3130 Wilshire
A targeted retrofit, using nonlinear analysis, saves over 80% of the cost of a conventional approach.
Advanced Analytics
Nonlinear analysis revealed the strength of the existing fin columns, leading to efficient, targeted ground floor retrofit that met the mandatory Non-ductile Concrete Ordinance.
Integrated Value
Nonlinear analysis reduced retrofit costs from $13.8M to $2.5M by utilizing inherent strength of existing fin column facade.
Seismic Resilience
Substantial cost savings, coupled with comprehensive owner education on the building's performance, allowed for the implementation of enhanced retrofit measures.
Constructed in 1968, the six-story office building at 3130 Wilshire in Santa Monica was subject to the city's Mandatory Non-ductile Concrete Ordinance. The Swig Company purchased the building in 2022. During a pre-purchase due diligence period, Swig asked Tipping to review a retrofit concept based on what we presume was a conventional linear analysis that had been commissioned by the seller, and was considered necessary to meet the Non-ductile Concrete Ordinance. Tipping was tasked to develop a less invasive solution that would avoid displacing tenants and avoid impacts to leasable space, circulation, and views to the extent possible.
Project Challenge: Non-ductile Concrete
The building’s gravity system consisted of a slab and joist system spanning to four girder lines. On the upper floors, an array of exterior fin columns supported the exterior girders, while interior girders were supported by square columns. At the first floor, these fin columns were transferred by a girder at level two, with discreet columns supporting the transfer girder. The basement perimeter was enclosed with concrete walls.
Typical of its age, the building lacked a discrete lateral system. The assessment provided to Swig and Tipping by the seller identified insufficient lateral strength at all levels, brittle transfer girder responses, and brittle column behavior.
The retrofit scheme, which we presume was based on a linear analysis, suggested FRP wrapping of all columns and the level two girders. It also proposed four full height interior steel frames to help supplement the story stiffness. This solution, however, was expensive and highly disruptive, impacting the building layout, window placement, and necessitating the displacement of a significant number of tenants within the largely occupied building.
Tipping’s proposed retrofit needed to adopt a more strategic and targeted approach, while ensuring the building’s overall seismic performance was not compromised and the requirements of the Non-ductile Concrete Ordinance were satisfied.
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Location
Santa Monica, CA
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Square Footage
98,000
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Cost
Confidential
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Completion Date
2024
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Owner
The Swig Company
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Architect
Gensler, Los Angeles
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Contractor
BNBuilders
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Photography
©The Swig Company
Exterior Fin Columns Offer Inherent Strength
From the onset, Tipping recognized that the closely spaced fin columns possessed well detailed shear ties, suggesting significant and reliable strength. To validate this, a column tree nonlinear analysis was conducted during the three week due diligence assessment undertaken to align with the purchase timeline. This analysis effectively summed the inherent strength of all fin columns and subjected them to nonlinear ground motion simulations. The results confirmed these elements were capable of resisting a significant portion of the lateral load. With this confidence, a detailed nonlinear model of the entire building was developed and used to justify a limited intervention primarily located at the exterior walkways of the ground floor for ease of access and cost efficiency.
Detailed Nonlinear Model
The model incorporated fiber sections for the fin columns, shear and moment hinges for the exterior and level two transfer girders, and detailed modeling of the interior columns.
The analysis revealed a predictable weak story at the level one slab and potential shear failures at the level two girders. To address these vulnerabilities, the level two drift was reduced by installing six Buckling Restrained Brace (BRB) frames at the perimeter of the level one space.
These BRBs were carefully tuned to prevent a weak story mechanism while avoiding excessive acceleration increases at the upper levels. This approach achieved collapse prevention under BSE-2E and life safety under BSE-1E. To protect the transfer girders, vertical epoxy dowels were installed at four locations. These interventions met the Santa Monica Basis of Design (BOD) requirements.
Careful Placement of Six new BRBs
The ground floor facade’s three-foot setback from the upper levels allowed the new BRBs and their connection to be placed outside of the occupied level one office spaces, minimizing disturbance to tenants. Concrete shear walls were installed below the BRBs in the garage, ensuring continuity down to the foundation.
The Importance of Tailored Retrofits for Non-ductile Concrete
Non-ductile concrete buildings have garnered significant attention due to their collapse vulnerability and prevalence across California communities. Multiple jurisdictions have mandatory non-ductile concrete ordinances, with others encouraging voluntary retrofitting. It’s crucial to recognize that seismic retrofit solutions for non-ductile buildings are not a one size fits all approach with many retrofits being highly invasive and costly.
Effective retrofitting requires leveraging existing strength and mechanisms, ensuring solutions are tailored to project specific guidelines. Utilizing advanced tools like nonlinear analysis is essential for gaining a deeper understanding of building behavior. The 3130 Wilshire project exemplifies this approach, demonstrating the value of thoughtful, targeted strategies. The initial retrofit estimate of $13.8 million was significantly reduced to a final cost of $2.5 million.
