3D Modeling Environment for Curved Structures
For curved structures with a smaller radius of curvature, it is appropriate to consider a 3D model for time dependent analysis as well as other transverse and torsion behavior. For this type of structure and analysis, IDS offers BD3.

BD3 is essentially a 3D version of Bridge Designer. In BD3 the members are 6 degree of freedom beams and the displacement field is 3-dimensional (3 translations and 3 rotations).

Main Differences between 2D and 3D:
BD3 is very similar to BD2, although it operates in 3D. All components (nodes, supports, segments, tendons, etc…) are defined in a 3-dimensional space. Because of this, there are key differences between the two products that should be considered. Even though great care was taken to provide a high level of compatibility at the input level (close to 100%), there are implications unique to a 3D model:

• Results are provided by BD3 in a 3-dimensional displacement field. Therefore there are 6 degrees of freedom per node (as opposed to 3 degrees of freedom in BD2).
  
• The stiffness matrix of each element is a 144-term matrix (as opposed to 36 terms in 2D). Even with bandwidth optimization the overall stiffness matrix is much larger, and so are all the components needed in the analysis. Therefore, BD3 uses more memory than BD2.
  

• Because of the 6 degrees of freedom, the internal forces resulting form the 3D analysis which now has 6 components, 3 force components (Fx, Fy and Fz), and 3 bending moments (Mx, My and Mz). These forces are produced for all the active members, for all load-cases and all construction phases. There is therefore a large amount of information produced, and this encourages the use of a graphical environment to view results.
  
• In order to be meaningful, results are shown either in local or general systems. For example it is appropriate to show the Mx moment (torsion) expressed in local.
  
• Supports and Restraints may be defined in local systems as well as in the general system, as dictated by the bridge model (horizontal and vertical curvature).
  
• Post-Tensioning tendons are defined in 3D as they are in BD2 as well. However, in BD3 there is a difference between “internal” and “external” tendons. Because of the overall model geometry (for example horizontal curvature), “external” tendons are assumed linear between deviation points (anchors and deviation blocks), and “internal” tendons, being inside the concrete elements, follow the overall model curvature.

Compatibility between 2D and 3D Models
Overall, there is excellent compatibility between BD2 and BD3. At the input level, any BD2 model is usable in BD3 with some very limited minor adjustments. BD3 will generate a full 3D post-tensioning layout based on the input provided in a BD2 model input file, saving a large amount of time in model preparation.