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SECTION 2106A SEISMIC DESIGN

2106A.1 Seismic design requirements for masonry.


Masonry structures and components shall comply with the requirements in Section 1.14.2.2 and Section 1.14.3, 1.14.4, 1.14.5, 1.14.6 or 1.14.7 of ACI 530/ASCE 5/TMS 402 depending on the structure's seismic design category as determined in Section 1613A. All masonry walls, unless isolated on three edges from in-plane motion of the basic structural systems, shall be considered to be part of the seismic-force-resisting system. In addition, the following requirements shall be met.

2106A.1.1 Basic seismic-force-resisting system.


Buildings relying on masonry shear walls as part of the basic seismic-force-resisting system shall comply with Section 1.14.2.2 of ACI 530/ASCE 5/TMS 402 or with Section 2106A.1.1.1, 2106A.1.1.2 or 2106A.1.1.3.

2106A.1.1.1 Ordinary plain prestressed masonry shear walls.


Not permitted by OSHPD and DSA-SS.


2106A.1.1.2 Intermediate prestressed masonry shear walls.


Not permitted by OSHPD and DSA-SS.


2106A.1.1.3 Special prestressed masonry shear walls.


Not permitted by OSHPD and DSA-SS.


2106A.2 Anchorage of masonry walls.


Masonry walls shall be anchored to the roof and floors that provide lateral support for the wall in accordance with Section 1604A.8.2.

2106A.3 Seismic Design Category B.


Structures assigned to Seismic Design Category B shall conform to the requirements of Section 1.14.4 of ACI 530/ASCE 5/TMS 402 and to the additional requirements of this section.

2106A.3.1 Masonry walls not part of the lateral-force-resisting system.


Masonry partition walls, masonry screen walls and other masonry elements that are not designed to resist vertical or lateral loads, other than those induced by their own mass, shall be isolated from the structure so that the vertical and lateral forces are not imparted to these elements. Isolation joints and connectors between these elements and the structure shall be designed to accommodate the design story drift.

2106A.4 Additional requirements for structures in Seismic Design Category C.


Structures assigned to Seismic Design Category C shall conform to the requirements of Section 2106A.3, Section 1.14.5 of ACI 530/ASCE 5/TMS 402 and the additional requirements of this section.

2106A.4.1 Design of discontinuous members that are part of the lateral-force-resisting system.


Columns and pilasters that are part of the lateral-force-resisting system and that support reactions from discontinuous stiff members such as walls shall be provided with transverse reinforcement spaced at no more than one-fourth of the least nominal dimension of the column or pilaster. The minimum transverse reinforcement ratio shall be 0.0015. Beams supporting reactions from discontinuous walls or frames shall be provided with transverse reinforcement spaced at no more than one-half of the nominal depth of the beam. The minimum transverse reinforcement ratio shall be 0.0015.

2106A.5 Additional requirements for structures in Seismic Design Category D.


Structures assigned to Seismic Design Category D shall conform to the requirements of Section 2106A.4, Section 1.14.6 of ACI 530/ASCE 5/TMS 402 and the additional requirements of this section.

2106A.5.1 Loads for shear walls designed by the working stress design method.


When calculating in-plane shear or diagonal tension stresses by the working stress design method, shear walls that resist seismic forces shall be designed to resist 1.5 times the seismic forces required by Chapter 16A. The 1.5 multiplier need not be applied to the overturning moment.

2106A.5.2 Shear wall shear strength.


For a shear wall whose nominal shear strength exceeds the shear corresponding to development of its nominal flexural strength, two shear regions exist.

For all cross sections within a region defined by the base of the shear wall and a plane at a distance Lw above the base of the shear wall, the nominal shear strength shall be determined by Equation 21A -1.

Vn
= An rn fy   ( Equation 21A-1)

The required shear strength for this region shall be calculated at a distance Lw /2 above the base of the shear wall, but not to exceed one-half story height.

For the other region, the nominal shear strength of the shear wall shall be determined from Section 2108A.

2106A.5.3 Modifications to ACI 530/ASCE 5/TMS 402


2106A.5.3.1 ACI 530/ASCE 5/TMS 402, Section 1.14.6.3.


Replace Section 1.14.6.3 as follows:


1.14.6.3—Minimum reinforcement requirements for masonry walls. The total area of  reinforcement in reinforced masonry walls shall not be less than 0.003 times the sectional area of  the wall. Neither the horizontal nor the vertical reinforcement shall be less than one-third of  the total. Horizontal and vertical reinforcement bars shall be spaced at not more than 24 inches (610 mm) center to center. The minimum reinforcing shall be No. 4, except that No. 3 bars may be used for ties and stirrups. Vertical wall steel shall have dowels of  equal size and equal matched spacing in all footings. Reinforcement shall be continuous around wall corners and through intersections. Only reinforcement which is continuous in the wall shall be considered in computing the minimum area of  reinforcement. Reinforcement with splices conforming to ACI 530/ASCE 5/TMS 402 Section 2.1.10.7, as modified by Section 2107A, shall be considered as continuous reinforcement.


Horizontal reinforcement shall be provided in the top of  footings, at the top of  wall openings, at roof  and floor levels and at the top of  parapet walls. For walls 12 inches (nominal) (305 mm) or more in thickness, reinforcing shall be equally divided into two layers, except where designed as retaining walls. Where reinforcement is added above the minimum requirements, such additional reinforcement need not be so divided.


In bearing walls of  every type of  reinforced masonry, there shall not be less than one No. 5 bar or two No. 4 bars on all sides of, and adjacent to, every opening which exceeds 16 inches (406 mm) in either direction, and such bars shall extend not less than 48 diameters, but in no case less than 24 inches (610 mm) beyond the corners of  the opening. The bars required by this paragraph shall be in addition to the minimum reinforcement elsewhere required.


When the reinforcement in bearing walls is designed, placed and anchored in position as for columns, the allowable stresses shall be as for columns.


Joint reinforcement shall not be used as principal reinforcement in masonry designed by the strength design method.


2106A.5.3.2 ACI 530/ASCE 5/TMS 402 Section 1.14.6.5.


Replace Section 1.14.6.5, as follows:


1.14.6.5—Minimum reinforcement for masonry columns. The spacing of  column ties shall be as follows: not greater than 8 bar diameters, 24 tie diameters, or one half  the least dimension of  the column for the full column height. Ties shall be at least 3/8 inch (10 mm) in diameter and shall be embedded in grout. Top tie shall be within 2 inches (51 mm) of  the top of  the column or of  the bottom of  the horizontal bar in the supported beam.


2106A.5.4 Lateral support.


Lateral support of  masonry may be provided by cross walls, columns, pilasters, counterforts or buttresses where spanning horizontally or by floors, beams, girts or roofs where spanning vertically. Where walls are supported laterally by vertical elements, the stiffness of  each vertical element shall exceed that of  the tributary area of  the wall.


The clear distance between lateral supports of  a beam shall not exceed 32 times the least width of  the compression area.


2106A.6 Additional requirements for structures in Seismic Design Category E or F.


Structures assigned to Seismic Design Category E or F shall conform to the requirements of Section 2106A.5 and Section 1.14.7 of ACI 530/ASCE 5/TMS 402.