Applies To Product(s): AutoPIPE, Version(s): 2004, XM, & V8i Environment: N/A Area: Modeling Subarea: Original Author: Bentley Technical Support Group Dec 2014, AutoPIPE V8i 09.06.01.10 Problem: how to best model the bending stiffness and torsional stiffness of the ball joints. The manufacturer has given the following values for the ball joints; Axial stiffness = Rigid Transverse shear stiffness = Rigid Torsional resisting moment = 4610 ft -lbs Bending resisting moment = 2305 ft - lbs The problem is, that Autopipe accepts a "stiffness" for torsional and bending resistance but the ball joint friction is more a constant "friction torque" during movement and not proportional to the angular displacement. The only solution we can think of is to estimate the amount of angular displacement of the ball joint and scale the stiffness value accordingly so that the friction torque specified above is not exceeded. Can you provide any suggestion on the best way to model this type of joint? Answer: The approach suggested above is reasonable. Another possible solution is to calculate the moment in the joint and if the moment exceeds the limit friction moment. If exceeded, apply a constant moment (using force/moment) on one end of the joint to counter the friction and set rotational stiffness to zero. If the moment is not exceeded, set the stiffness to rigid. (So set to rigid first to estimate the moment and if exceeded, set back to zero with constant moment). 1. Set stiffness to rigid. 2. Analyze to calculate moment in the joint (for all basic load cases, not combinations) 3. If moment does not exceed friction moment, you are done, rigid is valid for that load case 4. If moment exceeds friction. Set stiffness to zero (unless one is provided) and apply a constant moment equal to friction moment on one end of the joint. The moment is applied in the load case evaluated. Make sure the moment sign is correct. The problem with this approach is that you cannot set stiffness to zero for one load case and rigid for different load case. See Also Bentley AutoPIPE External Links Bentley Technical Support KnowledgeBase Bentley LEARN Server Comments or Corrections? Bentley's Technical Support Group requests that you please submit any comments you have on this Wiki article to the "Comments" area below. THANK YOU!
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