SEC. 91.9108. ANALYSIS AND DESIGN.
(Amended by Ord. No. 171,175, Eff. 7/25/96.)
For the purpose of this section, “anchorage system(s)” shall mean all structural elements, which supports the wall in the lateral direction, including wall anchorage and continuity tie (cross-tie) connectors in subdiaphragms and main diaphragms for retrofit and repairs.
91.9108.1. Wall Panel Anchorage. Concrete walls shall be anchored to all floors and roofs which provide lateral support for the wall. The anchorage shall provide a positive direct connection between the wall and floor or roof construction capable of resisting a horizontal force equal to 30 percent of the tributary wall weight for all buildings, and 45 percent of the tributary wall weight for essential buildings, or a minimum force of 250 pounds per linear foot of wall, whichever is greater. The required anchorage shall be based on the tributary wall panel assuming simple supports at floors and roof.
EXCEPTION: Alternate design may be approved by the Superintendent when justified by well established principles of mechanics.
91.9108.2. Special Requirements for Wall Anchors and Continuity Ties. (Amended by Ord. No. 186,488, Eff. 12/27/19.) The steel elements of the wall anchorage systems and continuity ties shall be designed by the allowable stress design method using a load factor of 1.7. The 1/3 stress increase permitted by CBC Section 1605.3.1.1 shall not be permitted for materials using allowable stress design methods.
The strength design shall be per Section 17.2.3 of ACI 318-14, unless modified by CBC Section 1905.1.8.
Wall anchors shall be provided to resist out-of-plane forces, independent of existing shear anchors.
EXCEPTION: Existing cast-in-place shear anchors may be used as wall anchors if the tie element can be readily attached to the anchors and if the engineer or architect can establish tension values for the existing anchors through the use of approved as-built plans or testing, and through analysis showing that the bolts are capable of resisting the total shear load while being acted upon by the maximum tension force due to earthquake. Criteria for analysis and testing shall be determined by the Superintendent.
Expansion anchors are not allowed without special approval of the Superintendent. Attaching the edge of plywood sheathing to steel ledgers is not considered as complying with the positive anchoring requirements of the Code; and attaching the edge of steel decks to steel ledgers is not considered as providing the positive anchorage of this Code unless testing and/or analysis are performed, which establish shear values for the attachment perpendicular to the edge of the deck.
91.9108.3. Development of Anchor Loads into the Diaphragm. (Amended by Ord. No. 188,797, Eff. 12/31/25.) Development of anchor loads into roof and floor diaphragms shall comply with CBC Section 1616.33 and Section 12.11 of ASCE 7.
EXCEPTION: If continuously tied girders are present, then the maximum spacing of the continuity ties is the greater of the girder spacing or 24 feet (7315 mm).
In wood diaphragms, anchorage shall not be accomplished by use of toe nails or nails subject to withdrawal, nor shall wood ledgers, top plates or framing be used in cross-grain bending or cross-grain tension. The continuous ties required by CBC Section 1616.3.3 and Section 12.11 of ASCE 7 shall be in addition to the diaphragm sheathing.
Lengths of development of anchor loads in wood diaphragms shall be based on existing field nailing of the sheathing unless existing edge nailing is positively identified on the original construction plans or at the site.
At reentrant corners, continuity collectors may be required for existing return walls not designed as shear walls, to develop into the diaphragm a force equal to the lesser of the rocking or shear capacity of the return wall, or the tributary shear, but not exceeding the capacity of the diaphragm. Shear anchors for the return wall shall be commensurate with the collector force. If a truss or beam, other than rafters or purlins, is supported by the return wall or by a column integral with the return wall, an independent secondary column, is required to support the roof or floor members whenever rocking or shear capacity of the return wall is governing.
Seismic deflection shall be determined at the return walls, and fins/canopies at entrances, to ensure deflection compatibility with the diaphragm, by either seismically isolating the element or attaching the element and integrating its load into the diaphragm.
91.9108.4. Anchorage at Pilasters. (Amended by Ord. No. 172,592, Eff. 6/28/99, Oper. 7/1/99.) Anchorage of pilasters shall be designed for the tributary wall anchoring load per Section 91.9108.1 of this Code, considering the wall as a two-way slab. The edge of the two-way slab shall be considered “fixed” when there is continuity at pilasters, and considered “pinned” at roof or floor levels. The pilasters or the walls immediately adjacent to the pilasters shall be anchored directly to the roof framing such that the existing vertical anchor bolts at the top of the pilasters are by-passed without causing tension or shear failure at the top of the pilasters.
EXCEPTION: If existing vertical anchor bolts at the top of the pilasters are used for the anchorage, then additional exterior confinement shall be provided.
The minimum anchorage at a floor or roof between the pilasters shall be that specified in Section 91.9108.1 of this Code.
91.9108.5. Symmetry. Symmetry of connectors in the anchorage system is required. Eccentricity may be allowed when it can be shown that all components of forces are positively resisted and justified by calculations or tests.
91.9108.6. Minimum Roof Member Size. Wood members used to develop anchorage forces to the diaphragm must be at least 3x for new construction and replacement. All such members must be checked for gravity and earthquake as part of the wall anchorage system. For existing buildings, the member check shall be without the 1/3 stress increase per Section 91.9108.2.
91.9108.7. Combination of Anchor Types. To repair and retrofit existing buildings, a combination of different anchor types of different behavior or stiffness shall not be permitted. The capacity of the new and existing connectors cannot be added.
91.9108.8. Prohibited Anchors. Usage of connectors that were bent and/or stretched from the intended use shall be prohibited.
91.9108.9. Crack and Damage Repairs, Evaluation of Existing Structural Alterations. The engineer shall report any observed structural conditions and structural damage that have imminent life safety effects on the buildings and recommend repairs. Evaluations and repairs shall be reviewed and approved by the Department. The engineer shall also evaluate the effects of alterations such as openings cut in existing wall panels without a permit, that may present immediate life safety hazard and correct when necessary.
91.9108.10. Miscellaneous. Existing mezzanines relying on the tilt-up walls for vertical and/or lateral support shall be anchored to the walls for the tributary mezzanine load. Walls depending on the mezzanine for lateral support shall be anchored per Sections 91.9108.1, 91.9108.2 and 91.9108.3.
EXCEPTION: Existing mezzanines that have independent lateral and vertical support need not be anchored to the walls.
Existing interior masonry or concrete walls not designed as shear walls, that extend to the floor above or to the roof diaphragm shall also be anchored for out-of-plane forces per Sections 91.9108.1, 91.9108.2 and 91.9108.3 of this Code. In the in-plane direction, the walls may be isolated or shall be developed into the diaphragm for a lateral force equal to the lesser of the rocking or shear capacity of the wall, or the tributary shear but not exceeding the diaphragm capacity.
