Performing a seismic retrofit can strengthen any building’s structure so that it can effectively resist the damaging effects of earthquakes. The United States has been tectonically active since the Pangea subcontinent separated nearly 200 million years ago and majorly because of its proximity to the North American tectonic plate. As a result of the formation of the San Andreas Fault system, the juxtaposition of the Pacific and North American tectonic plates has made California extremely susceptible to lateral motion.
The Great 1906 San Francisco Bay Area earthquake of magnitude 7.9 (Mw) is one of California’s most famous and largest earthquakes. San Francisco earthquakes cause liquefaction and landslides which undermines building support structures. In this blog, we will discuss the meaning of seismic retrofitting, its importance, advantages and disadvantages, as well as common types of upgrades and techniques used in the seismic retrofitting process.
Retrofitting in construction refers to the process of upgrading existing systems with new technology or features. It is performed for multiple reasons, for example, retrofitting equipment makes them more advanced and long-standing. While retrofitting in structures like residential and commercial buildings, bridges, and historical buildings is done either to update them according to changing codes, protect them against seismic activity, or increase their energy efficiency.
Seismic Retrofitting is the modification and strengthening of existing buildings to improve their resistance to the reverberations of seismic activity, especially earthquakes. Seismic retrofitting of a new or existing building also safeguards it from the hazards of seismic events like volcanoes, floods, thunderstorms, landslides, and other natural disasters.
Every structure requires two load-resisting systems, a vertical load-resisting system for transferring the vertical load to the ground and a horizontal load-resisting system for transferring the horizontal load to the vertical load system. Retrofitting buildings brings regularity to the load transfer of seismic forces from the horizontal framing system to the vertical lateral resisting system and vice versa.
Seismic retrofitting protects structures from collapse or damage due to seismic effects, ground motion, or soil failure. The National Earthquake Information Center records about 20,000 earthquakes worldwide each year or approximately 55 per day. These numbers are enough to address the need for seismic retrofitting as many buildings might not have been designed to be earthquake-proof. Also, some buildings may get structurally weak after some time and might have to be reinforced.
When seismic engineers studied the earthquakes in Turkey and Syria, they found that non-ductile concrete construction, a common building design in California and other parts of the U.S., was the main reason behind the destruction. Many non-ductile buildings in California remain unassessed or seismically retrofitted, making them vulnerable to collapse during a heavy earthquake. Hence, officials of US cities have started implementing ordinances that require seismic retrofits on thousands of non-ductile, soft-story, or both types of buildings.
Photo by Ivan Henao
The prime benefits of performing seismic retrofits in buildings are as follows:
Seismic retrofit techniques are used to strengthen existing buildings and achieve earthquake resilience. Some popular seismic retrofit techniques used by contractors include adding steel braces or shear walls, strengthening foundations and footings, thickening existing walls, installing dampers, or setting up base isolation systems. The selection of specific seismic retrofitting techniques and upgrades depends on the type and condition of the building, as well as the seismic hazard and local building codes and regulations.
A few seismic retrofitting steps can be taken to help fix a building’s weakness from earthquake damage.
Base Isolation is an extensively used seismic retrofit technique around the world to reinforce structures against seismic activity. Base isolators connect the building to the foundation and act as suspension. These isolators make use of materials and techniques that absorb a large part of the vibrations from the shaking ground. This seismic retrofitting method may involve the use of roller balls and bearings, sliders, or elastomeric bearings.
In case of an earthquake, the vibrations transferred from the ground to the building are considerably reduced when compared to structures that are built directly on foundations without employing isolators. This reduces the oscillation or movement of the building and lowers the chances of seismic damage. The San Francisco City Hall was retrofitted with a base isolation system in the 1990s, which included installing 768 base isolators to protect it from earthquakes.
As the name suggests, wall thickening is a seismic retrofitting technique that increases the thickness of existing walls of a building by adding bricks, concrete, and steel. For example, if a wall is 250mm, using thickening technique we can extend that to 300mm. The increased dimensions allow the walls to withstand more vertical, horizontal, and transverse loads without sudden failure as well as reduce the lateral-loading effect in a structure. It is important that steel reinforcement is properly covered in mortar, otherwise, rust can develop.
To increase the strength of a building and prevent it from shaking, columns made from reinforced concrete may be constructed on the exterior of the building. The addition of these columns to an already existing building is one of the most practical seismic retrofitting techniques. It encases the building in a protective frame and ensures higher resistance to damage from seismic activity.
Sometimes, a structure needs additional support to withstand lateral forces during an earthquake but does not have enough interior space to accommodate new columns. In such cases, reinforced concrete columns are strategically placed at the corners of the building and along its sides to provide maximum support.
There are multiple points in a building that are under constant pressure from the load they are supporting. Some of these may be overstressed and might have weakened over time. Their symptoms include bending, distortions, or cracks in the structure. Buildings with these are already under undue pressure and any earthquake activity will have a greater impact on them.
To make the establishment more durable, these parts are seismically retrofitted with post-tensioning systems that help to share the load. Generally, steel bars, cables, and strands may be installed externally or inside the original structure to provide post-tensioning. They may also exert a force in the opposite direction to normalize the distorted shape. These also prevent the development of cracks and increase the load-carrying capacity.
Suppose a commercial office building is to be retrofitted for seismic resistance, contractors and seismic engineers will first install steel anchors in the building’s foundation and the upper floors. This will create a secure base for post-tensioning cables. The steel cables will be then installed between the anchors and tensioned to a specific limit. In this way, post-tensioning cables will compress the structure which will improve its ability to resist seismic forces.
Shear walls and braced frames are powerful seismic retrofitting techniques that provide support to the building and counter and stabilize any motion or vibration resulting from earthquake shocks. These act as a brace for the walls of a building. While braced frames are constructed of steel, shear walls are made from wood or concrete. These braces are highly ductile and stretch whenever they are under pressure due to lateral movement in the building.
For example, the triangular Torre Reforma is an open book shaped tower in the earthquake-prone region of Mexico City. The unique structure has two walls of reinforced shear concrete and a glass facade which provides it stability. Additionally, the concrete walls sink 50 meters deep into the ground, enabling the building to withstand any earthquake.
Jacketing is a widely-used technique in seismic retrofitting. It is a concrete reinforcement method to strengthen columns and beams in building construction. It involves the addition of a reinforced concrete, steel, or fibre-reinforced polymer composite (FFPC) jacket to the existing columns or beams of a building.
The jacket with longitudinal and transverse reinforcement is typically installed around the perimeter of the existing columns. This increases the axial and shear strength of the columns and allows them to carry lateral loads without failing. Jacketing is a seismic retrofit strategy that also improves the confinement of concrete in circular columns, thereby preventing the formation of cracks in the concrete itself.
For example, the Royal Ontario Museum located in Toronto, Canada is a historic structure that underwent seismic retrofitting with concrete jacketing in the early 2000s. Due to an ice storm, the building suffered damage and was found to be vulnerable to seismic effects. To enhance its structural integrity, a layer of reinforced concrete was added to the exterior walls as other required areas.
The external plate bonding technique of seismic retrofitting has been in practice for decades to strengthen reinforced concrete beams with external plates or strips. When external steel plates are partially or completely wrapped around the joints of a potentially cracked beam, additional shear strength is achieved. The level of this additional shear strength depends on the beam geometry, current concrete strength, and the applied wrapping method.
The Grand Central Tower is a 40-story commercial building situated in a high-seismic zone. After a structural assessment, the seismic retrofitting team recommended external plate bonding technique to strengthen the building. Steel plates were fabricated off-site and cut to fit around columns and beams. They were carefully secured in place using a high-strength adhesive without compromising the building’s functionality or aesthetics.
In old concrete buildings, with time, the walls might develop cracks or the plaster might deteriorate and its binding strength will decrease. It may come loose in places and this will hamper the structural integrity of the building. To counter this, Gunite may be applied for effective seismic retrofitting. It is a combination of cement and sand supplemented with water resulting in concentrated concrete that is sprayed at high pressure over the surface to be repaired. It bonds strongly to the existing structure and increases its structural integrity making it more resilient to earthquakes.
The seismic retrofitting project of the famous San Francisco Bay Bridge was carried out in the 1990s in which gunite was applied to the bridge’s concrete piers. The gunite was applied in multiple layers using a pneumatic spraying process and created a strong protective shell for the structure.
This seismic retrofit technique involves the removal of one or more stories from the building structure. Removing mass from the structure is an effective way to decrease its weight and increase its strength. For example, if a building is G+4 (ground floor + 4 stories) with higher loading conditions, mass reduction can lower it to G+3 or G+2 to match the strength of the building.
In place of structural elements, seismic dampers are used like diagonal braces to numb seismic reverberations in structures. Dampers absorb motion energy and convert it to heat, thus decreasing the resonant effects of rigidly attached structures.
Besides adding energy dissipation capacity to the structure, supplementary damping has the ability to reduce the motion of the buildings. Supplementary dampers act similarly to shock absorbers in automobile suspension. Dampers such as viscous, viscoelastic, friction, yielding, tuned mass, and magnetic types are used for seismic retrofit projects.
Though seismic retrofitting methods are effective in improving strength and ductility of reinforced concrete buildings, they have challenges associated with constructability, serviceability, ensuring integrity, and cost. Below are the biggest disadvantages of seismic retrofitting:
Fortunately, these challenges can be mitigated by partnering with an experienced seismic retrofitting contractor who has a qualified team of professionals. Hire a company providing high-quality structural engineering, metal fabrication, and construction services under a reasonable budget.
In recent years, seismic retrofitting has been considered the most reliable technology for the protection of a broad range of structures from earthquakes. Though there is no such thing as ‘earthquake-proof construction’, a seismic retrofit makes pivotal structural improvements that shield the building from damage by seismic waves.
To determine which is the most effective seismic retrofit for your building, you need to understand the different optimization techniques used by professionals. In seismic zones. retrofitting for new and existing buildings would require compliance with updated seismic standards. Make sure you hire an experienced and qualified seismic retrofit contractor to handle all your project’s requirements.
Constructive Solutions, Inc. has successfully completed commercial seismic retrofits in San Francisco and San Jose, California, and is dedicated to providing high-quality structural engineering and optimization services to its clients.
Contact us today to discuss your building’s seismic retrofitting needs.
Seismic design category (SDC) is a classification system assigned to buildings based on the severity of seismic forces they are expected to face and determines their requirement for seismic design and construction. SDC ranges from “A” to “F” with “A” category buildings requiring basic seismic design while “F” category buildings require a high level of seismic retrofitting and care.
Usually in California, the cost for seismic retrofitting projects ranges from about $3,000 to $7,000.
ASCE SEI 41-17 is a standard published by the American Society of Civil Engineers (ASCE) and the Structural Engineering Institute (SEI) that outlines procedures for seismic evaluation and retrofitting of existing buildings. It guides on how to assess the seismic performance of structures, identify deficiencies, and determine retrofitting measures to improve their earthquake resilience.
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This post was last modified on June 3, 2023 3:14 am
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