Fremont Municipal Code Art. III Design and Construction

(a) Buildings within the scope of this chapter shall be analyzed, designed and constructed in conformance with the Building Code, except as modified by this chapter, and shall conform to Sections 15.75.150 and 15.75.170.

EXCEPTION: The seismic retrofit of buildings that meet the limitations and additional conditions for prescriptive measures under Section 15.75.160 may be constructed according to those prescriptive measures without complying with Section 15.75.150.

(b) No alteration of the existing force-resisting or vertical load-carrying system shall reduce the strength or stiffness of the existing structure.

(c) When any portion of a building within the scope of this chapter is constructed on or into a slope steeper than one unit vertical in three units horizontal, the lateral-force-resisting system at and below the base level diaphragm shall be analyzed for the effects of concentrated lateral forces at the base due to this hillside condition. (Ord. 10-2007 § 19, 5-1-07. 1990 Code § 7-10301.)

(a) Scope of Analysis. This chapter requires the alteration, repair, replacement or addition of structural elements and their connections to meet the strength and stiffness requirements herein. The lateral load path analysis shall include the resisting elements and connections from the wood diaphragm above any soft, weak or open front wall lines to the foundation soil interface or the upper level of a rigid substructure such as concrete subterranean or podium below. The top story of any building need not be analyzed. The lateral load path analysis for added structural elements shall also include evaluation of the allowable soil bearing and lateral pressures in accordance with the Building Code.

EXCEPTION: When an open front, weak or soft wall line exists due to parking at the ground level of a two-level building and the parking area is less than 20 percent of the ground floor level, then only the wall lines in the open, weak or soft directions of the enclosed parking area need comply with the provisions of this chapter.

(b) Design Base Shear. The design base shear in a given direction shall be 75 percent of the base shear required for similar new buildings as set forth in the Building Code.

(c) Vertical Distribution of Forces. The total seismic force shall be distributed over the height of the structure based on the Building Code.

(d) Weak Story Limitation. Weak story construction shall be strengthened to the lesser of:

(1) Ω0 times the story shear prescribed by subsections (b) and (c) of this section.

(2) In two-story buildings up to 30 feet in height, 65 percent of the strength of the story above. In all other buildings, 80 percent of the strength of the story above.

(e) Story Drift Limitation. The calculated story drift for each retrofitted level shall not exceed the allowable deformation compatible with all vertical-load-resisting elements and be no more than 0.025 times the story height. The calculated story drift shall not be reduced by the effects of horizontal diaphragm stiffness but shall be increased when these effects produce rotation.

The effects of rotation and soil stiffness shall be included in the calculated story drift when lateral loads are resisted by vertical elements whose required depth of embedment is determined by pole formulas such as Formula (6A-1) and (6A-2) in Building Code Section 1806.8.2. The coefficient of variation of subgrade reaction used in the deflection calculations shall be provided from an approved geotechnical engineering report or other approved methods.

(f) P-Delta Effects. The requirements of the Building Code regarding P-delta effects shall apply except as modified herein. All structural framing elements and their connections, not required by design to be part of the lateral-force-resisting system, shall be designed and/or detailed to be adequate to maintain support of design dead plus live loads when subjected to the expected deformations caused by seismic forces. The stress analysis of cantilever columns shall use a buckling factor of 2.1 for the direction normal to the axis of the beam.

(g) Ties and Continuity. All parts of the structure included in the scope of this section shall be interconnected and the connection shall be capable of resisting the seismic force created by the parts being connected.

(h) Cripple Walls. Cripple walls braced with nonconforming structural materials shall be braced in accordance with this chapter. When a single top plate exists in the cripple wall, all end joints in the top plate shall be tied.

(i) Collector Elements. Collector elements shall be provided which can transfer the seismic forces originating in other portions of the building to the elements within the scope of this section.

(j) Horizontal Diaphragms. The strength of an existing horizontal diaphragm sheathed with wood structural panels or diagonal sheathing need not be investigated unless the diaphragm is required to transfer lateral forces from vertical elements of the seismic-force-resisting system above the diaphragm to elements below the diaphragm because offset in placement of the elements. Wood diaphragms with stories above shall not be allowed to transmit lateral forces by rotation or cantilever except as allowed by the Building Code; however, rotational effects shall be accounted for when unsymmetric wall stiffness increases shear demands.

EXCEPTION: Diaphragms that cantilever 25 percent or less of the distance between lines of lateral-load-resisting elements from which the diaphragm cantilevers may transmit their shears by cantilever; provided, that rotational effects on shear walls parallel and perpendicular to the load are taken into account.

(k) Wood-Framed Shear Walls. Wood-framed shear walls shall have strength and stiffness to resist the seismic loads and shall conform to the requirements of this section.

(1) Gypsum or Plaster Products. Gypsum or cement plaster products shall not be used to provide lateral resistance in a soft or weak story or in a story with an open-front wall line, whether or not new elements are added to mitigate the soft, weak or open-front condition.

(2) Wood Structural Panels.

(A) Drift Limit. Wood structural panel shear walls shall meet the story drift limitation of this section. Conformance to the story drift limitation shall be determined by approved testing or calculation, not by the use of an aspect ratio. Calculated deflection shall be determined according to International Building Code Equation 23-1 and shall be increased by 25 percent. Contribution to the shear wall deflection from the anchor or tie-down slippage shall also be included. The slippage contribution shall include the vertical elongation of the metal components, the vertical slippage of the connectors to framing members, localized crushing of wood due to bearing loads and shrinkage of the wood elements because of changes in moisture content as a result of aging. The total vertical slippage shall be multiplied by the shear panel aspect ratio and added to the total horizontal deflection. Individual shear panels shall be permitted to exceed the maximum aspect ratio, provided the allowable story drift and allowable shear capacities are not exceeded.

(B) Openings. Shear walls are permitted to be designed for continuity around openings in accordance with the building code. Blocking and steel strapping shall be provided at corners of the openings to transfer forces from discontinuous boundary elements into adjoining panel elements. Alternatively, perforated shear wall provisions of the building code are permitted to be used.

(C) Wood Species of Framing Members. Allowable shear values for wood structural panels shall consider the species of the framing members. When the allowable shear values are based on Douglas fir-larch framing members, and framing members are constructed of other species of lumber, the allowable shear values shall be multiplied by the following factors: 0.82 for species with specific gravities greater than or equal to 0.42 but less than 0.49, and 0.65 for species with specific gravities less than 0.42. Redwood shall use a factor of 0.65 and hem fir shall use a factor of 0.82 unless otherwise approved.

(3) Substitution for Three-Inch (76 Millimeter) Nominal Width Framing Members. Two two-inch (51 millimeter) nominal width framing members shall be permitted in lieu of any required three-inch (76 millimeter) nominal width framing member when the existing and new framing members are of equal dimensions, are connected as required to transfer the in-plane shear between them, and when the sheathing fasteners are equally divided between them.

(4) Hold-Down Connectors.

(A) Expansion Anchors in Tension. Expansion anchors that provide tension strength by friction resistance shall not be used to connect hold-down devices to existing concrete or masonry elements. Expansion anchors that provide tension strength by bearing (commonly referred to as “undercut” anchors) shall be permitted.

(B) Required Depth of Embedment. The required depth of embedment or edge distance for the anchor used in the hold-down connector shall be provided in the concrete or masonry below any plain concrete slab unless satisfactory evidence is submitted to the building official that shows that the concrete slab and footings are of monolithic construction.

(C) Required Preload of Bolted Hold-Down Connectors. Bolted hold-down connectors shall be preloaded to reduce slippage of the connector. Preloading shall consist of tightening the nut on the tension anchor after the placement but before the tightening of the shear bolts in the panel flange member. The tension anchor shall be tightened until the shear bolts are in firm contact with the edge of the hole nearest the direction of the tension anchor. Hold-down connectors with self-jigging bolt standoffs shall be installed in a manner to permit preloading. (Ord. 10-2007 § 20, 5-1-07. 1990 Code § 7-10302.)

(a) Limitation. These prescriptive measures shall apply only to two-story buildings and only when deemed appropriate by the building official. These prescriptive measures rely on rotation of the second floor diaphragm to distribute the seismic load between the side and rear walls of the ground floor open area. In the absence of an existing floor diaphragm of wood structural panel or diagonal sheathing, a new wood structural panel diaphragm of minimum thickness of three-fourths inch (19 millimeters) and with 10d common nails at six inches (152 millimeters) on center shall be applied.

(b) Additional Conditions. To qualify for these prescriptive measures, the following additional conditions need to be satisfied by the retrofitted structure:

(1) Diaphragm aspect ratio L/W is less than 0.67, where W is the diaphragm dimension parallel to the soft, weak or open-front wall line and L is the distance in the orthogonal direction between that wall line and the rear wall of the ground floor open area.

(2) Minimum length of side shear walls equals 20 feet (6,096 millimeters).

(3) Minimum length of rear shear wall equals three-fourths of rear wall.

(4) No plan or vertical irregularities other than a soft, weak or open-front wall line.

(5) Roofing weight less than or equal to five pounds per square foot (240 N/m2).

(6) Aspect ratio of the full second floor diaphragm meets the requirements of the Building Code for new construction.

(c) Performance. The improved earthquake performance of the structure due to the proposed prescriptive measures varies and is greatly controlled by all of the following: proximity to the fault line, soil type, weight of roof and floor above, quality of existing walls, posts and columns and their connections to the floor diaphragm, and the quality of construction provided in order to comply with the prescriptive measures. The implementation of the proposed measures is not intended to improve the earthquake performance of the building above the first story.

(d) Minimum Required Retrofit.

(1) Anchor Bolt Size and Spacing. The anchor bolt size and spacing shall be a minimum of three-fourths inch (19 millimeters) in diameter at 32 inches (813 millimeters) on center. Where existing bolts are inadequate, new steel plates bolted to the side of the foundation and nailed to the sill may be used, such as an approved connector.

(2) Connection to Floor Above. Shear wall top plates shall be connected to blocking or rim joist at upper floor with a minimum of 18-gage galvanized steel angle clips four and one-half inches (114 millimeters) long with 12 8d nails spaced no farther than 16 inches (406 millimeters) on center, or by equivalent shear transfer methods.

(3) Shear Wall Sheathing. The shear wall sheathing shall be a minimum of fifteen-thirty-seconds inch (11.9 millimeters) five-ply structural I with 10d nails at four inches (102 millimeters) on center at edges and 12 inches (305 millimeters) on center at field; blocked all edges with three by four or larger. Where existing sill plates are less than three-by thick, place flat two-by on top of sill between studs, with flat 18-gage galvanized steel clips four and one-half inches (114 millimeters) long with 12 8d nails or three-eighths-inch diameter (9.5 millimeters) lags through blocking for shear transfer to sill plate. Stagger nailing from wall sheathing between existing sill and new blocking. Anchor new blocking to foundation as specified above.

(4) Shear Wall Hold-Downs. Shear walls shall be provided with hold-down anchors at each end. Two hold-down anchors are required at intersecting corners. Hold-downs shall be approved connectors with a minimum five-eighths-inch diameter (15.9 millimeters) threaded rod or other approved anchor with a minimum allowable load of 4,000 pounds (17.8 kilonewtons). Anchor embedment in concrete shall not be less than five inches (127 millimeters). Tie-rod systems shall not be less than five-eighths-inch (15.9 millimeters) in diameter unless using high strength cable. Threaded rod or high strength cable elongation shall not exceed five-eighths-inch (15.9 millimeters) using design forces. (Ord. 10-2007 § 21, 5-1-07. 1990 Code § 7-10303.)

(a) New Materials. All materials approved by the Building Code, including their appropriate allowable stresses and minimum aspect ratios, shall be permitted to meet the requirements of this chapter.

(b) Allowable Foundation and Lateral Pressures. The use of default values from the Building Code for continuous and isolated concrete spread footings shall be permitted. For soil that supports embedded vertical elements, Section 15.75.150(e) shall apply.

(c) Existing Materials. All existing materials shall be in sound condition and constructed in conformance to the Building Code before they can be used to resist the lateral loads prescribed in this chapter. The verification of existing material conditions and their conformance to these requirements shall be made by physical observation reports, material testing or record drawings as determined by the structural designer and approved by the building official.

(1) Horizontal Wood Diaphragms. Allowable shear values for existing horizontal wood diaphragms that require analysis under Section 15.75.150 may be taken from the following table. The values in the table shall be used for allowable stress design. Design forces based on strength design shall be reduced to allowable stress levels before comparison with the limiting values in the table.

For SI: 1 foot = 304.8 mm.

a. Material must be sound and in good condition.

b. A one-third increase in allowable stress is not allowed.

c. Shear values of these materials may be combined, except the total combined value shall not exceed 300 pounds per foot.

d. Stresses given may be increased for combination of loads as specified in the Building Code.

(2) Wood Structural-Panel Shear Walls.

(A) Allowable Nail Slip Values. The use of box nails and unseasoned lumber are permitted to be assumed. When the required drift calculations of Section 15.75.150(k)(2)(A) (drift limit for wood-framed shear walls) rely on the slip values for common nails or surfaced dry lumber, their use in construction shall be verified by exposure. The design value of the box nails shall be assumed to be similar to that of common nails having the same diameter. Verification of surfaced dry lumber shall be by identification conforming to the Building Code.

(B) Plywood Panel Construction. When verification of the existing plywood materials is by use of record drawings alone, the panel construction for plywood modulus “G” shall be assumed equal to 50,000 pounds per square inch (345 megapascals).

(3) Existing Wood Framing. Wood framing is permitted to use the design stresses specified in the building code under which the building was constructed or other stress criteria approved by the building official.

(4) Structural Steel. All existing structural steel shall be permitted to use the allowable stresses for Grade A36. Existing pipe or tube columns shall be assumed to be of minimum wall thickness unless verified by testing or exposure.

(5) Strength of Concrete. All existing concrete footings shall be permitted to be assumed to be plain concrete with compressive strength of 2,000 pounds per square inch (13.8 megapascals). Existing concrete compressive strength taken greater than 2,000 pounds per square inch (13.8 megapascals) shall be verified by testing, record drawings or department records.

(6) Existing Sill Plate Anchorage. Existing cast-in-place anchor bolts shall be permitted to use the allowable service loads for bolts with proper embedment when used for shear resistance to lateral loads. (Ord. 10-2007 § 22, 5-1-07. 1990 Code § 7-10304.)

(a) General. The plans shall show all necessary dimensions and materials for plan review and construction and shall accurately reflect the results of the engineering investigation and design. Details, specific to the actual condition found, shall be shown on the drawings to assure installation of all elements required for construction of the necessary complete load path.

(b) Existing Construction. The plans shall show the existing diaphragm and shear wall sheathing and framing materials, fastener type and spacing, diaphragm and shear wall connections, continuity ties, and collector elements. The plans shall also show the portion of the existing materials that needs verification during construction.

(c) New Construction.

(1) Foundation Plan Elements. The foundation plan shall include the size, type, location and spacing of all anchor bolts with the required depth of embedment, edge and end distance; the location and size of all columns for braced or moment frames; referenced details for the connection of braced or moment frames to their footing and referenced sections for any grade beams and footings.

(2) Framing Plan Elements. The framing plan shall include the width, location and material of shear walls; the width, location and material of frames; references on details for the column to beam connectors, beam to wall connections, and shear transfers at floor and roof diaphragms; and the required nailing and length for wall top plate splices.

(3) Shear Wall Schedule, Notes and Details. Shear walls shall have a referenced schedule on the plans that includes the correct shear wall capacity in pounds per foot; the required fastener type, length, gauge and head size; and a complete specification for the sheathing material and its thickness. The schedule shall also show the required location of three-inch nominal or two two-inch nominal edge members; the spacing of shear transfer elements such as framing anchors or added sill plate nails; the required hold-down with its bolt, screw or nail sizes; and the dimensions, lumber grade and species of the attached framing member.

Notes shall show required edge distance for fasteners on structural wood panels and framing members; required flush nailing at the plywood surface; limits of mechanical penetrations; and the sill plate material assumed in the design. The limits of mechanical penetrations shall also be detailed showing the maximum notching and drilled hole sizes.

(4) General Notes. General notes shall show the requirements for material testing, special inspection, structural observation and the proper installation of newly added materials.

(5) Engineer’s or Architect’s Statement. The responsible engineer or architect shall provide the following statements on the approved plans:

“I am responsible for designing this building’s seismic strengthening in compliance with the minimum regulations of this chapter,” and, when applicable, “The registered deputy inspector, required as a condition of the use of structural design stresses requiring continuous inspection, will be responsible to me.” (Ord. 10-2007 § 23, 5-1-07. 1990 Code § 7-10305.)

(a) Structural observation, in accordance with Building Code, shall be required for all structures in which seismic retrofit is being performed in accordance with this chapter. Structural observation shall include visual observation of work for conformance with the approved construction documents and confirmation of existing conditions assumed during design.

(b) Structural testing and inspection for new construction materials shall be in accordance with the Building Code, except as modified by this chapter. (Ord. 10-2007 § 24, 5-1-07. 1990 Code § 7-10306.)

The following checklists apply to the voluntary investigation and analysis of buildings under Section 15.75.060(a):

(a) Building Systems.

(b) Vertical Lateral Force Resisting System.

(c) Connections.

(d) Horizontal Lateral Force Resisting System.

1 The basic structural checklist shall be completed prior to completing this supplemental structural checklist.

(Ord. 10-2007 § 25, 5-1-07. 1990 Code § 7-10307.)