Building Code Guides (BCGs) and Building Official Determinations (BODs) provide answers about the application of building codes and outline optional alternates. BODs are typically smaller in scope and address simpler questions. Code Guides provide a comprehensive alternative to a complicated set of code requirements.
BCG 23-11: URM Seismic Retrofits
QUESTION: ASCE 41 and structural design standard of practice requires complex analysis and demonstration of compatibility between new lateral elements and existing masonry components during the seismic upgrade of a URM building. As these requirements are disproportionately burdensome, is there an alternate method to demonstrate compatibility?
RESPONSE: Design professionals may follow the requirements of the Alternate Method outlined in this document; provided the structures are within the scope limitations of this document.
I. Introduction
Where new vertical elements are being added to resist lateral forces as part of seismic upgrade of a URM building, analysis requirements of ASCE 41 and structural design standard of practice requires distribution of lateral forces along a wall line of resistance, in proportion to the relative rigidities. The level of complexity required to demonstrate this compatibility between new lateral elements and existing masonry components is very burdensome and not proportionate to the level of analysis required on most projects. Moreover, uncertainties in accurately modelling the properties of these archaic URM components often lead to analysis with low level of confidence and questionable results.
Therefore, in lieu of demonstration of compatibility between the URM components and new vertical elements of the Seismic Force Resisting System, it is acceptable for a design professional to follow the requirements of the Alternate Method outlined in this document; provided the structures are within the scope limitations of this document. When all the requirements of this document are provided for, it is not required to demonstrate the adequacy of the masonry components along the line of seismic force resistance under consideration, and equivalency to the deformation compatibility requirements of ASCE 41 are deemed satisfied. By using the Alternate Method, the URM walls will be classified as “Non-structural” components as defined in ASCE41 (Nonstructural component: element of a building that are not intended as part of the lateral or the gravity system, and still may pose seismic hazard due to seismic accelerations or induced displacements).
II. Scope
The Alternate method presented in this document is applicable to buildings:
In Risk Category I, II and III when the seismic improvement standard is either ASCE 41 BPOE or BPON.
Where new vertical elements of the Seismic Force Resisting system are provided and these new elements are designed to take 100% of the required seismic loads.
Exception: if the existing URM walls are in good condition, and the diaphragm meets ASCE41 criteria as flexible, then such walls may be used to resist lateral loads and shall be analyzed as Primary Elements and meet all requirements of ASCE 41 including deformation compatibility requirements.
The Alternate method is NOT applicable for structures in Risk Category IV structures. Deformation compatibility must be demonstrated per ASCE 41-17 §7.2.3.3.
This Alternate method applies only as an alternate to provisions of ASCE 41 that relate to compatibility of new vertical elements of the lateral system and existing URM walls. All other applicable requirements of ASCE 41 or other standards apply as required by those standards.
III. Requirements
URM wall/components and new vertical elements shall meet all of the following:
Back-up gravity support at existing URM walls, piers and spandrels shall be provided for rafters, girders and joists and all other gravity load elements that are supported by the URM wall.
All new vertical elements of the Seismic Force Resisting system shall be designed to take 100% of the required forces. Lateral loads shall be distributed to new Seismic Force Resisting systems based on stiffness considering diaphragm rigidity.
Exception: if the existing URM walls are in good condition, and the diaphragm meets ASCE41 criteria as flexible, then such walls may be used to resist lateral loads and shall be analyzed as Primary Elements and meet all requirements of ASCE 41 including deformation compatibility requirements.
New elements and systems shall conform with, at a minimum, the detailing requirements and strength design procedures as prescribed in the applicablematerial standards referenced in Chapters 9 through 12 of ASCE 41 including “Ordinary systems”. For reinforced concrete walls,90-degree,135-degree or180-degreehooks shall be provided on horizontal reinforcing at wall ends. Additionally, seismic force- resisting systems that are Not Permitted in accordance with ASCE 7 for structures assigned to Seismic Design Category C shall not be used for new vertical elements.
In-plane drift including diaphragm deformation, at existing URM walls shall be limited to 0.75% considering only the new Seismic Force Resisting System and only those URM walls that are part of the Seismic Force Resisting System as allowed under exception in section III item 2. Stiffness of URM walls that are not designed as primary elements and do not meet the acceptance criteria shall be neglected in determining the drift. Drift shall be evaluated using BSE-1E (as defined in City Code Title 24.85) ground motions when the seismic upgrade is required to meet the ASCE 41 Basic Performance Objective for Existing Buildings (BPOE). Structures where the seismic upgrade is required to meet the ASCE 41 Basic Performance Objective Equivalent to New Buildings (BPON), the 0.75% drift limit shall be evaluated using BSE-1N ground motions.
All URM walls/piers/spandrels shall be braced for out-of-plane loading per the following requirements. Use of h/t ratios from ASCE 41 or net flexural stress analysis to demonstrate adequacy of URM walls for out-of-plane loading is not allowed. Vertical bracing shall be provided that satisfies all of the following requirements;
Bracing shall be designed for forces determined using equation (7-13) but not less than forces calculated using equation (7-14) of ASCE 41-17 section 7.2.11.2.
Bracing members shall be attached to the floor diaphragm or foundation below and the floor or roof diaphragm above to resist force demands in accordance with requirement (a) above.
Out-of-plane deflection of vertical bracing members at force demands from requirement (a) above shall not exceed one-tenth of the wall thickness.
Horizontal spacing of vertical bracing members shall not exceed the lesser of (i) one-half of the unsupported height of the wall (ii) 8 x wall thickness and (iii)10ft for Risk Category I and II structures or 4 ft o.c. for Risk Category III structures.
Bracing members shall be anchored to the walls at spacing not to exceed4ft
The following components in structures that have Weak story irregularity, Out-of- Plane Discontinuity irregularity, In-Plane discontinuity irregularity, or Torsional Strength Irregularity (as defined in ASCE 41 section 7.3) in the un-retrofitted configuration shall be evaluated (Based on LSP or LDP Tier 2 procedures). o.c. vertically.
Elements supporting discontinuous URM walls shall be evaluated as force-controlled elements in accordance with section 5.2.5 of ASCE 41 to verify their adequacy to support the maximum loads that can be delivered to the elements based on initial stiffness of URM and their ability to initially resist lateral loads.
Diaphragms shall be evaluated to verify their adequacy to transfer lateral forces based on initial stiffness of URM wall and their ability to initially resist lateral loads. Transfer diaphragms shall be evaluated as force-controlled elements.
HISTORY: Implemented December 14, 2023.
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