- Block access,
- Open access,
- Divert traffic.
The simplest Blocking effect is a triggered tunnel collapse; variations on this effect is to collapse the tunnel floor (exposing a pit) or fill a tunnel with some material other than falling masonry (water, lava, a colossal pile of leeches.) Any of these effects are basically one-shots; a more controlled Blocking effect is the portcullis trap, which can be dropped or raised again. Similarly, a lever can lower a bar across a door, to prevent it opening. A Blocking effect trap may injure a victim caught in its path as if it were a Material effect trap, and some materials used to block access may have side effects, or may only block certain kinds of access; a water-filled tunnel will only block access to those who need air to breath and can't make it to the other side while holding their breath.
The reverse is an Opening effect, which enables access, usually for a hostile opponent. The first thing that comes to mind is probably a wild animal or monster enclosed in an adjacent room; activating a trigger releases a latch, causing a false stone wall to drop, allowing the beast to enter the area and attack. Secret doors are sometimes used in this way; an alarm can alert intelligent dungeon occupants, allowing them to use secret passages to maneuver behind an unsuspecting party of adventurers.
A Diverting effect is, in a sense, a combination of the other two structural effects. Before the trap is triggered, the adventurers have access to one area; after a wall shifts to one side, that access is blocked and a second route is opened. If the diversion is designed to make detection difficult, such as rotating or shifting walls in a maze, the adventurers may become lost and confused, unable to find their way back to a safe zone. A more obvious diversion would be a covered chute trap that opens, dropping unsuspecting adventurers to another level.
In general, structural effects have to be fairly large scale, involving entire walls, floors, or ceilings that move to block, open, or divert routes. Thus, the moving structural element will probably have visible seams (Vis/S, Vis/P if concealed.) Covered pits and chutes can be disguised with sand, mud, or other materials to prevent visible examination, but the ten-foot pole will be useful here, as might pebbles or rocks (Tact or Aud/C.) One-time effects powered by gravity, such as a stone block that seals a passageway, probably use very simple triggers which will be difficult to detect; repeating or continuous effects, such as walls that slowly rotate, will probably require very large gear or pulley systems, which will make a detectable noise (Aud/S.)