SECTION 408B – ALLOWABLE DESIGN VALUES

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.