Structural Engineering Strategies for Hybrid Mass Timber: San Mateo County’s COB3 Project Achieves 85 Percent Embodied Carbon Reduction

Structural engineers working on low-carbon civic buildings face a critical challenge: achieving dramatic embodied carbon reductions without compromising structural performance, safety, or serviceability. San Mateo County’s County Office Building 3 (COB3), designed by Skidmore, Owings and Merrill (SOM) and currently under construction in Silicon Valley, provides a compelling case study in hybrid mass timber structural engineering. With an embodied carbon intensity of just 110 kgCO2e per square meter representing an 85 percent reduction over conventional steel and concrete construction, COB3 demonstrates how strategic material hybridization can push beyond the typical 65 to 75 percent reduction range of standard mass timber structures. The building rises 21.9 meters (72 feet) across 19,231 square meters (207,000 square feet) of gross floor area. This article examines the structural engineering decisions that made this performance possible, from gravity and lateral system design to connection detailing, vibration serviceability, and fire resistance engineering.

Structural System Design for Hybrid Mass Timber

Gravity Load Path Optimization

The gravity load-resisting system at COB3 uses a hybrid approach that places each structural material where it delivers the highest structural efficiency per unit of embodied carbon. The team reduced the total volume of timber through careful optimization of member sizes and spans, and diminished the piece count of cross-laminated timber panels by using larger panelization with simplified grid layouts.

Key design decisions in the gravity system include:

• Column grid rationalization: Column spacing was optimized to allow standard CLT panel widths without excessive waste, reducing both material volume and fabrication complexity
• Glulam beam sizing: Beam depths were selected based on deflection-controlled spans rather than strength, a common approach in mass timber where serviceability governs member proportions
• Steel reinforcement at high-load areas: Steel beams and columns were introduced at transfer zones and large-span areas, keeping timber sections within practical dimensions
• Concrete core integration: The concrete cores that house stairs and elevators serve dual roles as vertical load-bearing elements and lateral force-resisting components

Lateral Force-Resisting System

For a building in seismically active California, the lateral system design was a defining engineering challenge. The project employs concrete shear walls at the building cores and steel moment frames at select perimeter locations, with the mass timber floor diaphragms transferring lateral loads to these stiffer elements.

The lateral load distribution strategy involves:

1. CLT floor diaphragms collect seismic and wind forces from each level
2. Forces are transferred through engineered connections at the timber-to-concrete interface
3. Concrete cores resist the majority of lateral load through shear wall action
4. Perimeter steel moment frames provide additional redundancy and torsional resistance
5. The combined system achieves the stiffness required to control inter-story drift under design-level seismic events

This approach allows the mass timber to participate in lateral resistance where it performs best as a diaphragm, while relying on concrete and steel elements for higher stiffness demands at the core and perimeter.

Connection Design and Detailing for Hybrid Structures

Timber-to-Concrete Interface Details

The connections between mass timber floor panels and concrete cores represent one of the most technically demanding aspects of the hybrid system. These interfaces must transfer significant shear forces while accommodating differential movement between materials with different creep and moisture-response characteristics.

Standard connection details developed for COB3 include:

• Steel embed plates cast into concrete cores: These provide welded connection points for timber brackets, allowing precise load transfer at each floor level
• Slotted connections for differential movement: Elongated holes in steel brackets allow vertical movement between timber floors and concrete cores as each material responds to environmental conditions and long-term creep
• Fire-rated penetration seals: All connections maintain the required fire-resistance rating through intumescent wraps and mineral wool packing
• Structural screw clusters: Self-tapping screw groups transfer diaphragm shear forces from CLT panels to steel ledger angles

Steel Reduction Through Optimized Load Paths

One of the defining structural engineering achievements at COB3 is the reduction of steel content below what is typical for hybrid mass timber buildings. By distributing lateral loads through the concrete cores and using timber as the primary gravity system wherever possible, the structural engineers reduced the total steel tonnage by approximately 40 percent compared to a conventional hybrid design. Every kilogram of steel eliminated represents approximately 2.5 kg of CO2e savings, making steel reduction a high-leverage strategy for meeting the project’s aggressive carbon targets.

Steel Moment Frame Connections to Timber Elements

Where steel moment frames interface with timber beams and columns, the connection detailing must address the anisotropic nature of engineered wood products. Steel-to-timber connections rely on dowel-type fasteners such as self-tapping screws and glued-in rods rather than the bolted connections typical of steel-to-steel assemblies.

The design team specified concealed connection systems that maintain the architectural intent of exposed timber interiors. This required close coordination between structural engineers and architects to ensure that connection hardware remained hidden within timber sections while still providing the required strength and stiffness. Building professionals studying mass timber material specifications for CLT and glulam performance will find that connection detailing in hybrid systems requires fastener design values specific to the timber product being used, as dowel bearing strengths vary significantly between CLT face laminations and edge-glued glulam sections.

Floor Vibration and Serviceability Engineering

Vibration Criteria for Long-Span CLT Floors

Long-span CLT floor panels present unique serviceability challenges. The low mass and high stiffness-to-weight ratio of timber mean that floors can be susceptible to vibration from pedestrian traffic, mechanical equipment, and other rhythmic excitations. For a government office building where occupant comfort is paramount, the structural team established stringent vibration criteria during the design phase.

Vibration Control Strategies

To meet these targets, the engineering team employed several strategies:

• Increased panel thickness: CLT panel depths were specified at the upper end of what gravity load design required, providing additional stiffness to raise natural frequencies above 8 Hz
• Composite topping slab: A thin concrete topping on the CLT panels added mass and damping while improving acoustic isolation between floors
• Span-to-depth ratio limits: Beam spans supporting CLT panels were kept within a 20:1 ratio to control both deflection and vibration response
• Strategic beam placement: Secondary glulam beams were introduced at mid-span of the longest CLT panels to break up long vibration wavelengths

The industry shift toward mass timber tall wood building codes has created updated guidance on vibration serviceability for timber floors, and the COB3 project extends these principles to the hybrid system context.

Fire Resistance Engineering and Structural Safety

Exposed Timber Fire Design Approach

One of the architectural advantages of mass timber is the ability to leave structural elements exposed as finished interior surfaces. At COB3, the exposed CLT ceilings and glulam beams serve as a signature design feature. From a structural engineering perspective, exposed timber requires a char layer calculation approach to fire resistance.

The fire design methodology follows these principles:

1. The char layer forms at a predictable rate, typically 0.65 mm per minute for CLT under standard fire exposure, with the charred material providing thermal insulation to the remaining uncharred section
2. The residual uncharred cross section must carry all design loads at ambient temperature capacity during the required fire resistance period
3. Sacrificial timber thickness is added to account for char depth over the required duration, typically 60 to 90 minutes for office buildings of this occupancy classification
4. Connection hardware must be protected or located outside the char zone to prevent premature failure, since steel connections lose strength rapidly above 400 degrees Celsius
5. Fall-off resistance of CLT laminations must be verified to prevent delamination that would accelerate the char rate

Code Compliance with NFPA Tall Mass Timber Provisions

COB3 was designed in accordance with the latest adopted building codes, including the tall mass timber provisions that permit exposed timber structures up to 18 stories. The hybrid nature of the building with concrete cores and limited steel elements simplifies some fire resistance requirements, because these non-combustible components provide inherent fire separation at egress paths and building service zones. Engineers familiar with tall mass timber code provisions will recognize that the prescriptive fire resistance requirements for Type IV-C and Type IV-A construction under the International Building Code create a clear pathway for projects like COB3, where exposed timber is a deliberate design choice rather than a waiver-dependent exception.

Sprinkler systems serve as the primary active fire protection, and all timber elements are designed with additional sacrificial thickness beyond the calculated char depth to ensure the structure maintains load-bearing capacity for the full fire duration without relying on sprinkler activation. This redundant approach provides inherent protection even when active systems are compromised. The lessons from pre-engineered steel structures for civic facilities regarding compartmentation and fire-resistive construction apply equally to hybrid mass timber buildings, where the interaction between combustible and non-combustible elements at floor-to-wall junctions requires careful detailing to prevent fire spread through concealed spaces.

The structural engineering decisions at San Mateo County’s COB3 project demonstrate that hybrid mass timber systems can achieve carbon reduction targets well beyond what typical mass timber buildings deliver. By optimizing gravity load paths, engineering robust lateral force-resisting systems with concrete cores and steel moment frames, designing precise connections between dissimilar materials, and addressing vibration serviceability and fire resistance through analytical methods, the project team created a structural solution that performs at 110 kgCO2e per square meter.

Several transferable lessons emerge for structural engineers undertaking similar projects:

• Material hybridization requires early coordination: The decision to combine mass timber with steel and concrete must be made during conceptual design, as the gravity system layout, lateral load path, and connection strategy are interdependent variables
• Serviceability often governs timber member sizing: Vibration and deflection criteria for mass timber floors are typically more restrictive than strength requirements, making occupant comfort rather than ultimate capacity the governing design condition
• Carbon optimization is a systems-level problem: Reducing steel tonnage by 40 percent through load path optimization had a greater carbon impact than specifying the lowest-carbon timber products, because steel emissions are approximately four times those of engineered wood per kilogram
• Fire resistance and structural design must be integrated: Char depth allowances affect member sizing from the earliest design stages, and exposed timber interiors require coordinated engineering from the outset

For structural engineers designing low-carbon civic buildings, the COB3 approach of strategic material hybridization supported by rigorous structural analysis provides a replicable model for achieving ambitious environmental targets without compromising safety or performance. As embodied carbon limits become mandatory in more building codes across North America, the engineering methods demonstrated at San Mateo County’s hybrid mass timber headquarters will become increasingly relevant to the profession.