A screenshot from Requiem Map13. The same as the previous 3D bridge, but now two bridges crossing eachother in midair. In this example, you can walk on both bridges, but you cannot walk under the lower-most bridge (on the poison in this case). It is possible to make a triple bridge, which allows you do to just that, but I haven't tried this yet myself.

[dbridge.jpg] SECTORS
1 : ceiling/floor 320/-64;
2 : ceiling/floor 320/64;
3 : ceiling/floor 320/192;
4 : ceiling/floor 320/64;
5 : ceiling/floor 320/192;
6 : ceiling/floor 320/96;
7 : ceiling/floor 192/64;
8 : ceiling/floor 320/64;
9 : ceiling/floor 64/64;
10 : ceiling/floor 320/192
LINEDEFS
a : no lower/upper textures, both sides reference to sector 4, trigger 91 for sector 8;
b : no upper/lower textures, both sides references back to sector 5, trigger 83 to sector 8;
c : both sides references to sector 7, no upper/lower textures;
d : no upper/lower textures, both sides references to sector 6;
e : no upper/lower textures, both sides references back to sector 4;
f : no textures, both sides references back to sector 8

A fine example of a double bridge. If you haven't done so already, check out how a normal 3D bridge is constructed, because that facilitates understanding this one. The effect can be duplicated with warm, dmapedit and BSP.

Again, it is sector 8 acting as the actual bridge, with triggers 91 and 83 taking care of lowering/raising it to the appropriate floor level in time. The dummy sectors 9 and 10 attached to the dummy part of sector 8 contain the information about the exact floor levels it should conform to. The blocking effect of the railing in the actual bridge crossing is done by sector 6 (floor above floor level lower bridge) and 7 (lowered ceiling blocking player from upper bridge) while still enabling the player to see and shoot from all directions.

The bright yellow boxes indicate the vertices where linedefs meet.... note that linedefs 'e' cross without contact. "Steptop" or other textures as middle textures for the 'a', 'b', 'c', 'd' and 'e' linedefs act as visible railings and/or floor crossbeams on which the player appear to be walking.