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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.
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