Thank you so much for your reply.
By following your instructions, I created a local coordinate system called "beam" coinciding with the global coordinate system and used this local cooridinate for the element orientation, as marked in the snapshot. I just wanna make sure I understand it corrctly. So after the force is applied, the element coordinate will rotate accordingly and thus the orthotropic properties will be interpreted in the directions with the rotating, rather than the original ones. Is that right?