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Pile Spring

This document discusses the use of node springs (ks) to model lateral pile behavior in finite element analysis. It provides an equation (9-10) to calculate ks values based on depth and presents examples of using a ks profile to compute first four node springs for a pile. The program allows input of individual ks values for stratified soil or adjustment of an overall ks equation. Piles can be modeled with fixed or intermediate springs to represent attachments like on offshore drilling platforms. Sensitivity analyses show pile response depends more on the range of ks values than the exact values.

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Azizul Khan
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114 views

Pile Spring

This document discusses the use of node springs (ks) to model lateral pile behavior in finite element analysis. It provides an equation (9-10) to calculate ks values based on depth and presents examples of using a ks profile to compute first four node springs for a pile. The program allows input of individual ks values for stratified soil or adjustment of an overall ks equation. Piles can be modeled with fixed or intermediate springs to represent attachments like on offshore drilling platforms. Sensitivity analyses show pile response depends more on the range of ks values than the exact values.

Uploaded by

Azizul Khan
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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tions are usually obtained as well as, sometimes, bending moments in the top 1 to 3 m of

the embedded pile. From these one might work back using one's favorite equation for lateral
modulus (or whatever) and obtain values to substantiate the design for that site.
Node values (or an equation for node values) of ks are required in the FEM solution for
lateral piles. Equation (9-10), given in Chap. 9 and used in Chap. 13, can also be used here.
For convenience the equation is repeated here:
^ = A , + BsZn (9-10)
If there is concern that the ks profile does not increase without bound use Bs = 0 or use
Bs in one of the following forms:

Bs ( | j = ^ Z " = B'sZn (now input B's for B5)

or use B5(Z)" where n < 1 (but not < 0)

where Z = current depth from ground surface to any node


D = total pile length below ground

The form of Eq. (9-10) for ks just presented is preprogrammed into program FADBEMLP
(B-5) on your diskette together with the means to reduce the ground line node and next lower
node ks (FACl, FAC2 as for your sheet-pile program). You can also input values for the
individual nodes since the soil is often stratified and the only means of estimating ks is from
SPT or CPT data. In this latter case you would adjust the ground line ks before input, then
input FACl = FAC2 = 1.0.
The program then computes node springs based on the area Ac contributing to the node,
as in the following example:

Example 16-9. Compute the first four node springs for the pile shown in Fig. El 6-9. The soil
modulus is ks = 100 + 50Z 05 . From the ks profile and using the average end area formula:

Summary,

,etc.
ks = Profile

Projected pile width, m


Figure E16-9

Example 16-9 illustrates a basic difference between this and the sheet-pile program. The
sheet-pile section is of constant width whereas a pile can (and the pier or beam-on-elastic
foundation often does) have elements of different width.
This program does not allow as many forms of Eq. (9-10) as in FADSPABW; however,
clever adjustment of the BS term and being able to input node values are deemed sufficient
for any cases that are likely to be encountered.
In addition to the program computing soil springs, you can input ks = 0 so all the springs
are computed as Ki = 0 and then input a select few to model structures other than lateral piles.
Offshore drilling platforms and the like are often mounted on long piles embedded in the soil
below the water surface. The drilling platform attaches to the pile top and often at several
other points down the pile and above the water line. These attachments may be modeled as
springs of the AE/L type. Treating these as springs gives a partially embedded pile model—
with possibly a fixed top and with intermediate nonsoil springs and/or node loads—with the
base laterally supported by an elastic foundation (the soil).
Since the pile flexural stiffness EI is several orders of magnitude larger than that of the
soil, the specific value(s) of ks are not nearly so important as their being in the range of 50 to
about 200 percent of correct. You find this comparison by making trial executions using a Ic5,
then doubling it and halving it, and observing that the output moments (and shears) do not
vary much. The most troublesome piece of data you discover is that the ground line displace-
ment is heavily dependent on what is used for ks. What is necessary is to use a pile stiff enough

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