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CBC 2016

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Section 91.9601 Purpose

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This section has been amended at the state or city level.
The purpose of this division is to promote public safety and welfare by reducing the risk of death or injury that may result from the effects of earthquakes on reinforced concrete and masonry wall buildings with flexible diaphragms designed under the building codes in effect prior to January 1, 1995. These buildings are potentially hazardous and prone to significant damage, including possible collapse, in a moderate to major earthquake. These structures typically shelter large numbers of persons and property for retail, food markets, food distribution centers, warehousing, aerospace, industrial/manufacturing and general business and office use. Their continued use after an earthquake is also essential to the local economy and its post-earthquake recovery.

The provisions of this division are minimum standards for structural seismic resistance established primarily to reduce the risk of loss of life or injury on both subject and adjacent properties and will not necessarily prevent all earthquake damage to an existing building which complies with these standards. This division shall not require existing electrical, plumbing, mechanical or fire-safety systems to be altered unless they constitute a hazard to life or property.

This division provides voluntary retrofit standards for deficient wall anchorage systems on structures that are not subject to the mandatory provisions of Division 91, Article 1, Chapter IX of the LAMC. When fully followed, these standards will strengthen the portion of the structure that is most vulnerable to earthquake damage.

Section 91.9602 Scope

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This section has been amended at the state or city level.
The voluntary provisions of this division shall apply to existing buildings of the following types:

  1. Cast-in-place reinforced concrete or masonry wall buildings with flexible diaphragms designed under building codes in effect prior to January 1, 1995.
  2. Tilt-up concrete wall buildings with flexible diaphragms designed under the building codes in effect prior to January 1, 1995, but after January 1, 1976.
All tilt-up concrete wall buildings designed under the Building Code in effect prior to January 1, 1976, are subject to the mandatory provisions of Division 91, Article 1, Chapter IX of the LAMC. All existing reinforced masonry or concrete buildings with flexible diaphragms, including tilt-up concrete wall buildings, designed under the Building Code in effect on or after January 1, 1995, shall be designed in conformance with Division 16, Article 1, Chapter IX of the LAMC.

Section 91.9603 Definitions

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This section has been amended at the state or city level.
For the purposes of this division, the applicable definitions in CBC Sections 1602, 1613.2, 1902 and 2302; ASCE 7 Sections 1.2, 3.1, 4.1 and 11.2; and the following shall apply:

ANCHORAGE SYSTEM is the system of all structural elements and connections, which support the concrete or masonry wall in the lateral direction, including diaphragms and subdiaphragms, wall anchorage and continuity or cross tie connectors in subdiaphragms and main diaphragms.

COMMENCED CONSTRUCTION is construction pursuant to a valid building permit that has progressed to the point that one of the called inspections as required by the Department has been made and the work for which the inspection has been called has been judged by the Department to be substantial and has been approved by the Department.

EXISTING BUILDING is an erected building for which a legal building permit and a certificate of occupancy have been issued.

FLEXIBLE DIAPHRAGM is any diaphragm constructed of wood structural panel, diagonal or straight wood sheathing, metal decking without a structural concrete topping, or horizontal rod bracing.

HISTORICAL BUILDING is any building designated or currently in the process of being designated as a historical building by an appropriate federal, state or City jurisdiction.

REINFORCED CONCRETE WALL. A concrete wall that has 50% or more of the reinforcing steel required for reinforced concrete in Division 19, Article 1, Chapter IX of the LAMC.

REINFORCED MASONRY WALL is a masonry wall that has 50 percent or more of the reinforcing steel required by Section 2.3 or Section 3.3 of ACI 530-05/ASCE 5-05/TMS 402-05 (MSJC).

RETROFIT is strengthening or structurally improving the lateral force resisting system of an existing building by alteration of existing or addition of new structural elements.

TILT-UP CONCRETE WALL is a form of precast concrete panel construction either cast in the horizontal position at the site and after curing, lifted and moved into place in a vertical position, or cast off-site in a fabricator's shop.

Section 91.9604 Analysis and Design

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This section has been amended at the state or city level.

91.9604.1 Wall Panel Anchorage

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This section has been amended at the state or city level.
Concrete and masonry 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.

EXCEPTION: Using 75% of the design force as specified in Section 12.11 of ASCE 7 and completely complying with all the requirements as specified in Section 12.11 of ASCE 7 is considered equivalent to the requirements specified in this LAMC section and LAMC Subsection 91.9604.2.

The required anchorage shall be based on the tributary wall panel assuming simple supports at floors and roof.

EXCEPTION: An alternate design may be approved by the Superintendent of Building when justified by well established principles of mechanics.

91.9604.2 Special Requirements for Wall Anchors and Continuity Ties

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This section has been amended at the state or city level.
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 specified in CBC Section 1912, using a load factor of 2.0 in lieu of 1.4 for earthquake loading, shall be used for the design of embedment in concrete.

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 seismic loading. Criteria for analysis and testing shall be determined by the Superintendent of Building.

Expansion anchors are not allowed without special approval of the Superintendent of Building. 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 analysis are performed, which establish shear values for the attachment perpendicular to the edge of the deck.

91.9604.3 Development of Anchor Loads Into the Diaphragm

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This section has been amended at the state or city level.
Development of anchor loads into roof and floor diaphragms shall comply with 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 LAMC Subsection 91.1613.5.3 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.

91.9604.4 Anchorage at Pilasters

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This section has been amended at the state or city level.
Anchorage of pilasters shall be designed for the tributary wall anchoring load per LAMC Subsection 91.9604.1, 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 LAMC Subsection 91.9604.1.

91.9604.5 Symmetry

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This section has been amended at the state or city level.
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.

91.9604.6 Minimum Roof Member Size

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This section has been amended at the state or city level.
Wood members used to develop anchorage forces to the diaphragm shall be of minimum three-inch nominal width for new construction and replacement. All such members must be designed for gravity and earthquake forces as part of the wall anchorage system. For existing structural members, the allowable stresses shall be without the 1/3 stress increase per LAMC Subsection 91.9604.2.

91.9604.7 Combination of Anchor Types

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This section has been amended at the state or city level.
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.

91.9604.8 Prohibited Anchors

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Usage of connectors that were bent or stretched from the intended use shall be prohibited.

91.9604.9 Crack and Damage Repairs, Evaluation of Existing Structural Alterations

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This section has been amended at the state or city level.
The engineer or architect shall report any observed structural conditions and structural damage that have imminent life safety effects on the buildings and recommend repairs. This includes alterations such as openings cut in existing wall panels without a building permit. Evaluations and repairs shall be reviewed and approved by the Department.

91.9604.10 Miscellaneous

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This section has been amended at the state or city level.
Existing mezzanines relying on the concrete or masonry walls for vertical 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 LAMC Subsections 91.9604.1, 91.9604.2 and 91.9604.3.

EXCEPTION: Existing mezzanines that have independent lateral and vertical support need not be anchored to the concrete or masonry walls.

Existing interior masonry or concrete walls not designed as shear walls, which extend to the floor above or to the roof diaphragm shall also be anchored for out-of-plane forces per LAMC Subsections 91.9604.1, 91.9604.2 and 91.9604.3. 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.

91.9604.11 Historical Buildings

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This section has been amended at the state or city level.
Qualified historical buildings shall be permitted to use alternate building standards or deviations from this division in order to preserve their original or restored architectural elements and features. See LAMC Section 91.2.1200 for these alternatives.

Section 91.9605 Materials of Construction

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This section has been amended at the state or city level.
All materials permitted by this Code, including their appropriate allowable stresses and those existing configurations of materials specified in Division 88, Article 1, Chapter IX of the LAMC, may be utilized to meet the requirements of this division.

Section 91.9606 Information Required on Plans

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This section has been amended at the state or city level.

91.9606.1 General

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In addition to the seismic analysis required elsewhere in this division, the licensed engineer or architect responsible for the seismic analysis of the building shall record the information required by this section on the approved plans.

91.9606.2 Information Required

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This section has been amended at the state or city level.
The plans shall accurately reflect the results of the engineering investigation and design and show all pertinent dimensions and sizes for plan review and construction. The following shall be provided:

  1. Floor plans and roof plans shall show the existing framing construction, diaphragm construction, proposed wall anchors, cross-ties and collectors. Existing nailing, anchors, ties and collectors shall also be shown on the plans if these are part of the design, and these structural elements need to be verified in the field.
  2. At elevations where there is alterations or damage, the details shall show the roof and floor heights, dimensions of openings, location and extent of existing damage, and proposed repair.
  3. Typical concrete or masonry wall sections with wall thickness, height, and location of anchors shall be provided.
  4. Details shall include the existing and new anchors and the method of development of anchor forces into the diaphragm framing; existing and new cross-ties, existing and new or improved support of the roof and floor girders at pilasters or walls.

91.9606.3 Engineer's or Architect's Statement

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This section has been amended at the state or city level.
The responsible engineer or architect shall state on the approved plans, the following:
  1. "I am responsible for this building's seismic strengthening design of the tilt-up concrete wall anchorage system in compliance with the minimum seismic resistance standards of Division 96, Article 1, Chapter IX of the LAMC."
or when applicable:

  1. "The Registered Deputy Inspector, required as a condition of the use of structural design stresses requiring continuous inspection, will be responsible to me as required by LAMC Section 91.1704."
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