Wood Frames And Earthquakes

Wood-Frame Construction Meeting the Challenges of Earthquakes

Introduction

  • North American single-family homes are often seen as safe during earthquakes.

  • Wood-frame construction is lightweight and absorbs energy, enhancing safety during seismic events.

  • Historical performance shows that well-built wood-frame buildings provide occupant safety.

Key Concepts in Earthquake Impacts on Wood Buildings

Fundamentals of Earthquakes

  • Earthquake originates at the junction of tectonic plates, causing energy release in seismic waves.

  • The focus is the point of energy release; the epicentre is the surface point above the focus.

Earthquake Magnitude and Effects

  • Richter Scale measures earthquake magnitude, indicating potential damage:

    • M4: Felt at epicentre, minimal damage.

    • M5-M6: Moderate size, potential considerable damage.

    • M7-M8: Widespread severe damage likely.

    • Most effects are felt from seismic wave ground motions rather than ground failure.

Wood-Frame Construction Characteristics

Advantages of Wood-Frame Design

  1. Redundant Load Paths:

    • Connections in wood-framed houses allow force distribution; if one connection fails, others can compensate.

  2. Light Weight:

    • Lower weight reduces inertia forces during seismic activity, advantageous for earthquake resilience.

  3. Flexibility:

    • Nailed wood connections allow buildings to flex during quakes, absorbing and dissipating energy.

  4. Shear Wall and Diaphragm Construction:

    • Use of structural panels (plywood or OSB) for shear walls and diaphragms aids lateral force resistance.

Building Performance and Lessons Learned

Performance During Past Earthquakes

  • Statistics reveal low fatalities in wood-frame buildings during significant earthquakes (e.g., 1964 Alaskan Earthquake).

  • Damage assessments post-Northridge Earthquake (1994) indicated good performance of wood-frame schools and homes. Most damage was non-structural or easily repairable.

Common Deficiencies in Wood-Frame Structures

  1. Weak First Storey:

    • Buildings with soft first storey (like those with garage openings) can be prone to collapse.

  2. Foundation Connections:

    • Structures without proper anchor bolts risk relocating off their foundations during seismic events.

Lessons from Earthquakes for Future Design

  • Improvements have been made post-assessment, including:

    • Enhanced wall bracing and sheathing.

    • Better connections between framing elements to withstand ground movements.

Research & Development

Advances in Seismic Research

  • Research continues into improving and understanding wood-frame performance during earthquakes.

  • Use of shake tables for testing designs and materials.

  • Engineering guides and building codes evolve based on observational data from past earthquakes.

Global Interest in Wood-Frame Construction

  • Increasing global interest in wood-frame construction for residential buildings due to its proven earthquake resistance and safety.

  • Countries like Japan and Taiwan are adopting North American wood construction principles to improve seismic safety.

Conclusion

  • Wood-frame construction is recognized for its lightweight nature and energy absorption characteristics, making it effective in earthquake-prone regions.

  • Ongoing advancements in building codes and engineering practices aim to enhance seismic resilience further.