Testing – Top Loading

This post is continued from How to Test Your Model Bridge

Loading a bridge by placing weights on top of it has become my method of choice. This is because I have access to a weight set with ample weights to to break any bridge I have created so far. This method is quite and easy, and requires a minimum of equipment. In fact, I do not even need a scale and can simply add up the total of the weights. This allows me to know the running total of force on the bridge, and I can add smaller weights when I think it is close to maxing out.

However, that brings up another point. Unlike the hanging bucket method with sand, I can only add weights in larger increments (2.5 pounds, for example). This means that I may break the bridge without knowing the precise amount it could have held. If I add a 30 pound weight and the bridge collapses, I don’t know if it could have held 25 more pounds or just 1 more pound.

You could substitute free weights with a bucket on top and fill that up with sand or water, but this creates a high center of gravity and you definitely will be cleaning up a large mess when the bridge does break. With anything that you add on top of the bridge, you need to be careful to get out of the way quickly when it breaks. 300 pounds of free weights will not feel good if it lands on your feet.

Instead of free weights you could use any heavy and dense object, such as bricks, heavy textbooks, etc.

You do need to elevate the ends of the bridge in order to get a good test. You can use scrap boards, books, or anything that won’t get crushed by heavy weight.

Pros

• Easy setup
• Minimal equipment
• Fun
• Potentially no dynamic forces

Cons

• Not precise
• Potentially dangerous to feet
• Requires heavy objects

Top Loading Tips

If you are going to test your bridge by putting textbooks on the top, like many people do for popsicle bridges:

• Make sure the first textbook is perfectly centered over the bridge
• Line up all the over textbooks with the first one
• Put each textbook on the bridge gently

If you aren’t going to test your bridge with any of the above methods, here is another simple one.

Take a bathroom scale and place it on the top of your bridge. Simply push down on the scale until the bridge breaks. Of course, only use this method if you think the bridge is not going to hold very much. I will tell you from experience, it gets very hard to push perfectly straight down over 100 pounds. You can end up breaking your bridge pre-maturely by accidentally pushing to one side.