What is a non-ductile concrete building?
A nonductile concrete building is a reinforced-concrete structure designed and constructed without adequate detailing to ensure ductile behavior during strong earthquakes. Typically built before modern seismic codes, these buildings often lack sufficient transverse reinforcement (such as closely spaced ties or stirrups) in columns, beams, and beam-column joints. As a result, they are prone to brittle failure modes—including shear failure, column-side blowout, and sudden loss of lateral load-carrying capacity—when subjected to intense ground shaking. Because they cannot undergo large deformations without losing strength, nonductile concrete buildings are considered highly vulnerable in seismic events and are a priority for evaluation and retrofit in earthquake-prone regions.
A non-ductile concrete building is one that is constructed of concrete in such a way that makes it more vulnerable to substantial damage or collapse in a large earthquake. The word “ductile” means that something can deform without breaking; it can be considered the opposite of the word “brittle.” Modern building design, especially in seismic-prone regions, focuses on ensuring that buildings are ductile—that they can move easily as the earth sways beneath them without breaking in a way that could injure or kill the people inside.
The 1976 Uniform Building Code, which was adopted by jurisdictions in the late 1970s and 1980s, ensures that the reinforcing steel within a concrete structure is designed to hold the concrete around it intact when the building moves in an earthquake (I.e. provide ductility). This is the first version of the building code that focused on providing ductility in concrete structures. Due to the lack of these requirements before 1976, many concrete structures built prior to the 1980s do not have the reinforcing steel to allow ductile behavior. These buildings are called “non-ductile concrete,” or “NDC” buildings.
Why are non-ductile concrete buildings problematic?
Lacking ductility means that the structure cannot easily stay intact while it undergoes significant movement. In other words, the problem with non-ductile concrete buildings is that they crack and collapse as they move, such as occurs during a large earthquake.
What are some strategies to retrofit non-ductile concrete buildings?
Based on knowledge of old building code provisions and lessons learned from building damage and failures due to earthquakes over time, the pitfalls of non-ductile concrete buildings are well known and can therefore be remedied.
Retrofits typically involve increasing the stiffness and strength of buildings to ensure that there are structural elements in place that can resist seismic forces and prevent the degree of movement that would crack or collapse the original concrete structure. Some common strategies to retrofit non-ductile concrete buildings include installing diagonal steel braces between floors, adding reinforced concrete walls (freestanding or installed directly adjacent to existing concrete walls), strengthening concrete elements with fiber-reinforced polymer (FRP), and improving connections between structural elements.
Another way that non-ductile buildings can be retrofitted is to reduce the effect of seismic forces on the building. This can be done by isolating the building from the earth below it—called base isolation—or by installing structural elements called dampers that function like shock absorbers. These retrofit options require extensive structural analysis and specialized products but can provide solutions that are less invasive to an existing building.
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ZFA’s experience with non-ductile concrete buildings
While non-ductile retrofit strategies are often similar for different buildings, each building is unique. Similarly, the goals and priorities of each building owner are unique. Therefore, the approach to a retrofit begins with listening and understanding owner concerns, priorities, and hurdles as well as gaining a thorough understanding of the structural system and deficiencies to be remedied. ZFA has successfully performed evaluations and provided designs for various non-ductile concrete building retrofits throughout California.
Temple Beth Hillel Seismic Retrofit
VALLEY VILLAGE, CA
ZFA assisted the Congregation Beth Hillel of San Fernando Valley with compliance of the Los Angeles Department of Building and Safety (LADBS) Non-Ductile Concrete Retrofit Program. Two of the Temple Beth Hillel campus’ four structures are included in the program: a two-story, concrete-framed sanctuary/administration building, and a two-story concrete-framed school facility that is connected to, but seismically separated from, the sanctuary/administration building.
The retrofit includes FRP strengthening of existing columns, shear walls, and floor/roof diaphragms; addition of new concrete shear walls and shotcrete strengthening of existing shear walls; out-of-plane anchorage of existing concrete walls to existing floor diaphragms; and new floor/roof diaphragm ties and drag elements to ensure that lateral loads have a path to the shear walls.
Pine Street Seismic Retrofit
SAN FRANCISCO, CA
ZFA is providing structural engineering services for a mandatory seismic retrofit of the 111 Pine Street building in San Francisco. Our scope of work addresses known deficiencies that will satisfy a mandatory non-ductile concrete ordinance that is expected to be adopted in the future.
Built in 1963, the 252,000-square-foot building consists of 19 stories over a single-story subterranean basement. The building is a typical cast-in-place mid-rise building with a 7 ¾”-thick floor slab spanning to interior concrete columns and walls. The perimeter of the building is constructed with concrete beams spanning to perimeter columns, and the whole structure is clad with precast concrete panels.
The approach to the full seismic retrofit is to utilize viscous and hysteretic dampers throughout the building to decrease story drifts and reduce the building’s overall torsional response, keeping seismic demands in existing elements below their capacity. The viscous dampers would offset additional forces due to the increase in stiffness provided by the hysteretic dampers. These viscous dampers (similar to shocks in a vehicle or mountain bike – but much larger), are dependent on velocity, hence do not add forces to the existing structure. Rather, they absorb/dissipate earthquake energy without distress to the existing structure. Damper locations at all levels will consider impacts to existing architectural and MEP systems to minimize cost.
In addition to dampers, some beams, columns, and beam-to-column joints will be strengthened using FRP and/or steel jackets. New concrete shear walls will be added at select locations in the basement and ground floor levels to mitigate a discontinuous shear wall condition at these floors. New micropile foundations will be used to augment the existing foundations to support new elements as required.
University of San Francisco – Masonic East Seismic Retrofit
SAN FRANCISCO, CA
This project involves a mandatory seismic retrofit of a non-ductile concrete building that was originally constructed in 1954. ZFA completed a condition assessment and seismic retrofit design of the 75,000-square-foot building, with roughly 25,000 square feet each at the basement, ground level, and second levels. The building is a cast-in-place, reinforced concrete structure.
The existing concrete shear walls and collectors were found to be inadequate. ZFA designed new interior concrete shear walls, strengthened existing shear walls, and designed new collectors in multiple locations. Additionally, strengthening was added to existing beams and columns supporting discontinuous shear walls above. Since the existing columns were found to have inadequate ductility, FRP jackets were designed to allow the columns to displace to the required horizontal drift while maintaining their vertical load carrying capacity.
Broadway Street Seismic Retrofit
OAKLAND, CA
ZFA performed a seismic evaluation and subsequent seismic retrofit of a 1960s-era, 12-story concrete moment frame building in downtown Oakland. The evaluation and retrofit were in accordance with ASCE 41-17 for three discrete levels of seismic performance as selected by the client. ZFA selected 11 ground motions from the NGA-WEST2 database and spectrally matched them to the target seismic hazard of the BSE-2E for use in Non-Linear Response History Analysis. The first retrofit design employed a targeted combination of fluid viscous dampers and exterior column strengthening to achieve the desired performance.
Subsequent to this effort, ZFA provided two reduced-scale retrofit concepts and performed incremental dynamic analysis, reducing the scale factors on time histories (earthquake records) little by little until the retrofit approach met the acceptance criteria. Following this, in an effort to convey the level of seismic protection to the client, ZFA translated this level of shaking back into a Moment Magnitude of earthquake using the NGA-WEST2 ground motion prediction equations. ZFA then coordinated a cost estimate of all three retrofit concepts with a general contractor to develop ROM pricing, which enabled visualization of benefit in terms of dollars of retrofit per level of earthquake.
King Estates Middle School Seismic Retrofit
OAKLAND, CA
The King Estates Middle School project consisted of a full seismic retrofit of three nonductile concrete buildings. The buildings are low-rise concrete structures supported on concrete drilled pier foundations with concrete walls and precast double tee roofs. The buildings were evaluated to the requirements established in DSA Procedure 08-03 and ASCE 41-06. The retrofit scheme included FRP roof diaphragm strengthening, wall-to-diaphragm connection strengthening, wall-to-foundation connections, and new reinforced concrete grade beams. The project was completed during the summer of 2012 and finished in time for the new school year without any closures.
Sylvia Mendez Elementary School Seismic Retrofit
BERKELEY, CA
ZFA is providing structural engineering services for the renovation, revitalization, and expansion of the Sylvia Mendez Elementary School campus. The 50,000-square-foot, one- and two-story facility was built in 1949 and constructed largely of reinforced concrete walls, floors, and roofs. Renovations to the donut-shaped building are extensive and include upgrades to all classrooms and support spaces. The structure of the building will be updated to meet 2019 building code requirements using ASCE 41-17, and the retrofit involves the addition of shotcrete shear walls, new and strengthened collector elements, and associated new foundations. Two new elevators and one new staircase are added, and a new seismic joint will be installed to separate the one- and two-story portions of the building for improved seismic performance. The work is being completed over the course of one school year and multiple summers.