Expert: Jamie Yocono - 1/12/2008

Question
I'm interested in building a bookcase/stair combo called Tansu.  It involves a series of cabinets placed in an arrangement that allows someone to use it as steps to a loft.  My question is what type of material would be best to build these with so that the cabinets support not only the weight of those above them, but also that of an average adult?

Answer
Hi Stephanie,

While I don't know a lot about Tansu chests, other than being familiar with their concept, I can tell you what I would want to think about if I was designing one.

First, I'll address your questions regarding material.

I don't think your choice of material is as important as is your use of proper joinery, which brings me to my second point...proper joinery.  Most woodworkers I know primarily build with hardwoods, and since those woods are basically dense and durable, I think the bigger question is how to put them together so they support all the loads that they are designed to carry. I think the key to designing something that will carry a heavy load is to build it so that the two sides (vertical members) support the top. Better yet, I would probably create each individual Tansu box as a square, with rugged corner joints like dovetails or locking rabbets/dados. I would want a lot of gluing surface, so dovetails seem ideal.

Next, I would also consider the span of these boxes, because if the span is too long, the center of the top will sag. There is a VERY cool website that helps you calculate sag deflection, here's a link: http://www.woodbin.com/calcs/sagulator.htm

It's called a Sagulator... isn't that great?

Anyway, using this sag calculator, you can play with spans, thickness of material, and types of woods, to determine the amount of deflection.  Besides using this to help you (me, actually) determine the sizes of the individual boxes, I like to use the Golden Rectangle (or Golden section, or Golden rule) when designing pieces. This is a ratio that creates pleasing shapes, and I use it quite often.  The correct ratio is 1:1.618.   

OK, I'm on a roll, and I like to ALSO keep in mind that most architectural standards call for a maximum step height of 8" or so.  Most building departments won't let a set of stairs have a step higher than 8", it's just too steep. So I would design some sort of a modular system based on various heights of 8", 16", 24", and so on... all multipliers of an 8" dimension.   

Using the "golden" rule, some "harmonious" sizes would be 8" x 13", 16" x 25 7/8",  24" x  38 3/4" and so on. I don't follow it to the nearest 1/64th... but I follow it fairly closely.

Finally, I've mentioned modularity; I'm a big fan of it. I'm also big on designing pieces using simple CAD programs. So I would use one of my drawing programs, and play around with modular shapes, mixing and matching them up for a design that not only stays true to the principles I've outlined (golden ratios, sag calculations, material constraints) but also fits the basic dimensions of the piece that I want to build.

There are ways to make your design work, like adding center dividers in long spans, so that long boards don't sag. I think the bottom line is- there are many variations of Tansu chests, you just have to use some common sense and engineering principles when designing one.

Whew! I hope that made sense!

OK, I hope this helps. Write back if you have more questions after reading this. You might want to do a search for Hawaiian woodworkers. The last time I was in Hawaii, I attended a woodworking show and it seemed like every other piece was a Tansu. So perhaps you'll get some inspiration from their websites. Good luck,

Jamie Yocono
Wood It Is! Custom Cabinetry
Las Vegas, NV
www.wooditis.com