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# How to Retrofit a Building

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### Purpose or Problem

The purpose is to find the best placement of braces to strengthen a square wooden frame.

### Overview

The science discipline of engineering has many branches: chemical, electrical, aeronautical, biochemical, nuclear, and mechanical.

Mechanical engineering requires knowledge of the behavior of materials and their physical properties to design buildings, machines, aircraft, bridges, skyscrapers, overpasses, dams, and other structures. Mathematics, physics, experimentation, and testing are important aspects of mechanical engineering.

Physical structures that undergo stress are often given extra support by using braces at key positions on the structure. Amazingly, braces sometimes don't need to be very strong to be effective. Their placement in the structure is more critical than their strength. Yet, they can give tremendous strength to the structure they are bracing.

This project requires wood to be cut to specific lengths and holes to be drilled in the wood. If power tools are to be used, make sure you have adult supervision. Observe all safety precautions using power tools.

### Hypothesis

Hypothesize which of three different bracing forms can best help to keep a square wooden structure true to form when an unbalanced force is applied.

### Materials' List

• One 5-foot piece of rope
• Pulley
• Two 2-inch-long bolts with nuts
• Several plastic gallon milk/water jugs
• Four large screw hooks
• Lumber: five 8-foot-long pieces of ½-inch pine of any width (1 to 4 inches)
• Lumber: several 3-foot-long shims
• Lumber: 2-foot piece of 2×4
• Several dozen nails about 1½ inches long
• Several buckets of sand
• Towel
• Scale
• Wood saw
• Wood drill
• Hammer
• Use of a table
• Adult supervision, if using power tools

### Procedure

Make four 2-foot-square wooden frames using ½-inch pine boards. The boards can be of any width, from 1 inch to 3 or 4 inches. Use only one nail at each corner.

Stand the frames vertically, as shown. Drill two holes in the bottom of each frame. The holes must be placed 12 inches apart.

Put one screw hook into the end of each frame at the top of one side, as shown above.

Take one of the frames and add exterior X braces from corner-to-corner on both sides. These braces need not be made out of strong wood. Thin wood, such as shims or wood used for making latticework, will suffice.

Take another frame and cut a horizontal brace out of the pine wood. Use one nail through each side to mount it horizontally across the interior center.

Using the pine wood, cut four 4-inch pieces and nail them into the inside corners as shown.

This completes the four structures to be tested. Next, construct a jig, so each structure can be tested.

Cut a 3-foot-square piece of ½ or thicker plywood. At one edge, securely mount a 2-foot vertical piece of 2×4 wood. You may need to strengthen it by using a large metal elbow angle brace behind it. Mount a pulley at the top of the board.

Near the middle of the plywood, drill two holes spaced 12 inches apart. The holes must be in line behind the 2×4 piece, as shown. Push a 2- or 3-inch bolt up through each hole. This completes our test jig.

Position the basic wooden structure on the test jig, so the bolts come up through the holes in the structure. The hook should be on the side facing the pulley. Screw the nuts down onto the bolts and tighten. The structure is now secured to the jig.

Set the jig on a table. Place a towel or something soft underneath the jig to be sure the heads of the bolts do not scratch the surface of the table.

Tie a piece of rope onto the hook. Drape it over the pulley and let it hang down. Tie the other end onto several empty plastic gallon milk or water jugs, so the jugs hang suspended and do not touch the floor.

Begin adding sand to the jugs until the square wooden structure begins to show signs of skewing. If the test jig begins to tip, place some heavy weights on the plywood base or have a friend sit on it! Weigh the jugs of sand.

Repeat the procedure for each of the three structures that have braces added. Record the weights.

Were all the brace structures able to sustain more lateral force? Which brace method worked best? Which was the least beneficial?

### Results

Write down the results of your experiment. Document all observations and data collected.

### Conclusion

Come to a conclusion as to whether or not your hypothesis was correct.

Something More

Compare the strength of a simple wooden square structure where the top and bottom lengths of wood are equal, and the two shorter side pieces are equal, to a structure where all sides are equal, with each corner overlapping the next, as shown in this illustration.