# Design Guides 3.10.1 - LRFD Pile Design

Document Sample

```					Design Guides                                                   3.10.1 - LRFD Pile Design

3.10.1 LRFD Geotechnical Pile Design Procedure

Reference the AASHTO LRFD Bridge Design Specifications third addition with ’05 and 06’
interims.

1. The Geotechnical Engineer evaluates the subsurface soil/rock profile and develops a
pile design table provided to the structural engineer in the Structure Geotechnical Report
(SGR). The pile design table will contain a series of Nominal Required Bearing (RN)
values, the corresponding Factored Resistances Available (RF) for design, and the
Estimated Pile Lengths. When multiple pile types are being considered, a pile design
table will be provided for each pile type to allow the structural engineer to select the most
economical and feasible pile type, size and layout.

a. The Nominal Required Bearing (RN) represents the sum of the nominal tip
resistance (qPAP) and nominal side resistance (qSASA) the pile will experience
during driving (RN = qPAP + qSASA)

i. Nominal Tip Resistance is determined by multiplying the nominal unit end
bearing resistance (qP) of the soil or rock layer below the pile by the pile
end bearing area (AP).

The nominal unit end bearing resistance should be determined as follows:

For granular soils, the qP may calculated as:

0.8N160 D b
qp =               ≤ ql
D

⎡          ⎛ 40 ⎞⎤      q l = 8N160    (sands)
where:                         ⎜ σ ' ⎟⎥ N & q = 6N1
N160 = ⎢0.77log10 ⎜     ⎟
⎢
⎣          ⎝ v ⎠⎥  ⎦      l      60    (nonplastic silt)

D=      pile diameter or width (ft.)
Db=     depth of penetration into soil (ft.)
N=      field measured SPT blow count (blows/ft.)
N160 = SPT blow count corrected for overburden pressure
and 60% hammer efficiency (blows/ft.)

Nov. 2006                                                                             Page 3.10.1-1
Design Guides                                                 3.10.1 - LRFD Pile Design
σv’ =   effective vertical soil pressure (ksf)

For cohesive soils, the qP should be computed as:

qP = 9 QU

where:        QU =    average unconfined compression strength of the
soil (tsf)

Note that QU is input in tsf and qP is output in ksf.

For rock, the qP should be computed as:

For shale and sandstone qP = 27 ksi = 3888 ksf
For limestone and dolomite qP = 36 ksi = 5184 ksf

These values are to be used for driven H-piles only, not for design of
piles set in rock.

The pile end bearing area should be determined as follows:

Steel piles           AP = the cross-sectional area of steel member
Metal Shell piles AP = the horizontal end plate area
Precast piles         AP = width squared end area
Timber piles          AP = Assume a 7 in. diameter tip to calculate end
area

Do not make any adjustment to the above end areas when pile shoes
are used.

ii. Nominal Side Resistance is determined by multiplying the nominal unit
side resistance (qS) of the soil layer by the effective surface area of the
pile (ASA)

The nominal unit side resistance should be determined as follows:

Page 3.10.1-2                                                                         Nov. 2006
Design Guides                                             3.10.1 - LRFD Pile Design

For granular soils, the qS should be computed as:

For Hard Till,

qs = 0.07N160                                      (N160 < 30 )
q s = 0.00136N12 − 0.00888N160 + 1.13
60                                  (30 ≤ N160 )

Very Fine Silty Sand,

q s = 0.1N160                                      (N160 < 30 )
⎡ (N160 −175.05 )2 ⎤
⎢                  ⎥
−7944
q s = 42.58 e   ⎣                  ⎦
(30 ≤ N160 < 74)
q s = 0.297N160 − 10.2                             (74 ≤ N160 )

Fine Sand,

q s = 0.11N160                                     (N160 < 30 )
182
q s = 0.3256N160 +              − 12.51            (30 ≤ N160 < 66)
N160
q s = 0.329N160 − 9.91                             (66 ≤ N160 )

Medium Sand,

q s = 0.117N160                                    (N160 < 26)
q s = 0.00404N12 − 0.0697N160 + 2.13
60                                  (26 ≤ N160 < 55)
q s = 0.356N160 − 9.1                              (55 ≤ N160 )

Clean Medium to Coarse Sand,

q s = 0.128N160                                    (N160 < 24)
q s = 0 .00468 N12 − 0 .0693 N160 + 2.05
60                                (24 ≤ N160 < 50)
q s = 0.394N160 − 9.42                             (50 ≤ N160 )

Nov. 2006                                                                 Page 3.10.1-3
Design Guides                                                3.10.1 - LRFD Pile Design

Sandy Gravel,

q s = 0.15N160                                      (N160 < 20)
q s = 0 .00861 N12 − 0 .217 N160 + 3 .91
60                                 (20 ≤ N160 < 40)
q s = 0.6N160 − 15.0                                ( 40 ≤ N160 )

For cohesive soils, the qS should be computed as:

−1    3          2
qs =        Q u − 0.177Q u + 1.09Q u                (Q u ≤ 1.5 tsf)
2500

3          2
q s = 0.0495Q u − 0.347Q u + 1.278Q u − 0.068       (1.5 tsf < Q u < 2 tsf)

q s = 0.470Q u + 0.555                              (2 tsf ≤ Q u < 4.5 tsf) *
qs = 2.67 ksf                                       (4.5 tsf ≤ Q u ) *

*If QU > 3 tsf and N > 30, treat as granular and use Hard Till
equations.

Note that QU is input in tsf and qS is output in ksf.

The effective surface area of the pile (ASA) should be determined as the
follows:

Steel piles         ASA = 2 x (flange width + web depth) x length (for
cohesive layers) or 1 x (flange width + web
depth) x length (for granular layers)
Metal Shell piles ASA = the shell circumference x length
Precast piles       ASA = 4 x (width) x length
Timber piles        ASA = the circumference (assume 12 in. dia.) x
length

iii. The Maximum Nominal Required Bearing (RN MAX.) that can be specified
for the standard pile types are limited by empirical relationships
developed to provide reasonable confidence that the dynamic pile

Page 3.10.1-4                                                                           Nov. 2006
Design Guides                                             3.10.1 - LRFD Pile Design
stresses caused by properly functioning pile hammers will not cause pile
damage when installed in appropriate subsurface conditions.            These
empirical relationships are as follows:

1. Metal Shell Piles:      RN MAX. = 0.85xFYAS

where: 0.85 = IDOT factor for Metal Shell piles relating acceptable
dynamic stress to static nominal steel capacity
FY =    yield strength of the steel shell (45 ksi)
AS =    the steel shell area (in.2)

2. Steel Piles:             RN MAX. = 0.54xFYAS

where: 0.54 = IDOT factor for Steel H-piles relating acceptable
dynamic stress to static nominal steel capacity
FY =    yield strength of the steel (50 ksi)
AS =    the steel cross-sectional area (in.2)

3. Precast Piles:           RN MAX. = 0.3xf’cxAg

where: 0.30 = IDOT factor for Precast piles relating acceptable
dynamic stress to static nominal concrete capacity
f’c =   compressive strength of concrete (4.5 ksi for
precast or 5 ksi for prestressed)
Ag =    area of 14 in. x14 in. square pile (196 in.2)

4. Timber Piles:            RN MAX. = 0.5xFcoxAP

where: 0.50 = IDOT factor for Timber piles relating acceptable
dynamic stress to static nominal timber capacity
Fco =   base resistance of wood in compression parallel to
grain (2.7 ksi)
AP =    average cross-sectional area near top of timber pile
(assuming 12 in. dia =113 in.2 )

Nov. 2006                                                                      Page 3.10.1-5
Design Guides                                                   3.10.1 - LRFD Pile Design
Using the above relationships and material strengths results in the
following maximum nominal required bearings that can be safely specified
in the pile design table and on the Contract plans:

Maximum Nominal
Pile Designation
Required Bearing (RN MAX.)
Metal Shell 12”φ w/0.179” walls                 256      kips
Metal Shell 12”φ w/0.25” walls                  355      kips
Metal Shell 14”φ w/0.25” walls                  416      kips
Metal Shell 14”φ w/0.312” walls                 516      kips

Steel HP 8x36                           286      kips
Steel HP 10x42                           335      kips
Steel HP 10x57                           454      kips
Steel HP 12x53                           419      kips
Steel HP 12x63                           497      kips
Steel HP 12x74                           589      kips
Steel HP 12x84                           664      kips
Steel HP 14x73                           578      kips
Steel HP 14x89                           705      kips
Steel HP 14x102                          810      kips
Steel HP 14x117                          929      kips

Precast 14”x14”                           265      kips
Precast Prestressed 14”x14”                    294      kips

Timber Pile                           153      kips

When the Nominal Required Bearing (RN) is specified to exceed 600 kips,
the use of the Standard Specifications Gates formula cannot provide
sufficiently accurate predictions of Nominal Driven Bearing as well as
assurance against pile damage during driving (See LRFD AASHTO Art.
10.7.3.2.3 and 10.7.3.8.5). In these cases, i.e. RN > 600 kips, General
Note #24 (See Section 3.1.3 of the Bridge Manual) shall be included on
the Contract plans. This note requires the contractor to conduct a wave
equation analysis to establish the pile driving criteria.

b. The Factored Resistance Available (RF) represents the net long term axial
factored pile capacity available at the top of the pile to support factored structure

Page 3.10.1-6                                                                            Nov. 2006
Design Guides                                              3.10.1 - LRFD Pile Design
after driving such as scour, downdrag (DD), or liquefaction (Liq.) and reflects the
construction control resistance factor used to verify the nominal required bearing
(RN).

i. The Factored Resistance Available (RF) shall be calculated using the
following equation:

RF= RN(φG) - (DD+Scour+Liq.)x(φG) x(λG) – DDx(γp)

where:      φG =   the geotechnical resistance factor related to pile
installation control
λG =   the Bias factor between Gates Resistance and the
IDOT Resistance Equations (1.0)
γp =   the load factor for DD applied to the pile by soil

Appling the resistance factor (φG) to the geotechnical losses may appear
unconservative. However, AASHTO LRFD Article 10.7.3.7 requires the
factored loads (RF + γpDD) be ≤ the factored resistance below the DD
layers. Thus, the factored resistance below the DD layers must be equal
to the RF + γpDD. The pile must be driven to a Nominal Required Bearing
RN equal to the nominal DD resistance to install the pile to below the DD
layer plus (RF + γp DD)/φG. Solving for the RF, the geotechnical losses
and RN are multiplied by φG.

ii. The Nominal values of the downdrag (DD), Scour and Liquefaction (Liq.)
shall be calculated using side resistance equations provided above.

1. The value of DD is used twice (as shown in the equation for RF
above). Once to represent the decrease in side resistance and
nominal value DD is only the resistance lost or the load applied
(since they are of equal magnitude), not the sum of both.

2. The nominal value of Scour represents the decrease in resistance
in the layers above the design scour elevation. The Scour term

Nov. 2006                                                                        Page 3.10.1-7
Design Guides                                             3.10.1 - LRFD Pile Design
shall be taken as zero when calculating the Factored Resistance

3. The nominal value of Liq. represents the decrease in resistance in
the layers expected to liquefy due to the design seismic event.
However, non-liquefied layers above layers expected to liquefy will
settle and result in DD in those layers. Thus, the value of Liq.
shall be added to the value of DD (to represent the total decrease
in resistance) in the RF equation. The value of DD is used again
when it is multiplied by the DD load factor (to represent the
increase in load) in the RF equation. The term Liq. shall be taken
as zero unless calculating the Factored Resistance Available (RF)

iii. Geotechnical Resistance Factor (φG) and Bias factor (λG)

The geotechnical resistance factor (φG) relates to the method selected to
establish the final Nominal Driven Bearing resistance of the pile. The
engineering news formula (ENR) was used in the past for this purpose
but now the more accurate Gates formula will be relied on to indicate the
nominal resistance. The 0.4 resistance factor recommended for Gates in
AASHTO results in either an increase in the number of piles or much
larger hammers sizes. The 0.4 represents a factor of safety of 2.4 which
when combined with the average increase in load (due to larger HL-93
and load factors) which approaches 72% and results in an overall factor
of safety just over 4.0. Although the formal factor of safety on the ENR is
6.0, research has shown the IDOT resistance equations and the ENR to
only provide on average overall factor of safety of about 2.4. To make
use of the Gates formula and maintain only modest increases in
foundations (mainly due to the new live load model) a geotechnical
resistance factor (φG) of 0.50 will be used. This value also corresponds to
the ratio between the increase in load demand 1.72 and the ASD factor of
safety 3.5 recommended for Gates (1.72/3.5=0.49 say 0.50).

When scour, downdrag or Liq. reduce the geotechnical Factored
Resistance Available, the Bias factor shall be used to “adjust” the

Page 3.10.1-8                                                                      Nov. 2006
Design Guides                                                   3.10.1 - LRFD Pile Design
geotechnical resistance factor to correctly capture the reliability of the
side resistance equation predictions.        It is also important to properly
account for bias when using the nominal side resistance and end bearing
equations to estimate the length where the Gates formula will indicate
Nominal Required Bearing.       Since the factor of safety used with the
resistance equations was 3.0 and the Gates factor of safety (using .50
should be between 3.0 and 3.5) a bias factor of 1.0 is recommended until
further research can be conducted.

If it becomes clear during the planning process that earthquake forces
may govern the foundation design, the SGR pile tables should report both
setting the geotechnical resistance factor (φG) to 1.0, and for Strength

c. The Estimated Pile Lengths shall be calculated and provided in the pile design
table.

i. Initially, the geotechnical engineer will evaluate the Nominal Required
Bearing (RN) values that would be expected as the pile is being driven
through each soil layer.       The Nominal Required Bearing typically
increases linearly as it passes though a soil layer while it may suddenly
increase or decrease as it enters a new layer (due to changes in end
bearing). Thus, the initial pile lengths investigated are commonly located
just above and just below each major soil layer to get an idea of how the
pile will drive and what Nominal Required Bearings and/or pile types are
feasible.   To calculate these lengths correctly, the bottom of footing,
bottom of pile encasement, and pile cutoff elevations should be known
with reasonable accuracy.      This is because the estimated pile length
includes portions of the pile which will be incorporated in the substructure.

ii. Then, layers expected to settle, scour or liquefy are identified and the DD,
Scour or Liq. is calculated such that the RN can be reduced to determine
the Factored Resistance Available (RF) at each preliminary penetration
investigated above.

Nov. 2006                                                                             Page 3.10.1-9
Design Guides                                                     3.10.1 - LRFD Pile Design
iii. The geotechnical engineer should discuss the preliminary estimated pile
length vs. Factored Resistance Available (RF) relationship with the
narrow the range of pile types and loadings to be included in the SGR pile
design table(s).        This usually involves obtaining total factored
demand per pile, from which an expanded range of RF values can be
supplied.

iv. Using the preliminary pile length vs. Factored Resistances Available
calculated above that fall near the range of preliminary RF values
calculated above, the final pile design table of estimated pile lengths vs.
both RN and RF, can be developed and provided in the SGR for use by the
structural engineer.

2. The Structural Engineer calculates the controlling loadings applied to the foundation,
evaluates the cost and structural feasibility of using various pile types and sizes
recommended in the SGR while evaluating various pile layouts/spacings to select the
most cost effective and feasible foundation design.

Calculate the factored loadings as described in Section 3 of LRFD for all
states need not be evaluated for most typical highway structures as the strength
limit state pile design, following this procedure, will result in less than ⅛ in. axial
deflection at the top of the pile. In some cases, it may be clear which group
resolved into a factored shear, a factored moment, and a factored vertical loading
applied to the center of the foundation along both the longitudinal and transverse
axes of the foundation.

In some cases, the SGR pile table will not include the Factored Resistances
Available to resist Extreme Event I seismic considerations unless it becomes

Page 3.10.1-10                                                                             Nov. 2006
Design Guides                                                 3.10.1 - LRFD Pile Design
clear during the planning process that earthquake forces may govern the
foundation design. A method to convert the factored resistances available in a
pile design table from non-seismic to seismic is provided in Section 3.15 of the
Bridge Manual.

It should also be noted that the factored loadings (QF) are at the top of piling and
shall not include any factored downdrag (DD) which may be present. The LRFD
standard practice to require that the DD loading and DD reduction in resistance
for a pile (as well as other reductions in resistance such as scour and
liquefaction) be taken into account by the geotechnical engineer and
incorporated in the SGR pile design table. Consequently, it shall not be added to

Evaluate various pile layout arrangements, using the preferred number of pile
rows and pile spacing.    Using each of the group limit states that may control,
identify the maximum and minimum factored loading applied (QF) per pile in the
group.

zero. In cases where this cannot be accomplished using an economical pile
layout, such as for seismic design, the Factored Resistance Available (RF) in
pullout should be calculated using a geotechnical uplift resistance factor of 0.20
for non-seismic loadings and 0.8 for seismic, and the nominal side resistance
equations. This calculation will provide the minimum tip elevation to be included
on the plans, to ensure pullout resistance. The pile anchorage into the footing or
substructure should also be modified and designed to carry the factored pull out

The maximum Factored Loading Applied (QF) shall be less than or equal to the
Factored Resistance Available (RF) provided in the SGR pile design table for the
pile type and size under consideration.

Nov. 2006                                                                          Page 3.10.1-11
Design Guides                                                    3.10.1 - LRFD Pile Design
The pile size or type should be adjusted as necessary to ensure QF < RF or the
pile layout (rows, spacing, number) may be modified and reanalyzed to
determine the most economical pile type, size and layout to be used.

c. Check Structural Resistance of pile:

Completion of step 2b ensures that the piles will have the geotechnical factored
a nominal required bearing (RN) which is below the maximum RN            MAX   values
above will ensure that the pile has the structural resistance to withstand the
driving stresses caused by a correctly functioning hammer. However, the static
structural resistance of the pile shall also be calculated to verify it can carry the
above finished ground surface or above the design scour or liquefaction depth.
The structural resistance factors and equations to check this are provided in the
LRFD Specification. Only a portion of the necessary information is provided
below for reference:

i. The structural resistance factors (φS) to be used with the nominal
structural resistance equations are provided below.

Metal Shells    φS =   0.8 in compression and 1.0 in flexure [Art. 6.5.4.2,
LRFD]
H-piles         φS =   0.7 in compression and 1.0 in flexure [Art. 6.5.4.2,
LRFD]
Precast         φS =   0.75 to 1.0 in both compression and flexure [Art.
5.5.4.2, LRFD]
Timber          φS =   0.9 in compression and 0.85 in flexure [Art. 8.5.2.2,
LRFD]

ii. The Nominal Structural Resistance (RNS) of piles below ground or
confined by concrete encasement is typically calculated assuming them
to be continually braced. For portions of piling that are not continually
braced (extending above ground, subject to scour, Liq., etc.), the Nominal

Page 3.10.1-12                                                                            Nov. 2006
Design Guides                                             3.10.1 - LRFD Pile Design
Structural Resistance should be calculated taking the unbraced length
into account.      The following equations are provided in LRFD to
accomplish this check.

Metal Shell and Steel Piles

RNS=0.66λFYAS                              (Eq. 6.9.4.1-1, LRFD)

Precast Piles

RNS=0.85(0.85f’c(Ag-As)+fyAs)              (Eq. 5.7.4.4-2, LRFD)

Prestressed Piles

RNS=(0.85f’cAg)-(AsE(εpe-(εcu-εce)))       (Com. C5.7.4.4, LRFD)

Timber Piles

RNS=CcoAgCp                                (Eq. 8.8.2-1, LRFD)

Normally the Factored Geotechnical Resistance (RF) will control over the
factored structural resistance (RNS x φS).       If the factored structural
resistance is less than the factored loadings applied (QF), either the pile
size or the number of piles shall be increased such that RNS x φS > QF.

1. If the size of the pile is increased, both the nominal required
bearing (RN) and the factored resistance available (RF) shown on
the plans should not change, since the QF demand would not
change.

2. If the number of piles is increased, both the nominal required
bearing (RN) and the factored resistance available (RF) shown on
the plans should be decreased, since the QF demand would
decrease.

Nov. 2006                                                                   Page 3.10.1-13
Design Guides                                                     3.10.1 - LRFD Pile Design
Both approaches, either an increase in pile size or an increase in the
number of piles, results in RNS x φS ≈ QF , RF ≈ QF and RN          MAX   > RN.
However, if the end bearing resistance of the piles is much larger than the
side resistance, it is recommended that the piles be driven to their
maximum nominal required bearing (RN          MAX)   since the added installed
geotechnical resistance can normally be obtained with minimal additional
penetration/cost and may also be needed for future bridge rehabilitations.
The modified load-resistance relationships would then be RNS x φS ≈ QF ,
RF > QF and RN MAX = RN.

d. The Pile Data to be included in the Contract plans shall include: i. the pile type
and size, ii. the nominal required bearing, iii. the factored resistance available, iv.
the estimated pile length, v. the number of production piles, and vi. the number of
test piles. In some cases, other information shall be provided and is discussed.

i.   The Pile Type and Size shall be provided so the contractor can bid and
furnish the piles required at each foundation location. Examples of typical
pile type and size callouts are as follows:

Metal Shell –___ in. dia. x ____ in. walls with pile shoes
Steel – HP ___x ___ with pile shoes
Precast Concrete – 14 in. square prestressed
Timber – 12 in. dia. treated

Note the items in bold are examples of parameters which may or may not
be specified for a project.

ii. The Nominal Required Bearing is provided in kips to instruct the
contractor as to the driven bearing the production piles shall be installed
to as well as assist the contractor in selecting a proper hammer size.

iii. The Factored Resistance Available shall be provided in kips.               This
value is not used by the contractor but documents the net long term axial
factored pile capacity available at the top of the pile for the current and
future design/rehabilitation work.       It documents any reductions in
geotechnical resistance that will occur after driving such as scour,

Page 3.10.1-14                                                                              Nov. 2006
Design Guides                                         3.10.1 - LRFD Pile Design
downdrag, or liquefaction.   It also reflects the resistance factor which
documents the accuracy in the method of construction control used at the
time of installation.

iv. The Estimated Pile Length is provided to give contractors a bid quantity,
helps determine the length of the test pile, and when no test pile is
specified, this length becomes the length furnished by the contractor. It
also is used as a reference by the inspectors to identify when the pile
problems, such as lack of set up or improper hammer performance, are
causing piles to stop short or run long. In some cases, a minimum tip
elevation will be specified in addition to the estimated pile length.
Normally, the minimum tip elevation will only be necessary when the piles
have the potential to stop shorter than estimated resulting in inadequate
lateral load strength or penetration below any geotechnical losses such
as scour.

v. The Number of Production Piles is the total number of production piles
required at the substructure or foundation covered by the pile data. When
test piles are specified, the number of production piles shall be decreased
by the number of test piles since they will be driven in production
locations.

vi. The Number of Test Piles shall always be stated. When no test piles
are required, the designer shall specify zero test piles to document that
length.

Nov. 2006                                                                   Page 3.10.1-15

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 218 posted: 8/9/2010 language: English pages: 15