Multi-Family Property Shoring in the Bay Area
- Joe Lazzaretti
- Jan 2
- 13 min read
Updated: Jan 2
Building multi-family properties in the Bay Area is challenging due to seismic risks, unstable soils, and urban density. Specialized shoring techniques are essential to address these issues. Here's what you need to know:
Seismic Risks: The Bay Area has a 72% chance of a magnitude 6.7+ earthquake by 2043, with soil conditions prone to liquefaction and settlement.
Soil Challenges: Sites often encounter weak materials like old bay clay, loose fill, and sand, requiring advanced foundation solutions.
Urban Constraints: Dense neighborhoods demand careful planning to avoid impacting nearby structures during construction.
Key Case Studies:
Millennium Tower Stabilization: $100M project corrected 18 inches of settlement and a 28-inch tilt by installing piles anchored into bedrock.
Soft-Story Retrofitting: San Francisco retrofitted 6,700 vulnerable buildings to improve earthquake safety, costing $20K–$25K per unit.
Tasman East Redevelopment: Used Deep Soil Mixing (DSM) and micropiles to stabilize a large-scale project with 4,500 units in Santa Clara.
Techniques Overview:
Shotcrete: Space-efficient, high seismic resistance.
Moment Frames: Reinforce soft-story buildings without blocking open spaces.
DSM & Micropiles: Mitigate liquefaction and stabilize deep foundations.
Bay Area shoring requires a mix of advanced engineering, compliance with strict regulations, and careful project sequencing to ensure safety and efficiency.
What Is Shoring In Construction? - Civil Engineering Explained
Case Study 1: Millennium Tower Stabilization Project
The Millennium Tower serves as a striking example of what can go wrong when foundation designs overlook the unique soil conditions of the Bay Area. This 58-story luxury residential building was constructed on approximately 950 precast concrete piles embedded in dense sand and clay, rather than being anchored into the bedrock below. The result? The tower’s immense weight squeezed water out of the underlying "Old Bay Clays", leading to 18 inches of settlement and a 28-inch westward tilt before any remediation efforts began [6][7]. Initial geotechnical predictions had estimated a uniform settlement of only 4 to 6 inches, a figure that was already exceeded during the construction phase [6].
To address this alarming situation, a $100 million stabilization project, spearheaded by Ronald Hamburger of Simpson Gumpertz & Heger, completely overhauled the building's foundation system [6][9]. The solution involved installing 250-foot-long, concrete-filled steel-pipe piles, each anchored 30 feet into bedrock, which lies 220–250 feet below the surface [6][7]. The revised plan reduced the number of piles from 52 to 18, designed to shift about 17% to 20% of the building’s weight - roughly 18 million pounds - from the compressible clay directly to the bedrock [6][7][10].
Challenges of Urban Space and Seismic Considerations
Working on a site that nearly fills its entire lot posed a host of logistical challenges. To make room for the drilling and stabilization work, crews secured easements to operate beneath public sidewalks along Mission and Fremont streets. Here, they dug trenches ranging from 15 to 25 feet deep to install a reinforced concrete mat that would connect the new piles to the existing foundation [7]. Remarkably, the building remained fully occupied throughout the project, with all work confined to these sidewalk trenches and a maintenance vault [7][9].
Despite the tower's dramatic tilt and settlement, a 2017 independent panel convened by the City of San Francisco determined that the building still met seismic safety standards under the City Building Code [9]. Stanford University’s Professor Gregory Deierlein noted, "The settlement has not appreciably affected the seismic safety of the building" [6]. The stabilization project was designed not only to halt further settlement caused by groundwater changes and clay consolidation but also to reinforce the building’s seismic resilience. This urban-constrained project showcased the precision and ingenuity required for retrofitting in dense city environments.
Engineering Approach and Key Milestones
Before permits for the retrofit could be finalized, the project underwent a rigorous, 7-month peer-review process conducted by an independent team of engineering experts [9]. Ronald Hamburger explained the innovative method used: "The system relies on loading each pile with 800,000 pounds using a permanent hydraulic jack that reacts against a new mat extension" [6]. This "bumper jack" technique allowed for incremental redistribution of the building’s weight without requiring major structural changes.
In January 2023, the team achieved a critical milestone by transferring 1,500 tons - or 1.5% of the building’s total weight - to six newly installed piles using hydraulic jacks in 100-kip increments [7]. This initial load transfer raised the building by 0.1 inches and reduced its westward tilt by 0.5 inches [7]. The project, led by prime contractor Shimmick Construction Co. Inc., with geotechnical monitoring by Dan Brown Associates, continued its success, and by August 2023, the sinking had been halted, and nearly one inch of tilt had been recovered [7][8][9][10].
Excavation played a key role in the stabilization process, with crews removing 3,500 tons of soil - 1,000 tons from Mission Street and 2,500 tons from Fremont Street. This reduced the load on the compressible clay, aiding in the recovery of the tilt [7]. The project highlights how a deep understanding of soil-structure interaction can lead to effective solutions in even the most challenging urban environments.
Case Study 2: Soft-Story Multi-Unit Apartment Retrofitting
Building on the techniques used in the Millennium Tower project, soft-story retrofits tackle another major seismic challenge by reinforcing vulnerable structures. San Francisco has about 6,700 soft-story buildings that are particularly at risk during earthquakes [13]. These buildings often feature large ground-floor openings, like those used for parking garages or retail spaces, and lack the internal walls needed for lateral strength. The San Francisco Department of Building Inspection describes the issue:
"The open condition makes the ground level significantly weaker and more flexible than the floors above it. During strong earthquake shaking, these 'soft' ground level walls cannot support the side-to-side or front-to-back movement of the stiff and heavy mass of the stories above them" [11].
The 1989 Loma Prieta earthquake brought this vulnerability into sharp focus. Nearly 50% of the homes rendered uninhabitable during the quake were soft-story buildings [12]. Without retrofitting, an estimated 43%–85% of these structures could be red-tagged, and up to 25% might collapse [11]. Today, around 58,000 people in San Francisco live in buildings identified as having soft-story conditions [11].
To address this, San Francisco adopted Building Code Chapter 34B (Ordinance 66-13). This law requires seismic retrofits for wood-frame buildings with five or more units and two or more stories over a soft or weak story, constructed before January 1, 1978 [11][13]. Buildings are divided into four compliance tiers based on their use and risk, with deadlines ranging from 2017 to 2018. Costs for retrofits typically fall between $60,000 and $130,000 per building, or about $20,000 to $25,000 per unit for straightforward projects. Under San Francisco Rent Board rules, landlords can pass 100% of these costs to tenants, though tenants can appeal based on financial hardship [11][13]. This case study explores retrofit solutions designed to address the structural weaknesses of these buildings.
Why Soft-Story Buildings Are Vulnerable
Soft-story buildings are particularly at risk because of the mismatch between their flexible ground floors and rigid upper stories. John Dal Pino, S.E., writing for Structure Magazine, explains:
"Linear parking configurations make transverse shear walls impossible, and the result was thousands of weak and soft-story buildings" [13].
During an earthquake, the upper floors' heavy mass shifts laterally, while the weak ground floor struggles to resist the movement. This can lead to lateral displacement, collapse, or the building being deemed uninhabitable. Retrofitting can reduce the collapse rate to less than 1%, turning these high-risk structures into safer housing options [11]. Given that 81% of housing units in San Francisco County were built before 1980, the need for retrofitting extends well beyond the buildings covered by the ordinance [14].
Reinforcement with Moment Frames
To address these vulnerabilities, engineers often rely on moment frame reinforcements. Steel moment frames are a popular choice because they add crucial lateral strength while preserving the open spaces needed for parking or retail use [13][14]. Unlike traditional shear walls, which would block garage entrances or storefront windows, moment frames use rigid beam-to-column connections to resist lateral forces through bending rather than shear.
An example of this approach is the 2019 retrofit of the Fox Plaza Tower, a 30-story mixed-use building. Engineers tackled issues with "Pre-Northridge" welded moment connections, which testing showed were prone to lateral-torsional buckling. By adding reinforcements like ribs and viscous dampers, they reduced story drifts to acceptable levels for a 475-year seismic hazard. Impressively, 95% of the building's residents were able to stay in their units during the retrofit [2].
For less complex retrofits, engineers often use guidelines like CEBC Appendix A4, FEMA P-807, or ASCE 41-13 [13]. Most projects take two to four months and are confined to the ground floor, allowing tenants to remain in their homes [11][14]. However, when moment frames create large overturning forces, engineers must also assess the building's foundation for additional strengthening [13].
Case Study 3: Tasman East Multi-Family Redevelopment Shoring
Tasman East showcases how integrated shoring can support large-scale, mixed-use developments. Located in Santa Clara, this project transformed four properties into a vibrant community with 4,500 apartment units and retail spaces. The development required advanced shoring techniques to handle deep excavations in unstable soils, all while considering the 72% likelihood of a magnitude 6.7 or greater earthquake by 2043 [15].
Phase I Reports and Soil-Structure Analysis
The shoring design for Tasman East was guided by a detailed analysis of the site’s subsurface conditions. Engineers faced a challenging mix of steeply dipping bedrock and large areas of liquefiable fill [4]. To stabilize the site for such a significant development, advanced ground improvement methods were essential.
Deep Soil Mixing (DSM) played a key role in reducing liquefaction risks and providing a stable foundation [4]. Additionally, uplift-resisting micropiles were combined with DSM panels to anchor shear walls and counteract seismic overturning forces [4]. During excavation, soldier piles and tiebacks supported depths ranging from 35 to 68 feet, enabling the construction of below-grade parking and retail spaces in this dense urban setting [4]. The complex soil conditions required a design that ensured both safety for residents and functionality for commercial spaces.
Integrating Retail and Residential Spaces in Seismic Zones
Beyond foundation strategies, the project tackled the unique challenges of blending retail and residential spaces in an earthquake-prone area. Incorporating ground-floor retail under residential towers created soft-story vulnerabilities, further complicated by the need for open floor plans in commercial spaces. These open layouts often conflict with traditional seismic reinforcements like interior shear walls [17].
The issue was particularly pronounced given the site’s unstable fill and soils prone to liquefaction and lateral spreading during seismic events [4][16]. Engineers carefully balanced the need for lateral strength with the open designs required for retail spaces. Steel moment frames provided the necessary stiffness to maintain structural integrity while preserving open layouts. To further enhance stability, micropiles were used to anchor the system and resist seismic overturning forces [4][17]. This approach ensured that the structure could meet both safety standards and the functional demands of a mixed-use development.
Advanced Shoring Techniques for Multi-Family Properties
Bay Area multi-family developments face unique challenges due to deep excavations, unstable soil conditions, and seismic risks. The region’s geology, which includes compressible deposits reaching depths of up to 60 feet, calls for specialized shoring methods. One effective solution is shotcrete technology, which creates monolithic structural elements. These elements not only enhance seismic resistance but also save space during construction. Another advanced method, Deep Soil Mixing (DSM) combined with micropiles, addresses liquefaction risks. As Wayne Magnusen explains:
"The DSM panels act both as a liquefaction mitigation mechanism and a foundation support system. Resistance to seismic overturning is provided by uplift-resisting micropiles" [4].
For soft-story buildings, moment frames and columns are particularly effective, as they redirect lateral forces to reinforced foundations. Additionally, underpinning techniques, such as slant-drill methods, are essential when working near existing structures or critical infrastructure like BART stations. These methods help prevent settlement and protect neighboring buildings during excavation. Below is a comparison of common shoring techniques and their trade-offs.
Shoring Methods Comparison Table
Technique | Cost | Installation Efficiency | Seismic Resistance | Construction Impact |
Shotcrete | Moderate | High | Excellent (Monolithic) | Low (Small footprint) |
Moment Columns | High | Moderate | High (Lateral support) | High (Deep trenching) |
Shear Walls | Low to Moderate | High | Good (Rigid) | Moderate |
DSM & Micropiles | High | High | Superior (Liquefaction) | High (Heavy machinery) |
Soldier Piles | Moderate | High | Good (Lateral retention) | Moderate |
Adapting Techniques for Bay Area Conditions
In a region with a 72% probability of experiencing a magnitude 6.7 or greater earthquake by 2043 [15], shoring systems must be designed to handle intense lateral loads. For example, at 270 Brannan in San Francisco, engineers in 2017 used a combination of DSM panels and uplift-resisting micropiles to stabilize 60 feet of liquefiable soil. This approach enabled the use of conventional spread footings while anchoring shear walls to counter seismic overturning forces [4].
For occupied buildings, fluid viscous dampers (FVDs) offer a practical solution by reducing peak drift ratios without adding stiffness to the structure. This prevents the redirection of seismic energy to upper floors or overloading existing foundations [1][2]. Meanwhile, near transit infrastructure like BART, slant-drilled underpinning has proven effective. At Oakland's 1900 Broadway project, this method supported adjacent buildings during excavations deeper than 30 feet [18].
These advanced techniques, tailored to the Bay Area’s unique geological and seismic conditions, ensure that temporary supports can withstand both construction demands and potential earthquakes.
Best Practices and Compliance Requirements
Planning Construction Phases and Sequences
Phased construction helps reduce disruptions, especially in occupied buildings. A great example of this approach is the retrofit of Fox Plaza Tower by Plant Construction Company, which took place between May 2018 and March 2019. The work was confined to one apartment unit at a time, lasting only two to three days per unit, and impacted fewer than 5% of the residents at any given time[2].
Before any ground is broken, certain steps must be followed. For instance, design calculations and drawings for shoring systems over 14 feet in height or with a horizontal span exceeding 16 feet must be signed off by a California-registered civil engineer. Additionally, pre-placement inspections are required to ensure that the work aligns with approved drawings and that the base is stable and compacted to handle a minimum design load of 100 psf[19]. The process typically involves several phases: site assessment, design, municipal approval, bidding, construction, and final inspection[21]. These structured steps ensure that the project meets the stringent Bay Area building codes.
Meeting Bay Area Building Code Requirements
Careful planning must go hand in hand with strict compliance with Bay Area building codes. Engineers now use ASCE 41-13 standards, which focus on the seismic rehabilitation of existing buildings, moving away from older methods used between the 1960s and 1990s[1][2]. When employing advanced technologies like fluid viscous dampers, safety and compliance are ensured through independent peer reviews by firms such as Simpson Gumpertz & Heger[2]. Shoring systems, for example, need to withstand lateral loads of at least 100 pounds per linear foot of floor edge or 2% of the total dead load, whichever is greater[19].
For high-rise structures, performance-based assessments are crucial. These assessments often include Collapse Prevention objectives for seismic events expected to occur once every 475 years[2]. To address risks in vulnerable structures, California has allocated $250 million in funding to assist with retrofitting soft-story multifamily buildings in high-risk areas. Cities like San Jose have already enacted mandatory retrofit ordinances, which will take effect on April 1, 2025[21]. Meanwhile, in jurisdictions like Alameda, building owners must submit structural seismic adequacy analyses within 18 months of receiving a Notice and Order[20].
Conclusion: Safe and Efficient Shoring for Multi-Family Properties
Shoring for multi-family properties in the Bay Area demands precision, expertise, and adherence to high standards. The examples discussed in this article highlight three essential pillars for success: cutting-edge engineering, strict adherence to regulations, and thoughtful project planning.
Performance-Based Seismic Design (PBSD) has set a new standard for construction in the Bay Area. Unlike traditional codes, PBSD evaluates how buildings behave during earthquakes. As Ron Klemencic, Chairman and CEO of Magnusson Klemencic Associates, points out:
"Stronger buildings resist more force rather than absorbing the energy of the ground's shaking. In resisting these higher forces, shear stresses and foundation demands increase to undesirable levels and building performance can be compromised."[3]
This insight has led to innovations like hybrid retrofitting, which effectively dissipates seismic energy to protect structures and occupants[2].
Compliance with local regulations and independent peer reviews ensures that shoring systems meet the Bay Area's rigorous safety standards. For example, California law requires civil engineers to approve shoring systems that exceed 14 feet in height or span more than 16 feet horizontally[19].
On the ground, success hinges on more than just engineering. Careful project sequencing and execution are critical, especially when maintaining building occupancy during seismic upgrades. These strategies balance safety with the practical needs of property owners and residents.
Advanced analytical tools also play a key role, offering insights that go beyond standard code-based designs. These tools often uncover vulnerabilities, such as shear demands that are 2–4 times higher than traditional methods predict[5]. By combining advanced design techniques, regulatory compliance, and strategic execution, multi-family shoring projects in the Bay Area consistently achieve and surpass safety and performance goals.
FAQs
What are the key shoring methods used for multi-family properties in the Bay Area?
In the Bay Area, shoring for multi-family properties often involves a combination of earth-retaining walls and ground-improvement techniques to tackle seismic challenges and manage limited space. Common retaining structures include diaphragm (slurry) walls, soldier-pile and lagging walls, and secant or tangent pile walls. These systems are reinforced with materials like steel I-beams, timber lagging, or hydraulic struts to ensure excavation stability.
In areas with loose or liquefiable soils, particularly near the San Francisco Bay, engineers often turn to deep-soil-mixing (DSM) and micropiles. DSM strengthens the soil and reduces liquefaction risks, while micropiles add extra support to counter seismic forces. These techniques are particularly effective for multi-family buildings, balancing safety needs with minimal disruption to neighboring structures.
Shoring designs in this region also need to comply with strict seismic standards, ensuring they can endure anticipated ground movements. By combining advanced retaining walls, bracing systems, and soil-improvement methods, contractors can navigate the Bay Area’s complex construction environment with both safety and efficiency.
How do soft-story retrofits make buildings safer during earthquakes?
Soft-story retrofits are designed to strengthen the often-vulnerable ground floors of multi-family buildings, a common feature in the Bay Area, so they can better endure the forces of an earthquake. This is done by incorporating shear walls, steel moment frames, diagonal bracing, and securing the structure firmly to its foundation. These improvements create a sturdier and more adaptable system that can absorb seismic energy, reducing the risk of the ground floor collapsing and ensuring the upper levels remain stable.
Buildings that undergo these retrofits are far less likely to experience devastating damage, which translates to lower repair costs and enhanced safety for residents. Research indicates that retrofitting can save property owners from repair bills ranging anywhere from $10,000 to $200,000. Beyond the immediate safety benefits, adhering to initiatives like San Francisco's Mandatory Soft-Story Program also helps maintain the property's long-term value in earthquake-prone regions.
What challenges come with combining retail and residential spaces in earthquake-prone areas, and how are they addressed?
Combining retail and residential spaces in earthquake-prone regions, like the Bay Area, comes with its own set of hurdles. Retail podiums typically require open layouts and the ability to support heavy equipment, whereas residential towers above demand stiff lateral systems and effective vibration control for occupant comfort. These contrasting structural needs can cause uneven movement between the podium and tower during an earthquake, heightening the risk of damage. Additionally, older buildings often fall short of modern seismic standards, requiring updates to improve their resilience.
To tackle these challenges, engineers turn to advanced methods such as base isolation and supplemental damping. These techniques help minimize the seismic forces transferred between the retail and residential portions of the structure. For older buildings, retrofitting with moment-resisting frames, reinforced concrete cores, or post-tensioned slabs boosts safety while preserving functionality. Even foundational upgrades, like bolting structures to footings or reinforcing cripple walls, can make a big difference in stability. These measures work together to enhance the safety and performance of mixed-use buildings in areas prone to earthquakes.
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