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This article was originally created for SAP2000, but the concept applies also to CSiBridge.
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This page describes how to model pile lateral support based on P-y curves.

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h1. Introduction

P-y curves describe the lateral soil resistance along the depth of a pile or drilled shaft and are typically provided by geotechnical engineer. For each layer of the soil along the depth of a pile, the P-y curve describes lateral soil reaction _P_ per unit length along the pile mobilized by lateral pile displacement _y_. The relationship between _P_ and _y_ is nonlinear.

For piles modeled by [frame elements|kb:frame], the lateral support provided by the soil can be modeled by [nonlinear links|kb:link] distributed along the pile. The modeling of lateral soil support involves the following steps:

* Define the properties of links that will represent the lateral support provided by the soil.
* Assign to the links to the frame elements representing the piles.

h1. Link Definition

Nonlinear link properties can be defined (in SAP2000) via "Define > Section Properties > Link/Support Properties" as follows:

* Add new link property of "MultiLinear Elastic" type and specify nonlinear link stiffness for U2 degree of freedom to match the stiffness described by the P-y curve. Specifying the nonlinear stiffness for the U2 direction assumes that the the structure will be [pushed|kb:Pushover] along the Global X axis. For loading along other directions, the links local axes should be rotated to align the link stiffness with the direction of the loading.
* Enter the factor for [line spring|tp:Line and area springs] to represent the unit length along the pile. 

h1. Link Assignment

Assign the above-defined links to the frame elements representing the piles as [line springs|tp:Line and area springs]:

* Mesh the piles such that there is at least one frame element within each soil layer.
* Select the pile frame elements within a given soil layer and use the "Assign > Frames > Line Links" menu command to assign the P-y link with the properties corresponding to that layer. Repeat this for all layers.

h1. Miscellaneous

* Discrete springs could be used in lieu of line springs. In this case, the springs would be assigned to joints of the pile. Each joint should be located in the middle of a soil layer and the spring stiffness should represent the total stiffness provided by that layer.
* 2-joint links could be used instead of 1-joint links. In this case, one end of the link would be connected to a joint on the pile and the other end would be connected to a fixed joint. The nonlinear link stiffness would be defined for the U1 direction.

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Related Incidents:

* {incident:no=31866|comment=Inputting P-y curves, includes typical table with P-y curve data}
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