Overview of unreinforced masonry (URM) buildings
URM stands for unreinforced masonry. Built generally in the late 1800s until about the 1960s, URM buildings are constructed of walls (typically on the exterior) often made of brick, hollow concrete block, hollow clay tile or stone. Because of its durability, fire resistance and architectural character unreinforced masonry had often been the construction material of choice.
As the name implies, URM walls have little or no reinforcing steel. These walls typically support floor and roof framing commonly of wood construction. Typical construction practice of that era was to let the floors and roof framing simply bear directly on the exterior walls with no structural attachment necessary to resist earthquake forces.
How unreinforced masonry (URM) buildings behave during an earthquake
In general, URM buildings perform very poorly in earthquakes. There are numerous examples from around the world that show URM buildings are likely to suffer extensive damage and could partially or completely collapse during a seismic event.
A few recent examples of earthquakes include Italy (2016), Christchurch, NZ (2011), Napa, CA (2014), Scotts Mills, OR (1993), and Nisqually, WA (2001). Poor building performance poses three basic types of risk: injury, property damage and loss of use. All three kinds of risk are usually greater for unreinforced masonry buildings than for other building types.
URM buildings pose risk not only to occupants of the buildings but also those in adjacent buildings and pedestrians due to collapse and/or falling debris. Past earthquakes have shown that URM's are particularly vulnerable to falling parapets, chimneys, cornices or other exterior ornamentation and exterior walls collapsing which could be potentially lethal.
Why unreinforced masonry buildings perform poorly in an earthquake event
URM buildings were not specifically designed and built to resist seismic loads. Building codes requiring seismic design were not introduced until much later. This combined with the nature of the URM materials, age of the building and historical construction practices makes URM buildings more vulnerable than other building types.
- URM buildings tend to be a lot heavier. The heavier the building, the higher the seismic forces on the structure.
- These buildings are constructed of masonry which is a weak and brittle material. URM buildings are unable to absorb the energy from an earthquake. Modern buildings are designed to be “ductile” i.e to be able to absorb the energy from an earthquake. For example, compare the behavior of two materials, one a paper clip and another a piece of chalk subject to back and forth bending motion. A paper clip can bend back and forth without breaking for several cycles. This is an example of ductile behavior that modern buildings are designed to absorb that back and forth motion of an earthquake. On the other hand, when you bend a piece of chalk it can handle the bending action upto a certain point but then it breaks and it does so in a sudden brittle fashion. A URM building behaves similarly.
- Common practice in URM construction was, to let the floor or roof framing bear on the walls in beam pockets or niches with little or no positive connection between the walls and the floor or roof framing. When an unreinforced masonry building is subject to earthquake shaking, the roof or floor can pull away from the walls leading to partial or total collapse.
- Many URM buildings have parapets (that portion of the exterior wall that extends above the roof), chimneys, cornices and ornamentations. Historically these building components have proven to very vulnerable even in earthquakes of small magnitude. They break away from the building and pose a falling hazard to pedestrians.
- URM buildings are old and archaic. The masonry and the mortar that binds the masonry together is weak and has deteriorated over the years.
Find out if the structure you own, rent, or do business in is an unreinforced masonry building
There are several clues based on visual observation that may indicate that the building is a URM building:
- The first obvious sign is the presence of classic “red brick” on the walls. This is possibly the most common type of URM however some other types of URM include clay tile, or concrete or stone masonry. Refer to figures on this webpage.
- Another common characteristic of a URM brick building is the presence of a header course (see figure at the right). Typical brick units are usually 8” wide x 2” high x 4” thick and are laid in layers or courses bound by mortar between layers and between each unit. They are typically laid such that 8” face (sides) is visible. These layers are called stretcher courses. Approximately every 6th layer the brick is turned 90 degrees such that the 4” face(end) is visible. These layers are called header courses. The presence of header courses provides a strong clue that the wall is unreinforced.
Source
- In multi-story buildings with masonry construction, another indication of URM being present is if the wall thickness increases from the upper floors to the lower as URM walls need the increased thickness to resist the increase in loads as they accumulate from above floors.
- Many URM buildings have deeply recessed with flat or low arched lintels.
- Some URM buildings may have a row of bolts, tie plates or rosettes along floor lines that are visible from the exterior.
Some or all of the above characteristics may be present in any given building however presence or absence of one or many of these characteristics is not a guarantee that the building is a URM or not. It is highly recommended that the services of a licensed structural engineer be obtained to confirm if a particular building is a URM. The engineer can use various techniques to determine if a building is a URM, including field observations which may include removal of finishes to observe construction, examination of construction documents, and testing to confirm presence of any reinforcing steel.
The City of Portland maintains a list of potential Unreinforced Masonry (URM) buildings.
Unreinforced masonry (URM) building list and requesting public records
The City of Portland maintains an unreinforced masonry (URM) building list that contains information on buildings located in the City which are known or presumed to be of unreinforced masonry construction. Find more information about the list and submit a public records request.
Number of unreinforced masonry buildings in Portland
Based on an assessment completed in 2016, the Bureau of Development Services estimates Portland has over 1,600 URM buildings. It is estimated that less than 20% of the URM’s have either been demolished or have been fully or partially retrofitted.
Most of the 1,600+ URM’s are single-story (approx. 56%). The average age of these buildings is 90 years. These structures house more than 7,000 residential units in approximately 250 buildings.
Retrofitting unreinforced masonry buildings
URM buildings pose a danger to public safety during an earthquake, risking death or injury, property damage, and loss of economic use. URM buildings are also part of the historic and cultural character of many Portland neighborhoods. Some are designated as historic structures. Retrofitting or seismically upgrading these buildings will save lives, reduce injuries, and ensure some will remain following an earthquake.
Seismic retrofits can vary in scope and provide different levels of safety. A limited scope (and lesser expense) such as bracing parapets will provide some pedestrian and occupant protection from falling debris but may not prevent a partial or complete building collapse. A complete code-compliant retrofit can be very expensive but may prevent collapse and extensive damage from an earthquake.
The illustrations below indicate some of the retrofit measures and the resulting seismic protection that may be provided.
Retrofitting doesn’t guarantee a building can be occupied following an earthquake, but in some cases – especially smaller earthquakes – residents and businesses are able to quickly reoccupy the building. This adds to Portland’s resilience as a community.
What is Portland doing about retrofitting URM buildings?
Although Portland is in a high seismic area, scientists didn’t fully understand the risk until the 1990s. The original adopted building code classified different parts of the country into seismic zones 1, 2, 2b, 3, and 4 with Zone 1 having the lowest risk and Zone 4 having the highest seismic risk. When Oregon adopted a state building code in 1974, Portland was classified as Zone 2. In 1990 the building code increased Portland’s risk to Zone 2B. In 1993 the building code designated Portland and much of the western half of Oregon as a seismic Zone 3.
In recognition of the high risk of a seismic event, Portland City Council commissioned a task force in 1993 to address the danger posed by existing buildings in an earthquake and recommend policies to mitigate these hazards. The result of the work of this task force was the adoption in 1996 of City’s Title 24.85 “Seismic Design requirements for Existing Buildings” which requires seismic retrofits to existing buildings when a building undergoes a change of occupancy or other major renovation.
The task force also recognized the higher hazard posed by URM buildings and included specific triggers for seismic retrofit specifically for URM buildings. As part of the work of the seismic task force, a database of all commercial buildings was created based on a survey of buildings in the City. From this database a list of URM buildings was established. In 2004, City Council updated Title 24.85 with revised triggers for seismic upgrades.
Realizing that the current regulations have not been effective in reducing the hazards posed by URM buildings, City council directed City staff to develop recommendations to mitigate hazards posed by URM buildings. Three committees were formed to study this issue and a final report was presented to council in 2018. This report recommended minimal mandatory seismic upgrades along with closing loopholes in existing regulations. The reports of the three committees and the final recommendations can be found online.
City Council did not adopt the report but instead established a separate working group to reconsider the standards for seismic upgrades along with financial support that can be provided to URM owners. Council also adopted a resolution requiring all un-retrofitted URM buildings be placarded and tenants notified of the risk posed by URM buildings. This ordinance was however repealed based on the findings of a federal judge that the ordinance was illegal. The second working group was formed in 2019 but disbanded in 2020 due to the pandemic and city budget constraints.
Current regulations for unreinforced masonry (URM) buildings
Regulations for the seismic evaluation and retrofitting URM buildings in the City of Portland are contained in the Oregon Structural Specialty Code, the International Existing Building Code and City of Portland’s Title 24, Chapter 24.85 “Seismic Design requirements for Existing Buildings.”
A seismic upgrade for URM buildings (and other existing buildings) may be required if you invest in a major renovation, re-occupy a vacant building, or change the use or occupancy of a building. Chapter 24.85 of City Title 24 contains passive triggers which if exceeded will require a seismic evaluation or upgrade. These triggers include:
- A change in occupancy or use which results in an increase in occupant load of 150 or more occupants or where more than 1/3rd of the buildings net area has changed occupancy resulting in a higher seismic hazard classification, or
- The cost of alteration or repair exceeds certain cost triggers or
- More than 50% of the roof area is being re-roofed.
You can also review Chapter 24.85 Seismic Design Requirements for Existing Buildings. For more information, check out the related web pages.