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3CANDE, an acronym derived from Culvert ANalysis and DEsign, is a finite element program developed especially for the structural design, analysis, and evaluation of buried struc- tures including culverts made of corrugated metal, reinforced concrete, and plastic as well as other soil-structure interaction problems such as underground storage facilities and storm water chambers. CANDE was first introduced in 1976 under the sponsorship of the FHWA (1, 2). Over the years, FHWA has sponsored upgrades to the program culminating in 1989 with the CANDE-89 program (3, 4), which offers the follow- ing analytical features: ⢠Plane strain geometryâa 2-D finite element representation of the soil and structure under quasi-static vehicular load- ing conditions and based on small deformation theory. ⢠Incremental constructionâthe capability to simulate the physical process of placing and compacting soil layers, one lift at a time, below, alongside, and above the culvert as the installation is constructed. ⢠Interface elementsâthe ability to simulate the frictional sliding, separation, and re-bonding of two bodies origi- nally in contact. Typically these elements are used between the culvert and soil and between trench soil and in situ soil. ⢠Soil elements and modelsâsoil elements are high-order con- tinuum elements with a suite of soil models ranging from linear elastic to highly nonlinear. The so-called Duncan and Duncan/Selig soil models are very representative of the non- linear soil behavior in most culvert installations. ⢠Beam elements and pipe modelsâa culvert (or structure) is represented by a connected sequence of short beam- column elements that trace the culvertâs periphery. The material models of beam elements distinguish between different pipe types. Each material model includes design criteria, which provide measures of safety against potential modes of failure. Over the years, CANDE has enjoyed a great deal of popular- ity in the culvert community including state DOTs, consulting firms, manufacturers, universities, and research institutions in the United States and abroad who have used the program to design culvert installations, understand soil-structure behav- ior, interpret experimental results, and design new products. The popularity of CANDE is, in part, due to the rigorous adherence to the principle of good mechanics and to the trust- worthiness of the program, earned over years of testing (5). Equally important to its popularity is the availability of CANDEâs source code and documentation of the program- ming architecture. Accessibility to the source code is an extremely important feature for researchers who often want to test new theories and models as part of their research effort. Often, these research studies lead to new additions to the pro- gram that will ultimately become available to the entire culvert community. CANDE UpgradeâAASHTO In May 2005, TRB/NCHRP negotiated a contract with Michael Baker Jr., Inc. to modernize and upgrade CANDE-89 under the sponsorship of AASHTO. The project known as the CANDE Upgrade Project, designated as NCHRP Project 15-28, is the funding source for the enhancements that have been in- corporated into CANDE-2007 presented in this report. The CANDE Upgrade Project targeted the following three areas for enhancement: ⢠Pre- and post-processing with modern computer technol- ogy, ⢠New analysis capabilities and architecture for multiple structures, and ⢠Update design criteria for all culvert types to include LRFD methodology. Pre- and Post-Processing Previous versions of CANDE were in batch-mode input (i.e., ASCII text input file/output file). The input was provided C H A P T E R 1 Background
4via documented input instructions and an ASCII text editor without dedicated GUI software to aid the user in data prepa- ration. Similarly, output data was written to an ASCII text output file without any post-processing capability to graphi- cally display results. CANDE-2007 is now equipped with a Windows®-based, menu-driven format for interactive data input and real-time control of data output along with a context-sensitive help sys- tem and numerous graphical plotting options. New Analysis Capabilities The architecture developed for CANDE-2007 allows the use of multiple pipe groups (previous versions of CANDE permitted a single pipe group), thereby allowing an analy- sis of several culverts placed side-by-side, or a retrofit design (e.g., a plastic pipe inserted inside a corrugated steel pipe). In addition to the small deformation theory utilized in the original version of CANDE, CANDE-2007 has been updated to include a Lagrange formulation that provides an accurate and robust algorithm for predicting large deformations along with methodology for predicting buckling capacity at the end of each load step. CANDE has also been updated to include an automated bandwidth minimizer, thus relieving the user of the require- ment of assigning node numbers in a judicious manner to manually reduce the bandwidth of the system matrix. Design Criteria and LRFD Methodology The original CANDE program was based on the concept of working-stress design methodology. Accordingly, the previ- ous versions of CANDE reported safety factors against various modes of failure (design criteria) that have been traditionally associated with the type of culvert being considered. In recent years, AASHTO (6) has adopted the reliability-based LRFD methodology. Rather than using safety factors, the LRFD methodology uses load factors and resistance factors to achieve a safe design. As part of the CANDE Upgrade Project, a complete and compatible set of design criteria, applicable to both working- stress and LRFD methodologies, was identified for the common culvert materials: corrugated metal, reinforced con- crete, and thermoplastic pipe. These design criteria are now incorporated in the corresponding pipe subroutines of the CANDE-2007 analysis subsystem. The user now has the op- tion to choose either service load (working-stress evaluation of the design criteria) or factored loading (LRFD evaluation of the design criteria). Many other improvements have been incorporated into CANDE-2007 that are not mentioned above but are discussed in detail in the CANDE-2007 User Manual and Guideline and the CANDE-2007 Solution Methods and Formulations docu- ments. It should be noted, however, that all the capabilities and options that existed in the CANDE-89 version have been retained.