SECTION 412B – ANALYSIS AND DESIGN

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.

SECTION 413B – DETAILED SYSTEM DESIGN REQUIREMENTS

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.

SECTION 414B – IN-PLACE MASONRY SHEAR TESTS

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.

SECTION 415B – TEST OF ANCHORS IN UNREINFORCED MASONRY WALLS

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.

SECTION 416B – POINTING OF UNREINFORCED MASONRY WALLS

416B.1  Scope.  Pointing of deteriorated mortar joints when required by this chapter shall be in accordance with this section.

416B.2  Joint Preparation.  The old or deteriorated mortar should be cut out, by means of a toothing chisel or non-impact power tool, to a uniform depth of ¾ inch (19.1 mm) until sound mortar is reached. Care shall be taken not to damage the brick edges. After cutting is completed, all loose material shall be removed with a brush, air or water stream.

416B.3  Mortar Preparation.  The mortar mix shall be Type N or S proportions as required by the construction specifications. The pointing mortar shall be pre-hydrated by first thoroughly mixing all ingredients dry, and then mixing again, adding only enough water to produce a damp unworkable mix which will retain its shape when pressed into a ball. The mortar shall be kept in a damp condition for 1½ hours; then sufficient water shall be added to bring it to a proper consistency that is somewhat drier than conventional masonry mortar.

416B.4  Packing.  The joint into which the mortar is to be packed shall be damp but without freestanding water. The mortar shall be tightly packed into the joint in layers not exceeding ¼ inch (6.35 mm) in depth until it is filled; then it shall be tooled to a smooth surface to match the original profile.

FIGURE 4B-1 – ACCEPTABLE DIAPHRAGM SPAN

NOTE: To convert feet to meters, multiply by 0.3048.

   FIGURE 4B-2 – ANALYSIS OF UNREINFORCED MASONRY WALL IN-PLANE SHEAR FORCES

   TABLE 4B-A – ELEMENTS REGULATED BY THIS CHAPTER

ELEMENTS	SECTION	PROCEDURE
		BOLTS-PLUS	SPECIAL	GENERAL	301.22
Masonry Shear Strength	406B.3.3	X	X	X	X
Diaphragms	410B.1			X
	411B.4		X
	301.2			X
Diaphragm Shear Transfer	410B.1	X1
	411B.5	X1	X
	413B.2	X	X	X
Chords	411B.4			X
Diaphragm Capacity Ratios	SFBC 1604		X
Collectors	413B.3			X
	411B.4		X
	301.2				X4
Analysis of Vertical Elements	412B		X	X
Crosswalls	411B.3		X
Shear Walls	410B.3		X
	411B.6		X
	301.2				X3, 4
Out of Plane Wall Anchorage	413B.1	X	X	X
	301.2			X
Ties & Continuity	413B.4		X	X
	301.2			X	X
Wall Bracing	413B.5	X	X	X	X5
Parapets	413B.6	X	X	X	X
Veneer	413B.7	X	X	X	X
Nonstructural Masonry Walls	413B.8		X	X	X
Truss & Beam Supports	413B.9		X	X	X
Adjacent Buildings	413B.10		X	X	X
Subdiaphragms	301.2				X
	SFBC 1604.8.2
	ASCE 7-10 Section 12.11.24
1   Diaphragm shear transfer forces shall be calculated using the General Procedures unless the building qualifies for the use of the Special Procedure.
2   Retrofit procedure per SFEBC Section 301.2.
3   Wood shear walls allowed only for one- or two-story building per San Francisco Building Code Section 2305.1.
4   Only in-plane shear check required. (Rocking not allowed.)
5   Use (h/t) for “All other walls” from Table 4B-B.

 

TABLE 4B-B – ALLOWABLE VALUE OF HEIGHT-TO-THICKNESS RATIO  OF UNREINFORCED MASONRY WALLS

WALL TYPES	BUILDINGS WITH CROSSWALLS1	ALL OTHER BUILDINGS
Walls of one-story buildings	162,3	13
First story wall of multistory buildings	16	15
Walls in top story of multistory buildings	142,3	9
All other walls	16	13
1   Applies to the Special Procedure of Section 411B and the Bolts-plus Procedure of the last paragraph of Exception 1 to Section 409B.2 only. See Section 411B.7 for other restrictions.
2   This value of height-to-thickness ratio may be used only where mortar shear tests establish a tested mortar shear strength, vt, of not less than 100 psi (689.48 kPa). This value may also be used where the tested mortar strength is not less than 60 psi (413.69 kPa) and a visual examination of the collar joint indicates not less than 50 percent mortar coverage.
3   Where a visual examination of the collar joint indicates not less than 50 percent mortar coverage, and the tested mortar shear strength, vt, is greater than 30 psi (206.84 kPa) but less than 60 psi (413.69 kPa), the allowable height-to-thickness ratio may be determined by linear interpolation between the larger and smaller ratios in direct proportion to the tested mortar strength.

 

   TABLE 4B-C – HORIZONTAL FORCE FACTOR, Cp1

 

CONFIGURATION OF MATERIALS	Cp
Roofs with straight or diagonal sheathing and roofing applied directly to the heathing, or floors with straight tongue-and-groove sheathing	0.50
Diaphragms with double or multiple layers of boards with edges offset, and blocked plywood systems	0.75
1   Applicable to the Special Procedure of Section 411B only.

 

   TABLE 4B-D – ALLOWABLE VALUES FOR EXISTING MATERIALS

EXISTING MATERIALS OR CONFIGURATION OF MATERIALS1	ALLOWABLE VALUES
1.   HORIZONTAL DIAPHRAGMS2	(x 14.5939 for N/m)
a.    Roofs with straight sheathing and roofing applied directly to the sheathing	100 pounds per foot seismic shear
b.   Roofs with diagonal sheathing and roofing applied directly to the sheathing	250 pounds per foot seismic shear
c.    Floors with straight tongue-and-groove sheathing	100 pounds per foot seismic shear
d.   Floors with straight sheathing and finished wood flooring with board edges offset or perpendicular	500 pounds per foot seismic shear
e.    Floors with diagonal sheathing and finished wood flooring	600 pounds per foot seismic shear
2.    CROSSWALLS2,3	(x 14.5939 for N/m)
a.   Plaster on wood or metal lath	per side: 200 pounds per foot seismic shear
b.   Plaster on gypsum lath	175 pounds per foot seismic shear
c.   Gypsum wallboard, unblocked edges	75 pounds per foot seismic shear
d.   Gypsum wallboard, blocked edges	125 pounds per foot seismic shear
3.   EXISTING FOOTINGS, WOOD FRAMING, STRUCTURAL STEEL AND REINFORCE STEEL	(x 6.895 for kPa)
a.   Plain concrete footings	f ´c = 1,500 psi unless otherwise shown by tests4
b.   Douglas fir wood	Allowable stress same as D.F. No. 14
c.   Reinforcing steel	ft = 18,000 psi maximum4
d.   Structural steel	ft = 20,000 psi maximum4
1   Material must be sound and in good condition.
2   A one-third increase in allowable stress is not allowed
3   Shear values of these materials may be combined, except the total combined value shall not exceed 300 pounds per foot (2068.43 kPa).
4   Stresses given may be increased for combinations of loads as specified in this code

 

TABLE 4B-E – ALLOWABLE VALUES OF NEW MATERIALS USED IN CONJUNCTION WITH EXISTING CONSTRUCTION

NEW MATERIALS OR CONFIGURATIONS OF MATERIALS	ALLOWABLE VALUES1
1.   HORIZONTAL DIAPHRAGMS10	(x 14.5939 for N/m)
a.    Plywood sheathing nailed directly over existing straight sheathing with ends of plywood sheets bearing on joists or rafters and edges of plywood located on center of individual sheathing boards	225 pounds per foot seismic shear
b.   Plywood sheathing nailed directly over existing diagonal sheathing with ends of plywood sheets bearing on joists or rafters	375 pounds per foot seismic shear
c.   Plywood sheathing nailed directly over existing straight or diagonal sheathing with ends of plywood sheets bearing on joists or rafters with edges of plywood located over new blocking and nailed to provide a minimum nail penetration into framing and blocking of 1 5/8 inches (41.28 mm)	75 percent of the values specified in San Francisco Building Code Table 2306.2.1(1)
2.   SHEAR WALLS: (GENERAL PROCEDURE)
Plywood sheathing applied directly over wood studs. No value shall be given to plywood applied over existing plaster or wood sheathing.	100 percent of the value specified in San Francisco Building Code Table 2306.2.1(2) for shear walls
3.   CROSSWALLS: (SPECIAL PROCEDURE)
a.   Plywood sheathing applied directly over wood studs. No value shall be given to plywood applied over existing plaster or wood sheathing	133 percent of the value specified in San Francisco Building Code Table 2306.2.1(2) for shear walls
b.   Drywall or plaster applied directly over wood studs	100 percent of the values in San Francisco Building Code Table 2306.3(3)
c.   Drywall or plaster applied to sheathing over existing wood studs	The values in San Francisco Building Code Table 2306.3(3) reduced as noted in Footnote a of that table2
4.   TENSION BOLTS	(x 4.448 for N)
a.   Bolts extending entirely through unreinforced masonry walls secured with bearing plates on far side of a 3 wythe minimum wall with at least 30 square inches (19,355 mm2)* of area3,4,11	1,800 pounds per bolt 8
	900 pounds per bolt for 2 wythe walls8
b.   Bolts extending to the exterior face of the wall with a 2½-inch (63.5 mm) round plate under the head and drilled at an angle of 22½ degrees to the horizontal, installed as specified for shear bolts3,4,5	1,200 pounds per bolt
5.   SHEAR BOLTS
Bolts embedded a minimum of 8 inches into unreinforced masonry walls and centered in a 2½-inch diameter hole filled with dry-pack or non- shrink grout. Through bolts with first 8 inches as noted above and embedded bolts as noted in item 4b. 4,5,9	½ inch dia. = 350 pounds7,8
	5/8 inch dia. = 500 pounds7,8
	¾ inch dia. = 750 pounds7,8
6.   INFILLED WALLS
Reinforced masonry infilled openings in existing unreinforced masonry walls. Provide keys or dowels to match reinforcing.	Same values as for unreinforced masonry walls
7.    REINFORCED MASONRY
Masonry piers and walls reinforced per San Francisco Building CodeChapter 21	Same values as specified in San Francisco Building Code Section 21076
8.   REINFORCED CONCRETE
Concrete footings, walls and piers reinforced as specified in San Francisco Building CodeChapter 19 and designed for tributary loads	Same values as specified in San Francisco Building Code Chapter 19 6

 

1   A one-third increase in allowable stress is not allowed, except as noted.

2   In addition to existing sheathing value.

3   Bolts to be ½ inch (1.27 mm) minimum in diameter.

4   Drilling for bolts and dowels shall be done with an electric rotary drill. Impact tools shall not be used for drilling holes or tightening anchors and shear bolt nuts.

5   Embedded bolts to be tested as specified in Section 407B.

6   Stress given may be increased for combinations of load as specified in this code.

7   A one-third increase in allowable stress is allowed for short-term loading.

8   Other bolt sizes, values and installation methods may be used, provided a testing program is conducted in accordance with Section 415B. Bolt spacing shall not exceed 6 feet (1.83 m) on center and shall not be less than 12 inches (0.305 m) on center.

9   Tension and shear from seismic loads need not be assumed to act simultaneously.

10   Values and limitations are for nailed plywood. Higher values may be used for other approved fastening systems such as staples when approved by the Building Official.

11   Plate size may be reduced to not less than 9 square inches (5805 mm2), provided the bearing stress on the masonry at design load does not exceed 60 pounds per square inch, psi (414 kPa).

(Amended by Ord. 65-19, File No. 190136, App. 4/12/2019, Eff. 5/13/2019)

CODIFICATION NOTE

*   So in Ord. 228-16.