During an earthquake, buildings experience both lateral (side-to-side) forces and vertical loads. Buildings can dissipate this energy by incorporating design elements that resist lateral forces, such as a diaphragm.
The material used to construct a building has a huge impact on its resilience. Materials with high ductility—the ability to bend without breaking—are particularly effective.
Steel
There are many types of building materials that can withstand earthquake forces, but one of the most effective is cold-formed steel. This material is used in buildings designed to withstand seismic forces for two key reasons: its high ductility and its light weight.
During an earthquake, structural elements experience lateral and shear loading. These forces can snap, shatter and break critical connections between components. In addition, the force of an earthquake can be so great that it tries to move the foundation of a building off its ground.
Buildings designed to withstand these kinds of forces are constructed using construction methods that allow for flexing and deformation in the building structure. This helps to dissipate the energy of the earthquake without putting excessive stress on the building’s connection between the foundation and the ground.
The most important aspect of an earthquake-resistant design is incorporating a ductile element into the building structure. This is why steel is such an ideal material for earthquake-resistant buildings.
Steel is an extremely versatile construction material that can be used in a wide variety of building applications. It is found in the pots and pans that we use to cook, in cars and trucks that we drive, and even in buildings like our homes and offices. What makes steel so versatile is that it is not only extremely strong, but also lightweight and easy to work with.
Wood
Wood is a versatile and resilient building material that can be used to construct buildings that resist seismic forces, high winds, and even fire. In fact, a study of homes damaged in hurricanes in Florida and Texas found that those built with wood and to modern seismic codes suffered less damage than those built to older standards.
Wood’s ductility is especially useful during earthquakes, as it allows it to yield and deform without fracture under abrupt lateral stress, reducing the inertial force exerted on the structure. In addition, wood-frame buildings generally weigh less than structures made of concrete and steel, reducing inertial seismic forces. Also, the numerous nail- or metal-connections in light-frame wood construction provide several, redundant load paths for extreme forces, which can reduce the chance of a building collapse if any one connection fails.
Researchers are using wood to develop new methods for designing tall buildings that can withstand large earthquakes. For example, the NHERI Tall Wood project is investigating the performance of cross-laminated timber (CLT) panels, which are constructed from gluing together layers of solid-sawn lumbers in an orthogonal pattern. These panels could serve as a sustainable alternative to steel and concrete in building design up to 20 stories in height. The team recently tested a full-scale two-story mass timber building with post-tensioned CLT rocking walls on an indoor shake table, and the structure was able to withstand 14 powerful earthquakes with minimal damage.
Concrete
Concrete’s durability and strength have made it the backbone of buildings and infrastructure worldwide-houses, schools and hospitals, airports, highways and rail systems. It is the most-produced material in the world. Concrete has high compressive strength but low tensile strength, so it is reinforced with steel to handle tensile loads. This combination of a tough material and resilient steel creates structures that are exceptionally strong and safe during an earthquake.
The strength of concrete is influenced by how it is mixed, consolidated and cured. It also depends on the quality of the aggregate used and the proportions in which they are mixed with cement. Poor construction methods and inadequate quality materials can weaken the structural integrity of concrete buildings and increase the risk of damage during an earthquake.
Earthquakes in Guam (Richter scale 8.1), Manila, the Philippines (Richter scale 7.2) and Kobe, Japan (Richter scale 6.9) have shown that well-constructed concrete framing systems can withstand nature’s strongest forces. The 1994 Northridge earthquake (Richter scale 6.7) also demonstrated the capability of concrete shear walls to resist bending forces during an earthquake.
Researchers at the University of British Columbia have engineered a type of concrete called eco-friendly ductile cementitious composite (EDCC) that can withstand severe seismic activity. This engineered concrete combines conventional cement with fly ash and polymer-based fibers to make the material more ductile, meaning it can bend rather than break under pressure.
Metals
When a building is designed to withstand gravity, the weight of its occupants and above-ground forces, it must also be built to withstand underground vibrations and horizontal waves of force that can damage walls, support frames and foundations. These are forces that can cause buildings to collapse or, at the least, sway and shift significantly in different directions, making them unsafe for occupants.
Historically, buildings made from brick, stone and concrete have relied on steel reinforcement to be strong and deformable enough to withstand earthquakes. This is a key design principle in advanced earthquake-resistant structures called ductile buildings.
The secret behind a ductile building is the ability of its structural members to undergo large plastic deformations, or bending. Concrete and brick have low ductility, so their structures tend to fail when exposed to earthquakes. Steel-reinforced concrete, on the other hand, performs much better because its embedded steel increases the material’s ductility.
In addition to increasing a building’s ductility, seismic mitigation methods include base isolation, energy-dissipating devices and shear walls and bracing. Unlike wood, steel construction is lighter than concrete without sacrificing strength, so it needs less of a force to impose its own mass on the superstructure and foundations, reducing construction costs and allowing the use of more economical materials.