Why Some Masonry Buildings Fail: Understanding Unreinforced Masonry and Modern Seismic Design
Many of Portland's most recognizable historic buildings were constructed before modern seismic standards. Understanding the difference between unreinforced and reinforced masonry is an important part of responsible ownership and adaptive reuse.
If masonry buildings can survive for centuries, why do we sometimes see images of brick buildings collapsing during earthquakes? The answer lies in an important distinction: not all masonry buildings were designed the same way.
Many of the masonry structures that have endured for generations owe their longevity to durable materials, thoughtful detailing, and ongoing maintenance — topics we covered in the previous article in this series. But longevity under normal conditions does not guarantee performance under all conditions. Many older buildings were constructed before modern seismic requirements existed, and those structures — commonly referred to as unreinforced masonry, or URM buildings — can be particularly vulnerable during earthquakes. Understanding that distinction is especially important in the Pacific Northwest.
What is unreinforced masonry?
Unreinforced masonry buildings rely primarily on brick, stone, or concrete masonry units stacked together with mortar. While these materials perform exceptionally well under compression — supporting the weight of floors, roofs, and structure above — they contain little or no embedded steel reinforcement to resist lateral forces. Historically, this type of construction was common throughout the United States. Many churches, schools, commercial buildings, and downtown storefronts built before modern seismic codes fall into this category, and they often remain perfectly serviceable under normal conditions. The concern arises specifically when they are subjected to earthquake forces.
Why earthquakes present a different kind of challenge
Owner's Representation · Due Diligence · Feasibility Review
JR-DBA provides owner-side due diligence, existing building assessments, and early feasibility reviews to help clients understand potential code, seismic, and permitting considerations before moving forward. If you're evaluating an older masonry building — whether for purchase, renovation, or adaptive reuse — early assessment is the right starting point.
Masonry is remarkably strong when carrying vertical loads. Earthquakes create a very different set of demands. Rather than primarily downward forces, seismic events introduce rapid lateral and uplift movements that place masonry into tension and bending — conditions for which unreinforced assemblies are far less suited. Documented vulnerabilities in URM buildings include:
Exterior walls separating from floors or roofs
Out-of-plane wall failures
Falling parapets and decorative cornices
Chimney collapse
Cracking around openings
Progressive collapse of unsupported wall sections
These failure modes have been documented in earthquakes throughout North America and around the world — and they are precisely what modern seismic design requirements are intended to address.
The Pacific Northwest and the Cascadia Subduction Zone
For those of us living in Oregon and Washington, seismic risk is not a theoretical concern. The Pacific Northwest sits adjacent to the Cascadia Subduction Zone — a fault system extending from Northern California to British Columbia that scientists believe is capable of producing very large earthquakes, sometimes referred to as megathrust events. The exact timing of future seismic activity cannot be predicted, but the region's hazard profile has significantly shaped modern building codes and engineering practice. As our understanding of earthquake behavior has evolved, so too have the requirements governing masonry construction.
How modern masonry is different
Today's masonry buildings are fundamentally different from their historic predecessors. Modern systems are engineered to work together with reinforcing steel, grout, and anchored structural components as integrated assemblies rather than relying solely on the masonry units and mortar alone. Depending on the project, a modern reinforced masonry system may include:
Vertical reinforcing steel within wall cells
Horizontal reinforcement at designated intervals
Grouted masonry cores
Anchorage between walls and floor or roof diaphragms
Reinforced bond beams
Engineered connections designed to transfer seismic forces throughout the structure
Modern codes do not eliminate risk entirely — no building can be considered earthquake-proof — but they are specifically intended to improve life safety and reduce the most severe failure modes. The shift from unreinforced to reinforced masonry represents one of the more significant changes in how we design buildings in seismically active regions, and it's a story worth understanding whether you're an architect, an owner, or simply someone evaluating an older property.
"Longevity under normal conditions does not guarantee performance under all conditions."
What to consider with an existing masonry building
Owning or purchasing an older masonry building doesn't automatically mean the building is unsafe — many historic masonry structures continue to serve their communities well. But age alone should prompt questions. If you are considering purchasing, renovating, repurposing, or expanding an older masonry building, it's worth understanding:
When the building was constructed
Whether previous seismic upgrades have been completed — and when
Whether parapets or chimneys have been reinforced
How floors and roofs are connected to exterior walls
Whether a change of occupancy could trigger additional code requirements
What future improvements may be advisable to reduce risk
In many cases, a targeted assessment early in the process can help owners understand both the limitations and the opportunities of a building before significant investment decisions are made.
The value of early due diligence
Historic masonry buildings can be extraordinary assets — offering craftsmanship, character, and material quality that are difficult to replicate today. At the same time, understanding how those buildings were constructed, and how modern expectations differ from historic practices, is an essential part of responsible ownership. Early due diligence helps identify potential seismic considerations, avoids surprises during permitting, and supports informed decision-making before commitments are made.
Learn more about Portland Chapter 24.85
Masonry has earned its reputation for longevity — but context matters. Knowing the difference between unreinforced and reinforced masonry, and understanding what that means for a specific building in a seismically active region, is what separates a well-informed owner from one who discovers the distinction at the wrong moment.
JR-Design Build Architect | Portland, Oregon
Architectural Design · Owner's Representation · Construction Management Advisory
More from this series
Why Masonry Buildings Survive: Lessons from Lime Mortar and Moisture Management
Why Simple Rooflines Often Outperform Complex Ones
Why Old Timber Buildings Last: Lessons in Joinery and Movement
