Load it to the limit: How much can a bridge hold?

Text by Michele Regenold, Photos by Alison Weidemann

On August 1, 2007, a major freeway bridge in Minneapolis, Minnesota collapsed during rush hour. Investigators with the National Transportation Safety Board (NTSB) are looking at all sorts of possible causes, including

• possible design problems with the gusset plates—sheets of metal that connect steel pieces,
• the use of de-icing fluids (stuff to melt ice and snow that can cause steel to rust),
• the weight of 287 tons of construction materials and equipment that were left on the bridge, and
• the bridge’s maintenance and repair history.

Whew! That’s quite a variety.

One thing the NTSB is probably not seriously considering as a primary cause of the bridge's failure (though it hasn't ruled anything out) is the weight of the traffic on the bridge when it collapsed. Why? Because most of the traffic was cars and cars don’t weigh much.

A loaded semi weighs about 80,000 pounds compared to about 2,000 pounds for an average car. What if the bridge traffic had been bumper-to-bumper with semi tractor-trailers?

"Since bridges are designed to hold many trucks crossing at the same time, it’s unlikely that they would be the only cause of failure," says Terry Wipf, director of the Bridge Engineering Center at Iowa State University.

Basics of loads

Bridges have to carry many kinds and combinations of loads, including dead and live. Dead loads are basically permanent for the life of a bridge. They include the weight of the bridge itself, as well as the weights of the asphalt or concrete deck, the lights, and the barrier railings.

Live loads change over short periods and include the weights of vehicles that move across the bridge and some things you might not expect like temperature change, wind, and, if the bridge has piers in the water, the water itself.

So loads are things that exert force on the bridge and weight is one load.

Testing load capacities and bridge designs

To see the difference that bridge designs can have on load capacities, we’ll build 2 different kinds of bridges, a simple beam bridge and a suspension bridge.

Materials and tools you’ll need:

• 7 drinking straws (if they have flexible bends, cut them off)
• Masking tape
• Thread or dental floss
• 4 large paper clips
• Paper cup
• Pennies
• Ruler
• Scissors

Bridge 1: Simple beam bridge

1. Cut a drinking straw to make 2 short pieces that are each 3 centimeters (1.25 inches) long.



2. Lay 2 long straws (minus flexible bend pieces) on either side of one short straw and tape them together. Do the same with 2 more long straws and the other short straw. This creates the supports for the beam bridge and the towers for the suspension bridge.



3. On each tower, tape the 2 long straws together at the top.



4. Pick 2 surfaces that you can tape your towers to, such as 2 tables or desks pushed close together. Leave a few inches of empty space between them. (Your paper cup/load tester will need to dangle between the towers and hang below the tabletop or desktop.)

5. Without covering the short straw in your tower, tape one tower to the edge of one table or desk. Tape the other tower to a second table or desk.
6. If you’re using full-size straws (8.25 inches long), position the towers so they're 7 inches apart. If your straws are shorter because you cut out the joint, position the towers 5 inches apart.
7. Lay another straw (with its joint removed) between the 2 towers so that each end of it rests on top of the short straw.

Make a load tester

To test how much weight your bridge can hold,

1. Unbend a large paperclip and reshape into a rough “V.”
2. Bend the ends of the “V” about 90 degrees to make little hooks.
3. Poke each little hook through opposite sides of a paper cup, near the rim, to give the cup a handle.
4. Unbend a second large paperclip partway until you have a hook on each end.
5. Attach one end of the hook to the cup’s handle.
6. Hang the cup from the beam bridge using the other end of the hook.

Test the bridge

How many pennies can your simple beam bridge hold?

1. Add them one at a time until the bridge falls.
2. Record the number of pennies the bridge held before it fell.

Bridge 2: Suspension bridge

We’ll use the same tower supports for this bridge as we did for the beam bridge.

1. Cut a piece of thread or floss 100 cm (4 ft) long.
2. Tie the middle of the thread around the middle of a new straw.



3. Position the straw between the towers like you did for the beam bridge.
4. Lay each end of the thread over a tower (between the 2 straws taped together).



5. Wrap each end around a paperclip to anchor the bridge.



6. Slide each paperclip into position until the thread is taut.
7. Tape each paperclip firmly in place.



8. Test it the way you did for the beam bridge.

What were your results? How could you modify these designs to make them stronger?

Our results

We had to cut our straws because they had joints in them. So our bridges spanned 5 inches each. The beam bridge held 31 pennies and the suspension bridge held 64 pennies.

Why did the suspension bridge hold so much more weight than the beam bridge?

"The cables help take some of the load from the beam," Wipf says, "and spread the load out—to the towers and the cable supports too."

For more information

Learn more about the Minneapolis bridge collapse.