# Gravity Load

Gravity load is applied via the *LOAD_BODY_Z command (assuming the z-axis is vertical). Preloading due to gravity can be accomplished via a

- dynamic relaxation analysis (explicit or implicit), or
- as a separate implicit analysis, or
- during the early going of an explicit analysis.

To invoke implicit dynamic relaxation with velocity re-initialization,

- set idrflag=5 on *CONTROL_DYNAMIC_RELAXATION, and
- set a nonzero value of DRTERM on *CONTROL_DYNAMIC_RELAXATION, and
- set iphase=1 on *INITIAL_VELOCITY_GENERATION.

This will allow you to apply gravity (or other load) by implicit dynamic relaxation, then specify an initial velocity for your explicit impact analysis. The *CONTROL_IMPLICIT commands can be used to provide control to the implicit dynamic relaxation phase of the run.

(see also: implicit.dynamic_relaxation)

### Detail remarks:

There are different approaches to preloading the system with gravity. All involve using the *LOAD_BODY command:

- Use dynamic relaxation to run a precursor, quasi-static analysis wherein gravity is ramped up to preload the structure. This involves defining two curves of acceleration vs. time. For the curve designated in the LCID field of *LOAD_BODY, set (in define_curve) SIDR to 0 and prescribed a constant acceleration vs. time. For the curve designated in the LCIDDR field of *LOAD_BODY, set SIDR to 1 and linearly ramp the acceleration from zero to the constant (gravity) value over a short period of time (say 10 ms) and then hold it constant. A binary record of the dynamic relaxation phase is written when the command *DATABASE_BINARY_D3DRLF is included in the input deck (set DT/CYCL to 1.).
- Invoke mass damping (*DAMPING_GLOBAL) in the early portion of an explicit dynamic analysis to eliminate dynamic oscillations due to the gravity (*LOAD_BODY) load. After a steady state solution is attained, eliminate the damping and introduce the dynamic loads.

If you're unconcerned about the dynamic effects of applying gravity suddenly, you wouldn't need to use dynamic relaxation or damping or even need to ramp the load up over time. Just define the load_body curve as a horizontal line (ordinate value equal to the gravitational acceleration).