Author: Johanna Fischi
Boston is home to many historical buildings that date back to before the founding of the United States. But what would happen to these landmarks if a major earthquake occurred in the city today?
How do Earthquakes Damage Buildings?
Buildings are damaged by the strength of the earthquake shaking, length of the shaking, the type of soil and the type of building. Seismic building codes are meant to reduce seismic risk by offering structure design and construction guidelines which help limit the amount of damage or injury caused by a building shaking during an earthquake. Unsafe buildings can injure people if the entire structure collapses or if people are hit with falling debris.
Example of building shaking during the 2011 M6.3 Christchurch, New Zealand earthquake
The Problem with Older Buildings
The International Code Council and FEMA have different regulations for the construction of new buildings, and for the retrofitting of old, existing buildings. In Massachusetts, structures not built within the past 44 years can be very hazardous because they are not up to current the seismic safety standards. One of the greater seismic safety hazards with older buildings is that they are built with brick and mortar which often collapses easily during shaking because these materials are inflexible. In the United States, FEMA states that existing buildings only need to meet the seismic building codes present at the time the structure was built.
In order for older structures to meet modern seismic safety standards they must be retrofitted. Retrofitted means the existing structure is modified so that it will experience less damage during an earthquake. This process is often costly and done voluntarily causing older buildings to be the biggest contributor to seismic risk today.
Is Boston up to Code?
Earthquakes do occur in New England on a regular basis. A larger earthquake, like a magnitude 5.0 to 6.9 is only expected to occur about every 450 years. Many of the buildings in Boston were built in the 19th and 20th century and are built with brick and mortar. Areas of the city are also built on loosely compacted landfill meaning there is a liquefaction hazard.
Example of a brick and mortar building after the 2011 M6.3 Christchurch, New Zealand earthquake
In 1975, Massachusetts implemented seismic building codes for any structures built since then. However, 75% of Boston was built before 1975. This means older buildings may not be able to withstand a major earthquake. The city’s infrastructure, such as its gas and water lines have a similar problem.
On the Boston University campus, the Bay State brownstones are at risk of being damaged in an earthquake. Over 80% of these structures are unreinforced masonry. The brownstones are also built on top of underground timber pilings. The shifting watertable has caused the wooden pilings to be exposed to air making them rot. In an earthquake the pilings would provide no support for the building and the structure could collapse.
New England experiences 100 earthquakes every year with a magnitude 1 to 2. The last magnitude 6.0 earthquake occurred on Cape Ann in 1755. Another earthquake of that intensity is not expected for hundreds of years. However, in the event that a large magnitude earthquake does occur, we can minimize damages by retrofitting major buildings in the Boston area.
Structures built since 2000 following the ICC guidelines and will be built with multiple types of anti-seismic technology. Newer structures will be made from steel or wood that are flexible but also maintain the integrity of the structure under stress during an earthquake.
Examples of Anti-Seismic Technology
Base Isolation: The base of the building does not touch the surrounding bedrock. When an earthquake causes the ground to shake laterally, the seismic lateral shaking is transferred to the building possibly causing damage. A base isolator reduces the shaking the building experiences by allowing the foundation of the building to move with the shaking reducing the effect of shaking on the structure. An example of base isolation are oil dampers. The building stands on shock absorbers so that when the structure experiences lateral movement the shaking is transferred to large pistons under the building which are surrounded by oil. The mechanical energy of the shaking is converted into heat energy when it hits the oil reducing the structure’s shaking.
Tuned Mass Dampers: Towards the top of tall buildings a ball of huge mass is suspended with steel cables from the structure. The ball acts as a pendulum when the building shakes. The ball moves in the opposite direction of the structure counteracting the shaking. The mass is also tuned to the frequency of the building in order to better help stabilize the building.
Learn More About Building Damage During Earthquakes
An important aspect of anti-seismic technology is creating architectural designs that help prevent damage from occurring to the building. The National Earthquake Hazards Reduction Program offers 7 characteristics of strong design of a seismic reducing structure:
1) Stable Foundations: The structure’s foundation can withstand ground deformation, lateral shaking, and liquefaction. This is accomplished through deep foundations that are widely set.
2) Continuous Load Paths: All parts of the structure are connected and securely attached in order to prevent the building from pulling apart that can cause collapse.
3) Adequate stiffness and strength: A building must be sturdy enough in order to withstand vertical and lateral shaking. This shaking can cause a building to shift off its foundation.
4) Regularity: The structure’s mass, strength and stiffness is consistent throughout the building so that movement will be about the same through the building during an earthquake.
5) Redundancy: There are multiple earthquake withstanding reinforcements within a building so that some are able to fail and the building can still support itself and not collapse.
6) Ductility and toughness: The materials of a building are able to experience stress and still support the structure with little damage. This is why wood and steel are often good building materials because they are ductile and yield to stresses without losing the integrity of the structure.
7) Ruggedness: A building material has structural strength and maintains its integrity throughout shaking.
Text Modified from IRIS: BuildingsInEQs.pdf
University of Manchester: www.mace.manchester.ac.uk/project/teaching/civil/structuralconcepts/.../30.pdf
The Daily Star: https://www.thedailystar.net/spaces/top-3-earthquake-resistant-technologies-199750
The Boston Globe: https://www.bostonglobe.com/metro/2015/07/23/major-earthquake-will-hit-new-england-again-someday/3MAogFwcnqxY6jNU2qkwYP/story.html
Christine Regalla is a geology professor in the Earth and Environment Department at Boston University. See what she and her students do to study earthquakes by visiting their website or by following them on Twitter and Instagram (@Rengellia).