Some of the external forces that affect structures come from nature. When snow and ice build up on a structure, they add weight. The structure must be able to support that weight. An earthquake is an external force that occurs when large sections of Earth’s crust start to move. These sections are called tectonic plates. This movement causes the ground to shake. The shaking can make structures such as buildings collapse.

Strong wind can push against windows, causing them to shatter. Some people nail pieces of plywood over the windows to protect them.

A very strong wind can have enough force to lift the roof off a house. The structure of the house is much weaker once the roof is gone. The house may collapse. Some people use hurricane ties to attach the roof more securely to the house. Hurricane ties are strips of steel. They can withstand much more force than the nails that join the roof to the house.

Because of the threat of hurricanes or tornados, some buildings are made from different materials. They are not made from wood or bricks. Instead, they are built from reinforced concrete. Reinforced concrete has steel bars running through it. This material can withstand very strong winds.

Hurricanes form over oceans and they sometimes move over land. Hurricane winds can cause large waves. These waves can damage houses built along the ocean shore. One way to protect a house from wave damage is to build the house above the ground. Steel, concrete, or wood is used to create pillars. These pillars are called pilings. The house is built on top of the pilings so waves can travel under the house without damaging it.