LRFD Pile Design

Design Guides 3.10.1 - LRFD Pile Design

Nov. 2006 Page 3.10.1-1

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:

lqD

D0.8N1q b60

p ≤=

where: N '

400.77logN1v

1060⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛σ

= & silt)c(nonplasti6N1q

(sands)8N1q

60

60

=

=

l

l

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.)


Page 3.10.1-2 Nov. 2006

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:


Nov. 2006 Page 3.10.1-3

For granular soils, the qS should be computed as:

For Hard Till,

60s 1N07.0q = )301N( 60 < 13.11N00888.01N00136.0q 60

260s +−= )1N30( 60≤

Very Fine Silty Sand,

60s 1N1.0q = )301N( 60 < ( )

⎥⎦

⎤⎢⎣

⎡

−−

= 794405.1751N

s

2

60

e58.42q )741N30( 60 <≤ 2.101N297.0q 60s −= )1N74( 60≤

Fine Sand,

60s 1N11.0q = )301N( 60 <

51.121N

1821N3256.0q60

60s −+= )661N30( 60 <≤

91.91N329.0q 60s −= )1N66( 60≤

Medium Sand,

60s 1N117.0q = )261N( 60 < 13.21N0697.01N00404.0q 60

260s +−= )551N26( 60 <≤

1.91N356.0q 60s −= )1N55( 60≤

Clean Medium to Coarse Sand,

60s 1N128.0q = )241N( 60 < 05.21N0693.01N00468.0q 60

260s +−= )501N24( 60 <≤

42.91N394.0q 60s −= )1N50( 60≤


Page 3.10.1-4 Nov. 2006

Sandy Gravel,

60s 1N15.0q = )201N( 60 < 91.31N217.01N00861.0q 60

260s +−= )401N20( 60 <≤ 0.151N6.0q 60s −= )1N40( 60≤

For cohesive soils, the qS should be computed as:

u2u

3us 1.09Q0.177QQ

25001q +−

−= tsf)1.5(Qu ≤

0.0681.278Q0.347Q0.0495Qq u

2u

3us −+−= tsf)2Qtsf(1.5 u <<

0.5550.470Qq us += tsf) 4.5Qtsf(2 u <≤ *

ksf 67.2qs = )Qtsf(4.5 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


Nov. 2006 Page 3.10.1-5

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 )


Page 3.10.1-6 Nov. 2006

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:

Pile Designation Maximum Nominal 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


Nov. 2006 Page 3.10.1-7

loadings. It accounts for any reductions in geotechnical resistance that occurs 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 again to account for the added loading applied to the pile. The 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


Page 3.10.1-8 Nov. 2006

shall be taken as zero when calculating the Factored Resistance Available (RF) to resist Extreme Event I seismic loadings.

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) to resist Extreme Event I seismic loadings.

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


Nov. 2006 Page 3.10.1-9

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 the Factored Resistances Available for Extreme Event I loadings by setting the geotechnical resistance factor (φG) to 1.0, and for Strength Limit State loadings by setting φG to 0.5.

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.


Page 3.10.1-10 Nov. 2006

iii. The geotechnical engineer should discuss the preliminary estimated pile length vs. Factored Resistance Available (RF) relationship with the structural engineer and obtain any preliminary loading information to help narrow the range of pile types and loadings to be included in the SGR pile design table(s). This usually involves obtaining total factored substructure loadings and dividing by the maximum and minimum pile spacings to estimate the possible Factored Loadings Applied (QF) 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.

a. Find Controlling Factored Loading Applied to the foundation:

Calculate the factored loadings as described in Section 3 of LRFD for all applicable loading cases including extreme events. The factored service limit 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 loading will control the pile design while in others several load groups will need to be checked. Each factored loading group under consideration should be 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


Nov. 2006 Page 3.10.1-11

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 load groups specify that the portion of DD which applies a loading to the pile be included with loadings from other applicable sources. However, it is IDOT’s 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 the LRFD group loadings as suggested.

b. Pile Group Layout and Factored Loading Applied (QF) per pile:

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. The minimum Factored Loading Applied (QF) should normally be greater than 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 loading.

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.


Page 3.10.1-12 Nov. 2006

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 resistance available (RF) to support the factored loadings applied (QF). Showing 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 combined axial and lateral loadings, considering any unbraced length either 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]

See also the LRFD Specifications for applicable resistance factors for Extreme Event loadings such as earthquake.

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


Nov. 2006 Page 3.10.1-13

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.


Page 3.10.1-14 Nov. 2006

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,


Nov. 2006 Page 3.10.1-15

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 the estimated pile length was made with sufficient confidence or added length.

Documents

LRFD Pile Design