California’s Wildfire Perimeter Expands: Fire-Resistant Structural Systems That Work
As California’s wildfire perimeter continues to grow each year, building in the wildland-urban interface (WUI) presents a unique and urgent set of challenges for structural engineers. According to CAL FIRE, over 4.5 million California homes are now in wildfire-prone zones. With state codes tightening and fire behavior becoming more extreme, designing fire-resistant structural systems is no longer optional—it’s imperative. At GDI Engineering, we have designed, retrofitted, and evaluated structures in WUI zones across California. This blog outlines proven fire-resistant strategies for structural design, integrating compliance, materials, detailing, and real-world case data.
Understanding the WUI Threat Landscape
What is the WUI?
The WUI is the transitional zone between human development and unoccupied wildland. Structures in this zone face combined threats from embers, radiant heat, and direct flame contact.
Fire Behavior Trends
- Higher Wind Speeds: Driving embers several miles ahead of flame front.
- Longer Burn Seasons: Fires now occur year-round due to drought.
- Crown and Ground Fire Combinations: Increase radiant and convective heat exposure.
CAL FIRE mapping and NFPA 1144 identify areas of extreme fire hazard severity. Buildings in these areas must meet Chapter 7A of the California Building Code (CBC) and are typically subject to local fire hardening ordinances.
Key Structural Risks in Wildfire Events
1. Ember Intrusion and Ventilation Openings
- Open Eaves and Attic Vents: Act as ignition entry points.
- Soffit Collapse: Leads to accelerated flame spread into attic cavities.
- Gable-End Vents: Common weak spots without proper baffle or screen protection.
2. Structural Collapse from Heat Exposure
- Steel Loses 50% Strength at 1100°F.
- Wood Ignition: Begins at 572°F (300°C) and weakens framing.
- Post-Fire Rain Events: Lead to foundation undermining and slope failures.
3. Material Incompatibility
- Dissimilar materials expand at different rates, causing joint failure.
- Vapor barriers can trap moisture, which boils and delaminates under heat.
- Adhesives and foam insulations can ignite or melt, compromising wall assemblies.
Fire-Resistant Structural Systems: What Works
Heavy Timber Framing (Type IV Construction)
Why It Works:
- Charring Rate Predictability: Approximately 1.5 inches/hour for softwood.
- Structural Redundancy: Maintains load capacity even as surface chars.
- No Chemical Treatment Required: Unlike engineered wood products.
GDI Example:
- 3-story school building in Shasta County using glulam columns and heavy timber floors with encapsulated insulation—survived a perimeter brush fire in 2022 with only minor siding damage.
Insulated Concrete Forms (ICFs)
Performance Advantages:
- 2–4 Hour Fire Rating (ASTM E119)
- Monolithic Wall Assembly: No joints for fire to penetrate.
- Thermal Mass: Reduces internal temperature rise.
Limitations:
- Requires UV protection of foam.
- Additional detailing needed for tie-ins to conventional framing.
- Higher initial cost compared to traditional framing.
Steel Framing with Fireproofing
Best Practices:
- Spray-Applied Fire-Resistive Materials (SFRM) for exposed steel.
- Intumescent Coatings: For visible steel in architectural spaces.
- Fire-Rated Shaft Walls and Corridors: Per UL design listings.
GDI Implementation:
- Multifamily podium project in Lake Tahoe with steel podium and SFRM—enabled 2-hour rating continuity between commercial and residential occupancy layers.
Structural Insulated Panels (SIPs)
Features:
- OSB skins with EPS or polyiso cores.
- ASTM E84 flame spread rating <25.
- Code-approved for WUI walls with ignition-resistant cladding.
Design Cautions:
- Must prevent foam exposure to UV or radiant heat.
- Sealing around penetrations critical.
- Potential moisture buildup in panel cores post-event.
Windows and Glazing
- Tempered Dual Glazing: Withstand radiant heat better.
- Metal Frames: Better than vinyl in flame resistance.
- Exterior Shutters: Optional, but increasingly adopted in ember-prone zones.
- Edge-Sealing Gaskets: Critical to prevent flame ingress between panes.
Decks and Overhangs
- Must meet CBC 709A flame spread index ≤25.
- Heavy Timber or Non-Combustible Framing: Mandatory.
- Skirt Closures: Prevent debris and ember buildup below.
- Post Protection: Concrete piers or steel supports over wood.
Structural Fire Design Calculations
GDI provides:
- Time-Temperature Curves (ASTM E119): To validate system performance.
- Fire Load Density Models: Based on material mass and occupancy type.
- Intumescent Coating Thickness Calcs: For steel per UL 263.
- Cross-Sectional Reduction Calculations: For heavy timber char depth.
We model composite action and potential joint degradation using thermal-structural FEA tools like ANSYS and SAFIR.
Conclusion
As California’s wildfire perimeter expands, engineers must respond with resilient, fire-resistant structural designs. From heavy timber and ICF to SIPs and fireproofed steel, proven systems exist that stand up to flame, heat, and embers. At GDI Engineering, we go beyond code minimums to deliver structural systems that protect lives and preserve property under extreme conditions.
Whether retrofitting or designing new in WUI zones, GDI’s fire-adapted engineering approach combines performance, compliance, and innovation. With every degree of fire intensity and ember exposure, we help buildings not just survive—but remain operational.
