Shear wall and its design guidelines

Editor's Notes

1. The lateral stiffness (force/unit displacement) due to unit displacement. at a story is assumed to be the total lateral stiffness of that story. 
2. Seismic Performance Category (SPC). Varies from A to E, depending on how the structure is expected to behave during the event of an earthquake which in turn requires different level of detailing requirement. Seismic Design Category (SDC). Varies from A to F, depending on how design and detailing is carried out. Seismic Soil Type. Varies from A to F, depending on how the waves travel through the soil. Seismic Zone. 0, 1, 2A, 2B, 3 and 4, depending on maximum ground acceleration of a particular area. Low Seismic Risk. Corresponds to zones 1 and 2A of UBC-97, seismic design category (SDC) A and B and Seismic performance category (SPC) A and B. Moderate Seismic Risk. Corresponds to zones 2B of UBC-97, seismic design category (SDC) C and Seismic performance category (SPC) C. High Seismic Risk. Corresponds to zones 3 and 4 of UBC-97, seismic design category (SDC) D, E and F and Seismic performance category (SPC) D and E. Lateral Load Resisting System. Intermediate Moment Frame. A cast in place frame that can resist lateral loads by flexural action of its beams and columns and that can provide some energy dissipation, relatively better inelastic behavior and intermediate ductility in addition to the properties of ordinary moment frames. These frames must satisfy ACI 21.3. Ordinary Moment Frame. A cast in place frame that can resist smaller loads by flexural strength of columns. However, inelastic behavior in the event of severe earthquake is poor. It is not recommended for seismic zones with larger expected seismic activity. These frames must satisfy ACI 21.2. Special Moment Frame. A cast in place frame, having column and shear walls, that can resist lateral loads by flexure of its members and has excellent inelastic behavior. The design and detailing of special moment frames is controlled by ACI 21.5 to 21.8. Structural Walls. Walls proportioned to resist combination of shears, moments, and axial forces induced by earth quake motions. A shear wall is a structural wall. Intermediate Pre cast structural wall. A precast wall with intermediate inelastic deformation capabilities. Requirements for these walls are given in ACI 21.4. Ordinary reinforced concrete structural wall. It is used to resist lateral loads but very limited inelastic deformation capabilities. Ordinary structural plain concrete wall. It is used for basement walls for residential or light commercial buildings in areas where seismic risk is very low. These wall may be supported by soil, footings, grade beams or other structural members. Special precast structural wall. A precast wall complying with the requirements of ACI 21.8. in addition the requirements of ordinary reinforced concrete structural and requirements of ACI 21.2 is satisfied. Special reinforced concrete structural wall. A cast in place wall that can be used in high seismic activity regions and has good inelastic deformation capabilities. It fulfilled ACI 21.9.
3. SDC-D, E, F : Precast and in-situ structural diaphragms and trusses, foundations, frame members, in situ and precast special structural wall requirements, anchors and special moment resisting frame are satisfied. The design of this category may be used for zones 3 and 4 with sharp earth quake. SDC-A: Simple moment resisting frames or ordinary structural walls are used. Seismic zone 0 and 1 correspond to lowest seismic hazard. SDC-B: Ordinary moment resisting frame. Seismic zone 0 and 1 correspond to lowest seismic hazard. SDC-C: Precast structural wall, anchor requirements and moment resisting frame Structures. Seismic zone 2A and 2B correspond to moderately strong ground shaking.
4. SDC-A: Simple moment resisting frames or ordinary structural walls are used. Provisions of ACI chapter 21 is not applicable. The design of this category may be used for zones 0 and 1 corresponds to lowest seismic hazard. SDC-B: Analysis and design requirements and ordinary moment resisting frame provisions of ACI chapter 21 applicable. The design of this category may be used for zones 0 and 1. SDC-C: Analysis and design requirements, precast structural wall requirements, anchor requirements and moment resisting frame provisions of ACI chapter 21 applicable. Structures assigned to SDC-C may be subjected to moderately strong ground shaking. The design of this category may be used for zones 2A and 2B. SDC-D, E, F : Requirements for analysis and design, materials, precast and in-situ structural diaphragms and trusses, foundations, frame members not part of lateral load resisting system, in situ and precast special structural wall requirements, anchors and special moment resisting frame are satisfied. The design of this category may be used for zones 3 and 4. Almost all provisions of ACI chapter 21 is applicable. The design and detailing requirement must be compatible with the level of energy dissipation (toughness) assumed in the computation of response modification factor (R) required to calculate design earth quake forces.
5. Specific Material Requirements. According to ACI 21. 1. 4 to 21. 1. 7 the minimum value of the specified compressive strength of concrete, 𝑓^′ 𝑐 must be 21 MPa (only for special moment frames and special structural walls). The steel must either satisfy ASTM A 706 without any extra conditions or ASTM A 615 grades 280 and 420 with following requirements. The actual yield strength must not be greater than 𝑓_𝑦 by more than 125 MPa. 𝑓_𝑢/𝑓_𝑦 ≥ 1.25 The amount of confinement reinforcement must be calculated with the maximum value of 𝑓_𝑦𝑡 equal to 690 MPa in order to limit the width of shear cracks. ACI articles 21. 1. 6 to 21. 1. 7 deal with mechanical splices, welded splices and anchoring to concrete, respectively
6. Special Reinforced Concrete Structural Walls (Shear Walls) and Coupling Beams. (Interstory drift is the relative displacement of one story relative to the other) These are reinforced concrete walls that are used for the following purposes. To reduce the relative inter story drifts due to seismic forces acting on the multistory buildings. To reduce damage to the non-structural components by controlling the story drifts. To reduce the columns moments due to lateral loads and 2nd order effects. To provide the additional function of carrying axial loads. To be used as separate walls in two mutually perpendicular directions or to combine together as a lift wall, this can resist lateral loads in two mutually perpendicular directions. Shear wall are most common type of members used to resist lateral loads in tall buildings consisting of flat plate roof slab. In shear walls, in order to resist diagonal tension cracking shear reinforcement is provided and strength of such walls is controlled by flexural resistance. The length of wall acts as depth in providing the resisting stiffness. Such walls actually act as deep vertical cantilever beams providing shear and bending moment capacities for deflection in their own plan, which in turn provide lateral stability to whole of structure. The location of shear wall is carefully planned to make the structure symmetric and avoid any twisting of structure. The center of stiffness of the shear walls must coincide with their center of mass.
7. These are the element that are added along the edges of the structural walls to increase their strength and ductility as shown in Fig 24.6. These may entirely be placed within the thickness of the wall or may have a large cross-section. However these elements always have much larger longitudinal reinforcement and transverse hoops or spirals.
8. Coupling Beams. These are specially designed beams used to connect shear walls/ piers together to provide additional stiffness and energy dissipation. Coupling beams connecting structural walls can provide stiffness and energy dissipation.
9. Piers. A vertical wall segment bounded by two window opening is called a pier. If ℎ_𝑤 is clear height and 𝑙_𝑤 is horizontal length and 𝑏_𝑤 is the thickness of the web of the vertical wall segment, the categorization of walls in to simple walls or wall piers may be made by the criteria to follow. When ℎ_𝑤/𝑙_𝑤 < 2.0 when 𝑙𝑤/𝑏_𝑤 >2.0 but 𝑙𝑤/𝑏_𝑤 >6, the design of member is carried out as a wall. However when ℎ_𝑤/𝑙_𝑤 ≥ 2.0 and 𝑙𝑤/𝑏_𝑤 ≤6, the member is design as a wall pier where ACI 21.9.8.1 is to be additionally satisfied
10. Behavior of Shear Wall Depending on Height to Length Ratio. The shear wall having height to length ratio less than 2.0 is called low rise shear wall. In this type only shear strength of wall is utilized and mode of failure is shown in Fig 24.7. The shear wall having height to length ration greater than 2 is called high rise shear wall and it provides flexural resistance by acting as a cantilever beam as shown in Fig 24.8.
11. Requirement of Boundary Element of Special RC structural walls. There are two cases for the boundary element requirement, first is based on displacement and second is based on stress level. Case 1. Based on Displacement. For the wall or piers that are effectively continuous from the base of structure to top of wall and designed to have a single critical section for flexure and axial loads, boundary element must be provided if: C ≥ 𝒍𝒘/(𝟔𝟎𝟎(𝝏/𝒉𝒘)) Where, lw = length of wall hw = height of wall ∂= design displacement C= the largest N.A depth calculated from factored axial force and nominal moment strength consistent with the design displacement ∂. The minimum value of 𝝏/𝒉𝒘 is 0.007.
12. Case 2. Based on Stress Level. Special boundary elements must be provided at boundaries of walls and edges of piers where the maximum extreme fiber compressive strength corresponding to earth quake load combination, exceeds 0.2𝑓^′ 𝑐. these elements may be discontinued vertically where calculated compressive strength is less than 0.15𝑓^′ 𝑐.
13. Effective Flange Width Of Walls. flange width from the face of web is taken equal to smaller of: One half the distance to an adjacent wall web 25 percent of the total wall height = hw/4
14. Trial Required Area Of Shear Wall. The shear stress in the shear walls may be allowed up to ∅𝑥0.83√𝑓^′ 𝑐. A value of ∅𝑥0.6√𝑓^′ 𝑐 may be tentatively be taken to calculate the first trial area of the shear walls Acv. ∅ for shear = 0.60 Let V = total service shear of a story or base shear for the lowest floor, N then V x 1.1 = ∅𝑥0.6√(𝑓^′ 𝑐) x Acv Acv = (𝑉 𝑥 1.1)/(0.6 𝑥 0.6√(𝑓^′ 𝑐) 𝑥 〖10〗^6 ) 𝑚^2 For w = thickness of shear wall, mm Total length of shear walls required in one direction = (𝐴𝑐𝑣 𝑥 1000)/𝑤 𝑚
15. Minimum Recommended Length of Shear Walls. For length the value of factor ῥ is calculated which is between 1 and 1.5 𝑝=2− 6.1/(𝑟_𝑚𝑎𝑥√𝐴_𝐵 )   For ῥ ≤ 1.0𝑙𝑤 ≥ 0.5 𝑉𝑤/𝑉𝑠√𝐴_𝐵 For ῥ ≤ 1.25 𝑙𝑤 ≥ 0.375 𝑉𝑤/𝑉𝑠√𝐴_𝐵 For ῥ ≤ 1.5𝑙𝑤 ≥ 0.25 𝑉𝑤/𝑉𝑠√𝐴_𝐵   Vs = total storey height Vw= wall shear 𝐴_𝐵 = ground floor area of structure, 𝑚^2 𝑟_𝑚𝑎𝑥 = maximum element story shear ratio (ri). 𝑟𝑖= maximum value of (3.05 𝑥 𝑉𝑠)/(𝑙𝑤 𝑥 𝑉𝑠) .
16. Shear Strength of Special Structural Walls. Design forces 𝑉𝑢 is obtained from the lateral load analysis in using the factored load combinations. Shear strength 𝑉𝑛 of structural wall is calculated as follow. 𝑉𝑛=𝐴𝑐𝑣 ( 𝛼 λ √(𝑓^′ 𝑐) +𝑝𝑡 𝑓𝑦)   ≤0.83 𝐴𝑐𝑤 √(𝑓^′ 𝑐) for any one of the individual wall piers ≤0.83 𝐴𝑐𝑤 √(𝑓^′ 𝑐) for horizontal wall segments and coupling beams. ≤0.83 𝐴𝑐𝑣 √(𝑓^′ 𝑐) for all wall piers sharing a common lateral force.
17. Axial and Flexural Strengths of Special Structural Walls. The combined axial and flexural strengths of special structural walls are calculated just like an ordinary eccentrically loaded column, except that ACI 10.3.6 and the non-linear strain requirements of ACI 10.2.2 need not to apply.
18. Minimum Vertical and horizontal Reinforcement. If 𝑉𝑢≥0.83 λ 𝐴𝑐𝑣 √(𝑓^′ 𝑐) the distributed minimum web reinforcement ratios, (𝑃𝑙)𝑚𝑖𝑛 𝑎𝑛𝑑(𝑃𝑡) min⁡〖𝑎𝑟𝑒 𝑒𝑞𝑢𝑎𝑙 𝑡𝑜 0.0025.〗 Maximum reinforcement spacing in each direction must be equal to 450 mm. Reinforcement contributing to Vn must be continuous and must be distributed across the shear plane. If 𝑉𝑢>0.17 λ 𝐴𝑐𝑣 √(𝑓^′ 𝑐) at least two curtains of reinforcement must be used in the wall. 𝑉𝑢 ≤ 0.83 λ 𝐴𝑐𝑣 √(𝑓^′ 𝑐) 𝑡ℎ𝑎𝑛 (𝑃𝑙)𝑚𝑖𝑛=0.002 𝑓𝑜𝑟 𝑑𝑒𝑓𝑜𝑟𝑚𝑒𝑑 𝑏𝑎𝑟𝑠 𝑢𝑝 𝑡𝑜 𝑁𝑜.16 𝑎𝑛𝑑 𝑓𝑦 not less than 420 Mpa and 0.0015 for other deformed bars. (𝑃𝑡)𝑚𝑖𝑛=0.0012 𝑓𝑜𝑟 𝑑𝑒𝑓𝑜𝑟𝑚𝑒𝑑 𝑏𝑎𝑟𝑠 𝑢𝑝 𝑡𝑜 𝑁𝑜.16 𝑎𝑛𝑑 𝑓𝑦 not less than 420 Mpa and 0.0025 for other deformed bars.
19. Development of Steel Reinforcement. Reinforcement in structural walls must be developed or spliced for fy in tension in accordance with following modifications. The effective depth of the member referenced in ACI 12.10.3 can be taken equal to 0.8 𝑙𝑤 for walls. The requirement of ACI 12.11, 12.12 and 12.13 need not be satisfied. At location where yielding of longitudinal reinforcement is likely to occur as a result of lateral displacements, development lengths of longitudinal reinforcement must be 1.25 times the values calculated for 𝑓𝑦 in tension. For normal weight concrete the value of development length with hook is 𝑙_𝑑ℎ= (𝑓𝑦 𝑥 𝑑𝑏)/(5.4 √𝑓′𝑐) ≥8 𝑑𝑏 ≥150𝑚𝑚 Development length of bars without hook for bar Nos. 10 to 36 is 𝑙_𝑑ℎ=2.5 𝑙_𝑑ℎ 𝑤ℎ𝑒𝑛 𝑐𝑜𝑛𝑐𝑟𝑒𝑡𝑒 𝑏𝑒𝑙𝑜𝑤 𝑏𝑎𝑟𝑠≤300𝑚𝑚 𝑙_𝑑ℎ=3.5 𝑙_𝑑ℎ 𝑤ℎ𝑒𝑛 𝑐𝑜𝑛𝑐𝑟𝑒𝑡𝑒 𝑏𝑒𝑙𝑜𝑤 𝑏𝑎𝑟𝑠>300𝑚𝑚
20.   Design of Boundary elements. In case of special boundary element following requirements must be satisfied. Horizontal dimensions of boundary element from the extreme compression fiber = larger of (𝑐−0.1𝑙𝑤)𝑎𝑛𝑑 𝑐/2 Where c = the largest neutral axis depth calculated for the factored axial force and nominal moment strength consistent with design displacement. The height 𝑙𝑜 from the joint face on both side and extend up to the points where flexural yielding is expected due to in-elastic lateral displacement. 𝑙𝑜 = largest of following three 𝑙𝑤 at the joint face where flexural yielding is expected One sixth of the clear height of the wall 450mm In flanged sections, the boundary element includes the effective flange width in compression plus at least 300 mm extension in to web. Horizontal reinforcement in the wall web is anchored to develop fy with in the confined core of the boundary element. The transverse reinforcement at the wall base must extend in to the support at least equal to the development length of largest longitudinal reinforcement in the special boundary element.