Rebar constraints lock rebar handle planes to planar references.
The planar references are as follows:
- Concrete element surfaces
- Stirrup bar handle planes. (standard style bars only)
Note: Arc-shaped rebar can form constraints to concentric host surfaces.
In most cases, the rebar handle plane and the reference plane must be parallel. However, bar end handles can constrain to planes at angles up to 60 degrees. 
You can specify the following types of constraints:
- Rebar Cover
- Other Rebar (Stirrup Handle Plane)
- To Host Surface
In the following figure, a stirrup and four simple, straight bars apply different constraint types. 
- Bar A: Constrains to the rebar cover at the top surface of the beam.
- Bar B. Constrains to stirrup of other rebar.
- Bar C. Constrains to the nearest parallel host surface (S).
- Bar D. A special constraint case. Longitudinal straight bar placed near a bend in the stirrup can constrain themselves to distinct locations along bends in stirrup bars. For example the points at 45 degree increments around each bend such as 0°, 45°, 90°, 135° and so on. An offset is applied to the constraints to rest the longitudinal bar against the inside of the stirrup's bend at the desired angular position.
Default Logic for Selecting Constraints References
The default constraint logic sequence for placed rebar is as follows:
- (Straight bars only) The bar seeks stirrup bar bend reference points to constrain its edge and planar position handles.
- The bar seeks the nearest host element cover surface. Standard style bars also look for stirrup bar handles, ignoring any host cover already occupied by a stirrup.
- If no cover surface or stirrup is found within the required tolerance, the bar seeks the nearest host surface, with or without cover. A constant distance lock constraint to that surface is formed.