My Character falls through the imported model, why?

I imported my model from SketchUp using 3ds format, some walls are missing and character falls through the model, Why?

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It probably doesn't have a collider, at least on the floor. You can add them as components.

What Dave said, and for your walls missing issue, it sounds like the normals might be inverted on the walls but it's really impossible to say without being able to look at the model itself.

lol is it a rigidbody? as for the walls are they made as a mesh renderer with the trigger on? if not thats probly why they cannot be seen

i believe you going through the walls means the walls are not physical and not reacting to both the terrain and character. there needs to be either a box collider on the walls or character collider i dont remember which. colliders make it so items react to certain things. i dont thing double walls has anything to do with it. if it does for some odd reason react after a second layer just delet the first one and see if it still is fixed. be sure to increment your work like alexgame_1 then alexgame_2 and so on and so forth just in case.

Ok guys I understood it, first in SketchUp you have to make your model staticmesh (only with physics plugin) from here http://code.google.com/p/sketchyphysics/

Next you have to make double walls, or box-like surface in other words, export as fbx and import it to Unity

Thanks guys for your help