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How To Build A Torsion Box Bookcase Seat

Using Airplane Technology to Build Bookcases

When designing an aircraft, it’s all about finding lightweight and strong material that is stiff enough to withstand the exceptional forces under which it has to operate.

Durability is an important factor especially when the wings come into play.  Aerodynamic forces not only bend the wing, they also twist it. To prevent this, aircraft’s often use two-spar wings joined and by strengthened opposing sections of skin, forming a torsion box.

Using A Torsion Box In Bookcases:


I recently had a customer ask if I would build two bookcases and connect them with a window seat.  She specifically requested that the seat span the area between the bookcases with no floor supports.  Oh yea, and she “preferred” a sleek and slender look.

Unlike standard window seats that have frames under them , this bookcase was being installed in a condo that had a concrete floor and hot water radiators running along the wall.  Recessing the hot water radiators into the floor was not an option this time so we needed to keep this area open, hence the window seat.

All of this meant one thing to me  – DEFLECTION!!! and a call-back.

In order to avoid deflection that this bench seat was doomed to have we decided on using torsion box technology.  this was the only way we were going to be able to span the distance as well as hold a persons weight.

What is a Torsion Box?

A torsion box is really nothing more than a grid-work with a skin on both sides. The shims and grid work together to create a strong, light, structure that resists warping and deflection.  Just like the concept behind our airplane wing analogy.

Torsion boxes are fairly common in our life you just may not recognize them.   A hollow core door is probably the most common example of a torsion box,   structural insulated panels for houses, some types of wooden tables, skis and  snowboards all utilize torsion box technology.

The size of your torsion box will depend on the types of projects for which you plan to use it. In general, smaller grid-squares result in greater strength and stability.

The stiffness in a torsion box comes from the two skins being separated by a fairly great distance while still being rigidly locked together by the filler structure. The stiffness of the filler can contribute some additional strength, but it isn’t what primarily makes a torsion box stiff.

By the way – when our project was complete two grown men could stand on this seat with no noticeable deflection.

Why is A Torsion Box So Strong?

The torsion box functions like a beam, but is considerably lighter than a solid beam of the same size without losing much strength.

The thicker the torsion box, the stronger it will be for example if you increase a torsion box’s thickness by 25%, for example, it will be 100% stronger.

If you doubled the thickness, it would be eight times stronger. Strength increases by the thickness cubed.

How To Build A Torsion Box:

The stiffness gained in building a torsion box comes from the two skins  separated and locked together by the grid filler structure.

The strength of a torsion box is highly dependent on establishing a good grid-to-skin glue joint over the entire surface of the grid, so you need to nail or screw the plywood skins to the grid right after applying the glue to ensure that good contact is maintained while the glue sets.

Determine Grid thickness and Assemble

Cutting the Grid Material

Attach the skins

Securing The Skins to The Grid


Photographer: a concord carpenter



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