MnDOT D e e p Fo Fo u n d a t i o n D e s i g n U s i n g L RF RFD Methodology LRFD Bridge Design Workshop June 12, 2007 David Dahlberg, P.E. LRFD Engineer
Pr e s e n t a t i o n O Ov verview
Previous Pile Design Method AASHTO LRFD Pile Design Method New MnDOT LRFD Method
Pile Downdrag Pile Lateral Load Capacity
Drilled Shaft Design
Pr e v i o u s P De s i g n M e t h o d Pii l e De
Based on Allowable Stress Design (ASD) ∑ Qi ≤ Qult / FS where
Q = service load Qult = ultimate capacity FS = factor of safety
Pr e v i o u s P Pii l e D e s i g n M e t h o d
Need to consider four things:
Capacity of soil Structural capacity of pile Driveability of pile (max driving stresses) Field verification during driving operation to ensure required resistance is obtained
Pr e v i o u s P De s i g n M e t h o d Pii l e De
Design soil allowable capacity determination based on combination of:
Static analysis w/ F.S (done by geotechs) Correlation of borings with field verification method (done by Regional Construction Engineer)
Pr e v i o u s P De s i g n M e t h o d Pii l e De
Typical pile was 12” dia. CIP w/0.25” wall
60 to 75 ton allowable maximum load (based on considering past practice, AASHTO, experience, and driveability of the pile)
Pr e v i o u s P De s i g n M e t h o d Pii l e De
Majority of pile capacities based on field measured initial drive capacity Soil/pile setup used when warranted by soil profile
Only in low initial capacity situations
Pr e v i o u s P De s i g n M e t h o d Pii l e De
Field verification during driving:
MnDOT Modified ENR Formula
CIP piles
H – piles
P
P
=
=
3.5E S + 0.2
3 .5 E S
PDA sometimes used
+ 0 .2
⋅
⋅
W + 0.1M W+M
+ 0 .2 M W +M
W
A A S H T O L RF RFD De s i g n M e t h o d
Requires use of factored loads & nominal resistance ∑ ηi ⋅ γi ⋅Qi ≤ φ⋅Rn where η = load modifier
= load factor Q = service load γ
φ = resistance factor Rn = nominal (ultimate) resistance
A A S H T O L RF R FD De s i g n M e t h o d
Need to consider four things:
Capacity of soil Structural capacity of pile Driveability of pile (max driving stresses) Field verification during driving operation to ensure required resistance is obtained
A A S H T O L RF R FD De s i g n M e t h o d
Capacity of soil:
Estimated by geotechnical engineer using static pile analysis Resistance factors φstat from LRFD Table 10.5.5.2.3-1
A A S H T O L RF RFD De s i g n M e t h o d
LRFD Resistance Factors for Piles LRFD Table 10.5.5.2.3-1
A A S H T O L RF RFD De s i g n M e t h o d
Structural capacity of pile:
CIP piles per LRFD 6.9.5.1 φc ·(Asf f y+0.85f’c·Ac) H piles per LRFD 6.9.4.1 φc ·Asf y Resistance factors for axial resistance per LRFD 6.15.2 and 6.5.4.2
A A S H T O L RF R FD De s i g n M e t h o d
LRFD Resistance Factors for Steel Piles found in LRFD 6.5.4.2
A A S H T O L RF R FD De s i g n M e t h o d
Driveability (max driving resistance):
Per LRFD 10.7.8: 0.9· φda·f y Resistance factor per LRFD Table 10.5.5.2.3-1 and LRFD 6.5.4.2
A A S H T O L RF R FD De s i g n M e t h o d
LRFD Resistance Factor for Driveability
LRFD Table 10.5.5.2.3-1
LRFD 6.5.4.2
A A S H T O L RF R FD De s i g n M e t h o d
Field verification during driving operation to ensure required resistance is obtained:
Verification by static load test, dynamic testing (PDA), wave equation, or dynamic formula Uses resistance factor φdyn from LRFD Table 10.5.5.2.3-1
A A S H T O L RF R FD De s i g n M e t h o d
LRFD Resistance Factors for Piles LRFD Table 10.5.5.2.3-1
N e w MnDOT L RF RFD M e t h o d
Capacity of soil:
Look in the Foundation Report Typical Foundation Report should include:
Project description Field investigation and foundation conditions
Foundation analysis Recommendations
Additional sections as needed
N e w MnDOT L RF RFD M e t h o d
Foundation analysis should include:
Nominal Resistance (ultimate capacity) estimates provided by Foundations Unit Initial drive and set-up graph which shows resistance as a function of depth
N e w MnDOT L RF RFD M e t h o d
N e w MnDOT L RF RFD M e t h o d
Pile Resistance φRn for design
Determined considering LRFD structural capacity of pile, maximum LRFD driving resistance, and past experience
Pile Capacity Table
N e w MnDOT L RF RFD M e t h o d
Field verification during driving
Typically will use MnDOT dynamic formula modified to provide nominal resistance as the output Will use PDA on larger projects by running a PDA on the test piles to calibrate the MnDOT dynamic formula for other piles
N e w MnDOT L RF RFD M e t h o d
Field Verification during driving:
MnDOT Nominal Resistance Pile Driving Formula (for both CIP & H-piles) R n
=
10.5E S + 0.2
⋅
W + 0.1M W+ M
Incorporated by special provision SB2005-2452.2
N e w MnDOT L RF RFD M e t h o d
LRFD Resistance Factors for Piles LRFD Table 10.5.5.2.3-1
N e w MnDOT L RF RFD M e t h o d
Resistance factors:
Compare LRFD to ASD LRFD: ∑ γQ ≤ φRn ASD: ∑ Q ≤ Rn /F.S. Then F.S.= γ / φ Average γ ≈ 1.4 For MnDOT formula, φdyn = 1.4/3.0 ≈ 0.45 For PDA, φdyn = 1.4/2.25 ≈ 0.60
N e w MnDOT L RF RFD M e t h o d
Comparisons made with MnDOT Formula, WEAP, Gates Formula, and PDA data
N e w MnDOT L RF RFD M e t h o d
Field verification
PDA
φdyn = 0.65
MnDOT Nominal Resistance Pile Driving Formula
φdyn = 0.40
N e w MnDOT L RF RFD M e t h o d
Monitoring method determines required driving resistance for the Contractor For example, assume a factored design load of 100 tons/pile:
PDA verification Rn = Qu/ φdyn = 100/0.65 = 154 tons
MnDOT Ultimate formula Rn = Qu/ φdyn = 100/0.40 = 250 tons
N e w MnDOT L RF RFD M e t h o d
Example
N e w MnDOT L RF RFD M e t h o d
N e w MnDOT L RF RFD M e t h o d
Pile Capacity Table
N e w MnDOT L RF RFD M e t h o d
N e w MnDOT L RF RFD M e t h o d
Bridge Plan Load Tables
I m p l e m e n t a t i o n f o r T .H .H .
MnDOT Foundation Unit (Maplewood Lab)
Regional Bridge Construction Engineers
Providing ultimate capacity estimates Provide pile type with maximum resistance Identify verification method(s) to use
Designers
Design with LRFD methods and loads Factored loads presented on plans Compare with past ASD designs
Im plem ent at ion for S Stt a t e A i d
Geotechnical Engineer
Providing ultimate capacity estimates
Designer
Provide pile type with maximum resistance Identify verification method(s) to use Design with LRFD methods and loads Factored loads presented on plans Compare with past ASD designs
Research
Two projects rolled into one:
Development of Resistance Factor for MnDOT Pile Driving Formula Study of Pile Setup Evaluation Methods
Research begins this year
Downdrag is the downward load induced in the pile by the settling soil as it grips the pile due to negative side friction Covered in LRFD 3.11.8, 10.7.1.6.2, 10.7.2.5, and 10.7.3.7
Estimated downdrag load will be given in the Foundation Report For piles driven to rock or a dense layer (end bearing piles), nominal pile resistance should be based on pile structural capacity
For piles controlled by side friction, downdrag may cause pile settlement, which will result in reduction of the downdrag load Amount of pile settlement difficult to calculate, so downdrag on friction piles to be considered on a case by case basis
Transient loads reduce downdrag, so do not combine live load (or other transient loads) with downdrag Consider a load combination with DC + LL and also a load combination that includes DC + DD, but do not consider LL and DD within the same load combination Discuss with Regional Construction Engineer before using battered piles
Past Practice Using ASD
Service loads resisted by: battered pile component + 12 kips/pile resistance
Current Practice Using LRFD
Factored loads resisted by: battered pile component + 18 kips/pile resistance
Parametric study conducted:
12” & 16” diameter CIP piles HP10x42, HP12x53 and HP14x73 Single layer of noncohesive soil with varied friction angles of 30 , 32 , 34 , 36 , and 38 ENSOFT program L-Pile 5.0.30 used for this study ˚ ˚
˚ ˚
˚ ˚
˚ ˚
˚ ˚
Piles under combined axial compressive load and moment due to axial and lateral loads at the top of piles LRFD 6.9.2.2 interaction equation: Pu
φ c Pn
8 ⎛ M u
⎞ ⎟⎟ ≤ 1.0 + ⎜⎜ 9 ⎝ φ f M n ⎠
Inserting known values for Pu, φcPn, φf Mn, interaction equation solved for Mu Lateral load applied at top of pile and increased until the calculated maximum Mu was reached in the pile
Results: Pile Type
Fy
Wall t
φRnh
(k s i)
(in.)
(k ips )
12" CIP 16" CIP
45
all
24
45
1/ 4
28
16" CIP
45
5/ 16
40
16" CIP
45
3/ 8
40
16" CIP
45
1/ 2
40
HP 10x 42
50
NA
24
HP 12x 53
47. 8
NA
32
HP 14x 73
43. 9
NA
40
Results:
Max deflection due to factored loads was approximately 0.5”
Serviceability does not govern
Design process is interactive Designer, Regional Construction Engineer, and geotechnical engineer need to discuss:
Proposed construction method
Permanent vs. temporary casing
Shaft diameter
Vertical & horizontal loads for multiple row shaft foundation
Loads & moment for single shafts
Rock sockets
Resistance factors vary:
Tip/side resistance
Load tests
Base grouting
Existing foundation load tables given in MnDO Mn DOT T Bri Bridg dge e De Desi sign gn Ma Manu nual al Appendix 2-H do not include drilled shafts Spread footing load tables were used in the past New load tables to be created for drilled shafts
Questions
