## Forum

### GEBT Initial Rotations and Deflections as Input

1. Hello,

If I want to run the time-marching dynamic simulation mode of GEBT to obtain the deflections and rotations at given keypoints and then want to use this result to run a new time marching case with the deflections and rotations specified as an initial condition what would be the best way to do this?

It does say at the end of the file that that if there are a total of nelem elements in the structure I can provide the initial positions and rotations for each element.  Does this mean that GEBT assumes that each element is rigid and therefor the displacements and rotation for each point in that element is the same? And thus if I set each member to have only 1 element, I should theoretically only have one deflection and rotation for all of the keypoints inside that element?

Thank You,

Lance

2. Also if I did this,

for the deformed beam, would I input new key points with the deflection and rotations applied? Or use the original keypoints to define the members and elements?

3. Hi Lance,

The key points are used to define the beam geometry. It is independent from the initial displacements/rotations. So I don't think you need to change the key point info.

Qi

4. Ok, thank you!

5. If I run GEBT from t =0 to t = t2

Then want to observe results and now continue at the next time

t =t2 to t=t3

How would I do this?

I can input the deflections easy enough.  But for the derivative terms?  The deflection derivatives I used the linear momentum divided by the mass per unit length to get the velocities.

But the theta terms are in Wiener Milenkovic Paramters correct ?

6. I think this is a restart function. GEBT doesn’t have a well-defined restart function yet, but I think you can treat these as two separate cases. For the latter one, from t=t2 to t=t3, use the results from the former, t=0 to t=t2, as the new initial conditions.

Qi

Emailed forum response from aaron.qi.wang@gmail.com
7. Yes, that's exactly how I would like to treat it.
However how should I obtain the initial time derivative terms from the previous output of GEBT?  Since GEBT outputs linear and angular momentum P and H.

I simply did: u_dot = v= P / mu    (where mu are the first three diagonal terms from the mass matrix)

then omega = H/i (where i is the mass moment of inertia)

However how do I get from Omega to theta_dot since theta are in the WM parameters ?

8. Yes, that's exactly how I would like to treat it.
However how should I obtain the initial time derivative terms from the previous output of GEBT?  Since GEBT outputs linear and angular momentum P and H.

I simply did: u_dot = v= P / mu    (where mu are the first three diagonal terms from the mass matrix)

then omega = H/i (where i is the mass moment of inertia)

However how do I get from Omega to theta_dot since theta are in the WM parameters ?

9. Lina Shang

I think I know the answer: Your treatment for the initial time derivatives is wrong. To get the initial time derivatives you need, you must first know how GEBT treat time derivative terms. The following is the equation used by GEBT to express time derivatives:

dot{A(t+dt)}= 2/dt*A(t+dt)-2/dt*A+dot{A} For the time marching step(init_flag=2), 2/dt*A(t)+dot{A(t)} is stored in the array init_cond, so you can find the time derivatives you need in the array init_cond (you need to output init_cond by yourself, it is not a output in GEBT). However, for the initial step, u, theta, udot and thetadot are known, CTCabPdot and CTCabHdot are treated as unknowns and obtained by solving the equations for initial conditions. After all, it is difficult for you to get the initial time derivatives correctly unless you know clearly how GEBT treat time derivatives for initial step and time marching step.

10. Lina Shang

sorry, the correct equation should be: dot{A(t+dt)}= 2/dt*A(t+dt)-(2/dt*A+dot{A}).