Why Older Buildings Collapse in Earthquakes: Structural Vulnerabilities Explained

During the 1989 Loma Prieta earthquake in San Francisco, a dramatic pattern emerged in the Marina District. Block after block of older unreinforced brick buildings collapsed into rubble—roofs caved in, walls exploded outward, entire structures pancaked. Meanwhile, adjacent modern buildings from the 1980s stood intact with only cosmetic damage. Same earthquake, same soil conditions, same neighborhood. The only difference? Construction date and building code standards. That 30-year age gap meant the difference between total collapse and survival.

This pattern repeats in every major earthquake. The 1995 Kobe earthquake killed over 6,000 people—most crushed in older wooden houses that collapsed, while modern buildings largely survived. The 2011 Christchurch earthquakes devastated unreinforced masonry buildings from the early 1900s while post-1980 structures performed well. In Turkey's devastating 2023 earthquakes, pre-1999 buildings collapsed by the thousands while newer code-compliant buildings fared dramatically better.

Understanding why older buildings fail in earthquakes isn't just academic curiosity—it's critical life-safety information. Millions of people worldwide live and work in older buildings that are fundamentally vulnerable to earthquake forces. This comprehensive guide explores why pre-code buildings are death traps, the specific structural deficiencies that cause collapse, how building codes evolved after deadly disasters, the most dangerous building types and construction periods, unreinforced masonry dangers, soft-story collapse mechanics, non-ductile concrete failures, lack of structural connections in older construction, retrofit options that can save vulnerable buildings, and how to identify whether the building you're in is at risk.

The Evolution of Building Codes: Learning from Disasters

Why Building Codes Matter

Building codes are prescriptive rules specifying how buildings must be constructed. They cover everything from materials to structural design to construction details. For earthquake safety, building codes determine whether buildings survive or collapse.

The fundamental reality: Buildings constructed before modern seismic codes weren't designed to resist earthquake forces. Architects and engineers of the past weren't incompetent—they simply didn't understand earthquake forces or didn't have codes requiring earthquake resistance. Many older buildings were only designed for gravity loads (supporting their own weight), not lateral loads (resisting sideways pushing from earthquakes).

Code compliance timeline: This creates a clear age-risk relationship. The older the building, the less likely it incorporated seismic design. And each major earthquake revealed new vulnerabilities, leading to improved codes.

Critical Code Development Milestones

Geographic variability: Code development varied by region. California led the world due to frequent earthquakes providing harsh lessons. Other seismic regions (Japan, New Zealand) developed parallel improvements. Low-seismic regions often had no earthquake provisions until recently—buildings from 1970 in California are better than buildings from 2000 in non-seismic regions.

Unreinforced Masonry: The Most Dangerous Construction Type

What Is Unreinforced Masonry?

Definition: Buildings with walls made of brick, stone, or concrete block stacked with mortar but containing no steel reinforcement. The masonry alone resists all forces.

Construction era: Dominant from 1800s through 1933 in California, through 1950s-1960s elsewhere. Millions of these buildings still exist.

Typical examples:

• Historic downtown commercial buildings (3-6 stories)
• Old apartment buildings and row houses
• Churches and institutional buildings
• Warehouses and industrial buildings
• Victorian-era residential buildings

Why URM Buildings Fail

Fundamental weakness: Masonry is strong in compression but weak in tension.

Gravity loads (normal conditions):

• Building weight pushes down on walls
• Walls experience compression
• Masonry is excellent in compression
• Building stands indefinitely

Earthquake loads (lateral forces):

• Ground shakes horizontally
• Upper portions of building try to stay in place (inertia)
• Creates bending and tension stresses in walls
• Masonry has almost no tensile strength
• Walls crack, separate, and collapse

Common URM Failure Modes

Out-of-plane wall collapse (most deadly):

• Walls perpendicular to shaking direction fall outward like dominos
• Entire wall face can separate from building and crash to street
• Kills pedestrians, crushes vehicles, traps occupants
• Happens suddenly with little warning

Parapet failure:

• Parapets (decorative walls extending above roofline) are particularly vulnerable
• Tall, heavy, poorly connected to roof
• Break off and fall to sidewalk below
• One of most common causes of earthquake deaths in historic districts

Floor diaphragm separation:

• Wood floors in URM buildings aren't strongly connected to masonry walls
• During shaking, floors pull away from walls
• Without floor support, walls collapse
• Results in pancaking collapse

In-plane shear cracking:

• Walls parallel to shaking develop diagonal cracks (X-pattern)
• Mortar joints fail in shear
• Walls lose strength and stiffness
• Can lead to complete collapse in strong shaking

Historical URM Collapse Examples

1933 Long Beach Earthquake:

• 120 school buildings destroyed—mostly URM construction
• Struck at 5:54 PM—if during school hours, casualties would have been catastrophic
• Led to immediate ban on URM school construction in California
• Field Act passed within weeks requiring seismic safety for schools

1989 Loma Prieta Earthquake:

• San Francisco's Marina District: Entire blocks of URM buildings collapsed
• 68 people died, many in URM building collapses
• Buildings from 1906 San Francisco earthquake rebuild—ironically built just before modern codes
• Demonstrated that URM buildings remain deadly decades later

2011 Christchurch Earthquakes:

• 185 people killed
• URM buildings caused majority of central city deaths
• Parapets and facades fell onto streets and pedestrians
• Many victims were on sidewalks outside collapsed buildings
• Led to massive URM demolition program

Soft-Story Buildings: Collapse Waiting to Happen

What Is a Soft-Story Building?

Definition: Building with open first floor (fewer walls, more openings) supporting heavier upper floors. The "soft" first floor is much weaker than floors above.

Common examples:

• Apartment buildings with ground-floor parking and no walls (just columns)
• Buildings with ground-floor retail (large storefront openings) and residential above
• Buildings with first-floor garages in hillside areas
• Older apartment buildings where parking was added by removing first-floor walls

Why they were built:

• Parking requirements—cars need open space
• Retail visibility—storefronts need large openings
• Economic optimization—maximizing usable space
• Not realizing earthquake vulnerability

Why Soft-Story Buildings Collapse

The mechanics of collapse:

1. Stiffness discontinuity: Upper floors have many walls (stiff). First floor has few walls (flexible). During earthquake, soft first floor deforms much more than upper floors.
2. Concentration of damage: All earthquake deformation concentrates in soft first floor. Upper floors move as rigid block sitting on flexible first floor.
3. Excessive drift: First floor can deform several feet horizontally while upper floors barely move.
4. Column failure: First-floor columns weren't designed for this much deformation. They fail in shear or buckling.
5. Progressive collapse: As columns fail one by one, remaining columns become overloaded. Finally, all columns fail simultaneously.
6. Pancaking: Upper floors drop onto collapsed first floor. Occupants on first and second floors are crushed.

Speed of collapse: Soft-story collapse happens in seconds. No time to evacuate. Occupants on first floor have no warning and no chance of escape.

Historical Soft-Story Collapses

1989 Loma Prieta Earthquake:

• San Francisco Marina District: Multiple soft-story apartment buildings collapsed
• Northridge Meadows apartment complex: 16 people killed when building collapsed
• All three upper floors pancaked onto first floor
• Victims were residents sleeping in ground-floor and second-floor units

1994 Northridge Earthquake:

• 16 deaths in collapsed apartment buildings—almost all soft-story failures
• Northridge Meadows: 16 deaths in single building collapse
• Hundreds of soft-story buildings suffered severe damage
• Led to mandatory soft-story retrofit ordinances

1995 Kobe Earthquake:

• Over 6,000 deaths—many in soft-story wooden buildings
• Traditional Japanese construction had open first floors for shops
• Upper floors collapsed onto occupants sleeping on first floor
• Most deaths occurred in pre-1981 buildings (before improved codes)

Identifying Soft-Story Buildings

Visual clues:

• First floor is mostly open (garage doors, large storefronts)
• Upper floors have many windows with walls between them
• Clear difference in appearance between first floor and upper floors
• Building on slope with first floor partially exposed on one side
• First floor converted from residential to parking

Especially dangerous indicators:

• First floor taller than upper floors (increases soft-story effect)
• Heavy upper floors (concrete or masonry) on wood-frame first floor
• First floor with corner columns only (no interior walls at all)
• Visible modifications where walls were removed to create openings

Non-Ductile Concrete: Brittle Failures in "Modern" Buildings

The Problem with Pre-1970s Concrete

What is non-ductile concrete? Reinforced concrete buildings constructed before modern ductile design requirements (generally pre-1971 in California, pre-1980s elsewhere). These buildings have concrete and steel reinforcement, but the detailing doesn't allow ductile (bending without breaking) behavior.

Why they were considered safe: When built, these were "modern" buildings using reinforced concrete—the latest technology. Engineers knew concrete needed steel reinforcement. What they didn't understand yet was that connection details and reinforcement patterns determine whether concrete behaves ductility or brittlely.

Specific Deficiencies in Non-Ductile Concrete

Inadequate beam-column joint reinforcement:

• Joints where beams meet columns are critical stress concentration points
• Pre-1971 buildings often had minimal stirrups (ties) in joints
• Without confinement, concrete crushes and fails suddenly
• Modern design requires closely-spaced stirrups through joint

Lap splice vulnerabilities:

• Reinforcing bars must be spliced (connected end-to-end)
• Old practice: lap bars side by side with minimal length of overlap
• During earthquake: bars can pull apart at splices
• Especially problematic at column bases (maximum stress location)

Lack of confinement:

• Concrete gains strength and ductility when confined by steel ties
• Old columns had widely-spaced ties (12-18 inches apart)
• Modern columns have closely-spaced ties (2-6 inches apart in critical regions)
• Without confinement, concrete spalls (breaks off) and columns fail

Strong-beam-weak-column design:

• Pre-1971 buildings sometimes had stronger beams than columns
• During earthquake: columns fail before beams
• Column failure = loss of vertical support = pancake collapse
• Modern design ensures columns are stronger (beam failure is preferable)

How Non-Ductile Concrete Fails

Shear failure of columns:

• Most common and most dangerous failure mode
• Column develops diagonal cracks (X-pattern)
• Concrete fails suddenly—no ductile warning
• Column loses vertical load-carrying capacity
• Floor above drops down
• Can trigger progressive collapse of entire building

Joint failure:

• Joint region where beam and column meet cracks and crushes
• Reinforcement can poke through failed concrete
• Loss of connection between beam and column
• Floor system loses support

Splice failure:

• Lap splices pull apart
• Reinforcement becomes ineffective
• Section fails as if unreinforced
• Particularly dangerous in columns

Notable Non-Ductile Concrete Failures

1971 San Fernando Earthquake:

• Olive View Hospital: "Modern" reinforced concrete hospital severely damaged
• Multiple buildings collapsed or near-collapse
• Columns failed in shear, joints crushed
• Hospital built in 1969—only 2 years old when earthquake struck
• Revealed that "reinforced concrete" didn't guarantee earthquake safety
• Led to complete overhaul of concrete design codes

1985 Mexico City Earthquake:

• Hundreds of reinforced concrete buildings collapsed
• Many were "modern" buildings from 1950s-1970s
• Non-ductile details plus resonance effects = catastrophic failure
• 10,000+ deaths, most in collapsed concrete buildings

2010 Haiti Earthquake:

• Over 200,000 deaths
• Most casualties in collapsed concrete buildings
• Poor construction quality plus non-ductile design = mass collapse
• Demonstrated that modern materials (concrete, steel) don't guarantee safety without proper design

Other Critical Vulnerabilities in Older Buildings

Tilt-Up Buildings with Roof-to-Wall Connection Failures

What are tilt-up buildings? Construction method where concrete wall panels are cast on ground, then tilted up into place. Common for warehouses, big-box retail, light industrial buildings from 1950s-1980s.

The vulnerability:

• Roof (often wood or steel) sits on top of concrete walls
• Connection between roof and wall is critical
• Pre-1980 buildings often had minimal connections
• During earthquake: walls can separate from roof and fall outward
• Roof collapses inward, crushing occupants

1971 San Fernando earthquake: Multiple tilt-up buildings collapsed. 64 people died in collapsed buildings. Led to connection requirements in codes.

1994 Northridge earthquake: Despite improved codes, several older tilt-up buildings collapsed. Demonstrated need for retrofit of existing buildings.

Pre-Northridge Steel Buildings

The hidden vulnerability discovered in 1994:

• Steel buildings from 1970s-1994 used welded beam-to-column connections
• Design looked perfect on paper—strong steel, rigid connections
• Northridge earthquake revealed catastrophic problem: welds were brittle
• Hundreds of buildings had cracked welds in beam-column connections
• Many buildings appeared undamaged from outside but had hidden structural damage

Why welds failed:

• Welding creates heat-affected zone in steel—locally brittle material
• Connection geometry created stress concentrations
• Low-toughness weld material couldn't handle stress
• Cracks initiated at weld, propagated through connection

Post-Northridge changes:

• Complete redesign of steel moment connections
• New connection types (reduced beam section, bolted connections)
• Improved weld procedures and inspection
• Full-scale testing required before approval

Legacy: Pre-1994 steel buildings may have vulnerable connections even if they appear modern and were "code-compliant" when built.

Weak Stories and Vertical Irregularities

The problem: Buildings where one floor is significantly weaker or more flexible than floors above or below create weak-story collapse risk.

Common examples:

• Commercial building with residential tower above (transition floor is vulnerable)
• Building where one floor has much larger rooms/fewer walls
• Building with heavy upper floors on lighter lower structure
• Penthouses or mechanical floors with different construction

Pounding Between Adjacent Buildings

The problem: In dense urban areas, buildings are built close together or share party walls. During earthquake:

• Buildings sway at different frequencies
• Adjacent buildings can collide (pound into each other)
• Pounding creates concentrated damage at impact points
• Floor-level misalignment makes this worse (one building's floor hits another's column)

Modern codes require separation or structural connection, but older buildings often have inadequate gaps.

Geographic Patterns: High-Risk Buildings by Region

California

Highest risk buildings:

• Pre-1933 unreinforced masonry: Found in every city, downtown historic districts particularly dangerous
• Pre-1971 concrete buildings: Non-ductile design, common in 1950s-1960s construction boom
• Soft-story wood buildings: Abundant in San Francisco, Los Angeles, Oakland—thousands identified
• Pre-1980 tilt-up warehouses: Throughout state

Mandatory retrofit programs: Los Angeles, San Francisco, and other cities have ordinances requiring URM and soft-story retrofits. However, many buildings remain unretrofitted due to cost and enforcement challenges.

Pacific Northwest (Washington, Oregon, British Columbia)

Unique challenge: Major seismic threat (Cascadia Subduction Zone) but no major earthquake in modern era. Building codes didn't include strong seismic provisions until 1990s-2000s.

Highest risk buildings:

• Pre-1990 unreinforced masonry: Extensive—every downtown has vulnerable URM buildings
• Pre-1994 concrete buildings: Designed for much lower seismic forces than California equivalents
• Soft-story buildings: Common in Seattle, Portland, Vancouver

Major concern: When Cascadia megaquake occurs (M9.0+ expected within next 50 years), building stock is far more vulnerable than California's. Casualties projected in tens of thousands.

Central and Eastern United States

New Madrid Seismic Zone (Missouri, Illinois, Arkansas, Tennessee, Kentucky):

• Last major earthquakes 1811-1812 (before modern construction)
• Buildings designed without seismic provisions until very recently
• Extensive URM building stock in Memphis, St. Louis, other cities
• Future major earthquake would cause catastrophic damage

Eastern seismicity characteristics:

• Earthquakes are rare but can be strong
• Ground shaking propagates farther than West Coast (harder bedrock)
• Buildings completely unprepared for seismic forces

International Patterns

Developing nations: Often have highest vulnerability due to:

• Lack of building codes or poor enforcement
• Unreinforced masonry construction continuing into present
• Poor construction quality even when codes exist
• Economic pressure leads to substandard construction

Examples: Turkey's devastating 2023 earthquakes killed 50,000+ people—most in collapsed buildings that violated building codes. Haiti 2010, Pakistan 2005, Nepal 2015—all had massive casualties from building collapses.

Identifying At-Risk Buildings

Questions to Ask About Any Building

1. When was it built? Construction date is single most important factor.
2. What is it made of? Unreinforced masonry? Concrete? Wood? Steel?
3. Where is it located? High seismic zone? Building codes reflect this?
4. Has it been retrofitted? Seismic upgrades can transform dangerous building into safe one.
5. What's the building configuration? Soft-story? Irregular? Symmetrical?

Visible Warning Signs

Exterior indicators:

• Solid masonry walls (brick or stone) with no visible steel
• Open first floor with parking or retail
• Decorative parapets extending above roofline
• Heavy building (concrete or masonry) on hillside with exposed lower floor
• Older building adjacent to newer buildings (pounding risk)
• Existing cracks in walls or foundation (pre-existing damage)

Interior indicators:

• Thick masonry walls without visible reinforcement
• Concrete columns that appear small for loads they carry
• Open plan first floor with minimal interior walls
• Signs of previous repairs or modifications

Retrofit Solutions and Their Effectiveness

URM Building Retrofits

Wall anchors:

• Steel rods drilled through walls, connected to floors
• Prevents walls from falling outward
• Relatively inexpensive ($10-30 per square foot)
• Dramatically reduces collapse risk

Parapet bracing or removal:

• Brace parapets to roof or remove them entirely
• Eliminates most dangerous falling hazard
• Cost-effective safety improvement

Diaphragm strengthening:

• Strengthen floor and roof structures
• Allows forces to distribute to walls
• Combined with wall anchors, creates complete load path

Soft-Story Retrofits

Add shear walls:

• Install new concrete or wood shear walls on first floor
• Most effective solution but impacts parking/retail space
• Cost: $100-150 per square foot typically

Steel moment frames:

• Add steel frames in strategic locations
• Maintains open floor plan better than shear walls
• More expensive than shear walls

Non-Ductile Concrete Retrofits

Column jacketing:

• Wrap columns in steel plates or fiber-reinforced polymer
• Provides confinement and shear strength
• Can transform brittle columns into ductile ones

Joint strengthening:

• Add steel jackets or FRP wraps at beam-column joints
• Prevents joint crushing
• Critical for buildings with inadequate joint reinforcement

Conclusion: The Deadly Legacy of Pre-Code Construction

The harsh reality is that millions of people live and work in buildings that are fundamentally unsafe in earthquakes. These aren't defective buildings—they were constructed to the standards of their time. But those standards didn't account for earthquake forces we now understand are inevitable in seismic zones.

Key takeaways:

• Age matters enormously. Pre-1971 buildings in California, pre-1994 buildings in Pacific Northwest, pre-2000 buildings in many other regions lack modern seismic protection.
• Unreinforced masonry is the most dangerous construction type. These buildings kill people in every major earthquake. If you occupy a URM building regularly, understand the risk.
• Soft-story buildings collapse catastrophically. Upper floors pancake onto lower floors in seconds. No warning, no escape.
• Non-ductile concrete buildings fail suddenly. "Modern" reinforced concrete from 1950s-1970s can collapse just like older buildings.
• Retrofits work. Engineering solutions exist to dramatically improve older building safety. The challenge is cost and political will to mandate retrofits.
• Building codes evolve through tragedy. Every major earthquake reveals new vulnerabilities. Today's "safe" buildings may have hidden weaknesses we haven't discovered yet.

What this means for you:

Don't assume buildings are safe just because they're standing. Age and construction type determine earthquake safety more than appearance. A beautiful historic brick building may be a death trap. An ugly 1990s concrete building may be far safer.

If you have choice in where to live or work, prioritize building age and construction type. If you're stuck in an older building, understand the risks and have evacuation plans. Support retrofit programs in your community—they save lives.

The next major earthquake will kill people. Most of those deaths will occur in older buildings that collapse. We know which buildings will fail. We have the engineering knowledge to fix them. What we often lack is the political will and economic resources to retrofit buildings before disaster strikes.

Understanding why older buildings collapse isn't just academic knowledge—it's life-safety information that could save your life in the next earthquake.

For more earthquake preparedness resources, explore our guides on modern earthquake engineering, emergency preparedness, and high-rise building safety. Monitor real-time seismic activity using our earthquake tracking map.