Page 174 - Buried Pipe Design

148 Chapter Three

be called CANDE-2007 will soon be available which supports many new
features and can used for LRFD design.
A study completed at Utah State University in 1985 addressed some
of the problems of finite element modeling. The responses of flexible
pipes under various loading conditions, compaction conditions, and
groundwater conditions were analyzed. To calibrate the FEA technique,
results from physical tests were used for comparison. The actual model-
ing of some of the different loading schemes brought out the need for
additional development of FEA capabilities that had not been addressed
in any previous research efforts. These developments have greatly
increased the ability to more accurately model soil-structure interac-
tion, particularly for very flexible pipes.
A computer code SSTIPN was obtained and modified by the Utah
State University researchers. This program has a structure similar to
58
the program SAP originally developed by Wilson. Modifications to SAP
to include soil modeling were implemented by Ozawa and Duncan. 38
Further enhancements including interface elements and improved soil
59
models were included (Duncan et al. and Wong and Duncan ). The
6
SSTIPN program has been significantly enhanced at Utah State
University and is currently called PIPE5.
The FEA research and program development that was performed
included the addition of nonlinear geometric analysis; an improved
iteration scheme; modifications to the soil model to include primary
loading, unloading, and reloading analysis; and improved output files
for pipe response analysis and plotting and currently runs in the
Microsoft Windows environment. Applications of the enhanced model
included compaction simulation, initial ovalization of the pipe, unsym-
metric compaction and bedding analysis, and pseudotime effects due
to saturation and soil structure collapse.
Laboratory testing for the soil properties was also performed. The
testing included grain-size analysis, Atterberg limits, compaction, con-
fined compression, and triaxial testing for stress-strain properties of
each soil type. The results of the triaxial testing were used to analyze
the pipe response for several soil types in PIPE5.

A general description of FEA solutions
for buried pipe
A finite element method for stress analysis in solid mechanics is a
mathematical technique whereby a continuum is idealized by dividing
it into a number of discrete elements. These elements are connected to
their adjacent elements at nodes which are typically located at the
ends or corners of the elements. Special shape functions are used to
relate displacements along the element boundaries to the nodal dis-
placements and to specify the displacement compatibility between adja-
cent elements. Once the continuum has been idealized, as shown in