meyerhof 0.53 0.29 0.35 0.19

α-API Method 0.40 0.35 0.28 0.24

β-Method 0.35 0.33 0.24 0.23

Mixed

SPT-Meyerhof 0.42 0.24 0.27 0.16

α-API Method 0.32 0.27 0.21 0.18

β-Method 0.34 0.35 0.24 0.24

Nordlund 0.34 0.28 0.22 0.18

Typical R.F. Values


Failure

Region, Pf

βσβ : Reliability Index

σ : Standard Deviation of PDF

Pf : Probability of Failure

Resistance factors for H-piles in different soils using various static methods

Determining the reliability index = probability of failure Depending on:1) Pile group redundancy2) Importance of structure3) Allowable settlement4) Budget

2.33

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Probability of Failure %

Eff

icie

mcy

Fac

tor

(ϕ/λ

)

Eff

icie

mcy

Fac

tor

(ϕ/λ

)

Reliability Index (β)

BlueBook

SPT-Meyerhof

α-API Method

β-Method

Nordlund

2.33

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Probability of Failure %

Res

ista

nce

Fac

tor

(ϕ)

Res

ista

nce

Fac

tor

(ϕ)

Reliability Index (β)

BlueBookSPT-Meyerhofα-API Methodβ-MethodNordlund

100 70 10 1 0.1 0.01 0.001 <0.001

100 70 10 1 0.1 0.01 0.001 <0.001

Design charts with wider range of β (or probability of failure)


Typical R.F. Values cont’d

sand

clay

WSD vs. LRFD

4.54.03.53.02.52.01.51.00.50.0

3.0

2.5

2.0

1.5

1.0

0.5

0.0

K = Davisson/BlueBook

Fre

qu

ency

1.116 0.2826 5

1.395 0.3533 5

1.786 0.4522 5

Mean StDev N

K1 (Nominal)

K2 (LRFD)

K3 (WSD)

Variable

K = SLT / SPT Meyerhof

130

201

313

244

216

92 90

123

96

125 132

43

67

104

8172

0

50

100

150

200

250

300

350

SPT-Meyerhof α-method β-method Nordlund-method BlueBook SLT

Ca

pa

cit

y (

kip

s)

Nominal Capacity

Factored Capacity

Allowable Capacity

LRFD is more consistent among different static methodsLRFD is more efficient, cost effective

Reliability is known and constant



Outline





Pile Design Example


11

Deep Foundations Practices

Pile Analysis Methods

LRFD Implementation

Regionally Calibrated R.F.

`

MT

WY

ID

WA

OR

NV UT

CA

AZ

ND

SD

NE

CO

NM

TX

OK

KS

AR

LA

MO

IA

MN

WI

IL

IN

KY

TN

MS AL GA

FL

SC

NC

VA

WV

OH

MI

NY

PA

MD

DE

NJ

CTRI

MA

ME

VTNH

AK

HI

150 m

DS, CIDH,

dia. 1.5 to 3 m

Nord/CPT

15 to 60 m

DP, OEP,

dia. 30 to 50 cm

α/β/Nord

> 45 m

DP, OEP,

dia. 1 to 1.5 m

α/β/Nord

> 15 m

DS, CIDH,

dia. 10 to 30 cm

α/β/Nord/SPT

DP, HP,

α/λ/Nord

DP, OEP,

dia. 40 to 60 cm

10 m

DP, HP,

310x79 mm

α/β/λ/Nord/SPT

Deep

DP and DS

β-method 15 m

DS, CIDH,

SPT

Deep

DP, HP,

360x132 mm

β/SPT

Deep

DS, CIDH,

dia. 1.2 to 3 m

60 m

DP, HP,

360x152 mm

DP, HP,

all sizes

< 15 m

DS, CIDH,

No response

No response

No response No response

No response

No response

Nord/SPT/ In-house

> 25 m

DS, CIDH,

dia. 1 to 1.5 m

α/SPT

Deep

DP, PSCP,

dia. 40 to 60 cm

α/Nord

No response

20 m

DP, HP,

250x63;

250x85mm

In-house

No response

60 m

DP, HP,

250x63;

360x109 mm

α/Nord

20 m

DP, HP,

250x63,

360x132 mm

In-house

No response

No response

30 m

DP, PSCP,

dia. 0.5 to 1.5 m

In-house

10 m

DP, HP,

310x79;

360x174 mm

PSCP, 30 to 40 cm

Nord/SPT

> 30 m

DP, PSCP,

30 to 60 cm

< 20 m

DP, HP,

310x79;

360x132 mm

SPT

5 to 60 m

DP, HP,

α/β/Nord

< 15 m

DP, HP,

250x85;

310x110 mm

Nord/SPT

< 15 m

DP, HP,

310x79 mm

Nord

< 20 m

DP, CEP,

dia. 30 cm

α/Nord

< 15 m

DP, DS,

Nord/SPT/CPT

No response

< 10 m

DP, HP,

250x85; 310x110 mm

α/Nord

Each State contains the following information (if available):

1) Depth to bedrock

2) Used pile categories and types (see map key)

3) Used pile sizes (see map key)

4) Used static analysis methods (see map key)

Map Key

DP = Driven Piles

DS = Drilled Shafts

HP = Steel H-piles

OEP = Open End Pipe piles

CEP = Closed End Pipe piles

CIDH = Cast In Drilled Hole piles

No response

No response

No response

No response

No response

No response

Glacial

Alluvium

Coastal Plain

Other soil types

Soil RegionsPSCP = Prestressed Concrete Piles

α = α-method

β = β -method

λ = λ-method

Nord = Nordlund method

SPT = SPT-methods

CPT = CPT-method

Deep Foundations Practices

East Coast

Soil in mainly composed of coastal plain and glacial tills

Pile types: steel H-piles and precast/prestressed concrete

West Coast

Soil is mainly composed of alluvium deposits

Pile types: open-end pipe piles

Specific areas use the CIDH shafts due to the seismic requirements

Midwest

Soil is mainly composed of glacial tills

Pile types: steel H-piles


0%

20%

40%

60%

80%

100%

6% 9% 11%

32%40%

45%

63%

0%

20%

40%

60%

80%

100%

OtherPDA

CAPWAPWEAP

16%

74% 74%

100%

0%

20%

40%

60%

80%

100%

14% 21%

43%57%

76%

18%

6%

Driven piles

Drilled shafts

Both

Pile Analysis MethodsStatic Methods

Dynamic Methods Dynamic Formulas


LRFD Implementation

MT

WY

ID

WA

OR

NV

UT

CA

AZ

ND

SD

NE

CO

NM

TX

OK

KS

AR

LA

MO

IA

MN

WI

IL IN

KY

TN

MS AL GA

FL

SC

NC

VAWV

OH

MI

NY

PA

MDDE

NJCT

RI

MA

ME

VTNH

AK

HIImplemented LRFDIn transition from WSD to LRFDCurrently using WSD

50%

40%

10%

Implemented LRFD

In transition from ASD to LRFD

Still Using ASD


15

Soil type

Pile type

Sand Clay Mixed soil

N ϕSt.

Dev.N ϕ

St.

Dev.N ϕ

St.

Dev.

Steel H-pile 11 0.45 0.11 12 0.48 0.15 8 0.55 0.13

CIDH 4 0.4 0.23 3 0.6 0.28 3 0.5 0.13

Open end pipe 2 0.65 0.04 2 0.67 0.04 2 0.67 0.04

Regionally Calibrated R.F.According to Soil-Pile (Shaft) types

Soil types

Static Analysis

Method

Sand Clay Mixed soil

N ϕ St. Dev. N ϕSt.

Dev.N ϕ St. Dev.

Nordlund 11 0.5 0.12 N/A N/A N/A 4 0.53 0.17

SPT method 3 0.45 0 N/A N/A N/A 3 0.53 0.11

α-method N/A N/A N/A 6 0.47 0.19 1 0.7 0

β-method 1 0.65 0 4 0.49 0.13 2 0.37 0.11

CPT method N/A N/A N/A 3 0.45 0.17 N/A N/A N/A

According to Soil-Static Analysis Methods



Outline





Pile Design Example


16

Calibration Requirements

Database Construction

LRFD Reliability-based Calibration

FOSM Equation

Verification and Comparison

Calibration Requirements

Adequate Database Contains large number of pile Static Load Tests (SLT) conducted in

Egypt and corresponding soil information.

Data Collection Survey questionnaire for code users (design engineers and consulting

firms) to stand on the current practices and future needs.

Calibration Framework To determine the most appropriate LRFD calibration framework for

Egyptian practice.

Full-Scale Testing Various pile SLTs accompanied by field and laboratory soil tests that

cover all possible soil types in the Egypt.


Database Construction

Large number of usable pile SLTs with sufficient soil data Alex.

20

Other20

Port-said15

Aswan10

Cairo100

Sand100

Clay40

Mixed25


Database Construction cont’d

CIDH (Bored Piles)

CFA

Timber

Steel piles

The database should also include information for different pile types


LRFD Reliability-based Calibration

Establish pile capacity from SLTs (QDavisson)

Calculate the pile capacity using Static methods (QAnalytical)

Calculate bias ratio λR = K = QDavisson/QAnalytical

Do the same for the entire database

Draw the PDFs

Calculate mean, st. dev., and COV

Assume probability of failure (ex. pf = 1%)

Calculate the resistance factor


FOSM EquationResistance factor

Dead load factor

Live load factor

Bias for dead loads

Bias for live loads

Bias for resistance

Dead load to live load ratio

Reliability index

Coefficient of variation for resistance

Coefficient of variation for dead loads

Coefficient of variation for live loads

3.02.52.01.51.00.50.0

3.02.52.01.51.00.50.0

12

10

8

6

4

2

0

12

10

8

6

4

2

0

Ksx = Davisson / BlueBook

Fre

qu

ency

Loc 0.1157

Scale 0.4180

N 35Lognormal

3.02.52.01.51.00.50.0

3.02.52.01.51.00.50.0

12

10

8

6

4

2

0

12

10

8

6

4

2

0

Ksx = Davisson / BlueBook

Fre

qu

ency

1

Loc 0.1284

Scale 0.4268

N 32

Lognormal

Sand

Different groups (PDFs):1) Pile type2) Soil type3) Pile design method (Static analysis methods)

Mean 0.95St. Dev. 0.24N 35

Clay

Mean 0.96St. Dev. 0.28N 32


FOSM Equation cont’d

Where:

Resistance factor

Dead load factor (see table)

Live load factor (see table)

Bias for dead loads

Bias for live loads

Bias for resistance

Dead load to live load ratio

Reliability index

Coefficient of variation for resistance

Coefficient of variation for dead loads

Coefficient of variation for live loads

Load typeLoad Factor

(γD, γL)

Load Bias

(λQD, λQL)

Load COV

( COVQD, COVQL)

Dead Load (D.L.) 1.25 1.05 0.1

Live Load (L.L.) 1.75 1.15 0.2

AASHTO LRFD Probabilistic characteristics of random variables for loads (after Nowak, 1999)

Figure 1: Probability of failure and reliability index (adapted from Withiam et al. 1998)

22


Assembling the load frame

Unloading and recording to DAS

Installing strain gauges and vibrating wires

Welding steel angles to protect the SGs

Monotonic loading on the test pile

Driving the test and the anchor piles

Verification – SLT

23


P. Cells

Atterberg Hydrometer CU-TriaxialClassification

SPT CPT BST/mBST

Verification – Soil Testing

24


0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300 350 400 450

φx D

esi

gn

Cap

acit

y (

kip

s)

φ x Davisson Capacity (kips)

BlueBook

SPT-Meyerhof

α-API

β-method

Nordlund

0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300 350 400 450

Desi

gn

Ca

pa

cit

y (

kip

s)

Davisson Capacity (kips)

BlueBook

SPT-Meyerhof

α-API

β-method

Nordlund

88

153

279

194 205

243

5280

98 93

143

194

0

50

100

150

200

250

300


Cap

acit

y (

kip

s)

Nominal Capacity

Factored Capacity

130

201

313

244216

165

58 65103

76 88

132

0

100

200

300

400


Ca

pa

cit

y (

kip

s)

Nominal Capacity

Factored Capacity

Clay

Mixed

Nominal

Factored

163

228193 199

167

127

70 73 63 6185

102

0

50

100

150

200

250

SPT-Meyerhof α-method β-method Nordlund-method Bluebook SLT

Ca

pa

cit

y (

kip

s)

Nominal CapacityFactored CapacitySand

Verification cont’d


Soil Type Static Method Egypt AASHTO NCHRP

Sand

SPT-Meyer. n/a 0.3 0.45

β-Method n/a N/A 0.3

Bluebook N/A

Clayβ-Method n/a 0.25 0.2

Bluebook N/A

Mixed

β-Method n/a 0.25 0.2

Nordlund n/a N/A 0.2-0.35

Bluebook N/A

Comparison between the Egyptian R.F. and the international codes


Verification cont’d


Outline





Pile Design Example


27

Design and Construction

Flowchart

Construction Control

Calibration for Static and

Dynamic Methods

Design and Construction Flowchart

Site Investigation andSoil Files Testing

Determine SoilParameters

Perform Static Analysis

Release BiddingDocuments

ConstructionStage

Determine StructuralLoads and

Requirements

Completed Substructure

DesignStage

Determine Type of DeepFoundations

Design VerificationModify the Design

(If Needed)


Construction Control


Construction control is achieved via:

Dynamic testing (PDA and CAPWAP)

Wave equation (WEAP)

Pile static load test (SLT)

Dynamic formulas (ENR, Gates, etc.)

Drilling to support on bedrock

Driving to bedrock or until refusal

Static analysis is only used during the design stage of a project, and mainly for releasing the bedding documents.

Dynamic analysis and/or SLT are conducted during the construction stage. The final capacity of the pile is established during this stage.

Perform Static Analysismethods

Determine DesignCapacity

Perform StatisticalAnalysis

Calibrate LRFDResistance Factors

using Reliability Method

Determine ResistanceFactors for StaticAnalysis Methods

Determine Pile Capacityfor Each Method

Perform StatisticalAnalysis

Calibrate LRFDResistance Factors

using Reliability Method

Determine ResistanceFactors for Dynamic

Analysis Methods

For Construction

Stage

For Design Stage

WEAP Analysis Dynamic Formulas

Proposed Field Tests

TR-573 DSPLTDatabase

Sort Data According toPile and Soil Types

Determine Pile NominalCapacity using

Davisson's Criterion

For StaticAnalysis

For DynamicAnalysis

PDA and CAPWAP


Calibration for Static and Dynamic

Methods


Outline





Pile Design Example


31

For the given soil profile, consider using CIDH piles (d=50 cm) designed for a maximum factored load of 100 ton/ pile.

End-bearing in a hard soil layer with length equal to 17 m.

Using WSD and LRFD, calculate the number of piles required under the pile cap.

Use various static and dynamic methods.


Design Example

Simply calculate the pile capacity as usual, then, instead of dividing by a FOS, multiply by a R.F.

Ultimate Capacity using different static and dynamic methods


Pile Design Example cont’d

0

20

40

60

80

100

120

21

52

117

89

37

49

76

111

76 75

67 69

63Pil

e C

ap

ac

ity,

to

n InconsistencyHigh variationOver-conservativeOver-estimating

Allowable Capacity (WSD) using different methods



0

10

20

30

40

50

60

70

8

21

47

35

1519

30

44

30 30

27 28

63

Pil

e C

ap

ac

ity,

to

n InconsistencyHigh variationOver-conservative



Factored Capacity (LRFD) using different methods

ConsistentLow variationEfficient

0

10

20

30

40

50

60

70

15

24 2529 31

35

4145

4954

44

5863

Pil

e C

ap

ac

ity,

to

n


Outline





Pile Design Example


36

In the USA, more than 90% of the States changed from WSD to LRFD for the design of

pile foundations, using AASHTO.

In Europe, the latest Euro Code was released using LSD with different combinations.

Using LRFD, at β = 2.33 (1% probability of failure), the design efficiency may increase up

to 80% relative to the actual field measurements (SLT), compared to the WSD.

The LRFD approach is superior compared to the WSD in terms of

reliability, consistency, efficiency, and cost-effectiveness.

Engineers can use the LRFD tables and charts to design pile foundations with a selected

reliability (chosen probability of failure), depending on the degree of conservatism.

Using the reliability-based LRFD for piles, the resistance factors can be regionally

calibrated for a specific soil type or region, pile type, or any particular condition, so that

to increase the design effectiveness.

Conclusions


The Egyptian code should shift to the LRFD to coupe with other international codes.

Changing to LRFD will lead to the following:

1. Constant and controllable degree of reliability

2. Consistent design for the entire structure

3. Improves the construction control process

4. Higher efficiency and cost-effectiveness

5. No assumptions, experience, or judgment needed

For the LRFD, the same design practice is followed, the only difference is replacing

the FOS by a R.F. for the specific soil/pile/method used; hence it is easy to use LRFD.

Recommendations


Thank You Discussion ?

39

lrfd lecture for the egyptian code (handouts)

Documents

api method

overlap area

various uncertainties

reliability index

values wsd

capacities x

factored capacities

wsd approach higher