The CAST Education and Design Tool |
3.1 |
Objective and Overview of CAST |
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The CAST program is being developed to provide both students and practitioners with a graphical design tool that makes the design process more efficient and transparent. The PI has had the opportunity to teach the STM to eight classes of students. It has been his experience that students readily take to STM due to their familiarity and confidence in the simple mechanics of a truss. They have also been able to grasp proposed design provisions for the strength of struts, ties, and nodal zones. However, once a specific design assignment is given, they can quickly become bogged down in the details of calculating truss member forces and ensuring that the dimensions of the struts and nodal zones are sufficient to support the imposed loadings. The CAST program is a graphically interactive design tool that has been under development for the last two years. This award would make possible the continued support of this work, and enable CAST to be developed into tool that allows the designer to quickly optimize his or her design, handle multiple load cases, and generate final drawings. This program would also serve as an instructional device, familiarizing students and practitioners with both the program and the strut-and-tie design philosophy.
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3.2 |
Design Sequence Using Program CAST |
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The user begins by defining the boundaries of the D-Region using a mouse, by numerical entry, or by selecting an object from a group of templates. The position of all of the nodes that define the shape of the object can be easily redefined. The applied loading, bearing plates, material properties, and support conditions can then be selected from toolbars or specified in dialogue boxes. To help the user in the selection of a truss, an elastic finite element analysis feature can be used to generate stress contours and principal stress trajectories. The designer defines the truss by first selecting the location of the center of the nodes and then forming truss members by interconnecting these nodes. The truss can then be analyzed, with the results being displayed alongside the truss member. Using a toolbar, the user can then select and position the estimated tension tie reinforcement as well as select the widths of the compressive struts. The capacity of the struts and ties as well as the applied stress on the faces of the nodes can then be displayed. The designer can use the nodal zoom edit tools to change features of the node that influence node and truss geometry, and thereby affect the stresses on the face of the nodes and design strengths. Once the user is satisfied with the design, the detailed design including member forces, capacities, stresses on the faces of nodal zones, and reinforcement details can be printed.
Some of these design steps are illustrated in Figure 14 below. Note that some of the images have been taken from the present version of program CAST while others were generated using a graphical design program.
Figure 14 Images that Describe the CAST Education and Design Tool
(Click here to view a larger image)
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3.3 |
Capabilities of CAST |
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While a few finite element based programs [31-34] have been written to help in the selection and evaluation of strut-and-tie models, it is believed that this is the first program with the capabilities that were summarized above and are more fully described below. Note that most aspects of capabilities C1 and C2, and a small number of C5 and C6 have been completed over the last two years.
C1 GUI for Drawing Capabilities
- Define structure boundaries either graphically or numerically
- Standard drawing features (cut, copy, paste, move, zoom, pan, unlimited redo and undo)
- Toolbars and dialogue boxes for defining common elements such as loading forces, material properties, supports, and bearing plates
- Define joints by mouse or in dialogue box
- Define struts and ties by interconnecting nodes
C2 Truss Solution and Member Definitions
- Analysis of truss at command
- Stability checked with warning messages
- Labeling of truss member values
- Selection of characteristics of struts and nodes
- Dialogue box for selection of tie reinforcement
- Illustrated dimensions and stress intensities
C3 Libraries and Truss Reconfigurations
- Templates for common D-Regions and trusses
- Autoselection of truss for design of B-regions
- User selected truss and node shapes
C4 Load Conversions and Multiple Load Cases
- Auto conversion of distributed and sectional forces to point load on truss elements
- Display of multiple dependent and independent windows
- Analysis of design for alternate load cases
C5 Advanced Drawing and Input/Output Capabilities
- Ability to print all images
- Displayed comparison of member loads and capacities
- Nodal zoom feature for additional details
- Easy adjustment of design details to adjust capacities
- CAD input/output features
- Create tab delimited files that save design details and results of design in spreadsheet format
C6 Automesh Generation and Elastic Stresses
- Automesh generation and finite element analysis
- Contour plots of stresses
- Plots of principal strain trajectories
C7 Non-Linear Truss Analysis
- Estimate non-linear characteristics of struts and ties
- Estimate forces in statically indeterminate trusses
- Predict load deformation response
- Option to use concrete ties
C8 Additional and Advanced Capabilities
- Help feature with information on benefits of confinement, fiber reinforcement, and improved anchorage
- Truss checker program (failure mode, dimensions of nodal regions, serviceability, suggestions)
- On-line information on allowable stresses/international codes
- Extension for the design of 3-dimensional structures
- Instructional tutorial for program CAST
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