http://web.archive.org/web/20050308151558/http://czg.spawnpoint.org/curv_tut/curv_tut.htm
There is a curve tutorial by CZG, where he shows you how to make pipes and stuff on the grid... in the archive version, the images are missing though. :-/
I guess I could whip something up as well for radiant, maybe I'll do it today, it's easy...
It's hard to explain without pictures. Let's start with an inward corner, like in doorways. Imagine a 90 degree corner of a doorway. To round it off, you basically put more angles in there, for example you can create a 45 degree corner by putting a little triangle shaped brush in there.
This is all that id usually did in the original Quake maps; you'll find lots of doorways that have little wedges in them.
To round it off more than 45 degrees, you'll have to put more brushes in. The next step is putting two little wedges in, and make it so they don't overlap (pull the edges around in Radiant until both little triangles look the same size, and always stay on grid). That will create two additional surfaces in the corner and make it *very* roughly rounded.
For each additional surface in the curve, you'll have to put in an additional little brush. This is the part that is easier to explain with pictures. It will usually look OK if you have three additional surfaces in a corner/curve (this translates to three little brushes), where the middle one is at a 45 degree angle and the outer ones are at a lesser angle.
A 45 degree angle is created in the editor by going one step down, one step sideways on the grid. OK?
Lesser / sharper angles are created by going two steps down, one step sideways. Or four steps down, one step sideways. It's done like this to ensure everything stays on the grid.
http://www.quaketastic.com/upload/files/screen_shots/24sided.jpg
Observe very closely how the angles are done in this 24-sided circle. You don't need so many sides in the beginning - doing the "three angles/brushes per corner" method will yield a 16 sided circle if you do it "all the way around", since 4 X 4 surfaces = 16. It's 16 and not 12 because you have to add the sides that aren't angled. ;-) hard to get without pictures, I know.
This 16-sided method is good enough usually, but sometimes you may want 24 sided to make something look even more smooth. More surfaces in a curve = it looks rounder, smoother.
Now for out facing corners, like the corner of a corridor, it is much the same method, only the curve goes in the other direction... the angles/brushes are on the inside of the curve in this case. The methodology is much the same.
Maybe someone has the pics from the CZG tutorial, but I guess I should put that in my radiant tutorial as well.
You can of course simply use (ex.) Radiant's functions to create cylinders. I'd strongly suggest to stay at a reasonable amount of sides, like a 16 sided cylinder, and be very sure that every corner is on the grid, snapping the thing to the grid (Ctrl-G in radiant) to make damn sure. You can then use the clipper to chop the cylinder into half-and quarter cylinders. This method works best if you set the grid to like 32, and make the cylinder from a relatively large brush like 256x256. For tighter curves, you'll need a smaller grid, or less sides in the curve.
If you look at handrails in games like Quake 4, the corners are often only using two sides. It looks crappy, but for small things in a large/detailed environment, you can get away with it. Also, curves eat performance.
A map full of curves will run slower and slower and take longer and longer to vis. The higher the number of sides in your circles, the slower it will be. Don't go overboard.