•The load-step file method (LSWRITE) for solving multiple load steps cannot be used with the birth-death option, because it will not write the status of deactivated or reactivated elements to the load step file. Birth and death analyses having multiple load steps must therefore be performed using a series of explicit SOLVE commands.

13.3.1. Changing Material Properties

You might be tempted to deactivate or reactivate elements by changing their material properties via the MPCHG command.

You must proceed cautiously if you attempt such a procedure. The safeguards and restrictions applying to "killed" elements do not apply to elements that have had their material properties changed in the solution phase of the analysis. (Element forces will not be automatically zeroed out; nor will strains, mass, specific heat, etc.) Many problems can result from careless use of the MPCHG command. For example, if you reduce an element's stiffness to almost zero, but retain its mass, it could result in a singularity if subjected to acceleration or inertial effects.

One application of the MPCHG command would be in modeling construction sequences in which the strain history of a "born" element is maintained. Using MPCHG in such cases will enable you to capture the initial strain experienced by elements as they are fitted into the displaced nodal configuration.

13.4. Using Birth and Death

You can apply element birth and death behavior to most static and nonlinear transient analyses using the same basic procedures described in the various analysis guides.

Modify your basic analysis procedure as follows to incorporate the element birth and death feature:

13.4.1. Build the Model

While in the PREP7 preprocessor (/PREP7), create all elements, even those that will not be activated until later load steps. You cannot create new elements after preprocessing.

13.4.2. Apply Loads and Obtain the Solution

For all analyses employing element birth and death, perform the following actions in the solution (SOLU) phase:

13.4.2.1. Define the First Load Step

In the first load step, choose the analysis type and all appropriate analysis options (ANTYPE).

For a structural analysis, activate large-deflection effects (NLGEOM,ON).

For all birth and death applications, set the Newton-Raphson option to full explicitly in the first load step (NROPT). (The program cannot predict the presence of an EKILL command in a subsequent load step.) Deactivate (EKILL) all of the initially inactive elements that you intend to add (reactivate) in later load steps.

Elements are deactivated (or activated) in the first substep of the load step, and maintain that status through the rest of the load step. The default reduction factor used as a stiffness multiplier might not suffice for some problems; sometimes, you may need to use a more severe reduction factor. To provide a new value for the reduction factor, issue the ESTIF command.