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7.2.2: Some Fundamentals—Inertia, Loads, and Ductility

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    6241
  • Overview

    Imagine for a moment that your house is anchored to a flatcar on a moving train. Suddenly the train collides with another train, and the flatcar stops abruptly. What happens to your house? If it’s a wood-frame house, as most houses in the Northwest are, it probably would not collapse, although your brick chimney might topple over. If your house is made of brick or concrete block, unreinforced by steel rebar, then the entire house might collapse.

     

    This analogy introduces an important concept. The jolt to your house during the train wreck is analogous to the shocks the house would receive during a large earthquake, except that the earthquake jolts would be more complicated and would last longer. The motion might be sharply back and forth for tens of seconds, combined with ups and downs and sideways motions. The response of the house and its contents (including you) to these jolts follows the principle of inertia.

     

    The principle of inertia says that a stationary object will remain stationary, or an object traveling at a certain speed in a certain direction will continue traveling at that speed and in that direction unless acted on by some outside force. Because of inertia, your body is pulled to the right when you turn your car sharply left. Inertia is the reason seat belts are necessary. If your car hits a tree and you’re not wearing a seat belt, your body’s inertia keeps you moving forward at the same rate as the car before it hit the tree, propelling you through the windshield.

     

    Stack some blocks on a towel on a table. Then suddenly pull the towel out from under the blocks and toward you. The blocks will fall away from you as if they were being propelled by an opposing force. This force is called an inertial force. The inertia of the blocks tends to make them stay where they are, which means that they must fall away from you when you pull the towel toward you.

     

    I saw a graphic illustration of inertia at the Los Angeles County Olive View Medical Center, which was destroyed by the Sylmar Earthquake in February 1971. Upper stories of the hospital seemed to weather the earthquake without damage. (In fact, glasses of water on bedside tables on the top floor weren’t even spilled.) But the walls on the ground floor—which had much more open space and, therefore, was much weaker than the upper floors—were tilted in one direction. The ground beneath the hospital had moved suddenly in a horizontal direction, but the inertia of the hospital building caused it to appear to move in the opposite direction (Figure 12-10). The inertial forces were absorbed in the weaker ground floor. (For an illustration of inertial forces affecting a garage, see Figure 11-6.)

     

    Engineers refer to the forces acting on a building as loads. The weight of the building itself is called a dead load. Other forces, such as the weight of the contents of the building—including people, snow on the roof, a wind roaring down the Columbia River gorge, or earthquakes—are called live loads. The building must be designed to support its own weight, and this is standard practice. It also must be designed to support the weight of its contents, and this is also standard practice—although occasionally the news media report the collapse of a gymnasium roof due to a load of snow and ice.

     

    Except for high winds and earthquakes, all the loads mentioned above are vertical loads, commonly accounted for in engineering design. But the wind load is a horizontal load. In designing buildings in a location subject to galeforce winds, horizontal wind loads are indeed taken into account. Earthquake loads are both vertical and horizontal. Massive structures attract more seismic forces; wooden buildings are lighter and respond better to earthquake forces. These forces are very complex, and in contrast to wind loads (except for tornadoes) they are applied suddenly, with high acceleration.

     

    I have already discussed acceleration as a percentage of the attraction of the Earth due to its gravity, or g. During a space shuttle launch, astronauts are subjected to accelerations of several g as they rocket into space. A downhill ride on a roller coaster temporarily counteracts the Earth’s gravity to produce zero g, and this accounts for the thrill (and sometimes queasy feeling) we experience. Acceleration during an earthquake is the Earth’s answer to a roller-coaster ride. If the shaking is enough to throw objects into the air, the acceleration is said to be greater than one g. High accelerations, particularly high horizontal accelerations, can cause a lot of damage.

     

    The deadweight of a building and its contents can be calculated fairly accurately and can be accounted for in engineering design. These loads are called static loads; they do not change with time. Wind loads and earthquake loads change suddenly and unpredictably; these are called dynamic loads. The engineer must design a structure to withstand dynamic loads that may be highly variable over a very short period of time, a much more difficult task than designing for static loads alone. Because the awareness of the potential for earthquake loads is only a few decades old, many older buildings were not designed to stand up against the dynamic loads caused by earthquakes.

     

    In Chapter 2, rocks of the crust were described as either brittle or ductile. Brittle crust fractures under the accumulated strain of the motion of tectonic plates and produces earthquakes. The underlying warm and pliable ductile crust deforms without earthquakes. Structural engineers use these terms to refer to buildings. A building that is ductile is able to bend and sway during an earthquake without collapsing. In some cases, the building “bounces back” like a tree swaying in the wind, and it isn’t permanently deformed. Deformation is elastic, as described earlier for balloons and boards. In other cases, the building deforms permanently but it still doesn’t collapse, so that people inside can escape, although, in the Northridge Earthquake, it turned out that the welds connecting steel frames were not ductile, and these welds failed. Wood-frame houses are also ductile. Fortunately, most of us live in wood-frame houses.

     

    In contrast, a brittle structure is unable to deform during an earthquake without collapsing. Brittle buildings include those made of brick or concrete block joined together with mortar but not reinforced with steel rebar. In an earthquake, your wood-frame house might survive, but your chimney, made of brick not reinforced with rebar, might collapse. Your house is ductile, but your chimney is not (Fig. 11-13).

     

    The reinforcing techniques described below are for a house that has already been built; this is called a retrofit. These techniques are also applicable to new construction, in which case they are a lot less expensive. This is immediately apparent in shoring up the foundation. It’s the difference between working comfortably on a foundation before the house is built on top of it and working in a confined crawl space.