Column Mantel Designs
Now that we're aware of the clearances needed for fireplace safety, it's time to look at some of the design possibilities. I should spell "possibilities" with a capital "P" because the possibilities are endless. I made a few sketches below to give you a few of my favorite design ideas. The sketches are rough and are meant just to get the design juices flowing. We'll look at mantel surrounds for now as opposed to mantel shelves which I'll sketch out later.
It should be noted that there are a number of mantel styles associated with historical periods such as Federal, Victorian, Georgian, Arts and Crafts, etc. If this is your interest, an internet search will yield satisfactory results I'm certain. But that is not is not my focus here.
First let's look at a mantel surround with whole columns. I'm partial to this one because of my background in wood turning. Some really magnificent mantel designs are possible by setting columns just in front of the mantel legs. The columns pictured below are roughly fashioned after a Doric style but all sorts of columns are possible.

I often get requests for barley twisted spirals. Pictured below is a pair of 6" X 50" twisted columns (called barley twist) that I made for the mantel of a friend of mine. The columns and mantel shelf are made of salvaged cypress taken from an old house or building. Old cypress has been popular for years in this area. The fireplace in this hundred plus year old house is not functional. Instead of tearing out, the owner decided to keep it as a point of interest in his dinning area. We wrapped the painted brick in recessed panels and applied a glazed finish.

Larger View
More often than not split colums are used to to avoid making the mantel surround too deep either because of fire code issues are simply for aesthetic reasons. Pictured below is a sketch of basically the same mantel as above but with the columns halved.

One little added bonus here is the opportunity to add corbels atop the split columns instead of the silly looking blocks I've sketched here.. Of course, I'm a little partial to the beautiful corbels I sell.
Below is a picture of a very nice pair of split fireplace columns made for a local cabinet maker.

Larger View
Alright, so We've taken a look at mantels with columns and mantels with split columns. Have you considered double columns. Below I've sketched such a one. This is only a rough sketch. Lots can be added to this design. Fluting the columns would be a niffty idea. We have some hand carved crown molding that would really make this one zing.

By the way the handsome guy standing behind some of these sketches is me. I'm always looking away for some reason.
OK, I'm probably going overboard with all of these sketches but I have one more idea for a column design. If you have a fireplace that bumps out into your room, a wrap around mantel may be in order. If that is your situation either by necessity or by design you may want to consider recessing your columns into the corners of the wrap around mantel. This can be quite a show stopper. Below is my sketch of one possibility.

click here for more mantel designs ideas.