鋼 vs コンクリート: 地震でどちらが優れているか?

The Two Dominant Structural Materials

Steel and concrete are the primary structural materials for multi-story buildings in earthquake-prone regions, each with distinctive characteristics that affect seismic performance. Comparing them for earthquake resistance requires examining not just the materials' intrinsic properties but how they are configured into structural systems, detailed at connections and joints, and assembled on-site. Neither material is universally superior; the best choice depends on building height, occupancy, seismic hazard level, construction context, and economic constraints.

Steel's Inherent Ductility

Structural steel's primary seismic advantage is inherent ductility. Hot-rolled steel can elongate 15-20% before fracturing, compared to essentially zero elongation for concrete in tension. This ductility allows steel members and connections to deform substantially under earthquake forces while maintaining load-carrying capacity — a critical behavior for preventing collapse. Steel frames can experience multiple cycles of yielding without fracture, dissipating large amounts of seismic energy through inelastic deformation.

モーメント抵抗フレーム（ラーメン構造）梁と柱を剛接合し、曲げによって地震の水平力に抵抗する構造システム。優れた靭性を持つが、他の構造システムより費用がかかる。 systems using steel beams and columns connected with fully restrained moment connections were long considered the gold standard of seismic design. In a properly designed steel moment frame, beams are designed to yield in bending — the "plastic hinge" — while columns remain elastic, a behavior called "strong column, weak beam" that ensures story mechanism rather than catastrophic collapse. The frame can sustain multiple plastic hinge formations without losing its ability to carry gravity loads.

The 1994 Northridge earthquake disrupted this confidence when detailed inspection of modern steel モーメント抵抗フレーム（ラーメン構造）梁と柱を剛接合し、曲げによって地震の水平力に抵抗する構造システム。優れた靭性を持つが、他の構造システムより費用がかかる。 buildings revealed brittle fractures in welded beam-to-column connections. These connections, believed to be ductile based on laboratory testing, failed in a brittle mode under the actual earthquake's ground motion characteristics without triggering collapse but without providing the expected ductility. The failures, invisible without detailed investigation, drove major research programs and complete revisions to steel connection standards. Post-Northridge connections — using specially detailed "pre-qualified" connection configurations — now provide substantially more reliable ductility.

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。 systems in steel buildings use steel plates — typically 1/4 to 1/2 inch thick — within framed bays to provide lateral resistance through plate shear and tension field action. Steel plate 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。 systems are extremely stiff and strong, suitable for buildings where drift control is critical. They are common in hospitals and emergency facilities where non-structural damage must be minimized.

Concrete's Characteristics

Reinforced concrete combines the compressive strength of concrete with the tensile strength of steel reinforcement to create a composite material capable of resisting forces in all directions. The combination is economical for large structures where the formwork costs are spread over large floor areas, fire resistance is required, and mass provides beneficial damping.

Concrete's seismic challenge is that the material itself is brittle in tension and shear. Without careful attention to reinforcement quantity and detailing, concrete structures fail in brittle modes that do not provide ductility warnings before collapse. Non-ductile concrete frame buildings — constructed before modern seismic detailing requirements were adopted — are among the most dangerous existing building types in seismic regions.

Modern ductile concrete construction achieves reliable ductility through specific detailing requirements: closely spaced transverse reinforcement (ties and spirals) that confines the concrete core, preventing explosive failure under combined compression and bending; continuous longitudinal reinforcement with proper lap splices; and joint reinforcement that maintains column-beam connection integrity under repeated cycling. Properly detailed ductile concrete frames can achieve ductility comparable to steel, with plastic hinges forming and cycling through multiple earthquake load reversals.

Concrete 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。 systems are extremely common in mid-rise and high-rise construction. Reinforced concrete core walls — surrounding elevator and stair shafts — provide enormous lateral stiffness and strength with architectural logic. Coupled shear walls — pairs of walls connected by link beams — are designed to yield in the coupling beams first, dissipating energy in replaceable elements while the main walls remain elastic. This deliberate energy dissipation strategy is analogous to the eccentrically braced frame concept in steel construction.

Height and Performance Range

For low-rise buildings (1-5 stories), both steel and concrete can perform excellently with appropriate detailing. Wood-frame construction often dominates this range for residential construction. Steel frames offer speed of erection; concrete offers mass and fire resistance. Neither has a clear seismic superiority at low heights.

For mid-rise buildings (6-20 stories), reinforced concrete 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。 systems are frequently the economical choice, combining structural efficiency with fire resistance. Steel moment frames and braced frames are competitive, with relative costs varying by local market conditions. Performance under large earthquakes depends strongly on the quality of detailing rather than the choice of material.

For high-rise buildings (above 20 stories), steel frames were historically dominant due to their light weight and rapid erection, but concrete core-plus-frame systems and composite construction have become competitive. The long natural periods of tall buildings reduce seismic acceleration demands somewhat, shifting concern toward drift control and secondary effects. 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 tall buildings increasingly uses 性能設計（性能規定型耐震設計）画一的な基準要件ではなく、地震の強さごとに異なる目標性能レベル（供用継続・人命安全・倒壊防止）を設定する高度な設計手法。 with advanced nonlinear analysis regardless of material choice.

The Composite Option

Modern construction increasingly uses steel and concrete together in composite systems that exploit the strengths of both materials. Composite columns — concrete-filled steel tubes or steel sections encased in concrete — are stronger and more ductile than either material alone. Composite floors — steel decking with concrete topping — provide efficient, flat floor plates with the steel deck acting as permanent formwork. Shear connectors between steel beams and concrete slabs ensure composite action, reducing beam depths and material quantities.

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 composite systems requires careful attention to force transfer at the steel-concrete interface and to the mixed behavior under seismic loading. AISC 341 provides composite structural system design provisions, and the performance of composite systems in major earthquakes has generally been favorable.

The Construction Quality Factor

Beyond material choice, construction quality is often the dominant factor in earthquake performance. Both steel and concrete require skilled workers and quality control programs to achieve design-intent performance. Steel requires precise fabrication and field welding or bolting with proper inspection. Concrete requires proper batching, placement, consolidation, and curing, with special attention to congested reinforcement zones in seismic detailing.

In earthquake disasters worldwide, poor construction quality — inadequate concrete strength, missing reinforcement, improper lap splice locations, substandard welding — has been a primary factor in collapse, often in buildings nominally designed to adequate standards. 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。 standard embodied in the code is only achieved if construction quality matches design intent — a chain where the weakest link determines structural performance.