Simulate Physics Grayed

Hello,

I currently working on a platform, witch can be destroyed by a bomb.
I did create a Mesh, add collision; Create a Destructible Mesh from it;
Then i create a Blueprint, add this Mesh as skeletal mesh. And try to activate physics, because i ■■■■ to be able to walk on it.

You can check my case on the image below.

11129-2014-07-25+11_12_02-sp_blockdirt.png

The DM is ok when i Drag and drop it to the level; i throw my bomb, and the platform is destroyed.
But in the blueprint context, the simulate physics boolean is grayed. So nothing append

I am on UE4.3

Any idea ?

Thanks in advance