When you're doing a 3-D cake the first thing you need to do is figure out what type of internal structure you're going to need. As we all know, some "cakes" are less cake and more styrofoam, rice krispy treats and PVC pipe. I like to do as much of the cake in actual cake, but sometimes you need a structure that won't keel over, and it's better to err on the side of caution.
When you're looking at what supports you need, the first thing is to think about the great equalizer, gravity. No matter how sturdy your structure, the weight of a cake can pull it over, and you need to decide what should be made out of something other then cake as a result.
This isn't a mystery, just think about how gravity + the weight of the cake will combine to affect the parts of the structure and go from there. Start by looking at this pinterest search that I did for gravity-defying cake tutorials. Once you get an idea about where supports need to go, it's pretty simple.
Keep a few things in mind. First, you want the actual cake to be supported by level plates as much as possible. If there's no reason to put a cake at an angle, don't. The goal here is to keep the final design from falling apart, and keeping things level is the best way to do that.
Second, when you're thinking about the structure, also think about the materials that you'll use to cover that structure. Try to use the sturdiest material that you can to achieve the goal you want without adding unnecessary weight. I like to use solid modeling chocolate as my first choice for thin pieces since it cools off to a solid piece, but it can get heavy. If it's not going to be structurally sound then you'll need to use dowels or copper tubing covered with a thin layer of the modeling chocolate or fondant.
Third, make sure that you're building a solid structure to begin with. Sticking a sharpened dowel into a cake drum won't give you enough support for a top-heavy design. For something that needs a lot of support at the top, you'll probably need to use a solid board as the base and screw a center supporting pipe into the board to guarantee that it's solid and isn't going to wiggle.
Last, the taller the structure, the sturdier it needs to be. If you're putting cake at the top of a tall structure it will automatically be top-heavy and you need to anticipate that movement might be an issue. Make sure that the structure itself moves as little as possible before the cake is added to it.
Take a look at the pinterest page again, and then go browsing the hardware store brace section and the plumbing materials for pipes and fittings. Flanges and pipe come in handy when building upright supports that have to hold cake in high places...
Tomorrow I'll take photos of some things and show how I would do the supports for them.
Kara Buntin owns A Cake To Remember LLC, online cake supplies at www.acaketoremember.com and www.acaketoremember.etsy.com