Screw-less Mounting Shelf for Dorm Room

After moving into the dorms, my roommates and I quickly realized that we had more stuff than we had space to put it in. There are lots of commercially available shelving solutions, but we are not allowed to anchor things to the wall, so I needed to design something freestanding and compatible with our other furniture.

Obviously the layout of the room influenced the design, but I also had to be able to get all of the materials from Home Depot back to the dorms. This meant that everything would need to fit into the Honda Civic Zip Cars we have access to on campus.

The shelf is nearly 14 feet wide, and there is nothing to support it in the middle. The easiest thing to do here would be to run a couple of 2x6s across the bottom, but there was no way to fit a board that long into the car. Aesthetics were also important because my roommates and I have will have to live in the same room as this for the next 8 months.

Fortunately, another type of structure exists that is supported by its endpoints and designed to hold weight in the middle. Bridges fulfil these criterion, and they are aesthetically pleasing. A cable suspension structure would be light, and could be fit into a zip car. The one remaining problem was that suspension bridge supports are typically above the road surface, but the shelf supports would need to be beneath it so that that we could put things on top.

In the case of a suspension bridge, the weight of the roadway is pulling straight down on the cables and the cables are pulling on anchors in the ground. The cables take the shape of a catenary. There is no compressive load on the roadway in this situation. In order to put the tensile member underneath the shelf surface and to avoid anchoring anything to the walls, the cables are anchored to the edges of the shelf. This means that both the shelf top and the vertical supports are in compression. The cable, still in tension, takes the form of an arc.

I do not know the math behind calculating the tension on a cable in the form of an arch, so I made an approximation by simplifying the problem to the vectors shown above. 1/8th inch steel cable gave me a safety factor of four with a load case of 200 pounds, which should be enough to make up for my spotty calculations.

With the cable suspension system figured out, the only other large challenge to tackle was how to hold the shelf up in the air. The wardrobe (right) and the bedframe (left) were the most convenient structures to mount to. The wardrobe side was easy because it lies forward of the shelf. The weight of the shelf and that of anything set on it will push the shelf towards the wall.

The bedframe side was harder because the shelf would have no natural tendency to stay against the wall. The way my solution works is that the vertical beam bears the majority of the weight of the shelf, and the diagonal beam keeps it from falling away from the wall. The diagonal beam it attached to the vertical one with a hinge, and there is a cable that pulls the bottom of the two beams together. The vertical beam is anchored to the bed frame via a preexisting bolt hole, and since that has nowhere to go, tightening the cable forces the shelf into the wall. There is a strip of foam tape across the back of the shelf to protect the wall.

Once these main systems were in place I began the CAD process, and the rest of the details came together along the way.

The following is a brief description of the different components and the purposes that they serve. To see how they go together please refence the assembly video in the introduction.

1. Shelf Surface: This is 3/4 inch plywood split into two 85" by 20" sections. The sections will get cut on the panel saw at home depot so that we can fit them into the car.
2. Cables: 1/8" stainless steel cables looped through two holes at each end of the shelf surface. The loops are crimped at one end and attached to turnbuckles at the other.
3. Spines: These are pieces of 5mm plywood that are laser cut into shape. They prevent the vertical supports from splaying inward or outward.
4. Vertical Supports: These are 1/2" PVC pipe, and they have holes drilled through the bottoms that the cable runs through. Crimps on either side of the vertical supports will prevent them from sliding up and down the cable.
5. Feet: The feet are plywood rings that the vertical supports sit inside of. These are glued to the shelf surface to prevent the vertical supports from sliding around.
6. Right Side Footing: The fixture that rests on top of the wardrobe that supports the right side of the shelf. It is also made of 3/4 inch plywood.
7. Left Side Footing Vertical: This takes all the weight of the left side of the shelf, and it bolts into a convenient preexisting whole in the bedframe underneath of it.
8. Left Side Footing Diagonal: This piece is connected by a hinge to the vertical footing, and they are pulled together in order to force the shelf into the wall.
9. Left Side Footing Tensioner: This cable is what pulls the two pieces of the left side footing together. It is crimped on one side and has a turnbuckle on the other.
10. Left Side Footing Bolt: This is what secures the left side footing to the bedframe.

I wish that I had separated the CAD models for the shelf and for the rest of the room into two discrete drawings. This would have simplified my feature tree and made the design less cumbersome to work on. I could then have used Fusion's insert derive tool to combine the models for perspective and rendering/animation.

Additionally, I am hoping to add colored lighting to the inside of all of the PVC vertical supports, so I might add some tracks in the shelf surface for the wiring before I begin construction.

Overall, this was a rewarding project in that I got some practice using the animation workspace. I find it a little bit difficult that you cannot animate joints using Fusion, but after using the move tools to do it for this project I started to feel more comfortable.