Chapter 24.85 - Seismic Design Standards for Existing Buildings - Information

Read about some frequently asked questions and clarifications requested on Chapter 24.85 - Seismic Design Standards for Existing Buildings.
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First adopted in 1994, Portland City Code Chapter 24.85 “Seismic Design Standards for Existing Buildings” contains requirements for upgrade to the lateral systems of existing buildings when the buildings are undergoing change of use or occupancy or are being altered. In addition, for Unreinforced Masonry Buildings (URM’s) undergoing alterations or repair, Section 24.85.065 has triggers associated with the cost of repair or alteration which, if exceeded, would require mandatory seismic strengthening.

In 2004, Chapter 24.85 underwent a major revision. This revised version adopted in October 2004, serves as the governing code for seismic design for existing buildings.

What are the limitations on the use of Sxs and Sx1 acceleration parameters associated with BSE-1E and BSE -2E ground motions in Chapter 24.85? 

BSE-1E is the Basic Safety Earthquake-1 for use with the Basic Performance Objective for Existing Buildings, taken as a seismic hazard with a 20 percent probability of exceedance in 50 years except that the design spectral response acceleration parameters Sxs and Sx1 for BSE-1E seismic hazard level shall not be taken as less than 75 percent of the respective design spectra response acceleration parameters obtained from BSE-1N seismic hazard level

BSE-2E is the Basic Safety Earthquake-2 for use with the Basic Performance Objective for Existing Buildings, taken as a seismic hazard with a 5 percent probability of exceedance in 50 years except that the design spectral response acceleration parameters of Sxs and Sx1 for BSE-2E seismic hazard level shall not be taken as less than 75 percent of the respective design spectra response acceleration parameters obtained from BSE- 2N Seismic hazard level and may not be greater than BSE-2N at a site.

If I am providing a new lateral system designed to resist 100% of the seismic force, can I exclude the existing elements from my model and evaluation of the building for lateral loads? 

Lateral forces are distributed to the various building components based on their initial stiffness and deformation compatibility regardless of whether they were specifically intended or not to be part of the lateral-force resisting system. Based on their stiffness, the existing elements may see loads or deformations that exceed their capacity resulting in failure if not considered in the design.

Whether elements are to be used in a mathematical model depends on the type of analysis procedure to be used.

In accordance with Section of ASCE 41, “components that affect the lateral stiffness or distribution of forces in a structure or are loaded as a result of lateral deformation of the structure.   shall be classified as primary or secondary components, even if they are not intended to be part of the seismic-force-resisting system.” Primary components are those that are required to resist seismic forces and accommodate deformations as opposed to secondary components that are not required to resist seismic forces but are designed /detailed to accommodate deformations to achieve the selected performance level. 

In accordance with Section of ASCE 41, when linear analysis procedures are used, secondary components may be neglected in the mathematical model, if however the total initial lateral stiffness of all the secondary components in a building exceed 25% of the initial stiffness of all the primary components then some secondary components shall be reclassified as primary to reduce the total stiffness of secondary components to less than 25% of the primary components or if the inclusion of secondary components increases the force or deformation demands on a primary component then the secondary component shall be re-classified as a primary element. Note that even if a secondary component is neglected in the mathematical model, they still need to maintain their strength at the required deformation in accordance with the acceptance criteria of of ASCE 41.

For non-linear procedures both primary and secondary components shall be included in the mathematical models. 

There is a building on a separate property adjacent to the building being seismically retrofitted and there is not enough separation between the two buildings to meet the minimum separation in accordance with the requirements of Section of ASCE 41. How can I address this deficiency? 

Where possible a seismic joint must be introduced between the two structures. Creation of the seismic joint shall not reduce the existing lateral or vertical load carrying capacity of the adjacent building unless the impacted adjacent building elements in line with the seismic joint are also independently retrofitted for vertical and lateral loads. The building being upgraded shall be provided with an independent lateral seismic-force-resisting system and vertical gravity support system.

When it is not possible to create a seismic joint or provide adequate separation between adjacent structures on different properties, pounding effects need to be considered on the building being upgraded. For structures in Risk category I and II where the Structural performance standard is Life Safety or lower, a simplified rational analysis acceptable to the City of Portland that demonstrates that the retrofitted structure and it’s components are capable of resisting and transferring all required forces due to pounding and that any damaged elements do not pose a falling hazard to occupants both inside or outside the building may be provided. In the past, such simplified analysis by engineering firms have included, among other things adding 25% of the adjacent building’s floor mass at each level where pounding is likely to occur to the seismic mass of the floor of the retrofitted building at those levels and also addressing any localized affects of impact of the adjacent floor. For example where floors of the two buildings do not align, strongbacks may need to be provided to resist the impact loads the adjacent building floors. The strongbacks are detailed to transfer such impact loads into the retrofitted building with a complete load path.

I am retrofitting an existing building that has multiple additions and different seismic resisting systems that are not separated. Can I treat each building/addition separately and retrofit them as independent structures? 

Buildings on the same property that are not adequately separated from each other or share common elements, other than foundation elements, shall either be thoroughly tied together so that they behave as an integral unit (Section of ASCE41) and be evaluated as one integral unit OR they shall be completely separated by introducing seismic joints between the structures  in accordance with Section and 7.2.13 of ASCE 41

I have a building for which an alteration and/or change of occupancy is being proposed. The building is composed of multiple structures/portions of different vintage and construction. What building area, occupancy and cost is to be used to determine if any seismic triggers per Chapter 24.85 are being exceeded? 

The structures/portions that are seismically separated from other portions with a seismic joint and have their own independent code-required means of egress are treated as  separate structures  and can be evaluated separately for the purposes of determining the occupant load, occupancy classification or cost to determine if any seismic upgrades per Chapter 24.85 are triggered. If an upgrade is triggered, then only the independent portion(s) that triggers the seismic upgrade will need to be retrofitted. Portions that are not separated by a seismic joint or do not have separate means of exits/egress shall be treated as one structure for the purposes of determining the occupant load, occupancy classification or cost to determine if any seismic upgrades per Chapter 24.85 are triggered. If an upgrade is triggered, then all the portions that are treated as one structure will need to be seismically upgraded.  

When an existing building is undergoing a change of occupancy or change of use and this change involves multiple or mixed occupancies, can you clarify how to determine whether a seismic upgrade is triggered due to this change of occupancy or use? 

Per Section 24.85.040 of PCC Chapter 24.85 a seismic upgrade is triggered when a building undergoes a change of use or change of occupancy and the resulting change results in either an increase in occupant load of greater than 149 OR if the occupancy of more than 1/3 the building’s net area is changed to a higher relative hazard classification as defined in Table 24.85-A. The changes in occupant load or occupancies in relative hazard classification are compared against the legal baseline occupancies as of October 1,2004. When establishing the change of occupancy count, the occupant load factors allowed under current code should be applied to both the baseline and proposed uses.

Consider the following example for a hypothetical building:  

Baseline Occupancy as of October 1, 2004 

Occupancy TypeSq. FootageHazard Level per Table 24.8-AOccupant count
B200032000/150= 13
M100031000/60= 17
S-2350023500/500= 7

Proposed Occupancy:

Occupancy TypeSq. FootageHazard Level per Table 24.85-AOccupant count
B200032000/150= 13
S-1150021500/300= 5
E200052000/20 = 100
R-2100041000/200= 5
Occupancies by Hazard level per Table 24.85-ASq Footage of 2004 Baseline occupancies  (sq.ft)Sq. Footage of Proposed occupancies (sq.ft)Net change (sq.ft)% Increase
502000+20002000/6500= 30.8%
401000+10001000/6500= 15.4%
32000+1000 =30002000-1000No net Increase
235001500-2000No net Increase
1000No net Increase
Totals65006500-46.2 %

Net increase in occupant count = 123-37 = 86 which is less than 150.  Therefore, a seismic upgrade is not triggered by the occupant load increase trigger

Percentage of building floor area changed to a Higher Hazard classification per Table 24.85-A = 46.2% which is greater than 33.3% of the building area therefore a seismic upgrade is triggered by the floor area trigger.

See next question for an example on how to determine the Standard of Improvement.

When an existing building undergoing a change of occupancy or change of use is required to be seismically upgraded due to provisions of Section 24.85.040, and this change involved multiple or mixed occupancies, what seismic improvement standard is required by Section 24.85.040?

 When seismic upgrades are triggered due to proposed alterations/improvements under Section 24.85.040 of PCC Chapter 24.85 in buildings with multiple or mixed occupancies the seismic improvement standard shall be determined as follows: 

a. Buildings classified as Occupancy Risk Category III or IV, per OSSC, shall meet the OSSC or ASCE 41 BPON Seismic Improvement Standard.

b. Buildings assigned to Risk Categories I and II shall comply with the improvement standard associated with the relative hazard category that has most of the occupants or most of the building area assigned to it after the change of occupancy, whichever is more restrictive. Occupant loads shall be calculated based on occupant load factors assigned to each space in accordance with OSSC Chapter 1004. 

Example. The example below explains how to determine the applicable upgrade standard when a building permit includes changing the existing occupancy to multiple or mixed classifications within the same structure.

Example: Three different occupancies, A, B, and R-1 are being proposed for an existing building (Risk Category II).  What seismic design standards apply?

In this scenario the applicant is proposing 200 occupants for the “A” occupancy with a cumulative building area of 3,000 sq. ft., The “B” occupancy will have of 100 occupants and include 10,000 sq.  R-1 will have 25 occupants with an associated building area of 5,000 sq. ft.

According to Table 24.85-A provided below, “A” occupancies have a relative hazard classification 5, “R-1” has a relative hazard classification 4 and “B” occupancy have a relative hazard classification 3.  Table below, illustrates the proposal and the corresponding hazard classification.

OccupancyTotal Building Area for Occupancy TypeOccupant Load FactorOccupantsHazard Classification Table 24.85 - A
A3,000 sq. ft.152005
B10,000 sq. ft.150673
R-15,000 sq. ft.200254

Provides a hypothetical situation to explain what seismic upgrade standards apply when a development proposal includes multiple occupancies with different hazard classifications.

In this example occupancies A and R-1 combined, have the highest number of occupants, 225 (200 from A occupancy + 25 from R-1).  Since A and R-1 have a hazard classification of 5 and 4 they would require an ASCE-41 BPON or current OSSC upgrade (Table 24.85-A).

Occupancy B will encompass the most building area, 10,000 square feet.  This is more than the combined square footage of A and R-1 occupancies, 8,000 sq. (3000 sq. ft. from A + 5000 sq. ft. from R-1).  Occupancy B has a relative hazard classification 3 and the most building area assigned. Based solely on building area, the upgrade standard would be ASCE 41-BPOE.  However, since the more restrictive seismic improvement standard applies, the greater occupant load with hazard classifications 4 and 5 applies.  Therefore, the upgrade standard for the entire building will be ASCE-41 BPON or current OSSC.

When Chapter 24.85 underwent a revision in 2004, it incorporated monetary triggers associated with the cost of repair or alteration of URM buildings which, if exceeded, would require mandatory seismic strengthening. Furthermore, these dollar figures are required to be adjusted annually. What are the current monetary triggers? 

When revised in 2004, Chapter 24.85 established cost triggers which if exceeded would require mandatory seismic strengthening. These triggers were set at $40 per square foot for a single story URM building and $30 per square foot for two stories or greater. For URM buildings with special hazards, the limit was set at $30 per sq. ft. regardless of the size. These triggers are usually adjusted annually in first quarter each year using the RS means construction cost index. To get the most updated cost information please visit the Commercial Structures webpage.

Section 24.85.065(B) states “....When the cost of alteration or repair work which requires a building permit in a 2 year period exceeds the following criteria, then the building shall be improved to resist seismic forces such that the entire building conforms to the ASCE 41 improvement standard.”  When does the two-year period to determine the costs of permits begin and end, and what permits need to be counted? 

At the time an application is submitted for a permit (any permit) on a Unreinforced Masonry Building,  the valuation of all “issued” permits from two years prior to the date of application of the current permit, including the valuation of the current permit, shall be added up to determine the total cost. These costs minus any costs allowed to be deducted per Section 24.85.065B(3) shall be used to determine if any of the triggers in Section 24.85.065 are exceeded. If revision permits are part of the issued permits in the two-year period described above and these revision permit valuations add to the value of the original permit, then, in addition to the valuation of the revision permit, the valuation of the original permit shall be included in the costs for the purpose of determining if any seismic triggers are exceeded.

Can exclusions allowed under Section 24.85.060 like mechanical, electrical, plumbing work be excluded from the costs used to determine if seismic upgrade is required under Section 24.85.065?

No. Exclusions listed in Section 24.85.060 mechanical, electrical, plumbing work do not apply to seismic triggers when determining if seismic strengthening of URM buildings is required under provisions of Section 24.85.065. The only exclusions are listed in Section 24.85.065B(3). Exclusions listed in Section 24.85.060 apply only to costs used to determine if a seismic evaluation of a building is required.

How is the valuation of the work confirmed for the purposes of determining if seismic strengthening is required by Section 24.85.065(B)? 

For all work in unreinforced masonry buildings that requires a permit, a cost estimate for the proposed work may be required to confirm the valuation. The estimate must provide a detailed estimated cost for the entire scope of work proposed including cost for the work that can be excluded per Section 24.85.065B (3) of Chapter 24.85. The final valuation will be the total value minus the excluded costs. The valuation should reflect the fair market value of all labor and material costs. At the discretion of the Bureau Director or their designee, the Bureau of Development Service may require confirmation of the estimate by an independent third-party cost estimator.  

Can Seismic strengthening or upgrades be phased over several years?

In accordance with Section 24.85.070, The Director at his or her discretion may approve a multi-year phased program of seismic improvements when the improvements are pre-designed, and an improvement/implementation plan is approved by the Director. Approval of request for phasing of seismic improvements is not guaranteed.

The maximum total time allowed for completion of phased improvements under Section 24.85.070 is ten years. However, based on several factors such as use and occupancy of the building, critical risk to life safety etc., it may be necessary to require completion of the strengthening work over a shorter time frame. In addition, strengthening of certain critical elements (such as the attachment of exterior walls in URM buildings to floor/roof diaphragms when there are no existing connections) may not be deferred, unless a compelling and extenuating circumstances are present.

More information on phasing of seismic upgrades and the process can be found  in the document Phased Seismic Upgrades - Information. 

Can a building be strengthened or upgraded to resist seismic forces on a voluntary basis?

A building may be strengthened to resist seismic forces on a voluntary basis. The owner may determine the scope of the voluntary strengthening. The scope can include partial or full seismic strengthening as long as the proposed upgrades do not decrease the seismic resistance of the building such that the building would be more hazardous than it currently is.  The drawings submitted for a permit for voluntary seismic strengthening shall contain a written narrative summarizing the building lateral system, scope of seismic strengthening, the standard used for strengthening and remaining deficiencies. Please see section 24.85.065 of the Chapter 24.85 of COP Title 24 for additional information.

Voluntary seismic upgrades or strengthening may be phased over a period of time under a Phased Seismic Agreement.