The actual depth of the stairs treads in the stairs, actual tread depth is equal to minimum tread depth by default. 
Namespace: Autodesk.Revit.DB.Architecture
Assembly: RevitAPI (in RevitAPI.dll) Version: 24.0.0.0 (24.0.0.0)
Since:  2013 
Syntax
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Exceptions
| Exception | Condition | 
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| Autodesk.Revit.ExceptionsArgumentOutOfRangeException | When setting this property: The given value for actualTreadDepth must be greater than 0 and no more than 30000 feet. |