Adding More Detail to a map (Black Mesa)
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Guide to Detailing
Adding detail to your Black Mesa map can make it look more convincing and it's pleasing to the eye. This can also apply to other 
Source games, but mostly 
Black Mesa.
Support Beams and Pillars
One of the most simplest ways to add a lot of detail, is to create pillars along corridor walls, or outdoor buildings. This makes it look like the room/building is actually being held up by something, rather than no supports at all. You can also add beams like a i-beam along ceilings of corridors/buildings, then you may add a light strip suspended on the i-beam. Make sure, if you are using brushes for either the pillars/supports, that you tie them to a func_detail. This will greatly help with VIS. There are i-beam static props in Black Mesa, that you may add instead of making detail brushes.
Decals
Probably one of the easiest ways to detail you map is by using decals. Just by adding waste or dirt decals into an industrial area, makes it look like people actually had a job there, or that the floor hasn't been mopped in a while. You can also add posters, signs, cracks, papers, and trash decals to add even more detail. Make sure not to overuse decals though, as this will make your map look too cluttered or over-detailed.
Props
The most obvious way to detail your map, is by using different props. You probably will mostly use the prop_static to detail. Static props are used for objects that will not move, and are just for looks and to create atmosphere. You might use static props for metal crates, shelves, inaccessible doors, forklifts, desks, tables, counters, computers, and other things that will detail your map. The prop_physics will also probably be used more frequently. Physics props encourage the player to explore the world around them, and making a fun world as well. You can use them for puzzles or debris from an explosion for example. They are used for radios, chairs, stools, crates, (see item_crate), computers, monitors, wooden boards, and much more. Another type of prop that can be used is the prop_dynamic, which is used for props with animations, for example, a security camera that moves from side to side.
Locked/Static Doors
Locked/static doors are seen a lot throughout Black Mesa. They are created to make it seem like Black Mesa is way bigger than the player's bounds. The static doors are static props and they are placed inside a door frame. The player cannot interact with the doors. There are 14 skins for the door, suiting it for all types of areas, the door can also be used to block off areas that the player can't get to or get to yet. Some of the doors seen in Black Mesa have signs next to them that say "AUTHORIZED PERSONNEL ONLY" or "MAINTENANCE PERSONNEL ONLY", etc.. Those signs can give the idea of where the doors lead to, along with other methods. The locked lab door/sliding door can also be used when in lab areas. You can give the lab door the "buzz" sound when the player tries to enter it when it's locked, indicating that they are not authorized to enter. They also use the locked/red texture on the doors, (the door is made from a func_door.)
Also see: Static Doors.