This is an object that comes from the original Tomb Raider 4 title wad. I used it in my level set "Seth Strikes Back" but I had a problem with it. Either the collision blocked Lara Croft from going through the opening or deleting the collision would allow her to go though it but also inside the walls and it was weird because she can totally disappear inside the arch.
For Seth Strikes Back V3 I wanted to fix this, so I decided to cut the arch in 3 parts (left, right and top). Each part have accurate collision and once you placed the 3 of them you get your full arch with Lara Croft able to go through the opening and also hit the collision box on those walls.
It's not perfect as it takes 3 animating slots but it works nicely and you won't notice it has been cut in 3 parts in game.
I hope someone else will found this usefull ! Feel free to change the textures to fit your level.
Historical / Normal / Remake
Tomb Raider The Last Revelation / Tomb Raider Classic