建物安全チェッカー

Evaluate your building's earthquake vulnerability and get personalized retrofit recommendations.

Assessment

建物タイプ

建築年

階数

地盤の種類

建物の建設方法が地震脆弱性に与える影響

建物の地震脆弱性は、構造システム、築年数、高さ、そして立地する地盤に依存します。現代の耐震建築基準は、1971年のカリフォルニア州サンフェルナンド地震後に大きく発展し始め、靱性設計により建物が地震動に耐えることを要求しています。つまり、突然の崩壊なく撓んでエネルギーを吸収できる構造です。これらの基準が採用される前に建設された建物は本質的により脆弱であり、特に無補強組積造（URM）構造は脆性的で、中程度の揺れでも壊滅的な破壊を起こしやすいです。

このツールはFEMA P-154に触発されています。これはエンジニアが多数の建物を迅速に評価するために使用する簡易目視スクリーニング（RVS）手法です。RVSは建物の種類（木造、鉄骨造、鉄筋コンクリート造など）に基づいて構造スコアを割り当て、地盤の種類、建物の高さ、耐震基準の採用時期に対する築年数などの要因による修正を適用します。閾値を下回る建物には詳細な工学的評価が必要と判定されます。地盤条件は極めて重要で、軟弱地盤や盛土は岩盤と比較して地震動を2〜3倍に増幅する可能性があります。これは1985年のメキシコシティ地震で悲劇的に実証されました。

建物の耐震安全性における重要な要因

構造種別：木造建築は柔軟性があるため、一般的に最も耐震性に優れています。無補強組積造やアドベ造が最も脆弱です。
築年数：耐震基準は1970年代以降に飛躍的に改善されました。1970年以前の建物は、耐力壁や基礎のアンカーボルトなどの基本的な耐震機能が欠けている可能性があります。
軟弱階層の脆弱性：1階が開放的な建物（例：マンション下の駐車場）は、1994年のノースリッジ地震で見られたようなパンケーキ崩壊を起こしやすいです。
地盤と構造物の相互作用：軟弱地盤上の建物は増幅された揺れを受け、建物の固有振動数が地震動の振動数と一致すると共振効果を受ける可能性があります。

主な用途

構造技術者に相談する前に、自宅や職場の予備的な脆弱性評価を得ること。
どの建物特性が地震リスクを増加または減少させるかの理解。
耐震補強の選択肢と、それが推奨される場合について学ぶこと。
教育・計画目的での異なる建物タイプ間の脆弱性比較。

How to Use

1
Describe Your Building
Select your building type (wood frame, unreinforced masonry, reinforced concrete, steel frame), approximate age, number of stories, and foundation type. Each characteristic directly influences seismic vulnerability.
2
Enter Your Seismic Zone
Provide your location to determine the applicable seismic design category (SDC) and peak ground acceleration (PGA) from national hazard maps. The tool references the USGS 2023 National Seismic Hazard Model for US locations.
3
Review Vulnerability Assessment
Read your building's estimated fragility classification, probable damage state at design-level shaking, and prioritized retrofit recommendations with estimated relative costs and benefit-cost ratios.

About

Building performance during earthquakes is determined by the interaction between seismic demand—the ground shaking imposed on a structure—and structural capacity—the building's ability to resist that shaking without collapse or severe damage. Seismic engineers characterize structural performance through fragility functions: probabilistic relationships between a ground motion intensity measure (such as peak ground acceleration or spectral acceleration) and the probability of reaching or exceeding specific damage states (slight, moderate, extensive, complete). HAZUS, FEMA's loss estimation methodology, incorporates fragility functions for dozens of building types to model community-scale earthquake losses.

The concept of seismic design categories (SDCs) in the International Building Code (IBC) organizes construction requirements based on both hazard level and occupancy classification. SDC A represents very low hazard and has minimal requirements; SDC D, E, and F represent high hazard and require full seismic design provisions including special moment-resisting frames, shear walls with boundary elements, and foundation ties. Critical facilities (hospitals, fire stations, emergency operations centers) are classified as Risk Category IV and face the most stringent requirements—designed to remain operational following the Maximum Considered Earthquake (MCE), a 2%-in-50-year ground motion level.

Base isolation represents the most advanced approach to seismic protection for new and retrofitted buildings. By inserting flexible bearings—typically layered rubber and steel discs or sliding friction pendulum systems—between the foundation and the structure, base isolation shifts the building's fundamental period to 2.5–4 seconds, far away from the 0.1–1 second periods of typical earthquake energy. The building essentially floats above the shaking ground. The Christchurch Women's Hospital (New Zealand) and numerous Japanese government buildings employ base isolation; during the 2011 Tohoku earthquake, base-isolated structures showed interior accelerations 3–5 times lower than comparable fixed-base buildings.

FAQ

Which building types are most vulnerable to earthquakes?

Unreinforced masonry (URM) buildings—brick, stone, or adobe construction without internal steel reinforcement—are universally recognized as the most seismically vulnerable common building type. Masonry is brittle and strong in compression but weak in tension and shear; lateral ground motion generates shear forces that cause diagonal cracking and out-of-plane collapse of walls. The 1999 Izmit (Turkey) and 2010 Haiti earthquakes dramatically illustrated this vulnerability. Soft-story wood-frame buildings (those with an open ground floor for parking or commercial space) are the second most critical concern in the US, as the weak ground floor concentrates drift demands during shaking. Tilt-up concrete buildings—common in industrial and retail settings—are vulnerable due to poor wall-to-roof connections.

How does building age relate to earthquake safety?

Building age is a strong proxy for seismic design standards because building codes have progressively strengthened over time following major earthquakes. In the US, the Uniform Building Code first incorporated seismic provisions in 1927, substantially revised them after the 1971 San Fernando earthquake, and again after the 1994 Northridge and 1989 Loma Prieta events. Buildings constructed before 1973 in California generally predate modern ductile concrete detailing requirements. Buildings constructed before 1940 predate most seismic provisions entirely. Japan's Building Standard Law was fundamentally revised after the 1981 Miyagi earthquake (new seismic design standard, or Shinseitai) and again after the 1995 Kobe earthquake to address soft-story and torsional irregularities.

What is a soft-story building?

A soft-story building has a floor level that is significantly weaker or more flexible than the stories above it, creating a 'weak link' where lateral deformation concentrates during an earthquake. The most common configuration is an open ground floor—used for parking, retail, or large open spaces—surrounded by lightweight wood-frame construction above. The 1994 Northridge earthquake caused the collapse of numerous soft-story apartment buildings in the San Fernando Valley, killing 16 people. Los Angeles implemented a mandatory retrofit ordinance in 2015 requiring seismic upgrades to approximately 13,500 soft-story wood-frame buildings. The retrofit typically involves adding steel moment frames or shear walls at the ground level to stiffen and strengthen the weak story.

What is seismic retrofitting and is it worth it?

Seismic retrofitting encompasses a range of structural interventions designed to improve a building's performance during earthquake shaking. Common techniques include: cripple wall bracing (adding plywood sheathing to the short stud walls between the foundation and first floor), anchor bolt installation (connecting the sill plate to the concrete foundation), soft-story retrofits (adding moment frames or shear walls), and base isolation (installing flexible bearings that decouple the building from ground motion). Cost-benefit analyses consistently show positive returns: FEMA's Benefit-Cost Analysis for residential retrofits typically yields ratios of 3:1 to 7:1, meaning each dollar spent on retrofit saves $3–7 in future expected losses. This ratio increases substantially in high-seismic zones and for URM buildings.

How do I know if my building needs a seismic evaluation?

Professional seismic evaluation should be considered for buildings in moderate to high seismic zones that exhibit risk indicators: construction before 1980 (US), unreinforced masonry or adobe construction, soft-story configuration (open ground floor), presence of 'falling hazard' elements such as unreinforced parapets, chimneys, or heavy facades, foundation on filled land or soft soil, or an irregular floor plan (L-shape, T-shape) that creates torsional vulnerability. The ASCE 41-23 standard provides a tiered evaluation framework: Tier 1 is a rapid checklist-based screening, Tier 2 is a deficiency-focused analysis, and Tier 3 is a full nonlinear structural analysis. For homeowners, FEMA's Plan for Natural Disasters (P-530) and the California Residential Mitigation Program (CRMP) provide accessible self-assessment resources.