On June 17, 2015, City Council passed amendments to Portland City Code 24.85, Seismic Design Requirements for Existing Buildings. The amendments will be effective on July 17, 2015.
In 2014, the primary external reference in Chapter 24.85, ASCE 31, was replaced by a new standard ASCE, 41-13. This amendment updates Chapter 24.85 to reflect the adoption of this new standard. The amendment adopts ASCE 41-13 in its entirety with one exception. The exception stipulates that the design base shear cannot be less than 75% of the base shear required for new buildings.
Based on studies, it was determined that when following the procedures in ASCE 41-13 for Basic Performance Objective for Existing Buildings using BSE-1 the base shears were about 50%-60% of that required for new buildings. This level of reduction in design base shear for existing buildings was determined to be un-conservative. This is inconsistent with current practice and the intent of Chapter 24.85. The amendments preserve the same maximum reduction allowed in base shear for existing buildings as was allowed prior to the adoption of the new ASCE 41-13.
Other amendments include:
- Defining the term Occupant Load to ensure uniform calculation of this base number which triggers the requirement for seismic upgrades;
- Revising the requirements for seismic upgrade of existing lateral load carrying elements impacted by additions or alterations to be consistent with the requirements of OSSC;
- Revising the definition of structural systems damaged by catastrophic events to be consistent with definition for structural systems damaged by an earthquake; and
- Adding definitions to previously undefined terms for clarity and ease of use.
Development of the amended code included outreach via publication on BDS website; notification to the Building Owners and Managers Association (BOMA); notification to the Structural Engineers Association of Oregon (SEAO); notification and meeting of the City of Portland’s Structural Engineering Advisory Board; and notification and meeting of the BDS DRAC (Development Review Advisory Committee).
For questions, please contact Amit Kumar, Senior Structural Engineer, at 503-823-7561, email@example.com.