Flexible: How do Earthquakes Affect Unbraced Frame Structures?

Problem

How do earthquakes affect unbraced frame structures?

Materials

• Masking tape
• 4 drinking straws
• Sheet of office paper
• Marking pen
• Helper

Procedure

1. Tape 4 straws together to form the outline of a square form.
2. Lay the sheet of office paper on a table.
3. Position the straw frame on the lower right-hand corner of the paper.
4. Use the marking pen to draw around the outside of the square frame.
5. Hold the bottom straw firmly against the paper with your left hand.
6. Use your right hand to push the top straw as far to the left as possible without breaking the frame.
7. Hold the frame in this leaning position while a helper marks around the outside of the frame.

Results

The bottom of the frame stays in place while the top and sides lean toward the left. The top of the frame can be as much as 2 inches (5 cm) to the left of the original positon.

Why?

The straw, as well as the tape joining the straws together, allows the structure to be flexible (able to bend without breaking). Frame structures with vertical (up-and-down) supports allow the most flexibility and movement, which can be dangerous to the occupants and damaging to the furnishings of the swaying building. The lateral (sideways) movement of a building during an earthquake can bend the structure's frame to a point that it breaks, causing the structure to collapse.

Let's Explore

1. Would a lateral brace increase the strength of the building and reduce the deforming flexibility? Repeat the experiment, placing a straw horizontally across the center of the frame.
2. How would a solid wall affect the strength and deforming flexibility of the structure? Solid walls connected to a frame are called shear-wall bracings. To test their effect, repeat the experiment while covering the original square frame with a piece of cardboard. Use tape to secure the cardboard to the frame. Science Fair Hint: As part of a project, display the shear-wall bracing model, as well as models representing a vertical- and a lateral-braced frame. Include diagrams indicating the amount of flexibility of each structure.

Show Time!

Buildings in earthquake areas that survive earthquakes must be able to bend without breaking. Test the flexibility of different strips of building material by securing each strip to an outdoor table with a C-clamp. (NOTE: If you use an indoor table, place a rug or towels beneath the pail.) Add weights to the end of the strip by placing rocks in a pail attached to the end of the strip. Continue to add the rocks to the pail until the pail is full or the strip cracks. Display the various materials tested with a comparison of their flexibility.

Check It Out!

In choosing a metal support for a building, the elasticity and tensile strength of the metal must be considered. Define tensile strength and elasticity, and explain why they are especially important when selecting building supplies in earthquake areas.

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