Portland Poised to Mandate URM Building Seismic StrengtheningAug 23 2016 · 0 comments · NISHKIAN DEAN, Seismic ·0
Read the latest developments on this subject at https://www.nishkian.com/portlands-urm-seismic-retrofit-requirements-on-hold-for-now/
The City of Portland is laced with seismic faults and is vulnerable to the looming Cascadia subduction zone earthquake, which could have a magnitude as high as 9.0. Despite this risk, Portland has one of the highest concentration of unreinforced masonry (URM) buildings in the Pacific Northwest. URM buildings are particularly vulnerable to potential catastrophic collapse in earthquakes. To alleviate this risk, the Portland Bureau of Emergency Management (PBEM) convened a series of committees to propose new URM seismic retrofit standards, which are currently under deliberation with the goal of passing the new standards in the City Council in 2017.
Current URM Seismic Strengthening Requirements
Oregon’s seismic risk was poorly understood until the early 1990s. The City of Portland put seismic strengthening regulations into effect in 1995 with Title 24.85, which was significantly updated in 2004. Currently, Title 24.85 only requires mandatory upgrade of URM buildings when significant modifications are made to the building, as outlined below:
Full seismic upgrade triggered when:
- More than 1/3 of the building area is changed to a higher occupancy classification;
- The occupant load is increased by 150 or more; or
- Building alterations exceed a certain cost per square foot, indexed to construction cost index.
Partial upgrade (brace roof parapet and attach roof to walls) triggered when:
- More than half the roof area is re-roofed.
In the 20 years since Title 24.85 went into effect, PEBM reports that about 5% of Portland’s URM stock has been fully upgraded, 9% partially upgraded, and 8% demolished, leaving 85% of the remaining URM building stock without any retrofit work completed. Under current city regulations, buildings that do not undergo any occupancy change or major renovation are never required to be retrofitted. For more information on these requirements see the article on our website, Seismic Upgrades of Existing Buildings.
Mandated URM Seismic Strengthening
In 2014, the PBEM began convening committees of architects, engineers, building owners, and the public to develop recommendations to reduce Portland’s URM risk. In August 2016, the URM Building Policy Committee publicly released a draft policy report outlining their proposed policy recommendations. The following is a broad outline of the policy recommendations in the August 2016 draft report, which may change before it is presented to the City Council for approval.
The proposed policy would institute mandatory URM upgrades within stipulated timeframes regardless of whether the building is to be renovated or not. In order to balance the need to mitigate seismic risk while minimizing hardship on building owners, URM buildings are categorized into five classes based on occupancy and risk level. All URM buildings would be required to have a seismic evaluation conducted within a 3 to 5 year timeframe using the ASCE 41-13 standard, and full seismic upgrades would have to be completed within a 10 to 25 year timeframe.
Proposed URM Classification System and Retrofit Requirements
|Classification||Approx. Num. of Buildings||Retrofit Requirements||Retrofit Timeline|
|Class 1: Critical buildings (critical facilities; police, fire, and emergency facilities; power, water, and utility facilities; hospitals; Risk Category IV structures||10||Full building evaluation (ASCE 41 Tier 3) to new building seismic force levelsStructural retrofit to Immediate Occupancy level for design earthquake, Life Safety for maximum considered earthquakeNon-structural retrofit to Operational level||3 years to assess 10 years for complete retrofit|
|Class 2: School buildings (Risk Category III structures)||85||Full building evaluation (ASCE 41 Tier 3) to reduced seismic force levels for existing buildingsStructural retrofit to Damage Control level for design earthquake, Limited Safety level for maximum considered earthquakeNon-structural retrofit to Position Retention level||3 years to assess 10 years to brace parapets, cornices, chimneys, and attach walls to roof 20 years for complete retrofit|
|Class 3: Larger URM Buildings (buildings with > 300 occupants; residential buildings with > 100 units; buildings with 4 or more stories)||220||Deficiency-based evaluation (ASCE 41 Tier 2) to reduced seismic force levels for existing buildingsStructural retrofit to Life Safety level for design earthquake, Collapse Prevention level for maximum considered earthquakeNon-structural retrofit to Life Safety level||3 years to assess 10 years to brace parapets, cornices, chimneys, and attach walls to roof 25 years for complete retrofit (+5 years for hardship)|
|Class 4: All other URM buildings not in Classes 1, 2, 3, or 5||1140||Deficiency-based evaluation (ASCE 41 Tier 2) to reduced seismic force levels for existing buildingsStructural retrofit to Life Safety level for design earthquake, Collapse Prevention level for maximum considered earthquakeNon-structural retrofit to Life Safety levelException: Class 4 buildings meeting certain requirements may be upgraded to the standards required of a Class 5 URM building||5 years to assess 10 years to brace parapets, cornices, chimneys, and attach walls to roof 25 years for complete retrofit (+5 years for hardship)|
|Class 5: Low occupancy structures (one- and two-story buildings with up to 10 occupants)||2780||Retrofit to Limited Performance Objective:
||5 years to assess 10 years for complete retrofit|
The vast majority of privately-owned URM buildings fall into classes 4 and 5 and are likely only to require the prescriptive upgrades outlined for class 5 URM buildings. Larger and more complicated buildings are likely to require ASCE 41 deficiency-based evaluation and retrofit. In both cases, building owners would be expected to conduct the ASCE 41 evaluations within 5 years (3 years for class 3 URM buildings) and complete the full seismic upgrades within 25 years (with the possibility of a 5 year extension for hardship).
The second-largest building owner in the city of Portland is Portland Public Schools, and they have a particularly large and old stock of URM buildings. Under the proposed policy recommendations, all of these URM school buildings would need a full-building ASCE 41 evaluation within 3 years, and a complete seismic upgrade within 20 years.
At present, these are draft policy recommendations that are subject to change. PBEM will be hosting two public forums in September 2016 to solicit public opinion on the proposed regulations. The final policy recommendation is expected to be submitted to the Portland City Council for adoption in May 2017.
Nishkian Dean has extensive experience in seismic evaluation and retrofit of URM structures. Feel free to contact us if you are a URM building owner facing upgrade requirements.
Edwin T. Dean, PE, SE is Vice President and Managing Principal of Nishkian Dean a structural engineering consulting firm in Portland, Oregon.
Chad Norvell, PE is a Project Engineer with Nishkian Dean a structural engineering consulting firm in Portland, Oregon.