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Chapter 4B
SEISMIC STRENGTHENING PROVISIONS FOR UNREINFORCED MASONRY BEARING WALL BUILDINGS
SEISMIC STRENGTHENING PROVISIONS FOR UNREINFORCED MASONRY BEARING WALL BUILDINGS
(NOTE: The time limits for compliance with the provisions of Chapters 4A and 4B have passed, but the ordinance and the time limits therein are still in effect.)
The purpose of this chapter is that stated in Section 401A.
402B.1 General. The seismic strengthening of unreinforced masonry bearing wall buildings shall comply with the provisions of this chapter when strengthening either is mandated by Chapter 4A or is done voluntarily under Section 405A.6. The elements regulated by this chapter shall be determined in accordance with Table 4B-A. Except as provided herein, other structural provisions of this code shall apply.
402B.2 Essential and Hazardous Facilities. The provisions of this chapter are not intended to apply to the strengthening of buildings or structures in Risk Categories III and IV of ASCE 7-10 Table 1.5-1. Such buildings or structures shall be strengthened to meet the requirements of this code for new buildings of the same occupancy category or to such other criteria as has been established by the Building Official.
402B.3 Unreinforced Masonry Private School Buildings. The strengthening of unreinforced masonry private school buildings shall comply with Sections 17320-17336 of the California Education Code.
402B.4 Qualified Historical Buildings. Qualified historical buildings shall be strengthened to comply with this chapter or the alternative provisions contained in Title 24, California Code of Regulations, Part 8, the State Historical Building Code.
402B.5 Party Wall Buildings. In buildings separated by party walls, all segments sharing the party walls shall be strengthened at the same time whenever feasible. When such action is not feasible, a party wall in any segment undergoing strengthening shall be provided with the capacity to resist a reasonable estimate of the shear forces generated by the adjacent unstrengthened segments.
402B.6 Buildings of Mixed Construction. When buildings having at least one bearing wall of unreinforced masonry also utilize other structural systems, the following requirements shall apply:
402B.6.1 Masonry-wood or steel mix. When the lower stories of the building are of unreinforced masonry bearing wall construction and the upper stories are of wood frame or steel stud construction, the unreinforced masonry stories shall be strengthened to meet the requirements of the general procedure of this chapter and the other stories need not be strengthened.
402B.6.2 Masonry-concrete mix. When a building is of mixed unreinforced masonry bearing wall construction and reinforced concrete or masonry construction, the entire building shall be strengthened in accordance with a program developed by the owner’s architect or engineer and approved by the Building Official.
For the purpose of this chapter, the applicable definitions in this code shall also apply.
COLLAR JOINT is the vertical space between adjacent wythes and may contain mortar.
CROSSWALL is a new or existing wall that meets the requirements of Section 411B.3. A crosswall is not a shear wall.
CROSSWALL SHEAR CAPACITY is the allowable shear value times the length of the crosswall, vcLo.
DIAPHRAGM EDGE is the intersection of the horizontal diaphragm and a shear wall.
DIAPHRAGM SHEAR CAPACITY is the allowable shear value times the depth of the diaphragm, vuD.
ESSENTIAL FACILITY is any building or structure classified in Risk Category IV of ASCE 7-10 Table 1.5-1.
HAZARDOUS FACILITY is any building or structure classified in Risk Category III of 7-10 Table 1.5-1.
NORMAL WALL is a wall perpendicular to the direction of seismic forces.
OPEN FRONT is an exterior building wall line, without vertical elements of the lateral force resisting system in one or more stories.
PARTY WALL is a wall common to two or more buildings located on separate parcels of land.
POINTING is the partial reconstruction of the bed joints of an unreinforced masonry wall as defined in Section 416B.
QUALIFIED HISTORICAL BUILDING is a building or structure as defined in the June 1, 1990, Edition of Title 24, California Code of Regulations, Part 8, Section 8-302.
UNREINFORCED MASONRY includes burned clay, concrete or sand-lime brick, hollow clay or concrete block, plain concrete and hollow clay tile. These materials shall comply with the requirements of Section 406B.
UNREINFORCED MASONRY WALL is a masonry wall in which the area of reinforcing steel is less than 25 percent of the minimum steel ratios required by this code for reinforced masonry. To qualify, reinforcing steel must have been installed in grouted cells within the masonry.
UNREINFORCED MASONRY BEARING WALL is an unreinforced masonry wall which provides the vertical support for a floor or roof for which the total superimposed load exceeds 200 pounds per linear foot (298 kg/m) of wall.
YIELD STORY DRIFT is the lateral displacement of one level relative to the level above or below at which yield stress is first developed in a frame member.
404B.1 For the purpose of this chapter, the applicable symbols and notations in this code shall apply.
A = cross sectional area of unreinforced masonry pier or wall, square inches.
Ab = total area of the bed joints above and below the test specimen for each in-place shear test.
Cp = numerical coefficient as specified in Table 4B-C for Special Procedure diaphragm shear transfer.
D = in-plane width dimension of pier, inches, or depth of diaphragm, feet.
DCR = demand-capacity ratio specified in Section 411B.4.2.
Fwx = force applied to a wall at level x, pounds.
H = least clear height of opening on either side of a pier, inches.
h/t = height-to-thickness ratio of an unreinforced masonry wall. Height, h, is measured between wall anchorage levels and/or slab-on-grade.
L = span of diaphragm between shear walls, or span between shear wall and open front, feet.
Lo = length of crosswall, feet.
Li = effective span for an open front building specified in Section 411B.8, feet.
PD = superimposed dead load at the location under consideration, pounds. For determination of the rocking shear capacity, dead load at the top of the pier under consideration shall be used.
pD+L = stress resulting from the dead plus actual live load in place at the time of testing, pounds per square inch (psi).
Pw = weight of wall, pounds.
Va = vaA, the allowable shear in any unreinforced masonry pier, pounds.
Vca = total shear capacity of crosswalls in the direction of analysis immediately above the diaphragm level being investigated, åvcLo, pounds.
Vcb = total shear capacity of crosswalls in the direction of analysis immediately below the diaphragm level being investigated, åvcLo, pounds.
Vp = shear force assigned to a pier on the basis of its relative shear rigidity, pounds.
Vr = pier rocking shear capacity of any unreinforced masonry wall or wall pier, pounds.
Vtest = load at incipient cracking for each in-place shear test per Section 414B, pounds.
Vwx = total shear force resisted by a shear wall at the level under consideration, pounds.
va = allowable shear stress for unreinforced masonry, pounds per square inch (psi).
vc = allowable shear value for a crosswall sheathed with any of the materials given in Table 4B-D or 4B-E, pounds per foot.
vt = mortar shear strength as specified in Section 406B.3.3.4, pounds per square inch (psi).
vto = mortar shear test values as specified in Section 406B.3.3.4, pounds per square inch (psi).
vu = allowable shear value for a diaphragm sheathed with any of the materials given in Table 4B-D or 4B-E, pounds per foot.
vuD = sum of diaphragm shear capacities of both ends of the diaphragm, pounds.
vuD = for diaphragms coupled with crosswalls, vuD includes the sum of shear capacities of both ends of diaphragms coupled at and above the level under consideration.
W = total seismic dead load as defined in San Francisco Building CodeChapter 16, pounds.
Wd = total dead load tributary to a diaphragm, pounds.
wd = total dead load to all the diaphragms at and above the level under consideration, pounds.
Ww = total dead load to an unreinforced masonry wall above the level under consideration or above an open front building, pounds.
Wwx = dead load of an unreinforced masonry wall assigned to Level x halfway above and below the level under consideration, pounds.
405B.1 General. All buildings shall have a seismic resisting system conforming with ASCE 7-10 Section 12.2, except as modified by this chapter.
405B.2 Alterations and Repairs. Alterations and repairs required to meet the provisions of this chapter shall comply with all other applicable structural requirements of this code unless specifically provided for in this chapter.
405B.3 Requirements for Plans. In addition to the requirements of San Francisco Building Code
Section 106A.3.3 of this code, the following construction information shall be included in the plans required by this chapter:
1. Dimensioned floor and roof plans showing existing walls and the size and spacing of floor and roof framing members and sheathing materials. The plans shall indicate all existing and new crosswalls and shear walls and their materials of construction. The location of these walls and their openings shall be fully dimensioned and drawn to scale on the plans.
2. Dimensioned wall elevations showing openings, piers, wall classes as defined in Section 406B.3.3.6, thickness, heights, wall shear test locations, and cracks or damaged portions requiring repairs. Where the exterior face is veneer, the type of veneer, its thickness and its bonding and/or ties to the structural wall masonry shall also be noted.
3. The type of interior wall and ceiling materials and framing.
4. The extent and type of existing wall anchorage to floors and roof when used in the design.
5. The extent and type of parapet and appendage corrections which were previously performed, if any.
6. Repair details, if any, of cracked or damaged unreinforced masonry wall walls required to resist forces specified in this chapter.
7. All other plans, sections and details necessary to delineate required retrofit construction.
8. The design procedure used shall be stated on both the plans and the permit application.
9. Details of the anchor prequalification program required by Section 415B, if utilized, including location and results of all tests.
10. In buildings with party walls, the details of construction on both sides of each party wall shall be shown. Where required by Section 411B.1, Item 5 the owners’ consent statements shall be included with the plans.
406B.1 General. All materials permitted by this chapter, including their appropriate allowable design values and those existing configurations of materials specified herein, may be utilized to meet the requirements of this chapter.
406B.2 Existing Materials. All existing materials utilized as part of the required vertical load-carrying or lateral force-resisting system shall be in sound condition or shall be repaired or removed and replaced with new materials. All unreinforced masonry materials shall comply with the following requirements:
1. The construction (lay-up) of the masonry units complies with Section 406B.3.2 and the quality of bond between the units has been verified to the satisfaction of the Building Official.
2. Concrete masonry units are verified to be load-bearing units complying with ASTM Standard Specification C 90 or such other standard as is acceptable to the Building Official.
3. Hollow clay tile units are verified to be structural load-bearing units complying with ASTM Standard Specification C 34 or such other standard as is acceptable to the Building Official.
4. The compressive strength of plain concrete walls shall be determined based on cores taken from each class of concrete wall. The location and number of tests shall be the same as prescribed for strength tests in Sections 406B.3.3.2 and 406B.3.3.3.
406B.3 Existing Unreinforced Masonry Walls.
406B.3.1 General. All unreinforced masonry walls utilized to carry vertical loads or seismic forces parallel and perpendicular to the wall plane shall be tested as specified in this section. All masonry that does not meet the minimum standards established by this chapter shall be removed and replaced with new materials, repaired or alternatively shall have its structural functions replaced with new materials and shall be anchored to supporting elements.
406B.3.2 Construction (lay-up) of walls.
406B.3.2.1 Multi-wythe solid brick. The facing and backing shall be bonded so that not less than 10 percent of the exposed face area is composed of solid headers extending not less than 4 inches (101.6 mm) into the backing. The clear distance between adjacent full-length headers shall not exceed 24 inches (609.6 mm) vertically or horizontally. Where the backing consists of two or more wythes, the headers shall extend not less than 4 inches (101.6 mm) into the most distant wythe or the backing wythes shall be bonded together with separate headers whose area and spacing conform to the foregoing. Wythes of walls not bonded as described above shall be considered as veneer. Veneer wythes shall not be included in the effective thickness used in calculating the height to thickness and the shear capacity of the wall.
406B.3.2.2 Grouted or ungrouted hollow concrete or clay block and structural hollow clay tile. These materials shall be laid in a running bond pattern.
Other lay-up patterns may be acceptable if their performance can be justified as being at least equal to those specified above.
406B.3.3 Mortar.
406B.3.3.1 Tests. The quality of mortar in all masonry walls shall be determined by performing in-place shear tests in accordance with Section 414B. Alternative methods of testing may be approved by the Building Official for masonry walls other than brick.
406B.3.3.2 Location of tests. The shear tests shall be taken at locations representative of the mortar conditions throughout the entire building, taking into account variations in workmanship at different building height levels, variations in weathering of the exterior surfaces, and variations in the condition of the interior surfaces due to deterioration caused by leaks and condensation of water and/or by the deleterious effects of other substances contained within the building. The exact test locations shall be determined at the building site by the engineer or architect in responsible charge of the structural design work. An accurate record of all such tests and their location in the building shall be recorded, and these results shall be submitted to the Department for approval as part of the structural analysis.
406B.3.3.3 Number of tests. The minimum number of tests per class shall be as follows:
1. At each of both the first and top stories, not less than two tests per wall or line of wall elements providing a common line of resistance to lateral forces.
2. At each of all other stories, not less than one test per wall or line of wall elements providing a common line of resistance to lateral forces.
3. In any case, not less than one test per 1,500 square feet (139.355 m2) of wall surface nor less than a total of eight tests.
406B.3.3.4 Minimum quality of mortar.
1. Mortar shear test values, vto, in psi shall be obtained for each in-place shear test in accordance with the following equation:
vto = (Vtest/Ab) - p
D+L (4B-1)
2. The mortar shear strength, vt, is the value in psi that, after discarding the lowest 20 percent of the mortar shear test values, vto, is the lowest of the remaining 80 percent of the mortar shear test values.
3. Any unreinforced masonry bearing wall with vto, or with mortar shear strength, vt, less than 30 psi (206.84 kPa) shall be either removed, entirely pointed and retested or have its structural function replaced and shall be anchored to supporting elements in accordance with Section 406B.3.1 and Section 413B.8. When existing mortar in any wythe is pointed to increase its shear strength and retested, the condition of the mortar in the adjacent bed joints of the inner wythe or wythes and the opposite outer wythe shall be examined for extent of deterioration. The shear strength of any wall class shall be no greater than that of the weakest wythe of that class.
(Amended by Ord. 65-19, File No. 190136, App. 4/12/2019, Eff. 5/13/2019)
406B.3.3.5 Collar joints. The collar joints shall be inspected at the test locations during each in-place shear test, and estimates of the percentage of the surfaces of adjacent wythes which are covered with mortar shall be reported along with the results of the in-place shear tests.
406B.3.3.6 Unreinforced masonry classes. All existing unreinforced masonry shall be categorized into one or more classes based on quality of construction, state of repair, deterioration and weathering. A class shall be characterized by the allowable masonry shear stress determined in accordance with Section 408B.2. Classes shall be defined for whole walls, not for small areas of masonry within a wall.
406B.3.3.7 Pointing. All deteriorated mortar joints in unreinforced masonry bearing walls shall be pointed according to Section 416B. Nothing shall prevent pointing of any deteriorated masonry wall joints before the tests are made, except as required in Section 407B.1.
407B.1 Pointing. All preparation and mortar pointing shall be performed with special inspection.
EXCEPTION: At the discretion of the Building Official, incidental pointing may be performed without special inspection.
407B.2 Masonry Shear Tests. In-place shear tests shall comply with Section 414B.
407B.3 Existing Wall Anchors. Existing wall anchors utilized as all or part of the required tension anchors shall be tested in pullout according to Section 415B. The minimum number of anchors tested shall be four per floor, with two tests at walls with joists framing into the wall and two tests at walls with joists parallel to the wall, but not less than 10 percent of the total number of existing tension anchors at each level.
407B.4 New Bolts. Twenty-five percent of all new embedded bolts resisting only shear forces in unreinforced masonry walls shall be tested using a calibrated torque wrench in accordance with Section 415B.
EXCEPTION: The number of bolts tested may be reduced to 10 percent when special inspection in accordance with Section 1704 is provided during installation but in no case shall less than two bolts per 500 square feet (46.45 m2) of wall or four bolts per wall be tested.
All new embedded bolts resisting tension forces or a combination of tension and shear forces shall be subject to periodic special inspection in accordance with San Francisco Building Code Section 1704 prior to placement of the bolt and grout or adhesive in the drilled hole. Five percent of all embedded bolts resisting tension forces, but not less than two bolts, shall be subject to a direct tension test and an additional 20 percent, but not less than three bolts, shall be tested using a torque calibrated wrench. Testing shall be performed in accordance with Section 415B.
New through bolts and existing bolts installed under the Parapet Safety Program need not be tested.
408B.1 Allowable Values.
408B.1.1 Existing materials. Allowable values for existing materials are given in Table 4B-D, and for new materials in Table 4B-E.
408B.1.2 Values not specified. Allowable values not specified in this chapter shall be as specified elsewhere in this code.
408B.2 Masonry shear. The allowable unreinforced masonry shear stress, va shall be determined for each masonry class from the following equation:
va = 0.1vt + 0.15P
D
/A (4B-2)
The mortar shear test value, vt, shall be determined in accordance with Section 406B.3.3, and shall not exceed 100 psi (689.476 kPa) for the determination of va.
The one-third increase in allowable values of this code for short-term loading is not allowed for va.
(Amended by Ord. 65-19, File No. 190136, App. 4/12/2019, Eff. 5/13/2019)
408B.3 Masonry Compression. Where any increase in dead plus live compression stress occurs, the allowable compression stress in unreinforced masonry shall not exceed 100 psi (689.476 kPa). The one-third increase in allowable stress of this code is allowed.
408B.4 Masonry Tension. Unreinforced masonry shall be assumed as having no tensile capacity.
408B.5 Unreinforced Masonry Materials Other Than Solid Brick. The provisions of this chapter are primarily intended for brick construction but are also applicable to other unreinforced masonry materials when the following conditions are satisfied:
1. The building does not exceed two stories in height.
2. In the case of hollow concrete and clay block, the shear stress is limited to that permitted by Equations 4B-1 and 4B-2 based on the net area in contact through the bed joints but not more than that calculated using a mortar shear strength, vt, of 100 psi (689.476 kPa).
3. In the case of plain concrete, the compressive strength (f´c) shall be not less than 900 psi (6,205.28 kPa) and the allowable shear strength is limited to not more than 0.02f´c.
4. In the case of all other unreinforced masonry materials, the shear stress is limited to 3 psi (20.684 kPa) based on the net area in contact through the bed joint.
Unreinforced masonry not meeting the above criteria shall have its structural function replaced and shall be resupported, if required, in accordance with Section 413B.8.
408B.6 Existing Tension Anchors. The allowable resistance values of the existing anchors shall be 40 percent of the average of the tension tests of existing anchors having the same wall thickness and joist orientation. The one-third increase in allowable value of this code is not allowed for existing tension anchors.
408B.7 Foundations. For existing foundations, new total dead loads may be increased over existing dead load by 25 percent. New total dead load plus live load plus seismic forces may be increased over existing dead load plus live load by 50 percent.
EXCEPTION: In buildings located in poor soil areas as defined in Chapter 4A, any increase in dead load shall require an evaluation of the existing foundation system.
Higher values may be justified only in conjunction with a geotechnical investigation. A foundation investigation shall be also submitted with the building permit application when:
1. A building has an existing full or partial pile supported, or similar foundation system or whenever the installation of such a system is proposed as part of the strengthening.
2. Whenever there is evidence of significant distress attributable to foundation or geotechnical conditions.
3. An investigation is required by San Francisco Building Code Section 1803 or 1804.
4. It is desired to prove that the building is not on poor soil as permitted by the exception to Section 403A.
409B.1 General. Except as modified herein, the analysis and design relating to the structural alteration of existing buildings shall be in accordance with this code.
EXCEPTIONS:
1. A building may be strengthened to the Bolts-plus level by complying only with the requirements for wall anchorage (tension bolts), diaphragm shear transfer (shear bolts) and out-of-plane wall and parapet and appendage bracing, provided the entire building complies with all of the following requirements:
(1) The building does not have any vertical irregularities of Types 1a or 1b (Soft Story), 4 (In-Plane Discontinuity) or 5a or 5b (Weak Story) as defined in ASCE 7-10 Table 12.3-2 or horizontal irregularities of Types 3 (Diaphragm Discontinuity) or 4 (Out-of-Plane Offset) as defined in ASCE 7-10 Table 12.3-1 or those irregularities are corrected.
(2) The building does not contain any Group A Occupancies with an occupant load of 300 or more, or Group E, Group I or Group H-1, H-2 or H-4 Occupancies.
(3) The building has a mortar shear strength, vt, as determined by Section 406B.3.3, of 30 psi (206.843 kPa) or more for all masonry classes.
(4) The building has wood or plywood diaphragms at all levels above the base of building.
(5) The building contains 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: In an otherwise qualifying building of greater than six stories, a maximum of six of the uppermost contiguous stories may be retrofitted using the Bolts-Plus Procedure, providing the building is not located on poor soil as defined in Section 403A. The masonry walls required by Item 7 below shall occupy not
less than 50 percent of the wall length in the lowest two of the uppermost six stories. Nonqualifying stories and stories below the uppermost six shall be retrofitted to any other procedure for which they qualify.
(6) The building has or will be provided with crosswalls as defined in Section 411B.3 at a spacing that does not exceed 40 feet (12.192 m) on center. Any story which does not have or is not provided with complying crosswalls and all stories below that story shall be analyzed using the General Procedure of Section 410B or, where applicable, the Special Procedure of Section 411B. The floor structure that separates the Bolts-Plus and General or Special Procedure stories shall be investigated for its adequacy to act as a diaphragm in accordance with Section 410B.1 or, where the Special Procedure is applicable, Section 411B.4.
(7) The building has or will be provided with a minimum of two lines of vertical elements of the lateral force resisting system parallel to each axis. Masonry walls shall have wall piers with a height-to-width ratio that does not exceed 2 to 1 and shall occupy not less than 40 percent of the wall’s length in order to be considered as providing a line of resistance. Existing moment frames and other lines of resistance added or altered to comply with this requirement shall fully comply with Section 412B. At least one line in each direction shall be a masonry or concrete shear wall.
(8) In buildings containing one or more party walls, the Bolts-Plus 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.
When the Bolts-Plus Procedure is applicable, the forces to be used for diaphragm shear transfer and irregularity correction shall be those specified in Sections 411B.5 and 411B.6 and h/t ratios shall be evaluated in accordance with Section 411B.7. When the intersection of the diaphragm span and demand capacity ratio falls outside the three regions of Figure 4B-1, the h/t ratios for “all other buildings” in Table 4B-B shall be used. The measures used to comply shall be part of, and be coordinated with, the complete strengthening scheme described in the engineering report required by Section 404A.2.3.
2. Buildings which are strengthened to conform to the requirements of SFEBC Section 301.2 in effect on or after May 21, 1973, are exempt from com- pliance with the provisions of this chapter.
410B.1 Minimum Design Lateral Forces. Buildings shall be analyzed to resist minimum lateral forces assumed to act nonconcurrently in the direction of each of the main axes of the structure in accordance with the following:
V = 0.10 W (4B-3)
EXCEPTION: The lateral forces need not exceed those prescribed by San Francisco Building Code
Section 1613.
For buildings more than one story in height, the total force shall be distributed over the height of the building in accordance with the procedures of San Francisco Building CodeChapter 16.
For the purpose of this chapter, a dynamic analysis need not be performed for those buildings with irregularities, as defined in ASCE 7-10 Table 12.3-2 and ASCE 7-10 Table 12.3-1 which would otherwise require such analysis. All other design and analysis requirements of those tables shall apply.
410B.2 Lateral Forces on Elements of Structures. Parts of structures shall be analyzed and designed as required in San Francisco Building Code
Chapter 16
.
EXCEPTIONS:
1. Unreinforced masonry walls for which height-to-thickness ratios do not exceed ratios set forth in Table 4B-B need not be analyzed for out-of-plane loading. Unreinforced masonry walls which exceed the allowable h/t ratios of Table 4B-B shall be braced according to Section 413B.5.
2. Parapets complying with Section 413B.6 need not be analyzed for out-of-plane loading.
3. Out-of-plane anchorage of the walls shall be designed to 0.3 times the mass of the wall.
410B.3 Shear Walls (In-Plane Loading). Shear walls shall comply with Section 412B.
410B.4 Chords. When required by the structural analysis, chord forces of horizontal diaphragms shall be developed in existing materials or by the addition of new materials.
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.
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