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Structural Advisory Committee Meeting June 17, 2020

Public Meeting

Available Online

Agenda

  1. Introductions
  2. Committee Members terms
  3. Update Title 24.85 to reference ASCE 41- 17 instead of ASCE 41-13.
  4. Introduction to proposed new simplified Alternate Method to Demonstrate Deformation Compatibility on URM Buildings with New Lateral System Using ASCE 41

Meeting Minutes for Structural Advisory Committee Meeting June 17, 2020

Committee members present: David Nilles, Kent Yu, Trent Nagele, Mark Tobin, Clinton Ambrose, Josh McDowell

City Staff present: Amit Kumar, Eric Thomas  

Meeting called to order at 2:04 PM

1)  Introductions

2)  Amit Kumar reminded the members that their terms currently expire on June 26,2021with the exception of David Nilles whose term expires August 31,2020. Members serve 3-year terms with a maximum of two consecutive terms.

3)  The committee discussed City’s proposal to update Title 24.85 to reference ASCE 41-17 in lieu of the current referenced standard ASCE 41-13. The committee also discussed the need to retain the current provision in Title 24.85 that limits the reduction in ground motions associated with BSE-1E and BSE-2E and requires that BSE-1E and BSE-2E acceleration parameters cannot be taken less than 75% of BSE-1N and BSE -2N accelerations respectively. This restriction was adopted on the advice of the pervious Structural Advisory committee as it was felt that the reduction in ground motions resulting from the use of BSE-1E and BSE -2E in ASCE 41-13 were too large and not intended by ASCE 41. The new edition of the standard, ASCE 41-17 that is being considered for adoption corrects this concern to large extent, but it was felt that there are some instances where this could still potentially be a concern. Some areas that this could potentially be an issue is with Risk category III and IV structures, URM  out-of-plane analysis and non-structural components. Amit Kumar informed the committee that this issue was also studied by the SEAO Seismic and Vintage building committee. SEAO recommended that the City maintain the current restrictions on BSE-1E and BSE-2E though there was a minority opinion among some members that these restrictions should be removed for simplicity.
After some discussion, the Structural Advisory committee voted unanimously to recommend (1) Update Title 24.85 to reference ASCE 41-17 in place of the current reference standard ASCE 41-13 and (2) Keep in place current restrictions on the reduction of BSE-1E and BS- 2E ground motions.

4) Amit Kumar introduced a document that the City is considering currently titled  “Alternate Method to Demonstrate Deformation Compatibility on URM Buildings with New Lateral System Using ASCE 41”. This document was developed in response to request from practicing engineers for a simplified process in lieu numerical modelling of existing and new elements to demonstrate compatibility of the URM and new elements as currently required by ASCE 41.  There is no or very limited guidance in properly modelling URM walls, spandrels and piers, their connections, degradation properties etc. Even if this was modeled, there is little confidence in the results. For many structures such a rigorous exercise may not be justified for projects of certain limited scope.  
The goal was to provide a simplified procedure to meet the intent of the code instead of requiring a full-blown numerical analysis. The engineer always has the option of doing the full-blown analysis if they wish.

In order to demonstrate that the seismic upgrade met the intent of ASCE 41 for life safety performance for Risk Category I and II structures intent of the code, the following must be demonstrated:

Buildings in Risk Category I or II

  1. Provide back-up gravity support at all URM walls.
  2. Limit drift at existing URM walls to 0.7% or less under BSE-1E (as defined in Title 24.85), considering only the new lateral system and neglecting contribution of the existing URM walls.
  3. URM walls/piers shall meet ASCE 41-17 §11.3.3.3.1 for Collapse Prevention (i.e. h/t ratios) for out-of-plane action, or the wall/pier and spandrel shall be retrofitted with a back-up system for out-of-plane bracing
  4. For BPON or Tier 3 BPOE upgrades, additional check shall be done to verify URM walls/piers meet ASCE 41-17 §11.3.3 for Life Safety, or the wall/pier and spandrel shall be retrofitted with a back-up system for out-of-plane bracing

Exception: When a new lateral system is provided that resists 100% of lateral load, and out-of-plane bracing is provided at all URM walls/piers/spandrels, items 2 through 4 need not be checked.

Buildings in Risk Category III

When the required upgrade standard per COP Title 24.85 is BPOE or BPON:

  1. Provide back-up gravity support at all URM walls.
  2. Limit drift at existing URM walls to 0.7% or less under BSE-1E (as defined in Title 24.85), considering only the new lateral system and neglecting contribution of the existing URM walls.
  3. Provide out-of-plane bracing for all walls/piers/spandrels.

Buildings in Risk Category IV

No prescriptive provisions—must demonstrate deformation compatibility per ASCE 41-17 §7.2.3.3.

The goal was to introduce this document to the committee for further discussion at the next meeting. Structural Engineers Association of Oregon’s ,  Seismic and Vintage Building sib-committee’s are also reviewing this document and will be providing recommendations to the City. Structural Advisory committee members expressed general initial support for the document and the need for simplified analysis at least for certain type of structures.

Meeting concluded at 3:05 PM