木造建物の地震対応

Wood's Surprising Earthquake Resilience

Wood-frame construction has a remarkable track record in earthquakes that often surprises those unfamiliar with structural engineering. Properly designed and built wood-frame buildings have survived major earthquakes with minimal structural damage, even as adjacent concrete and masonry structures failed catastrophically. The 1994 Northridge earthquake killed 57 people and caused $25 billion in damage — yet most of the 100,000+ wood-frame structures in the affected area performed well, with structural failures concentrated in specific vulnerable configurations and inadequately maintained buildings.

Wood's seismic resilience stems from several physical characteristics. Its high strength-to-weight ratio means that wood-frame buildings are relatively light, generating smaller inertial forces during earthquakes for the same ground acceleration. Its fibrous structure provides ductility under compression parallel to grain, bending, and shear. The multiple connections in a wood-frame building — thousands of nails, screws, and fasteners — collectively provide energy dissipation through friction, fastener yielding, and controlled slip. This distributed ductility is fundamentally different from the concentrated yielding in steel or concrete structures.

The Engineered Wood-Frame System

Modern wood-frame construction for earthquake resistance relies on engineered shear wall systems rather than the distributed bracing of older platform frame construction. Wood structural panels — plywood or oriented strand board (OSB) — are nailed to wood framing to create 耐震壁（せん断壁）地震の揺れによる水平方向の力に抵抗するよう設計された構造壁。多くのコンクリート造・組積造建築物において、水平力に抵抗する主要な構造システムである。 panels that resist lateral earthquake forces. The nails are the critical element: they yield and deform under seismic loading, providing ductility while the wood panels carry the shear forces in compression and tension diagonally through the panel.

The 耐震設計地震力に耐えられるよう構造物を設計する手法。現代の耐震設計は、大地震における構造被害はある程度許容しつつ、倒壊の防止と人命の保護を目指す。 of wood shear walls is explicitly addressed in the Special Design Provisions for Wind and Seismic (SDPWS) standard, which provides design values for shear walls based on panel thickness, nailing pattern, and framing size. For the highest seismic demands, 15/32-inch or 19/32-inch plywood with 3-inch on-center nail spacing at panel edges provides substantial shear capacity — enough to satisfy seismic design requirements for most California residential construction.

Hold-down anchors — metal connectors that attach wall framing to foundations or lower framing — prevent 耐震壁（せん断壁）地震の揺れによる水平方向の力に抵抗するよう設計された構造壁。多くのコンクリート造・組積造建築物において、水平力に抵抗する主要な構造システムである。 panels from overturning under large lateral forces. The tensile demand on hold-downs can be very large, and inadequate or missing hold-downs are a common deficiency in older construction that is often invisible without opening walls. Modern codes require hold-downs at all ends of shear wall segments, with calculated sizes based on the tributary seismic load.

The 耐震基準建物の最低限の耐震安全性を確保するための、設計・建設に関する法的要件の体系。大地震で新たな脆弱性が明らかになるたびに更新される。 History for Wood Frame

耐震基準建物の最低限の耐震安全性を確保するための、設計・建設に関する法的要件の体系。大地震で新たな脆弱性が明らかになるたびに更新される。 requirements for wood-frame seismic construction have evolved substantially over the past century. Early residential codes required minimal seismic provisions. The 1971 San Fernando earthquake revealed deficiencies in multi-story wood-frame apartment buildings, prompting research that led to systematic shear wall design methods. California's Health and Safety Code required plywood shear walls in new wood-frame construction beginning in 1973.

The 1994 Northridge earthquake identified specific vulnerabilities in nominally code-compliant wood-frame construction: inadequate connections between floor framing and walls, insufficient shear wall length at upper stories, and tuck-under parking creating soft-story conditions. Post-Northridge research programs, including the CUREE-Caltech Wood-Frame Project, produced improved design methods, testing protocols, and code provisions that substantially improved the seismic performance baseline for new construction.

Post-2000 wood-frame construction in California and other high-seismic states reflects these advances. Pre-engineered shear wall systems with factory-qualified hardware, pre-fabricated shear wall panels, and proprietary hold-down systems allow consistent quality that site-built construction often cannot achieve. These advances make new wood-frame construction highly reliable, even for three- to five-story residential and mixed-use buildings.

Apartment Buildings: The Critical Application

The most seismically critical wood-frame applications are mid-rise apartment buildings in dense urban areas, where failure affects large numbers of residents. Pre-1994 wood-frame apartments in California frequently have soft-story conditions, inadequate shear walls, and missing hold-downs — vulnerabilities that have driven mandatory retrofit programs in San Francisco and Los Angeles as described in the soft-story guide.

Post-2009, California allowed wood-frame construction for buildings up to 85 feet tall under certain conditions — a significant increase from the previous 65-foot limit. This "Type IIIA" and "Type VA" construction expansion acknowledged that properly engineered tall wood-frame buildings could achieve acceptable seismic performance and fire resistance. These buildings require carefully engineered lateral systems, often using proprietary mass timber or engineered wood products in conjunction with light-frame sheathing systems.

Cross-laminated timber (CLT) and mass timber systems are emerging as alternatives to conventional light-frame construction for mid-rise buildings. These systems offer different seismic characteristics — greater mass, different stiffness, and distinctive connection behavior — requiring specialized 耐震設計地震力に耐えられるよう構造物を設計する手法。現代の耐震設計は、大地震における構造被害はある程度許容しつつ、倒壊の防止と人命の保護を目指す。 approaches. Research on rocking CLT walls with replaceable fuse elements shows promise for highly ductile, low-damage performance in large earthquakes.

Common Failure Modes

Despite wood frame's general resilience, specific failure modes appear repeatedly in earthquake damage surveys. Inadequate anchor bolts allow houses to slide off foundations — a severe failure that breaks utility lines and may render the building uninhabitable even if the structure remains intact. Unbraced cripple walls collapse, dropping the building onto the foundation without allowing graceful deformation. Nail fatigue and withdrawal under repeated cycling can progressively reduce shear wall capacity over a long earthquake sequence.

Hillside homes present special challenges: the downhill side of the building may be supported on tall, unbraced posts while the uphill side sits directly on the slope, creating a dramatic soft-story condition. The 1994 Northridge earthquake destroyed numerous hillside homes in this configuration. Retrofit of hillside wood-frame construction is technically complex and requires site-specific engineering.

Non-structural failures in wood-frame buildings are often the primary source of injury: toppling furniture and water heaters, broken gas lines triggering fires, collapsed chimneys, and broken glass. The good news is that both structural and non-structural vulnerabilities in wood-frame construction are generally addressable through targeted retrofit at relatively modest cost — a favorable situation compared to concrete or masonry building types.