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(b) Existing Materials. (1) Unreinforced Masonry Walls. Unreinforced masonry walls analyzed in accordance with this Article may provide vertical support for roof and floor construction and resistance to lateral loads. The facing and backing of such walls shall be bonded so that not less than 4 percent of the exposed face area is composed of solid headers extending not less than 4 inches into the backing. The distance between adjacent full - length headers shall not exceed 24 inches vertically or horizontally. Where the backing consists of two or more wythes the header shall extend not less than 4 inches into the most distant wythe, or the backing wythes shall be bonded together with separate headers whose area and spacing conform to the foregoing. Tension stresses due to seismic forces normal to the wall may be neglected if the walls do not exceed the height to thickness ratio in Table 2 of this Article and the in -plane shear stresses due to seismic loads as set forth in Table 3 of this Article, and the in -plane shear stresses due to seismic loads as set forth in Table 4 of this Article. If the wall height - thickness ratio exceeds the specified limits, the wall may be supported by vertical bracing members designed in accordance with Section 7 -5 -855. The deflection of such bracing member at design loads shall not exceed one -tenth of the wall thickness. All vertical bracing members shall be attached to the 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 ten feet. The wall height may be measured vertically to bracing elements other than a floor or roof. Spacing of the bracing elements and wall anchors shall not exceed six feet. Bracing elements shall be detailed to minimize the horizontal displacement of the wall by components of vertical displacements of the floor or roof. (2) Existing Roof, Floors, Walls, Footings, and Wood Framing. Existing materials including wood shear walls utilized in the described configuration may be used as part of the lateral load resisting system, provided that the stresses in these materials do not exceed the values shown in Table 4 of this Article. (c) Strengthening of Existing Materials. New materials including wood shear walls may be utilized to strengthen portions of the existing seismic resisting system in the described configurations provided that the stresses do not exceed the values shown in Table 5 of this Article. (d) Alternative Materials. Alternative materials, designs and methods of construction may be approved by the Building Official in accordance with the provisions of the International Building Code. ORDINANCE NO. 2010 -018 6