Non-Ductile Concrete Building Retrofit FAQ
What is the Potential Cost of a Non-Ductile Concrete Building Retrofit?
The retrofit cost of construction can vary widely depending upon site-conditions, retrofit options and specific owner’s requirements. Based on various case studies, it is estimated that costs can be expected to vary between $50 to $100 per square foot with 10%-20% engineer and permitting fee.
What Is a Non-Ductile Concrete Building?
Many large non-ductile concrete buildings were designed and built prior to the Los Angeles City Building Code provisions. The Los Angeles Department of Building and Safety describes structures in need of retrofitting as “concrete buildings with a roof and/or floor supported by a concrete wall or concrete column” and whose plans were submitted before January 13, 1977. The detailing and construction of the reinforced steel in older concrete buildings can be insufficient against seismic forces and ground motions. A licensed civil or structural engineer needs to make a structural evaluation to decide if a building’s structure has details that qualify it as a non-ductile reinforced concrete building.
What is Non-Ductile Concrete Building Seismic Retrofitting?
Non-ductile concrete building seismic retrofitting is the alteration of an existing structure to make it more resistant to seismic activity, soil failure or ground motion due to an earthquake. It is critically important to identify what the specific issues are and conduct optimal retrofitting work to repair major deficiencies in concrete columns and concrete floor systems.
The seismic retrofitting can be done through different techniques such as deformation, expanding the load, and/or energy dissipation capacity of the structure. It can be poly reinforce fiber that hardens and strengthens existing columns, concrete shear walls that goes all the way to the roof from the ground floor, and steel brace frames that position heavy steel frames on the inside or outside of a building. The goal is to make the building become more rigid and improve the columns and floor connections so the building does not exhibit deformations and have structural weaknesses that makes it vulnerable to collapses during an earthquake.
Subject to structural analysis, typically non-ductile or steel frame buildings that fall under an ordinance, are retrofitted using one or a combination of reinforced concrete shear walls or special moment frame (steel frame) systems that tie the floors and wall systems together. These systems are supported by large isolated and/ or continuous reinforced concrete foundation systems.
What is the 2016 Safer Cities Survey?
The Structural Engineers Association of Southern California and Dr. Lucy Jones Center for Science and Society published a 2016 Safer Cities Survey that provides information about vulnerable building types and the development of seismic ordinances in six counties in California. It brings to light the hazards of concrete buildings constructed under outdated building codes following major earthquakes, including Mexico City in 1985, Northridge in 1994, Christchurch in 2011 and Kumamoto in 2016.
Does a Non-Ductile Concrete Building Have to be Retrofitted?
Yes, these buildings are very vulnerable to moderate and major earthquakes because of the lack of adequate confinement of the concrete core within the beams, columns and joints. During an earthquake, these cores can break up and result in the collapse of the building. Retrofitting corrects these inadequacies and strengthens the structure against future seismic catastrophes.
What is the Non-Ductile Concrete Retrofit Program?
The Los Angeles Department of Building and Safety has created the Non-Ductile Concrete Retrofit program to help reduce injury or loss of life from the effects of an earthquake on non-ductile concrete buildings. Throughout the world, recent earthquakes have shown the vulnerability of these types of buildings and, consequently, the devastating loss of lives. In California, non-ductile concrete buildings that were constructed prior to the Los Angeles City Building Code provisions are at risk of collapse and pose significant life safety hazards. These buildings can be brittle and have a limited capacity to absorb the energy of powerful ground shaking movements beyond their limited elastic range. This program provides a guide for property owners to meet the minimum standards in improving the performance of these buildings.
What Has to Be Done?
Non-ductile reinforced concrete buildings have to be inspected by a structural engineer or a qualified design expert to make an evaluation. There are many seismic ordinances that have already passed into law and many more on the way. How extensive the work depends on what the structure is made of and its complexities. Retrofitting corrects the lateral-force restraining system. But sometimes, retrofitting will not be enough and the structure needs to be demolished.
What Are the 3 Phases of Retrofitting?
- The preliminary building checklist identifies the building as a potential non-ductile concrete building and formally confirms whether it is in need of retrofitting
- An extensive and comprehensive engineering process includes material and soil testing, structural analysis, schematic design, and calculation of the actual design for the retrofitting system itself, i.e. what is going to be done. There is a need to explore the seismic load capacity, mapping the reinforcement of the building and what the structural elements of the building are made of (floor system, column system, gravity load and more). After the analysis is complete, it is sent to the local municipality retrofit program for review.
- Once the local city approves the analysis, the execution of the actual construction can begin. How long the construction will take depends on a number of factors, including whether the building is vacant or not.
What is the Timeline for Compliance?
Each city has its own schedule for assessment and the work involved. Please see your local municipality for more details. Here are the details for three cities who are leading the way for retrofitting.
Los Angeles
The property owner must comply with the ordinance within the following time limits, from the receipt of the Order to Comply:
- 3 years: Submit completed checklist for review to determine if building is a non-ductile concrete building
- 10 years: Submit proof of previous retrofit, or plans to retrofit or plans to demolish building
- 25 years: Complete construction
West Hollywood
The property owner must comply with the ordinance within the following time limits, from the receipt of the Order to Comply:
Phase 1: Engineering report and major deficiency mitigation
- 3 years: Submit engineering report and determine all structural deficiencies
- 5 years: Submit retrofit plans for major deficiency mitigation
- 7 years: Obtain building permit and commence construction
- 10 years: Complete major deficiency mitigation construction
Phase 2: Complete retrofit
- 13 years: Submit retrofit plans
- 15 years: Obtain building permit and commence construction
- 20 years: Complete construction
Santa Monica
The property owner must comply with the ordinance within the following time limits, from the receipt of the Order to Comply:
- 3 years: Structural evaluation report
- 12 years: Application for building permit and submission of plans
- 20 years: Final approval
What Types of Buildings are Candidates for Retrofit?
Historical Buildings
The California Historical Building Code provides the standards for the restoration, preservation, rehabilitation or reconstruction of all qualified historical buildings. The Los Angeles Non-Ductile Concrete Building Ordinance includes the following: “Qualified historical buildings shall comply with requirements of the California Historical Building Code established under Part 8, Title 24 of the California Code of Regulations.”
While the engineering and design principles for these buildings remain the same, the protection and preservation afforded to historic buildings can add design complexities that must be accounted for.
Here are some factors to be accounted for:
- Disruption of historical elements through material testing.
- Typical retrofit techniques that could alter or damage existing architecture/architectural features.
- Retrofit methods that alter the building layout.
Buildings impacted by the CHBC (California Historical Building Code) are not only ones currently designated as historical, but ones that may be deemed eligible for national, state or local historical inventories or registers. Thus, a qualified historical building may not be listed on any register or inventory but, if the building is eligible for this listing, it must follow the codes provided by the CHBC when retrofitting their non-ductile concrete building.
Los Angeles City lists current historical structures here
High Risk Buildings
Per the SEAOSC Design Guide, the following types of structures are deemed as high risk:
- Pre-1930 4+ story warehouse and manufacturing buildings, generally consisting of frame structures with shear-critical columns, frame with infill, or perforated walls with large widows, may have large ground floor openings for a soft first story.
- Pre-1930 4+ story commercial office buildings, generally consisting of perimeter perforated concrete shear walls.
- Pre-1960 4+ story apartment, hotel, nursing home buildings, generally consisting of perimeter perforated walls with interior columns of frame with URM infill.
- Pre-1977 8+ story high rise, generally consisting of moment frames, often with shear-critical columns of soft first story, usually with interior or core walls.
- Pre-1977 concrete frame buildings with URM infill.
- Pre-1977 lift slab buildings with non-ductile concrete lateral systems, limited slab-column connection deformation capacity.
- Pre-1977 precast construction with non-ductile interconnection details.
