Seismic Upgrade Requirements for Existing Buildings: Understanding the Building Code TriggersJun 27 2018 · 0 comments · NISHKIAN DEAN, Seismic ·0
By Edwin T. Dean, PE, SE
Existing buildings in the City of Portland are subject to unique requirements that govern what seismic strengthening improvements need to be implemented. These requirements have evolved with the heightened recognition of seismic risk within the region, advancements in seismic engineering technologies, and the movement to increased seismic resiliency, which is expected to provide improved performance of the buildings where we live, work, and shop when subjected to strong ground shaking.
Since its founding, Portland has not experienced the devasting earthquakes that have occurred in other parts of the country, such as the devastating 1906 San Francisco earthquake. For much of the city’s history, seismic design was not a significant consideration; in practice, prior to 1956 when the City adopted the 1955 Uniform Building Code (UBC) it was in many cases not a consideration at all. That all changed in the 1980’s when there was an increasing understanding and acceptance of geologic evidence that indicated there had once been great subduction earthquakes off of Oregon’s coast. The assumption is that those powerful quakes could have been felt strongly throughout what is now the Portland metro area, and there was coupled with increased concern about the potential for local faulting. A recent study (John M. Bauer, 2018) by the State provides background on the relative significance of these two types of potential events.
This all fed into changes in the level of seismic regulation as well as a more than doubling of the seismic design forces to which new buildings had to adhere in a series of code changes in the 1988 and 1991 UBC. The changes also meant that nearly all of the buildings that had been previously constructed, even potentially those newly constructed, were “dangerous” by the new building code definition. This would require that all of the existing building stock be either seismically strengthened and brought into compliance with now much-higher seismic requirements or to be abandoned. This was of course an economically and politically untenable position, so the City decoupled seismic requirements from the dangerous-building code and implemented interim seismic requirements. These requirements evolved and are now manifested in the City’s Code in 1995 as Chapter 24.85 Seismic Design Requirements for Existing Buildings. These regulations today define what is required to address the limited seismic strength that historic and older buildings in Portland possess.
When building owners, property developers, public leaders, and other stakeholders make decisions to purchase or make improvements to existing building properties, it is essential that they have a clear understanding of seismic-upgrade triggers under our current building codes, as they can have a significant economic impact on these decisions.
The current governing building code in the State of Oregon is the 2014 Oregon Structural Specialty Code (OSSC), which is based on the nationally-recognized 2012 International Building Code. This article is an update to one previously published. For the design of new buildings, minimum code design requirements are intended to ensure the life safety of the public. Requirements for existing buildings that undergo changes are covered separately under Chapter 34 of the OSSC, which includes the following seismic-upgrade triggers:
- Change of Use or Occupancy. If the use or occupancy classification of a building will undergo a change to a new classification of equal or higher hazard level, the building will require a seismic upgrade that meets the requirements of the current building code. The building official has the authority to waive part of this requirement if the proposed use is less hazardous based on life and fire risk.
- Addition or Modification. If there is an addition or modification to the building, the addition or modification will need to be in compliance with current code, along with any parts of the existing building that are also directly affected. An exception to this requirement takes place where the addition or modification does not affect or increase seismic forces by more than 10% to an existing building and its structural elements, in which case no upgrade is required. This generally occurs when structural modifications are minor and do not affect the main structural systems of a building.
The City of Portland has additional seismic design requirements for existing buildings within city limits under Chapter 24.85 of its Title 24 Building Regulations, Seismic Design Requirements for Existing Buildings. The goal of these regulations is to improve the level of seismic safety in Portland’s existing buildings by setting additional triggers while providing some flexibility to building owners. Under these regulations, the City also allows the use of the American Society of Civil Engineers (ASCE) 41-13, “Seismic Evaluation and Retrofit Rehabilitation of Existing Buildings,” as an acceptable improvement standard for seismic upgrades in certain cases, depending on factors such as occupancy classification, occupant load increases, and total floor area where occupancy changes occur. The ASCE standard provides a path to evaluate and address seismic deficiencies in existing buildings without meeting all requirements of the current building code.
The City of Portland also has additional triggers for upgrades to unreinforced masonry (URM) buildings under Chapter 24.85, which represent some of the oldest and highest-risk type of existing buildings during a seismic event. There are more than 1,200 privately and publicly owned URM buildings within the city, which includes many multi-story buildings and schools. The City maintains an inventory of URM buildings that is accessible to the public.
In addition to the requirements under Chapter 34 of the 2014 OSSC, the City of Portland has included in the Chapter 24.85 requirements the following adjustments and additions for seismic upgrade triggers:
- 1. Occupancy Change to a Higher Relative Hazard Classification. Where the occupancy change only applies to 1/3 or less of the total floor area and the occupant load increase is less than 150 people, a seismic upgrade is not required.
- Occupancy Change to Same or Lower Relative Hazard Classification. An occupancy change to the same or a lower relative hazard classification or a change in use within any occupancy classification will only require a seismic upgrade where the change results in an increase in occupant load of 150 people or more as defined by the OSSC.
- Required Seismic Evaluation. For buildings that were permitted with the City prior to 1974, a seismic study and report is required for any alterations or remodels that have a value exceeding $260,125 for work other than seismic upgrades, including mechanical, electrical, plumbing, permanent equipment, painting, fire extinguishing systems, site improvements, eco-roofs and finish works. Therefore, only a seismic study is required and not a seismic upgrade.
- Additional Triggers for URM Buildings
- Roof repair or replacement. Where a roof covering exceeding 50% of the roof area is repaired or replaced within a five-year period, the building structural roof system, roof anchorage to walls, and parapets will require a seismic upgrade based on ASCE 41 requirements for an existing building.
- Alterations or repairs. Where cost of work requiring a building permit exceeds $44.59 per square foot for buildings two or more stories and $59.46 per square foot for single-story buildings over any two-year period, the URM building will require a full seismic upgrade based on the requirements of the ASCE 41 requirements for an existing building.
- Special building hazards. For higher-hazard buildings designated Category 5, and where the existing structural system for seismic loads is classified as irregular, and the cost of alteration or repair exceeds $44.59 per square foot, the building will be required to be seismically improved.
All cost values associated with seismic upgrades under Chapter 24.85 are updated annually based on RS Means Cost of Construction Index to account for increased construction costs over time (costs listed are current as of publication). Certain costs are excluded for URM buildings, including site improvements, eco-roofs, and mandated ADA and elevator improvements costs. To provide some flexibility to building owners, the City may allow these upgrades to be phased over a maximum 10-year period.
Under the OSSC and City of Portland Title 24 Building Regulations, seismic upgrades are also required in certain cases when a building suffers major structural damage requiring repair or replacement from a catastrophic event such as an earthquake or fire. The level of repairs required is generally dependent on the amount of damage that has occurred and the age of the building.
If you have any questions or need to determine how these requirements apply to changes for your existing building, please contact us.
Edwin T. Dean, PE, SE is Vice President and Managing Principal of Nishkian Dean a structural engineering consulting firm in Portland, Oregon.