no slide title · this loss of stiffness can increase ... - the nonlinear foundation technique in...

2012 ANSYS, Inc. February 9, 2013 1 Release 14.5

14.5 Release

Lecture 7

Pile Analysis


Pile definition in Mechanical

- There are a number of methods that can be used to analyze piled foundations in ANSYS Mechanical:

SPLINTER (a standalone nonlinear solver that was originally developed as part of the ASAS suite)

Matrix definition (MAT27)

Spring definition (COMBIN14)

Direct Definition of P-Y and T-Z curves (COMBIN39)

The SPLINTER method will be described here


Soil-pile interaction

- Soils are classified in four different types based on their particle size and shape:

- The lateral resistance of the soil near the mud line is significant for many reasons, and the effects of this resistance of scour and soil disturbance during pile installation should be considered.

Soil Type Particle Size (mm) Particle Shape

Gravel 2 60 Granular

Sand 0.06-2 Granular

Silt 0.002 0.06 Granular

Clay < 0.002 Flat plate


Soil-pile interaction (cont.)

- The flexibility of the soil makes the soil-structure system less stiff than if the structure were on a fixed base. This loss of stiffness can increase f which can increase the dynamic response of the system (Watt et al. 1976)

- Three important features need to be considered:

a. Soil is a nonlinear material (stiffness decreases with increasing shear stress) b. Soil strength and stiffness is affected by repeated cycles of load c. Soil exhibits damping, which increases with increasing shear amplitude, when subjected to cyclic loading



- The nonlinear foundation technique in SPLINTER utilises a FE representation of the pile and soil system as shown below. The soil stiffness is modelled with spring elements in the lateral and vertical direction

- The soil springs are characterised by a nonlinear force-deflection curve. These curves are known as the P-Y and T-Z curves for lateral and axial (skin friction) behaviour respectively

- It is because of the nonlinear nature of the soil that SPLINTER carries out an iterative solution technique, each iteration taking an assumed, or calculated, value for the soil spring stiffness based on the previous iteration (see Newton-Raphson method)



- Soils are highly nonlinear in their behaviour, Youngs, Shear and Bulk Modulus varies with loading. The nonlinearity is caused by the soil particles moving relative to one another as the load is applied.

- For preliminary decisions the relevant values in BS5930, BS1377 and BS8004 may be adequate.

- In situ measurements are necessary with soil samples at later stages

Chakrabarti, SK (2005) Handbook Offshore Engring, Vol.1



- The generation of lateral soil resistance deflection (P-Y) curves should be constructed using stress-strain data from samples. Various methods of estimating P-Y curves for different soil types exist in the literature: Matlock (1970), Reese & Cox (1975), ONeill & Murchinson (1983)

LATERAL CAPACITY FOR SOFT CLAY

The p-y curves for the short-term static load case can be generated from the following table

p/pu y/yc

0.00 0.0

0.50 1.0

0.72 1.0

1.00 8.0

1.00

D

cXJXcpu 3 cpu 9and for RXX

where pu is ult resistance, psi c = undrained shear strength for undisturbed clay samples (psi) = eff unit wgt of soil (lb/in3) D = pile dia (in.) J = dimensionless empirical factor ranging from 0.25 to 0.5 X = depth below the sea bed (in.) XR = depth below sea bed to which the reduced resistance zone extends (in.) Min values approx. 2.5D p = actual lateral resistance (psi) y = actual lateral deflection (in.) c = strain which occurs at one-half the max stress on lab undrained compression tests of undisturbed soil samples

Dy cc 5.2



LATERAL CAPACITY FOR STIFF CLAY

The ultimate bearing capacity of stiff clay (c > 96kPa) would vary between 8c



Once the previous values are found the load deflection curves for sand are given by

y

AP

kHAPyP

u

u tanh)(

Where A = factor to account for cyclic or static loading Pu = ult resistance (psi) k = initial modulus of subgrade reaction (lb/in3) See fig below = depth (in) y = lateral deflection (in)

Relative density is an indication of the packing in comparison with the min (0% relative density) and max (100% relative density) densities achievable



- Two solution methods exist for determining the soil stiffness from P-Y and T-Z curves, namely Tangent and Secant stiffness. In general the tangent stiffness method (default) will converge faster.

- Soil properties are assumed to vary linearly between the defined points, for points outside the defined range linear extrapolation is assumed.

- Both symmetric and asymmetric curves may be defined for soil data

- The option to include/ignore pile self weight, select method to use in derivation of soil stiffness, include/exclude second order effects is given in PILEGEN command



- Extreme weather phenomena and the passage of waves induce pressure changes on the seabed that may cause remoulding and a reduction in shear strength. This in turn can lead to shear failure in soft sediments on sloping ground

- Mudslides (depressions in the seabed) should be considered in the design process and a piled structure should withstand a mudslide through the lowest plan bracing level

- This problem can be taken into account in SPLINTER when defining the soil properties (file_soil.txt). This ascii file should be in the solver files directory (RC on Solution>Open Solver Files Directory). For the syntax of this file see // Advanced Analysis Guide // 15. Soil-Pile-Structure Analysis // 15.2. Soil Data Definition and Examples


Nonlinear solution using linear solvers

- In a nonlinear analysis, the response cannot be predicted directly with a set of linear equations

- However, a nonlinear structure can be analyzed using an iterative series of linear approximations, with corrections

- ANSYS uses an iterative process called the Newton-Raphson Method. Each iteration is known as an equilibrium iteration

F

u Displacement

Load

1

2 3 4

A full Newton-Raphson iterative analysis for one increment of load. (Four iterations shown)

K1

K2 K3 K4



In the Newton-Raphson Method, the total load Fa is applied in iteration 1. The result is x1. From the displacements, the internal forces F1 can be calculated. If Fa F1, then the system is not in equilibrium. Hence, a new stiffness matrix (slope of red line) is calculated based on the current conditions. The difference of Fa - F1 is the out-of-balance or residual forces. The residual forces must be small enough for the solution to converge

This process is repeated until Fa Fi. In this example, after iteration 4, the system achieves equilibrium and the solution is said to be converged

F4

1

2 3

4

Newton-Raphson Method

F1

x1

The actual relationship between load and displacement (shown as a dotted line) is not known beforehand. Consequently, a series of linear approximations with corrections is performed. This is a simplified explanation of the Newton-Raphson method (shown as solid red lines)

F3 F2

x3

x2

x4



The difference between external and internal loads, {Fa} - {Fnr}, is called the

residual. It is a measure of the force imbalance in the structure.

The goal is to iterate until the residual becomes acceptably small; that is,

until the solution is converged.

When convergence is achieved, the solution is in equilibrium, within an

acceptable tolerance.

{Fa} {Fnr}

Fa

Fnr

u

{


SPLINTER

To use SPLINTER in Workbench Mechanical, the process required on a jacket model is:

Define named selections for each Pile cap node named Pile1, Pile2 etc.

Request approx. 5 loadsteps and set up with small time increment (the loadsteps are needed to allow iteration on soil curves; the small time step is needed as the ocean loading is time dependant)

Insert a command object to define the ocean

Insert a command object to define the piles

Insert a command object to solve each iteration


Workshop 5 Add piles to a jacket structure

Goal:

Use command objects & become acquainted with MAPDL

Use local parameters in command object

Study the necessary input for SPLINTER (strata, curves, mudslide etc.)

Get to know the analysis settings for a soil-pile analysis

Study the output from SPLINTER

Workshop 5 Pile Analysis
../Workshops/WB-Mech_120_WS_03.1.ppt../Workshops/WB-Mech_120_WS_03.1.ppt../Workshops/WB-Mech_120_WS_03.1.ppt../Workshops/WB-Mech_120_WS_03.1.ppt../Workshops/WB-Mech_120_WS_03.1.ppt

no slide title · this loss of stiffness can increase ... - the nonlinear foundation technique in...

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