The City of Portland developed regulations that will require mandatory seismic retrofit of all unreinforced masonry (URM) buildings. Headed by the Portland Bureau of Emergency Management, this effort is the Unreinforced Masonry Seismic Retrofit Project.
An unreinforced masonry building, or URM building, is a structure composed of clay masonry bricks joined by mortar without vertical or horizontal steel reinforcing. This type of construction was common from the late 1800s until the 1960s. The City of Portland has compiled an URM building database, including approximately 1880 URM buildings within city limits. Of these buildings, there are 45 schools, 35 churches and 270 multifamily residential structures. The remainder of the inventory consists of one or two-story commercial and industrial buildings.
URM buildings pose a significant threat to public safety in the event of an earthquake. Typical URM construction consist of heavy and brittle masonry walls with light and flexible wood floors. Often these floors, or diaphragms, are poorly connected to the walls. In the event of an earthquake, the walls pull away from the URM walls and the walls will collapse. Even during small earthquakes URM buildings are susceptible to having chimney, parapets and ornamental features fall from the building.
In 1995, the City of Portland added legislation (Title 24.85) addressing the seismic risk of URM buildings and subsequently updated the policy in 2004. The current code requires building owners to complete a seismic retrofit when there is a change in building use, a change in occupancy or when the cost of the improvements exceeds a certain cost threshold. In addition to these requirements, owners are required to provided parapet bracing and to provide cross ties at the roof when the building requires reroofing. Since implementing these requirements only a small percentage of the URM buildings have received either a partial or full seismic retrofit. Because current regulations have been ineffective in reducing the public risk during a seismic event, the City of Portland is considering a mandatory URM retrofit policy.
The mandatory URM retrofit policy has been developed over the last two years by several committees consisting of structural engineers, architects, developers, and employees from BDS, PBEM and PDC. The basic approach is to classify each URM building into a category based on its usage and number of occupants. Each category will have different retrofit requirements and timeframes for completion. Below is a table that summarizes each category and the proposed requirements.
While the proposed mandatory URM retrofit policy will increase the safety of these structures there can be other ramifications to the proposed legislation. In a future blog post, we will discuss these ramifications and discuss possible solutions that will increase public safety without causing undue hardship to building owners.