SEC. 91.9406. ANALYSIS AND DESIGN.
(Amended by Ord. No. 182,850, Eff. 1/3/14.)
91.9406.1. General. Every hillside building within the scope of this division shall be analyzed, designed, and constructed in accordance with the following provisions.
91.9406.1.1. Base for Seismic Design Defined. (Amended by Ord. No. 179,324, Eff. 12/10/07, Oper. 1/1/08.) The base for seismic design is defined as follows:
1. Downhill-Direction. For seismic forces acting in the downhill-direction, the base of the building shall be the floor at or closest to the top of the highest level of the foundation.
2. Normal to the Downhill-Direction. For seismic forces acting normal to the downhill-direction, the distribution of seismic forces over the height of the building using Section 12.8.3 of ASCE 7 shall be determined using the height measured from the top of the lowest level of the building foundation. Retrofitting, however, shall only be required at the base-level-diaphragm and below.
91.9406.1.2. Design Base Shear. The design base shear shall be that required at the time of the original building permit, or not less than 75 percent of the currently required by Section 12.8.1 of ASCE 7, but in no case not less than the following:
V= 0.133 W
Where:
V = The total design lateral force or shear at the base.
W = The total seismic dead load defined in Section 12.7.2 of ASCE 7.
91.9406.2. Base Shear Resistance in the Downhill-Direction.
91.9406.2.1. General. The base shear in the downhill-direction, including forces from the base-level-diaphragm, shall be resisted through primary anchors from diaphragm struts or collectors provided in the base level framing to the foundation.
91.9406.2.2. Location of Primary Anchors. A primary anchor and diaphragm strut or collector shall be provided in line with each foundation extending in the downhill-direction. Primary anchors and diaphragm struts or collectors shall also be provided where interior vertical lateral force resisting elements occur above and in contact with the base-level-diaphragm.
The base-level-diaphragm shall be provided with primary anchors designed for the tributary forces spaced at a maximum 30 feet (9145 mm) on center. Where the floor below the base extends to the uphill foundation, and the foundation at the base is not adequate to resist the forces from the primary anchors, the base shear may be transferred to that lower level and the primary anchorage made at that lower level. The connection shall be made directly to the foundation. The existing foundation shall be evaluated as specified in LAMC Subsection 91.9406.11. (Amended by Ord. No. 185,587, Eff. 7/16/18.)
91.9406.3. Seismic Forces on Floor Levels Below the Base in the Downhill- Direction.
91.9406.3.1. General. All floor diaphragm connections between floor diaphragms below the base level diaphragm shall be designed in accordance with the provisions of this section.
91.9406.3.2. Design. (Amended by Ord. No. 185,587, Eff. 7/16/18.) Each floor level below the base defined in LAMC Subdivision 91.9406.1.1 shall be designed for all tributary loads at that level using a minimum seismic force factor not less than the base shear coefficient.
91.9406.3.3. Direct Connections. (Amended by Ord. No. 185,587, Eff. 7/16/18.) Each floor level shall be directly connected to the foundation through a system of primary anchors at that level as required for the base as specified in LAMC Subdivision 91.9406.1.1.
91.9406.4. Secondary Anchors from Diaphragm to Foundation for Seismic Forces at and Below the Base in the Downhill-Direction.
91.9406.4.1. General. (Amended by Ord. No. 185,587, Eff. 7/16/18.) In addition to the anchors required by LAMC Subsections 91.9406.2 and 91.9406.3, the floor diaphragm for levels at and below the base shall be anchored to the uphill foundation at the level of the diaphragm, as specified in this section.
EXCEPTION: Secondary anchors are not required where:
1. the concrete or masonry foundations in the downhill-direction are spaced at not more than 30 feet (9145 mm) on center and extend up to and are directly connected to the base-level-diaphragm for at least 70 percent of the diaphragm depth; or
2. the diaphragm is separated from the mudsill at the uphill foundation by a cripple wall which has anchor bolts and is braced in the plane of the wall and constructed with studs that are no less than 12 inches (305 mm) in height and primary anchors are spaced a maximum of 20 feet (6096 mm) on center; or
3. the deflection of the plywood floor diaphragm between adjacent primary anchors is calculated to be less than 1/4 of an inch (6.4 mm).
91.9406.4.2. Diaphragm Anchors. Secondary anchors required by this section shall be provided at each level at and below the base of the building. Diaphragm anchors shall be fully developed into the diaphragm and be connected to the foundation at the uphill diaphragm edge to develop the forces required by this section.
91.9406.4.3. Anchor Spacing. Secondary anchors required by this section shall be uniformly distributed along the uphill diaphragm edge and shall be spaced a maximum of four feet (1219 mm) on center.
91.9406.4.4. Anchor Capacity for Floor Diaphragms at the Base and Below. (Amended by Ord. No. 185,587, Eff. 7/16/18.) Secondary anchors at the base and below shall be designed for a uniformly distributed mini-mum force equal to the total primary anchorage design force at that level divided by the length of the uphill diaphragm edge, but shall not be less than 300 pounds per lineal foot (4.37 kN/m). The existing foundation need not be checked to resist the additional forces induced by the system of secondary anchors; however, the existing foundation shall be evaluated as specified in LAMC Subsection 91.9406.11.
91.9406.5. Design of Anchorage.
91.9406.5.1. General. Primary and secondary anchors, and diaphragm struts and collectors, shall be designed in accordance with the provisions of this section.
91.9406.5.2. Anchorage. (Amended by Ord. No. 185,587, Eff. 7/16/18.) The structure shall be anchored to the foundation as specified in LAMC Subsections 91.9406.2, 91.9406.3 and 91.9406.4.
91.9406.5.3. Fasteners. All bolted fasteners used to develop connections to wood members shall be provided with square plate washers at all bolt heads and nuts. Washers shall be minimum 3/16 inch (4.8 mm) thick and two inch (51 mm) square for 1/2-inch (12.7 mm) diameter bolts, and 1/4 inch (6.4 mm) thick and 2-1/2 inch (64 mm) square for 5/8 inch (15.9 mm) diameter or larger bolts. Nuts shall be wrench tightened prior to covering.
91.9406.5.4. Fastening. The diaphragm to foundation anchorage shall not be accomplished by the use of toe nailing, nails subject to withdrawal, or wood in cross grain bending or cross grain tension.
91.9406.5.5. Size of Wood Members. (Amended by Ord. No. 185,587, Eff. 7/16/18.) Wood diaphragm struts, collectors, and other wood members connected to primary anchors shall not be less than three-inch (76 mm) nominal width members or doubled two-inch (51 mm) nominal width members. Fastening doubled two-inch (51 mm) nominal width members shall be designed in accordance to the provisions of CBC Chapter 23. Secondary diaphragm anchors as specified in LAMC Subsection 91.9406.4 may be developed through existing 2 inch (50.8 mm) nominal width framing members. The effects of eccentricity on wood members shall be evaluated as required per LAMC Subdivision 91.9406.5.9.
91.9406.5.6. Design. Primary and secondary anchorage, including diaphragm struts, splices, and collectors shall be designed for 125 percent of the tributary force.
91.9406.5.7. Allowable Stress Increase. (Amended by Ord. No. 179,324, Eff. 12/10/07, Oper. 1/1/08.) The one- third allowable stress increase shall not be permitted for materials using allowable stress design methods.
91.9406.5.8. Seismic Load Factor. (Amended by Ord. No. 185,587, Eff. 7/16/18.) Steel elements of the diaphragm anchorage systems and continuity ties shall be designed by the allowable stress design method using a load factor of 1.7. The strength design specified in CBC Section 1908 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.
91.9406.5.9. Symmetry. All seismic lateral force foundation anchorage and diaphragm strut connections shall be symmetrical. Eccentric connections may be permitted when demonstrated by calculation or tests that all components of force have been provided for in the structural analysis or tests.
91.9406.5.10. Load Path.
91.9406.5.10.1. Primary Anchors. The load path for primary anchors shall be fully developed into the diaphragm and into the foundation. The foundation must be shown to be adequate to resist the concentrated loads from the primary anchors and must be shown to comply with the following:
1. Soil maximum bearing capacity for conventional footings shall be limited to 3000 psf (143.7 kPa) unless an approved geotechnical report permits higher bearing values.
2. Conventional continuous footings shall be analyzed for uplift forces induced by primary anchors.
3. Soil capacities need not be investigated for grade beam and caisson or pile foundations.
4. Shear stress in grade beams and tie beams shall be investigated for vertical component of primary anchor forces. Unless otherwise known, a maximum 2000 psi (13.8 MPa) concrete strength shall be assumed.
91.9406.5.10.2. Secondary Anchors. The load path for secondary anchors need not be developed beyond the connection to the foundation.
91.9406.5.10.3. Above Base Uplift Forces. The load path for uplift forces generated from above the base shall be analyzed and fully developed into the below base structural system.
91.9406.6. Base Shear Resistance Normal to the Downhill-Direction.
91.9406.6.1. General. Lateral force resisting elements acting in the normal to the downhill-direction shall be designed in accordance with the requirements of following Sections.
91.9406.6.2. Base Shear. (Amended by Ord. No. 179,324, Eff. 12/10/07, Oper. 1/1/08.) The design base shear shall be 75% less than currently required by Section 12.8.1 of ASCE 7, but not less than the following :
V = 0.133 W
Where:
V = The total design lateral force or shear at the base.
W = The total seismic dead load defined in Section 12.7.2 of ASCE 7.
91.9406.6.3. Vertical Distribution of Seismic Forces. (Amended by Ord. No. 179,324, Eff. 12/10/07, Oper. 1/1/08.) The distribution of seismic forces acting normal to the downhill-direction shall be determined using Section 12.8.3 of ASCE 7. The height of the structure in Equation (12.8-12) of Section 12.8.3 of ASCE 7 shall be taken from the base, which shall be measured from the top of the lowest level of the building foundation.
91.9406.6.4. Drift Limitations. The interstory drift below the base-level-diaphragm shall not exceed 0.005 times the story height. The total drift from the base-level-diaphragm to the top of the foundation shall not exceed 3/4 inch (19 mm). Where the story height or the height from the base-level-diaphragm to the top of the foundation varies because of a stepped footing or story offset, the height shall be measured from the average height of the top of the foundation. The calculated story drift shall not be reduced by the effect of horizontal diaphragm stiffness.
91.9406.7. Lateral Force Resisting Systems at the Base and Below in the Downhill-Direction.
91.9406.7.1. General. (Amended by Ord. No. 185,587, Eff. 7/16/18.) As an alternative to providing primary anchor connections from diaphragms to foundation in the downhill-direction, the following systems may be used, provided their location and spacing is maintained as specified in LAMC Subdivision 91.9406.2.2 for primary anchors.
91.9406.7.2. Wood Shear Walls. Wood structural panels or existing wood diagonal sheathed shear walls may be used provided:
1. The minimum length of shear wall shall be eight feet (2438 mm).
2. The minimum level length between steps in the shear wall sill shall be eight feet (2438 mm) and the maximum step height between adjacent sills shall be two feet eight inches (813 mm).
3. Sill plates do not slope and they bear on a level surface.
4. The design lateral forces shall be distributed to lateral force resisting elements of varying heights in accordance with the stiffness of each individual element. The stiffness of a stepped wood structural panel shear wall may be determined by dividing the wall into adjacent rectangular elements, subject to the same top of wall deflection. Deflections of shear walls may be estimated by CBC Section 2305 and AWC SDPWS Section 4.3.2 or other equivalent methods. Sheathing and fastening requirements for the stiffest section shall be used for the entire wall. Each section of wall shall be anchored for shear and uplift at each step as an independent shear wall. (Amended by Ord. No. 184,692, Eff. 12/30/16.)
5. Actual configuration of steps shall be determined in the field at the time of pre-design investigation.
6. The drift limitations of LAMC Subdivision 91.9406.6.4 are not exceeded. (Amended by Ord. No. 185,587, Eff. 7/16/18.)
91.9406.7.3. Braced Frames. Structural steel braced frames with concentric connections may be used as part of the lateral force resisting system. All members in braced frames shall be designed to resist tension and compression forces. Seismic forces shall not induce flexural stresses in any member of the frame, in diaphragm struts, or in the collectors. Where existing anchor bolts are used for anchorage, existing confinement shall be verified and additional confinement provided where necessary. When the braced frame is not rectangular, distribution of forces to members shall account for the variations in slope.
91.9406.7.4. Rod-Braced Frames and Diaphragms. Existing tension only braces may be used provided they resist five times the design force, and the connections have the capacity to resist the yield strength of the braces. Tension braces and their connections shall be exposed for evaluation. Existing anchor bolts shall be tested in shear and tension to five times the design force.
91.9406.7.5. Cement Plaster and Lath and Gypsum Wallboard. (Amended by Ord. No. 185,587, Eff. 7/16/18.) The sheathing materials listed in Division 25, Article 1, Chapter IX of the LAMC are not permitted to resist seismic lateral forces below the base-level-diaphragm.
91.9406.8. Lateral Force Resisting Systems at the Base and Below and Normal to the Downhill-Direction. Lateral force resisting systems acting normal to the downhill-direction may include steel moment frames and those systems permitted under LAMC Subsection 91.9406.7, provided the drift limitations of LAMC Subdivision 91.9406.6.4 are not exceeded. (Amended by Ord. No. 184,692, Eff. 12/30/16.)
91.9406.9. Diaphragms.
91.9406.9.1. General. Diaphragms at the base and below may be of straight one- inch by six-inch (25 mm by 152 mm) or two-inch by six-inch (51 mm by 152 mm) sheathing, provided vertical lateral force resisting elements in the downhill-direction or primary anchors are spaced no more than 20 feet (6096 mm) apart and the diaphragm shear forces do not exceed 100 plf (1.46 kN/m).
91.9406.9.2. Existing Diaphragms. Existing plywood and diagonally sheathed diaphragms need not be investigated.
91.9406.9.3. Existing Cantilevered Diaphragms. Existing cantilevered wood diaphragms are acceptable provided they do not cantilever more than one-half the diaphragm backspan (anchor span).
91.9406.9.4. Wood Diaphragm Rotation. Diaphragm rotation is not permitted in resisting lateral forces.
91.9406.10. Steel Beam to Column Connections.
91.9406.10.1. General. All steel beam to column connections shall be braced at supports and locations of concentrated loads. The beam to column connection shall be designed to prevent rotation of the beam.
91.9406.10.2. Steel Beams. Steel beams shall have stiffener plates installed on each side of the beam web at the column supports and points of concentrated load. The stiffener plates shall be welded to each beam flange and beam web. This requirement applies at the base and below and only to those connections which are part of the lateral load resisting system or lateral load path.
91.9406.10.3. Column Bracing. All single length multi-level height columns shall be braced in each orthogonal direction at each diaphragm level.
91.9406.11. Foundations.
91.9406.11.1. Existing Foundations. Foundation soundness shall be verified by the engineer or architect. Foundation types such as unreinforced masonry, stone and ungrouted concrete block and unreinforced concrete shall be retrofitted to resist lateral loads applied through the diaphragm anchors.
91.9406.11.2. Damaged Foundations. Damaged foundations shall be evaluated by the engineer or architect. Cracks in excess of 1/8 inch (3.2 mm) or differential displacement in excess of 1/4 inch (6.4 mm) shall be further investigated and repaired where necessary. Specifications for the restoration of the earth to wood separation shall be included and be made a part of the plans.
91.9406.11.3. Stud Wall Attachment. Shot pinned anchors shall not be used to resist lateral forces. Lateral force resisting systems which utilize shot pins shall be retrofitted with approved drilled anchors.
91.9406.11.4. Existing Framing Connections. Deteriorated framing and connections shall be repaired or replaced.
91.9406.11.5. Metal Connectors. Metal connectors shall not be in contact with, or below earth unless the connectors are hot dipped galvanized and further protected from earth with four inches of concrete.
91.9406.12. Existing Materials.
91.9406.12.1. Allowable Stresses. (Amended by Ord. No. 185,587, Eff. 7/16/18.) Existing materials may be used as part of the lateral load-resisting system provided that the stresses in these materials do not exceed the values shown in LAMC Table 94-A in this division.
