To help clear up any confusion when I talk about the “efficiency” or “efficiency score” of the bridges I build. This is a mathematical equation to determine how well a bridge performed.

Simply take the mass that the bridge held, and divide that by the mass of the bridge.

Mass held

________ = Efficiency

**Mass of bridge
**

Some confusion comes up when I weigh the bridge in grams while add weight to the bridge in pounds. These units do not have the same base, and cannot be divided against each other. I usually convert pounds to kilograms (1kg = 2.2 pounds) and then multiply the number of kilograms by 1000 to get grams.

So if my bridge held 54 pounds, I would convert that to kilograms. 54 pounds equals 24.54 kilograms. That equals 24540 grams.

To get the efficiency, I take 24540 and divide that by the mass of the bridge, which was, say, 33 grams. The efficiency score is then 744.

Just wondering, wouldn’t the span of the bridge have some bearing on the efficiency score of a bridge? ie if the bridge spanned a great distance, but wasnt as strong (but was strong enough) as a solid bridge of more mass which spanned a small distance, how would you compare them?

Kim, yes. All other things staying the same, a longer bridge will have a smaller efficiency score than a shorter bridge. I don’t know a good way add the length of the bridge into the actual efficiency calculation, but I do know that a longer bridge with a high efficiency score is more impressive than a shorter one.

I feel that efficiency score should be compated for bridges tested under same rules such as fixed spacing of support when being loaded.

However, if you want to include the span of the bridge (distance between supports) then that could be multiplied to the current efficiency score thus the larger bridge will get a multiplying factor that is directly proportional to the length to offset light smaller bridges and light tested over shorter span.