FB Multipier v.4

Single Pile Example

Professor’s Driven Pile Institute

Utah State University – June 26, 2013

J. Brian Anderson (jbanders@auburn.edu)

Department of Civil Engineering Auburn University

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Deep Foundations – FB-MultiPier

Example 1a: Single Pile Axial Load

40 ft

Soil:

N = 15

= 32o

k = 100 pci

= 120 pcf

G = 7.5 ksi

= 0.33

f = 570 psf (from SPT97)

Qtip = 384 kips (from SPT 97

Pile:

Standard FDOT

24” Prestressed

Square Section

Default properties

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Start “FB-MultiPier” by selecting the

shortcut on your desktop, or find “BSI >

FB –MultiPier> FB – MultiPier” under

the Start Menu.

Select File>New from the menu at the

top to create a new file.

Fill in the appropriate project

information. Select, for this case,

choose Structure Type “Single Pile”

Finally set Units to “English (mixed

units)”. Click OK to continue.

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The editor windows will now appear.

The top left window is the Model Data

window where most info is entered. The

top right window is the Pile Edit window

shows the pile group in plan. The

bottom left window is the Soil Edit

window where the soil stratigraphy is

shown. A 3-D view of the pile group is

shown in the bottom right pane.

A default problem is automatically

loaded which you will need to change

Select “Pile” on the menu along the left

side of the Model Data pane.

Choose “Edit” under Cross Section Type

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Under “Database Section Selection,”

click on “Retrieve Section” and selection

Rectangular > 24” Square FDOT

Standard prestressed

Change the length of the pile to 40 ft.

Click OK to continue.

Select “Soil” from the Model Data pane.

Note that the “Soil Layer” drop down

box contains the layers shown in the Soil

Edit pane below.

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Since we will be using a single soil layer

for this example, select layer 2 then hit

the delete button to remove the layer.

Set the Soil Type, for Layer 1, to

“cohesionless.” Enter Total Unit Weight

equal to 110 pcf. Enter a friction angle

of 32o. Set the top of the layer at 0ft and

the bottom at –60ft (for proper

embedment). Since there is no water

table in this problem, use water table

depth of –100ft.

Under Soil Layer Models, use the drop

down box by “Lateral” to choose the

lateral soil type, in this case, Sand

(O’Neill).

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Click “Edit.” The unit weight and

friction angle will be copied from

before. Enter the value for subgrade

modulus, 100 pci. Click “OK” to

continue.

Under Soil Layer Models, use the drop

down box by “Axial” to choose the axial

soil type, in this case, Driven Pile.

Click “Edit.” The unit weight and

friction angle will be copied from

before. Enter the value for shear

modulus, 7.5ksi, Poisson’s Ratio, 0.3,

Vertical Failure Shear, 570psf. Click

“OK” to continue.

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Under Soil Layer Models, use the drop

down box by “Tip” to choose the end

bearing type, in this case, Driven Pile.

Click “Edit.” Enter the value for shear

modulus, 7.5ksi, Poisson’s Ratio, 0.3,

Axial Bearing Failure, 384kips. Click

“OK” to continue.

Soil properties can also be input in table

form. Click on the table button to bring

up the soil properties table.

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Soil Properties Input Table

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Lateral Properties for Sand:

Friction Angle,

Ne *0147.0*6034.27881.53

(Peck, Hanson, Thornburn, 1974)

(Gibbs and Holtz, 1967)

(Robertson and Campanella 1983)

Subgrade Modulus, rK

(Meyer and Reese, 1979)

(Terzaghi, 1955)

(Reese and Wang, 1993)

Lateral Properties for Clay:

Undrained Shear Strength, Su

VOUKC SNq

where: qc is cone tip resistance,

Nk is a factor ~17,

Su is undrained shear

strength,

and vo is the total

overburden stress. qc, Su

and vo are all in identical

units.

0

50

100

150

200

250

300

0 20 40 60 80 100

Dr (%)

k (

lb

/ i

nc

h 3

)

V ER Y

LOOS ELOOS E

M ED IU M

D EN S E

V ER YD EN S E

D EN S E

S A N D B ELOW

THE WA TER

TA B LE

S A N D A B OV E

THE WA TER

TA B LE

Relative Density of Sand

Parameters Loose Medium Dense

Blows/ft, N 4 to 10 10 to 30 30 to 50

, degrees 30 34 39

Dry or moist sand k lb/in3

8.1 24.3 64.8

Submerged sand k lb/in3

4.6 16.2 39.4

Relative Density of Sand

Loose Medium Dense

Dry or moist sand k lb/in3

25 90 225

Submerged sand k lb/in3

20 60 125

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Lateral Properties for Clay:

Undrained Shear Strength, Su

(EPRI Manual, 1990)

Subgrade Modulus, k

(Reese and Wang, 1993)

Strain at 50 % of the Failure Stress in

an Unconfined Compression Test, 50

(Reese and Wang, 1993)

(Reese and Wang, 1993)

Axial Properties for Sand and Clay

Unit Skin Friction, f

Plastic Clay: 6.4006

110**0.2)(

NNtsff

Clay-Silt-Sand:3.4583

110**0.2)(

NNtsff

Clean Sand: Ntsff *019.0)(

Limestone: Ntsff *01.0)(

(SPT97 Users Guide)

Shear Modulus, G

Sand/Sand with Fines

G(psi)= 500N [N<40]

G(psi) = 20000 [N>40]

Silt G(psi) = 500N [N<10]

G(psi) = 5000+175(N-10) [N>10]

Clay G(psi) = 500N [N<10]

G(psi) = 5000+100(N-10) [N>10]

(PLAID)

Ultimate Tip Resistance, QTIP

Plastic Clay: Ntsfq *73.0)(

Clay-Silt-Sand: Ntsfq *6.1)(

Clean Sand: Ntsfq *2.3)(

Limestone: Ntsfq *6.3)(

(SPT97 Users Guide)

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0

Su / Pa

SP

T N

Va

lue

Sowers (low PI)

Terzaghi & Peck, Sanglerat (sandy clay)

Sowers (medium PI)

U.S.B.R (Houston clay-Trend line)

Schmertmann (Chicago clay)

Sanglerat (silty clay)

Sowers (high PI)

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Next, we need to specify the loads on the

pile. Now, choose “Load” menu in the

model data window. Two default load

cases will be displayed.

Highlight “Load Case 2”, then click in

“Del” to delete it.

For this case, we will edit load case 1.

Click on “Node 1” under “Node

Applied.” Node 1 in this case is the top

of the pile. Make sure that the only load

listed is 310k in the Z direction.

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Alternately, you can edit load cases using the load table

You should see an arrow representing

this load in the “3dView” box.

This concludes the data input.

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Use the “save as..” command to save

your problem before the analysis.

The next step will be to perform the

analysis. Click on the lightning bolt icon

to run the analysis. You will get a

warning that you will force a file save,

accept and continue.

The iterative process of FB-MultiPier

will be shown while the analysis

progresses.

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Once the analysis is complete, click on

the 3D Results icon to see a graphical

depiction if the analysis.

In the tool bar area, you can select the

load case to view, in this problem there

is only load case 1. In the 3D Display

Control pane, you can select which node

to display displacements or rotations, as

well as the nodal coordinates. A 3

Dimensional representation of the results

is shown in the 3D View pane. The

deflection of node 1 (top of the pile

should be approximately 0.5 inches)

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Example 1b: Single Pile Lateral Load

Use same input as for axial loading presented above:

In the Model Data window, select

“Analysis” from the menu, click “user

defined” Phi ( ) factor and set = 1.0.

This removes AASHTO phi factors from

moment interaction diagram. Only use

this for hand calculation verifications.

In the “Load” Tab: click node # 1 and

set Z = 0.0, and X = 30kips.

Lat = 30 k

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Click on the lightning bolt icon to run

the analysis. You will get a warning that

you will force a file save, accept and

continue.

Click “Pile Results” (icon next to

lightning bolt) and see results below:

a. X deflection = 0.176”, soil lateral

load = 13.7 kips

b. Max moment = 136 ft-kips and

Ratio = 0.172

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Moment details: Click “Pile Interaction” and “Biaxial moment” and “component selection”

a. Maximum moment for 24” x 24” PSC pile is 795.4 to 795.6 ft-kips

b. 33

3

3 333.16

2

12

)(

()( ft

b

b

bh

axisneutralfromdistC

inertiaofmomentIftS

c. kipsftconcreteksiftMomentstressyield

MomentS 152,11446333.1; 3

max

d. However, more correctly is moment interaction shown by FB-Pier, where due to

prestressing Mmax ≈ 654 ft-kips

e. Moving up and down the segment selection finds “element #4 giving highest moment

(see on moment interaction diagram) Demand Ratio = 0.208 (4b above) = 136 ft-kips /

654 ft-kips

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Example 1c: Single Pile - Lateral Pushover Analysis

Return to “Edit” icon and then select

“Load” from the Menu. Add Load Case

# 2 and set X = 5 kips.

Click “Pushover” tab. Pushover will

increment lateral load in 5kip increments

until “failure”, either soil or maximum

moment.

Results of Pushover show: Lateral load producing “failure” = 120 kips (18 x 5 kips = 90 kips + original

30 kips load case #1 = 135 kips) Deflection = 2.17”, Max moment = 648 ft-kips, Ratio = 648 / 654 =

0.99

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