Tips & Tricks: Flex body simulation

“rigidified = ” – a useful flex body attribute


This article serves as an introduction to a recently introduced attribute called ‘Rigidified” for the Flex body statement in MotionSolve (XML). This is an optional Boolean attribute, which allows the user to convert the flex body into a rigid body.

It is recommended that the user has prior knowledge of MotionView Flex body simulation basics and some familiarity with the MotionSolve Solver input format (XML) to take full advantage of this article.

Topic details:

The solver deck (.xml file) for Altair MotionSolve is written in the Extensible Markup Language (XML) format.


In an xml file, Body_Flexible statement defines a flexible body object. Here is how the format of a Body_Flexible element looks like.


Fig.1 Body_Flexible format


The rigidified attribute highlighted in Fig.1 above is a Boolean Flag that has been introduced recently for the Body_Flexible element. It allows the user to convert the flexible body into a rigid body when the value is set to “TRUE”.  This can also be done by using the MotionSolve specific utility sub-routine c_modset.


The rigidified flag is optional. The default value for this flag is rigidfied = “FALSE”.




A rigidified flex-body is computationally more efficient than a pure flexible body, but less efficient than a pure rigid body, due to implementation differences.


Let us consider a scenario where the user is interested in the results of a flex body only after the multibody system reaches a certain configuration. This may sometimes imply that he can ignore the flex body results during the time taken by the system to reach the required configuration.


In such a case the user could choose to specify:

  • rigidified = “TRUE” during time taken by the system to reach the required configuration
  • rigidified = “FALSE” after the system has reached the configuration


This can also be specified within MotionView by defining a sequential simulation template in Templates panel.


The following lines show an example of a template for converting a flexible body into a rigidified flex body (0-3 seconds) and then back to a flex body (3-6 seconds).


Template showing switching between rigidified = TRUE and FALSE

Fig.2 Template showing switching between rigidified = TRUE and FALSE


When post processing, the user would be able to see the contour on the flex body only during the times when rigidified = FALSE.


Obviously, solving a rigid body takes lesser time than solving a flex body.

Hence, when the user chooses to use the rigidified option in an appropriate scenario, a template written like in Fig.2 would reduce the run time significantly!


Information on the rigidified option and Body_Flexible in general can be found in the online Help by navigating to Help > Other Applications > Radioss, MotionSolve and Optistruct > Reference Guide > XML Format > Model Statements.

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Dev Anand
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Dev Anand

About Dev Anand

Dev Anand joined Altair in 2004 and is currently the Senior Manager for Global Initiatives at Altair. He has over 16 years in the PLM industry focusing on global initiatives, business development, global customer support, program management, product design development and people management skills. He holds a Bachelor of Mechanical Engineering Degree from M S Ramaiah Institute of Technology, India, and a Post Graduate Diploma in Business Administration with Operations from Symbiosis, India. He is also Six Sigma Green Belt Certified from General Electric Power Systems, who has been trained to be Project Management Professional from PMI, USA and also attended the Leadership program.