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Dome Home May Provide Increased Structural Earthquake Protection

Dome Home May Provide Increased Structural Earthquake Protection

There is no method of accurately predicting when or where an earthquake will occur. Modern building methods have decreased the likelihood that traditional wood framed homes will collapse, but greater protection and structure survivability is inherent in the construction of thin shell concrete domes.

No Advanced Warning

The first warning of an earthquake could be when the ground starts moving. We can’t predict when or where they will occur (1). According to the U.S. Geological Survey, our current level of warning is in the form of whether it is probable that an earthquake will occur in a given geographical area in the next 12-50 years based on historical activity (2). The recovery and reconstruction cost in Japan because of the 2011 earthquake is estimated at $180 billion (3). Recent earthquakes have occurred in areas not usually considered danger zones, such as the 5.9 quake on the U.S. East Coast on Tuesday August 23, 2011(4),

Safer Residential Structures Needed

The main approaches to making structures safer during an earthquake are to strengthen structural members such as beams and columns and the connections between them, or to reduce the earthquake generated forces acting upon the structure. The weakest points of a structure are ninety-degree angles where walls, floors and ceilings meet. During movement of the earth when a quake occurs, these moment connections are likely to be stressed and sometimes fail. (1)

Many modern building methods are designed to lessen the destructive effects of the forces of acceleration and inertia a structure experiences during a quake. Much of the focus on strengthening these moment connections is to sustain building integrity long enough for the safe evacuation of the occupants (4). But what about the structure?

When the structure in question is a residence, it is vital that occupants be provided with time to get to safety. But a structure capable of surviving an earthquake would enable residents to have a habitable home when the event is over.

A traditionally designed wood frame home within the average price range has damage susceptible moment connections throughout the entire structure including junctions between walls, ceilings, floors and load bearing beams and columns. Most homes are designed and built to withstand collapse. These moment connections flex rather than separate. But during the flexing, the connectors, usually nails or metal plates are fatigued and their connection in the wood can be loosened. As a result, older structures may develop gaps in these moment connections. Successive tremors can cause these weakened connections to fail. (1)

How Earthquakes Damage Homes

The stress an earthquake causes on a building results from the inertia of the stationary building when the ground under it moves. Charles Ammon, Professor of Geosciences at Penn State University, said, the sudden movement “Can cause shearing of the structure which can concentrate stresses on the weak walls or joints in the structure resulting in failure or perhaps total collapse.” Three primary methods used to strengthen a building against earthquake damage are

1) secure the walls, floors, and foundation together and have the entire structure behave as a single stiff unit that moves with the ground,

2) take steps to strengthen buildings including using steel frame construction, and

3) adequately secure the structure to the ground through a solid foundation.

All three options can be cost prohibitive when retrofitting single family residential buildings. (6)

How Domes Provide Stronger Structural Earthquake Protection

In contrast, a steel reinforced thin shell concrete dome structure has no moment connections. It provides the advantages of the entire structure behaving as a single stiff unit with a steel frame construction. The entire structure is secured to the ground through a solid foundation at regular intervals.

David South, President of the Monolithic Dome Institute in Italy, Texas, explains the advantages to the domes they construct. “The (Monolithic) dome has no moment connections – those points at which a wall meets a roof or a floor attaches to a wall. With zero connections to fatigue or disconnect, in general, an earthquake will put no more pressure on a dome than a good snow load. There are no single components that carry large loads; there is nothing that can be knocked down like a beam or a column.”(7)

When an earthquake occurs, a thin shell concrete dome does not experience the damaging inertia of a traditional building. The earthquake forces are spread over the entire surface of the dome and it moves as a unit rather than the base shifting under the building and the building’s inertia causing it to accelerate at a different rate.

A thin shell concrete dome is continuously attached throughout the structure with steel reinforcement and rides the ripples in the earth’s surface during a quake, like an inverted bowl. The same dispersion of force that enables a dome to withstand forces of 400 pounds per square foot, as demonstrated during load testing at the Monolithic Dome Institute, also enables it to disperse earthquake generated forces usually without damage to the structure. A dome utilizes the entire structure to equalize the acceleration caused by the earth’s movement

Even With Stronger Structural Integrity, Precautions are Necessary

Since a dome will ride the ground movement there is still the likelihood that occupants and other contents will feel the movement. Despite the enhanced structural resistance to damage, occupants still need to practice earthquake safety and seek a protected location away from glass, furniture and furnishings that may be dislodged, propelled or toppled by the earth’s movement. If a structure is located on a fault that opens and the structure falls into the fissure, a dome may still retain its integrity, but the occupants and contents will shift catastrophically within the structure.

Dome Home Village Experience in Indonesia

When the earthquake is over, a dome resident will more likely have an undamaged habitable home. Such was the experience of Ngelepen, a village in Indonesia. An earthquake and subsequent mudslides leveled the village in 2006. Domes for the World, a nonprofit, organization that provides training, tools and methods for constructing superior shelters and community systems, for the world’s most needy rebuilt their village with dome structures. The village has withstood subsequent earthquakes without damage. (8)

Cost Effective Construction Technology

The technology to build thin shell concrete dome homes using inflatable air forms for about the same cost as traditional wood frame homes of comparable size has been available for about thirty years. Dome homes are still relatively uncommon, but their ability to survive earthquakes, fires, tornadoes, and other disasters seems to be increasing the demand for these unique looking structures.

Sources

(1) Advanced Earthquake Resistant Design Techniques, Multidisciplinary Center for Earthquake Engineering Research (MCEER), 2010

(2) Predicting Earthquakes, U.S. Geological Survey, Oct, 1997

(3) Natsuko Waki, Damage Estimate for Japan Quake is at $180B, Reuters News Service, Mar. 2011

(4) Lily Kuo and Malathi Nayak, Once a Century Earthquake Rattles East Coast, Reuters News Service, Aug. 2011.

(5) Andrew Kracht, Implementation of Moment Frame Connections Scaled to Residential Construction, Composite Materials and Engineering Center, Washington State University, Jul. 2010

(6)Charles J. Ammon, Earthquake Effects, Department of Geosciences, Penn State University, Aug. 2010

(7) David B. South, Eathquake Safety-it’s Yours in a Monolithic Dome, Monolithic Dome Institute, Jun. 2009

(8) Rebecca South and Andrew South, Final Report: New Ngelepen, Yogyakata, Indonesia, Domes For The World, Jun. 2010

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