Thank you gaieus, intersecting faces solved the problem with unity (that was my main goal, i've asked about twilight because, for me at least, when something is wrong in Twilight, it's wrong in Unity).
Also using easy 8 in twilight didn't showed any problem although with high+ it had some.
About the material, i was using the default material to avoid anyone saying that i was using a light emitting material.

i tested with textures too and the problem stills there; just didn't uploaded the pic with materials because it's almost the same as without materials.
i've also tried to make new stairs like this with different methods from scratch and this problem also shows up.

sorry mac1 but you just reduced the file size, cause the rendering problem stills there.
and it's based on a game staircase, that's why it doesn't have intermediate landings

hello there
i've made a staircase that looks to be a light emitter in twilight, and that also happens when there's not any material apllied to it, so that means it's a geometry problem, however i didn't figure out where or why.
Edit: It's not a problem with the render engine 'cause in Unity (game engine) it shows a similar effect.
rendered pics (left with materials, right without)


model http://hotfile.com/dl/109943295/a57154d/stair01.skp.html