Seismic Upgrades of Existing BuildingsOct 30 2013 · 0 comments · Educational, NISHKIAN DEAN ·0
Over the past few decades, earthquake knowledge and understanding has seen significant progress through scientific advancement, research and testing, and investigating the performance and failures of existing structural systems following major earthquakes. This progress has allowed building codes to further evolve by providing stricter requirements and requiring more resilient structural systems for addressing seismic safety in new buildings. Technology has also played a key role by allowing structural engineers to build advanced computer models that predict seismic behavior of buildings more accurately, and produce more robust structural systems for earthquake protection.
New buildings and those constructed more recently have already realized the benefits of this progress, however there are still many existing buildings in use with varying levels of seismic deficiencies that present a risk to life safety depending on the era they were constructed. Despite these issues, most national and local building codes do not require that existing buildings are brought to comply with current code for seismic requirements unless there is a change to the building that triggers a seismic upgrade.
When building owners, property developers, public leaders, and other stakeholders make decisions to purchase or make improvements to existing building properties, their understanding of seismic upgrade triggers under our current building codes is essential as it can have a significant economic impact on these decisions.
The current governing building code in the State of Oregon is the 2010 Oregon Structural Specialty Code (OSSC), which is based on the nationally recognized 2009 International Building Code. For design of new buildings, minimum code design requirements are intended to ensure 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 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.
Locally, the City of Portland has developed 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 using the American Society of Civil Engineers (ASCE) 31-03, “Seismic Evaluation 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. The State of Oregon has a large inventory of privately and publicly owned unreinforced masonry buildings, which includes many multi-story buildings and schools.
In addition to the requirements under Chapter 34 of the 2010 OSSC, the City of Portland has included the following adjustments and additions for seismic upgrade triggers:
- 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 $236,039 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 only required and not a seismic upgrade.
- Additional Triggers for Unreinforced Masonry (URM) Buildings
- - Roof repair or replacement: Where an area of roof covering exceeding 50% 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 31-03, Seismic Evaluation of Existing Buildings.
- - Alterations or repairs: Where costs of work requiring a building permit exceeds $40.46 per square-foot for buildings two or more stories and $53.95 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 31-03 improvement standard.
- - Special building hazards: For higher hazard buildings designated, and where the existing structural system for seismic loads is classified as irregular and the cost of alteration or repair exceeds $40.46 per square foot.
All cost values associated with seismic upgrades under Chapter 24.85 are updated annually based on RS Means Cost of Construction Index to account increased construction costs over time. 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 2010 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 to set up a meeting.