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The following items address frequently asked questions concerning structural design requirements within the jurisdiction of the City of Portland.
General Design Requirements
The following requirements are applicable to both commercial and residential projects located within the jurisdiction of the City of Portland.
The current state building code governing commercial and engineered residential construction is the 2014 Oregon Structural Specialty Code (OSSC), which is based upon the 2012 International Building Code (IBC) as amended by the State of Oregon.
Wind: Design wind pressures are to be determined using the 3-second gust wind speed and the procedures of the 2010 edition of Minimum Design Loads for Buildings and Other Structures (ASCE 7-10) or in accordance with the alternate method contained in the OSSC (if applicable). The design wind speeds 3-second gust for the City of Portland are:
Risk Category Vult (mph) I 115 II 120 III, IV 130
Wind exposure category is site dependent and must be determined by the Engineer of Record based upon site conditions.
- All locations within or administered by the City of Portland are classified as Seismic Design Category D in accordance with the procedures of the OSSC.
- Seismic design parameters for specific sites may be determined based upon zip-code or latitude and longitude using the web tool developed by the United States Geologic Survey available at http://earthquake.usgs.gov/designmaps/us/application.php.
- City of Portland Title 24.85 governs mandatory seismic upgrades for existing buildings. A seismic upgrade may be required for existing buildings undergoing a change of use or occupancy, addition, renovation, alteration, or URM building re-roof. Please refer to City of Portland Title 24.85 for additional information.
- Chapter 24.85, section 24.85.065 (B) of the Portland City code requires certain cost triggers be adjusted annually by the construction cost index. The following are the updated cost triggers.
Table 24.85-C Building Description Cost of Alteration or Repair Single story Unreinforced Masonry (URM) building $56.75 Unreinforced Masonry (URM) building two or more stories $42.56
The updated costs for required ASCE 31 evaluation report per section 24.85.060 is $248,000. See Frequently Asked Questions on Title 24.85.
Five Stories of Wood-framed Residential Construction over a Concrete Podium:
See Frequently Asked Questions on this topic.
The structural requirements for residential projects may be either prescriptive or engineered. In prescriptive design, the ORSC (Oregon Residential Specialty Code) defines a conservative method of construction to resist vertical and lateral loads. In engineered design, an engineer licensed in the State of Oregon prepares engineering calculations and drawings that demonstrate how the structure resists vertical and lateral loads.
View the current State Building Codes and Rules. Refer to Commercial Permits, above, for information regarding engineered residential design.
The following are design values may be used for prescriptive residential design:
Wind: Applicable design wind pressure is 18 psf.
Seismic: Applicable Seismic Design Category is D1.
Transmission and Cell Towers Design loads shall be determined per Structural Standards for Steel Antennae Towers and Antennae Supporting Structures (TIA/EIA-222 F-96). The structural design must account for atmospheric ice loads and increased wind load on ice covered sections.
Floating Structures Floating Structures are governed by the City of Portland Title 28.