You are viewing an archived code
Loading...
411B.1 Limits for Application. The Special Procedure of this section may only be applied to buildings with the following characteristics:
1. The building is not an essential or hazardous facility.
2. Wood or plywood diaphragms at all levels above the base of structure.
3. A maximum of six stories above the base of the building. The base shall be the ground level, and basement or basements shall be excluded from the story count.
EXCEPTION: An otherwise qualifying building of greater than six stories may also be retrofitted using the Special Procedure, provided the building is not located on poor soil as defined in Section 403A or does not contain any Group A Occupancies with an occupant load of 300 or more, or Group E, or Group I Occupancies.
4. Except for single-story buildings with an open front on one side only, a minimum of two lines of vertical elements of the lateral force resisting system complying with Section 412B parallel to each axis. At least one line in each direction shall be a masonry or concrete shear wall. Requirements for open front buildings are contained in Section 411B.8.
5. In buildings containing one or more party walls, the Special Procedure shall not be used unless each building sharing a party wall individually complies with all of the limitations set forth above, and the owner of each such building consents to the use of the procedure in writing.
411B.3 Crosswalls. Crosswalls when used shall meet the requirements of this section.
411B.3.1 Crosswall definition. A “crosswall” is a wood-framed wall sheathed with any of the materials described in Table 4B-D or 4B-E or other system as defined in Section 411B
.3.5. Spacing of crosswalls shall not exceed 40 feet (12.19 m) on center measured
perpendicular to the direction of consideration and shall be placed in each story of the building. Crosswalls shall extend the full story height between diaphragms.
EXCEPTIONS:
1. Crosswalls need not be provided at all levels in accordance with Section 411B.4.2(4).
2. Existing crosswalls need not be continuous below a wood diaphragm at or within 4 feet (1.219 m) of grade, provided:
(1) Shear connections and anchorage requirements, Section 411B.5 are satisfied at all edges of the diaphragm.
(2) Crosswalls with total shear capacity of 0.08åWd interconnect the diaphragm to the foundation.
(3) The demand-capacity ratio of the diaphragm between the crosswalls that are continuous to their foundations shall be calculated as:
DCR = (0.332Wd + Vca)/2vuD (4B-4)
and DCR shall not exceed 2.5.
(Amended by Ord. 65-19, File No. 190136, App. 4/12/2019, Eff. 5/13/2019)
411B.3.2 Crosswall shear capacity. Within any 40 feet (12.19 m) measured along the span of the diaphragm, the sum of the crosswall shear capacities shall be at least 30 percent of the diaphragm shear capacity of the strongest diaphragm at or above the level under consideration.
411B.3.3 Existing crosswalls. Existing crosswalls shall have a maximum height-to-length ratio between openings of 1.5 to 1. Existing crosswall connections to diaphragms need not be investigated as long as the crosswall extends to the framing of the diaphragm above and below.
411B.3.4 New crosswalls. New crosswall connections to the diaphragm shall develop the crosswall shear capacity. New crosswalls shall have the capacity to resist an overturning moment equal to the crosswall shear capacity times the story height. Crosswall overturning moments need not be cumulative over more than two stories.
411B.3.5 Other crosswall systems. Other systems, such as moment resisting frames, may be used as crosswalls, provided that the yield story drift does not exceed 1 inch (25.4 mm) in any story.
411B.4 Wood Diaphragms.
411B.4.1 Acceptable diaphragm span. A diaphragm is acceptable if the point (L,DCR) on Figure 4B-1 falls within Regions 1, 2 or 3.
411B.4.2 Demand-capacity ratios. Demand-capacity ratios shall be calculated for the diaphragm at any level according to the following formulas:
1. For a diaphragm without qualifying crosswalls at levels immediately above or below:
DCR = 0.332Wd/ΣvuD (4B-5)
2. For a diaphragm in a single-story building with qualifying crosswalls:
DCR = 0.332Wd/(ΣvuD + Vcb ) (4B-6)
3. For diaphragms in a multi-story building with qualifying crosswalls in all levels:
DCR = 0.332ΣWd /(ΣΣvuD + Vcb) (4B-7)
DCR shall be calculated at each level for the set of diaphragms at and above the level under consideration. In addition, the roof diaphragm shall also meet the requirements of Formula (4B-6).
4. For a roof diaphragm and the diaphragm directly below if coupled by crosswalls:
DCR = 0.332ΣW d /ΣΣvuD (4B-8)
411B.4.3 Chords. An analysis for diaphragm flexure need not be made and chords need not be provided.
411B.4.4 Collectors. An analysis of diaphragm collector forces shall be made for the transfer of diaphragm edge shears into vertical elements of the lateral force resisting system. Collector forces may be resisted by new or existing elements.
411B.4.5 Diaphragm openings.
411B.4.5.1 Forces. Diaphragm forces at corners of openings shall be investigated and shall be developed into the diaphragm by new or existing materials.
411B.4.5.2 Demand-capacity ratio. In addition to the demand-capacity ratios of Section 411B
.4.2, the demand-capacity ratio of the portion of the diaphragm adjacent to an opening shall be calculated using the opening dimension as the span.
411B.4.5.3 End quarter of diaphragm. Where an opening occurs in the end quarter of the diaphragm span, vuD for the demand-capacity ratio calculation shall be based on the net depth of the diaphragm.
411B.5 Diaphragm Shear Transfer. Diaphragms shall be connected to shear walls with connections capable of developing a minimum force given by the lesser of the following formulas:
V = 0.2CpWd (4B-9)
using the Cp values in Table 4B-C, or
V = vuD (4B-10)
411B.6 Shear Walls (In-Plane Loading).
411B.6.1 Wall story force. The wall story force distributed to a shear wall at any diaphragm level shall be the lesser value calculated as:
1. For buildings without crosswalls:
Fwx = 0.132 (Wwx + Wd /2) (4B-11)
but need not exceed
Fwx = 0.132Wwx + vuD (4B-12)
2. For buildings with crosswalls in all levels:
Fwx = 0.1 (Wwx + Wd /2) (4B-13)
but need not exceed
Fwx = 0.1 [Wwx + ΣWd (vuD/ΣΣvuD)] (4B-14)
and need not exceed
Fwx = 0.1W wx + vuD (4B-15)
411B.6.2 Wall story shear. The wall story shear shall be the sum of the wall story forces at and above the level of consideration.
Vwx = ΣFwx (4B-16)
411B.6.4 Moment frames. Moment frames used in place of shear walls shall be designed as required in San Francisco Building CodeChapter 16 except that the forces shall be as specified in Section 411B.6.1 and the story drift ratio shall be limited to 0.005, except as further limited by Section 412B.4.2.
411B.7 Out-of-Plane Forces – Unreinforced Masonry Walls.
411B.7.1 Allowable unreinforced masonry wall height-to-thickness ratios. The provisions of Section 410B.2 are applicable except the allowable height-to-thickness ratios given in Table 4B-B shall be determined from Figure 4B-1 as follows:
1. In Region 1, height-to-thickness ratios for buildings with crosswalls may be used if qualifying crosswalls are present in all stories.
2. In Region 2, height-to-thickness ratios for buildings with crosswalls may be used whether or not qualifying crosswalls are present.
3. In Region 3, height-to-thickness ratios for “all other buildings” shall be used whether or not qualifying crosswalls are present.
411B.7.2 Walls with diaphragms in different regions. When diaphragms above and below the wall under consideration have demand-capacity ratios in different regions of Figure 4B-1, the lesser height-to-thickness ratio shall be used.
411B.8 Open Front Design Procedure. A single-story building with an open front on one side and crosswalls parallel to the open front may be designed by the following procedure:
1. Effective diaphragm span, Li, for use in Figure 4B-1 shall be determined in accordance with the following formula:
Li = 2 [(Ww /Wd )L + L]x (4B-17)
2. Diaphragm demand-capacity ratio shall be calculated as:
DCR = 0.332 (Wd + W w ) / [(vuD) + Vcb ] (4B-18)
412B.1 Analysis of Vertical Elements of the Lateral Force-Resisting System. General. The following requirements are applicable to both the General Procedure and Special Procedure.
412B.2 Existing Unreinforced Masonry Walls.
412B.2.1 Flexural rigidity. Flexural components of deflection may be neglected in determining the rigidity of an unreinforced masonry wall.
412B.2.2 Shear walls with openings. Wall piers shall be analyzed according to the following procedure which is diagrammed in Figure 4B-2:
412B.2.2.1 For any pier:
1. The pier shear capacity shall be calculated as:
Va = vaAx (4B-19)
2. The pier rocking shear capacity shall be calculated as:
Vr = 0.5PDD/H (4B-20)
412B.2.2.2 Pier behavior. The wall piers at any level are acceptable if they comply with one of the following modes of behavior:
1. Rocking controlled mode. When the pier rocking shear capacity is less than the pier shear capacity, i.e., Vr < Va for each pier in a level, forces in the wall at that level, Vwx, shall be distributed to each pier in proportion to PDD/H.
For the wall at that level:
Vwx < ΣåVr (4B-21)
2. Shear controlled mode. Where the pier shear capacity is less than the pier rocking capacity, i.e., Vr < Va in at least one pier in a level, forces in the wall at the level, Vwx, shall be distributed to each pier in proportion to D/H.
For each pier at that level:
Vp < Va (4B-22)
and
Vp < Vr (4B-23)
If Vp < Va for each pier and Vp > Vr for one or more piers, such piers shall be omitted from the analysis, and the procedure shall be repeated for the remaining piers, unless the wall is strengthened and reanalyzed.
412B.2.2.3 Masonry pier tension stress. Unreinforced masonry wall piers need not be analyzed for tension stress.
412B.2.3 Shear walls without openings. Shear walls without openings shall be analyzed as for walls with openings except that Vr shall be calculated as follows:
Vr = (0.50PD + 0.25Pw) D/H (4B-24)
412B.3 Plywood Sheathed Shear Walls. Plywood sheathed shear walls may be used to resist lateral forces for buildings with wood diaphragms analyzed according to provisions of Section 410B. Plywood sheathed shear walls may not be used to share lateral forces with other materials along the same line of resistance.
412B.4 Combinations of Vertical Elements.
412B.4.1 Lateral force distribution. Lateral forces shall be distributed among the designated vertical resisting elements in a line in proportion to their relative rigidities except that moment frames shall comply with Section 412B.4.2.
412B.4.2 Moment-resisting frames. A moment frame shall not be used with an unreinforced masonry wall in a single line of resistance unless the wall has piers that are capable of sustaining rocking in accordance with Section 412B.2.2 and the frames are designed to carry 100 percent of the lateral forces, and the story drift ratio shall be limited to 0.0025.
412B.5 Shear Force. The shear force used in the design of any party wall shall be the sum of the shear forces contributed by each building sharing that wall.
413B.1 Wall Anchorage.
413B.1.1 Anchor locations. All unreinforced masonry walls shall be anchored at the roof and floor levels as required in Section 410B.2. Ceilings of plaster, gypsum board or similar heavier materials, when not attached directly to roof or floor framing, and abutting masonry walls, shall be either anchored to the walls at a maximum spacing of 6 feet (1.829 m) or removed.
413B.1.2 Anchor requirements. Anchors shall consist of bolts installed through the wall as specified in Table 4B-E, or by an approved equivalent at a maximum anchor spacing of 6 feet (1.829 m). All existing wall anchors shall be secured to the joists to develop the required forces.
413B.1.3 Minimum wall anchorage. Anchorage of masonry walls to each floor or roof shall resist a minimum force determined in accordance with San Francisco Building Code
Chapter 16
or 200 pounds per linear foot (298 kg/m), whichever is greater, acting normal to the wall at the level of the floor or roof. Anchor spacing shall not exceed 6 feet (1.829 m) on center. Existing through-the-wall anchors, if used, must meet the requirements of this chapter or must be upgraded.
413B.1.4 Anchors at corners. At the roof and floor levels, both shear and tension anchors shall be provided within 2 feet (0.609 m) horizontally from the inside of the corners of the walls.
413B.1.5 Anchors with limited access. When access to the exterior face of the masonry wall is prevented, wall anchors conforming to Item 4.b. in Table 4B-E may be used.
413B.1.6 Anchors at interior and party walls. When floor or roof framing aligns vertically at party and interior masonry walls, continuous anchors shall be utilized to directly connect the floor or roof framing on either side of the wall. Where the roof or floor framing is offset more than the least depth of any adjacent framing, the intervening wall section shall be investigated for cross wythe shear assuming that the diaphragm to wall tensions on either side of the wall are acting in opposite directions.
413B.2 Diaphragm Shear Transfer. Bolts transmitting shear forces shall have a maximum bolt spacing of 6 feet (1.829 m) and shall have nuts installed over malleable iron or plate washers when bearing on wood and heavy cut washers when bearing on steel.
413B.3 Collectors. Collector elements shall be provided which are capable of transferring the seismic forces originating in other portions of the building to the element providing the resistance to those forces.
413B.4 Ties and Continuity. Ties and continuity shall conform to SFEBC Section 301.2.
413B.5 Wall Bracing.
413B.5.1 General. Where a wall height-to-thickness ratio exceeds the specified limits, the wall may be laterally supported by vertical bracing members per Section 413B.5.2 or by reducing the wall height by bracing per Section 413B.5.3.
413B.5.2 Vertical bracing members. Vertical bracing members shall be attached to floor and roof construction for their design loads independently of required wall anchors. Horizontal spacing of vertical bracing members shall not exceed one-half the unsupported height of the wall nor 10 feet (3.048 m). Deflection of such bracing members at design loads shall not exceed one-tenth of the wall thickness.
413B.5.3 Intermediate wall bracing. The wall height may be reduced by bracing elements connected to the floor or roof. Horizontal spacing of the bracing elements and wall anchors shall be as required by design but shall not exceed 6 feet (1.829 m) on center. Bracing elements shall be detailed to minimize the horizontal displacement of the wall by the vertical displacement of the floor or roof.
413B.6 Parapets. Parapets and appendages not conforming to this chapter shall be removed, or stabilized or braced to ensure that the parapets and appendages remain in their original position.
EXCEPTIONS:
1. Parapets, appendages and roof-to wall-tension anchors which have already been removed, stabilized or braced to comply with Chapter 4C of this code or previous codes pursuant to an application filed before the effective date of this ordinance need not be reanalyzed or restrengthened.
2. Parapets whose heights do not exceed 3 times their thicknesses need not be removed, stabilized or braced, provided they are located either immediately adjacent to a normally inaccessible court or yard or another building. In the case of an adjoining building, the top of the parapet of the building under consideration shall not be more than 12 inches (0.305 m) above the top of the parapet of the adjoining building. In order to qualify for this exception, the owner must execute an agreement with the Department to voluntarily abate any hazard that may arise as a result of changed conditions such as demolition of the adjacent building or development or occupancy of the adjoining court or yard. The owner must record the agreement with the County Recorder on a form satisfactory to the Department and supply a copy of the recorded agreement to the Department.
Parapets previously exempted that would not be exempted under Exception 2 above shall be removed, or stabilized or braced when the building is strengthened.
The maximum height of an unbraced unreinforced masonry parapet above the lower of either the level of tension anchors or roof sheathing shall not exceed 1½ times the thickness of the parapet wall. If the required parapet height exceeds this maximum height, a bracing system designed for the forces determined in accordance with San Francisco Building CodeChapter 16 shall support the top of the parapet. Parapet corrective work must be performed in conjunction with the installation of tension roof anchors.
The minimum height of a parapet above any wall anchor shall be 12 inches (0.305 m).
EXCEPTION: If a reinforced concrete beam is provided at the top of the wall, the minimum height above the wall anchor may be 6 inches (170.44 mm).
413B.7 Veneer.
413B.7.1 Anchorages. Veneer shall be anchored with approved anchor ties, conforming to the required design capacity specified in this code and placed at a maximum spacing of 24 inches (610 mm) with a maximum supported area of 4 square feet (0.372 m2).
EXCEPTION: Existing anchor ties for attaching brick veneer to brick backing may be acceptable, provided the ties are in good condition and are corrugated galvanized iron strips not less than 1 inch (25.4 mm) in width, 8 inches (203.2 mm) in length and 1/16 inch (1.59 mm) in thickness or equal.
413B.7.2 Verification. The location and condition of existing veneer anchor ties shall be verified as follows:
1. An approved testing laboratory shall verify the location and spacing of the ties and shall submit a report to the Building Official for approval as part of the structural analysis.
2. The veneer in a selected area shall be removed to expose a representative sample of ties (not less than four) for inspection by the Building Official.
413B.8 Nonstructural Masonry Walls. Unreinforced masonry walls which carry no design vertical or lateral loads and are not required by the design to be part of the lateral force resisting system shall be adequately anchored to new or existing supporting elements. The anchors and elements shall be designed for the out-of-plane forces specified in San Francisco Building Code
Chapter 16
. The height or length to thickness ratio between such supporting elements for such walls shall not exceed 13.
413B.9 Truss and Beam Supports. Where trusses and beams, other than rafters or joists, are supported on masonry, independent secondary columns shall be installed to support vertical loads of the roof or floor members.
413B.10 Adjacent Buildings. Where elements of adjacent buildings do not have a separation of at least 5 inches (127 mm), the allowable height-to-thickness ratios for “all other buildings” per Table 4B-B shall be used in the direction of consideration.
414B.1 Scope. This section applies when this chapter requires in-place testing of the quality of masonry mortar.
414B.2 Preparation of Sample. The bed joints of the outer wythe of the masonry shall be tested in shear by laterally displacing a single brick relative to the adjacent bricks in the same wythe. The head joint opposite the loaded end of the test brick shall be carefully excavated and cleared. The brick adjacent to the loaded end of the test brick shall be carefully removed by sawing or drilling and excavating to provide space for a hydraulic ram and steel loading blocks.
414B.3 Application of Load and Determination of Results. Steel blocks, the size of the end of the brick, shall be used on each end of the ram to distribute the load to the brick. The blocks shall not contact the mortar joints. The load shall be applied horizontally, in the plane of the wythe, until either a crack can be seen or slip occurs. The strength of the mortar shall be calculated by dividing the load at the first cracking or movement of the test brick by the nominal gross area of the sum of the two bed joints.
415B.1 Scope. Shear and tension anchors embedded in existing masonry construction shall be tested in accordance with this section when and as required by this chapter.
415B.2 Direct Tension Testing of Existing Anchors and New Bolts. The test apparatus shall be supported on the masonry wall. The distance between the anchor and the test apparatus support shall not be less than one-half the wall thickness for existing anchors and 75 percent of the embedment for new embedded bolts. Existing wall anchors shall be given a preload of 300 pounds (136.4 kg) prior to establishing a datum for recording elongation. The tension test load reported shall be recorded at 1/8 inch (3.18 mm) relative movement of the existing anchor and the adjacent masonry surface. New embedded tension bolts shall be subject to a direct tension load of not less than 2.5 times the design load but not less than 1,500 pounds (682 kg) for five minutes (10 percent deviation).
415B.3 Torque Testing of New Bolts. Bolts which are embedded in unreinforced masonry walls shall be tested using a torque calibrated wrench to the following minimum torques:
1/2-inch-diameter bolts – 40 foot-pounds. | |
(12.7 mm) | (5.54 M-Kg) |
5/8-inch-diameter bolts – 50 foot-pounds. | |
(16 mm) | (6.93 M-Kg) |
3/4-inch-diameter bolts – 60 foot-pounds. | |
(19 mm) | (8.31 M-Kg) |
415B.4 Prequalification Test for Bolts and Other Types of Anchors. This section is applicable when it is desired to use tension or shear values for anchors greater than those permitted by Table 4B-E. The direct tension test procedure set forth in Section 415B.2 for existing anchors may be used to determine the allowable tension values for new embedded or through bolts except that no preload is required. Bolts shall be installed in the same manner and using the same materials as will be used in the actual construction. A minimum of 5 tests for each bolt size and type shall be performed for each class of masonry in which they are proposed to be used. The allowable tension value for such anchors shall be 40 percent of the average value of the tests for each size and type of bolt and class of masonry.
Shear bolts may be similarly prequalified. The test procedure shall comply with ASTM E 488-90 or such other procedure as is approved by the Building Official.
The allowable values determined in this manner may exceed those set forth in Table 4B-E.
415B.5 Reports. Results of all tests shall be reported. The report shall include the test results as related to anchor size and type, orientation of loading, details of the anchor installation and embedment, wall thickness and joist orientation.
Loading...