Underground Pipe Loads - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

Typical buried pipe displacements are considerably different than similar above-ground displacements. Buried pipe deforms laterally in areas immediately adjacent to changes in directions, such as those found in bends and tees. In areas far removed from bends and tees, the deformation is primarily axial. The optimal size of an element, that is, the distance between a single FROM and a TO node, is dependent upon which of these deformation patterns is to be modeled. Because there is no continuous support model, the software must locate additional point supports along a line to simulate this continuous support. These additional point supports can also be user-defined. For a given stiffness per unit length, one of the following must be added:

  • Several closely spaced, low stiffness supports

  • A limited number of distant and high stiffness supports

Where the deformation is lateral, smaller elements are needed to properly distribute the forces from the pipe to the soil. The length over which the pipe deflects laterally is called the "lateral bearing length" and can be calculated using the following equation:

Lb = 0.75(π) [4EI/Ktr] 0.25

Where:

E

=

Pipe modulus of elasticity

l

=

Pipe moment of inertia

Ktr

=

Transverse soil stiffness on a per length basis

CAESAR II places three elements in the vicinity of this bearing span to properly model the local load distribution. The bearing span lengths in a piping system are called the Zone 1 lengths. The intermediate lengths in a piping system are called Zone 2 lengths, and the axial displacement lengths in a piping system are called the Zone 3 lengths. To properly transmit axial loads, Zone 3 element lengths are computed using 100 x Do, where Do is the outside diameter of the piping. The Zone 2 mesh consists of four elements of increasing length; starting at 1.5 times the length of a Zone 1 element at its Zone 1 end and progressing in equal increments to the last which is 50 x Do long at the Zone 3 end. CAESAR II views a typical piping system element breakdown or mesh distribution as shown below. All pipe density is set to zero for all pipe identified as buried so that deadweight causes no bending around these point supports.

CAESAR II automatically puts a Zone 1 mesh gradient at each side of the pipe framing into an elbow. You must tell CAESAR II where the other Zone 1 areas are located in the piping system.

A critical part of the modeling of an underground piping system is the proper definition of Zone 1or lateral bearing regions. These bearing regions primarily occur:

  • On either side of a change in direction.

  • For all pipes framing into an intersection.

  • At points where the pipe enters or leaves the soil.

  • Using any user-defined node within or near Zone 1.

Data Conversion

CAESAR II converts the original job into the buried job by meshing the existing elements and adding soil restraints. The conversion process creates all of the necessary elements to satisfy the Zone 1, Zone 2, and Zone 3 requirements, and places restraints on the elements in these zones. All elbows are broken down into at least two curved sections, and very long radius elbows are broken down into segments whose lengths are not longer than the elements in the immediately adjacent Zone 1 pipe section. Node numbers are generated by adding “1” to the element’s FROM node number. The software checks a node number to make sure that is unique in the model. All densities on buried pipe elements are zeroed to simulate the continuous support of the pipe weight. A conversion log is also generated, which details the process in full.