Science Buddies: "Ask an Expert"

The project I am working on is building a lightweight roof truss.

The Truss Minimum Design Requirements:
• Truss must be able to CLEAR SPAN 6’-0” with a minimum of 4” bearing on
each of its two (2) ends. The truss maximum total weight is to be 6 lbs. (this
includes any connectors to hang load.)
• The truss will have to support a minimum of 150 lbs. from a centrally located
hanging point on the bottom of the truss.
• The truss must not exceed the maximum deflection of L/360, (which is
approx. 0.2”.)

The maximum weight of 6 pounds is my main problem. The traditional material of wood would have no problem holding much more weigh than that, but would weigh 20 pounds at the very least. The materials are going to be purchased by my group and the price will be divided between us. I have been thinking about using cardboard, aluminum, tin, or carbon fiber (if i can get a deal). I am unsure of several things such as the material I should use, the structural design of the truss, and what i will use to fasten it together. The material will have to be very rigid to maintain less than a 0.2" deflection.

*Bonuses are given for each pound (above 150 lbs) the truss can hold up too 200 pounds, and for the entire use of non-traditional materials (excluding fasteners).
Using the non-traditional materials is not very important to me, but i would like for it to hold at least 200 lbs. Any tips or advice would be great.

Be careful what you ask for, Angus MacGyver may strike again (see http://en.wikipedia.org/wiki/Angus_MacGyver). If this were my problem I would start out with two sticks about one inch square and five feet long, a roll of plumber's tape, and a few screws to make a triangle. Plumber's tape is that metal strip with a hole every two inches.

The sticks in compression will easily support the weight and the tape will prevent the bottom from spreading. The deflection may be marginal and may require a slight preload to measure accurately.

If you break this down into understanding which truss members are in compression and which are under tension and think about twisting forces on the truss members that are in compression, you can rule out a lot of designs quickly.

Roof truss designs with a high center point are going to twist or roll and either break or end up with the middle pointed down. z truss beams will do the same thing. If you look at the way steel z truss beams are used to support concrete floors and roofs in commercial buildings, you will see that they hang down below their resting point.

Once you figured that out, an upside down roof truss type design is a viable simple solution with few joints.

But now you have a problem in terms of the "test environment". An extremely light weight simple design would have a 6' 8" (horizontal beam in tension) with a rope attached that hangs down holding the load. If you had 20 feet of height, the horizontal beam would have very little tension on it and a 3/4" ID PVC pipe would easily hold a literal ton of weight. Coming up with 50 feet of light weight nylong rope with that will hold 2000 pounds is trivial.

Unfortunately, you probably won't be given 20 feet of hang distance so you will have to use trigonometry to calculate how the forces resolve. You really need to research the other dimensions and limitations of the test stand that will be used to test your design.

So how did the NASA space station engineers solve the problem on the solar panels? Or how did engineers building antenna masts for the Coast Guard and Navy solve the problem? The used multiple wires/ropes in tension runing from top to bottom (or end to end) spaced at 90 degrees off spacers in the center of the mast (or beam).

As for light weight materials, bamboo is hard to beat on a strength to weight basis.