Lecture 5: Shear Wall Analysis and Design PDF

Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code...

Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* A- Introduction: Horizontal forces acting on buildings, e.g., those due to wind or seismic action, can be resisted by different means. Rigid-frame resistance of the structure, augmented by the contribution of ordinary masonry walls and partitions, can provide for wind loads in many cases. However, when heavy horizontal loading is likely, such as would result from an earthquake, reinforced concrete shear walls are used. These may be added solely to resist horizontal forces, or concrete walls enclosing stairways or elevator shafts may also serve as shear walls. B- Behavior of Shear Walls: Shear walls are crucial structural elements in buildings, primarily designed to resist lateral forces from wind or seismic activity. Their behavior is determined by several factors, including geometry, material properties, loading conditions, and the boundary conditions at the top and bottom. Here's an overview of the key aspects of the behavior of shear walls: 1. Lateral Load Resistance: Shear walls resist lateral loads through two main mechanisms: Flexural action (bending): Dominates slender walls with a high height-to-length ratio. These walls behave similarly to vertical cantilever beams, with horizontal reinforcement resisting flexural forces, primarily near the base of the wall. Shear action: Dominates in squat walls with a low height-to-length ratio. In this case, the shear wall resists lateral loads primarily through diagonal compression and tension, with vertical and horizontal reinforcement playing a crucial role in resisting shear forces. The balance between these two mechanisms (flexural and shear) governs the overall response of the shear wall under lateral loading. 2. Deformation and Cracking Cracking in Flexure: In slender walls, cracks form primarily due to bending stresses at the base, where the moment is highest. These flexural cracks usually propagate vertically from the base of the wall. Cracking in Shear: For squat walls, diagonal cracking due to shear forces can occur. These cracks form along diagonal planes and may lead to a brittle failure if the wall is not properly reinforced. Diagonal Tension and Compression: Shear walls subjected to lateral loads experience diagonal tension and compression forces. Properly placed diagonal reinforcement or confined boundary elements help resist these forces. 1/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* 3. Axial Load Effects Axial loads (from gravity) affect the behavior of shear walls: Compression: An axial compressive load enhances the flexural and shear capacity of the wall by increasing the contribution of concrete. This is favorable for both flexure and shear, as it delays cracking and increases the wall's stiffness. Tension: In cases where the wall is subjected to tension (such as due to seismic overturning forces), the wall's flexural capacity decreases. In extreme cases, tension may cause the wall to crack and lose its ability to carry lateral forces. 4. Nonlinear Behavior and Ductility In seismic regions, shear walls are designed for ductility to ensure that they can absorb and dissipate energy during earthquakes. The behavior under large deformations is critical, and shear walls must exhibit nonlinear behavior to survive significant seismic events. Yielding: Under seismic loading, the reinforcement in the boundary elements and along the height of the wall may yield, allowing the wall to develop inelastic deformations without collapse. This provides the structure with ductility. Plastic Hinges: In taller walls, plastic hinges form near the base during intense seismic events, allowing for controlled flexural deformations. The design of boundary elements in these regions is crucial to avoid brittle failure. 5. Boundary Elements Boundary elements are the vertical, heavily reinforced sections at the ends of a shear wall. These regions experience the highest flexural stresses and need to be designed to handle high compression or tension forces. 2/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* Confinement: In seismic design, confinement reinforcement in boundary elements is essential to prevent buckling of longitudinal bars and crushing of concrete under cyclic loading. This improves the wall’s ductility and energy absorption capacity. 6. Interaction of Shear and Flexure: Shear walls experience a combination of bending moments and shear forces, which interact to define the overall behavior of the wall. Flexure-dominated walls (tall and slender) are designed primarily for bending, but they must also be checked for shear capacity to prevent brittle shear failure. Similarly, shear-dominated walls must also resist moments, especially at the base. The interaction diagrams (moment vs. axial force) help to determine the wall's capacity under combined loading. These diagrams are useful for ensuring that the wall has enough strength to resist both flexure and shear under the applied axial loads. 7. Torsional Behavior Shear walls may also be subjected to torsional forces if there is eccentricity between the center of mass of the building and the center of rigidity of the lateral load-resisting system. This can lead to twisting of the wall, introducing additional stresses. Proper detailing of the reinforcement is necessary to handle these forces, especially in buildings with irregular layouts. core Wind pressure P x x x y y y y y y y R P e x x x x e T R T T T Central Edge Half sides Sides e= 0 e= 20.3 m e= 9.18 m e= 0 : Centroid : Center of rigidity e: Eccentricity R: Rotation T: Translation 8. Coupled Shear Walls In some cases, two or more shear walls are connected by coupling beams (short horizontal elements that connect the walls). The behavior of this system is influenced by: 3/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* Coupling Action: The beams transfer forces between the walls, allowing the system to act together to resist lateral loads. Flexural and Shear Behavior: Coupling beams typically experience high shear and must be designed to prevent premature failure. Proper reinforcement, often including diagonal bars, is necessary for ductility. 9. P-Delta Effect The P-Delta effect refers to the additional moments and forces generated in a shear wall due to its lateral displacement under loading. As the wall deflects laterally, the vertical load causes a secondary moment, amplifying the lateral forces and deflections. This effect is more significant in taller walls, and it must be accounted for in the design to prevent collapse under high lateral displacements. 10. Seismic Behavior Stiffness Degradation: During an earthquake, the stiffness of the wall may degrade due to cracking, leading to larger displacements. Shear walls designed for seismic loads must be able to maintain adequate strength and stiffness under cyclic loading. Energy Dissipation: In seismic design, shear walls are expected to undergo inelastic deformations and dissipate energy through plastic hinging and yielding of reinforcement. Proper detailing ensures that the wall can survive multiple cycles of loading without significant strength loss. 4/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* C- Shear Walls Classifications A "shear wall" is a lateral force resisting system that essentially behaves like a really wide column. Shear walls must: 1- Have enough strength to resist the factored moments, shears and axial loads acting on it. 2- Have enough stiffness to limit lateral deflections. Shear walls are classified in two ways: I- Classification according to shape. - Rectangular shear walls - Flanged shear walls - Core walls - Coupled shear walls - Framed shear walls - Infilled shear walls II. Classification according to behavior - Shear-shear wall - Moment-shear wall -Ductile moment-shear wall b bb Xs 5/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* nl C n II- Classification According to Behavior 1. Shear-shear walls Deflections and strength are controlled by shear 2. Moment-shear wall Deflections and strength are controlled by moment 3. Ductile moment-shear wall Good energy dissipation characteristics under reversed cyclic loads For elastic behavior, any of these is OK. If we expect or are designing for inelastic behavior, we need to have ductile moment-shear wall. 6/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* D- Behavior of Lineal Walls We will be focusing on lineal walls to start with (simplest); concepts can be used for other wall types, Rectangular Shear Wall (Lineal Walls). Building with wind or seismic forces represented by arrows acting on the edge of each floor or roof. The horizontal surfaces act as deep beams to transmit loads to vertical resisting elements A and B. These shear walls, in turn, act as cantilever beams fixed at their base to carry loads down to the foundation. They are 7/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* 8/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* 9/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* E- Design Considerations: hw: Shear wall height lw: Shear wall length h: Shear wall thickness lw: Shear wall length d: Distance from the extreme compression face to the tensile reinforcement center (≈ 0.8 lw) As: Vertical flexural reinforcement area Av: Horizontal shear reinforcement area S: Spacing of horizontal shear reinforcement vertical 10/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* In-plane shear Vn will be calculated by Acv :Wall gross area of concrete section bounded by web thickness and length of section in the direction of shear force. 𝐴𝐴𝑐𝑐𝑐𝑐 = 𝑙𝑙𝑤𝑤 ∗ ℎ Limit : Vn at any horizontal section shall not exceed 𝑉𝑉𝑛𝑛 ≤ 0.66 𝑓𝑓𝑐𝑐′ 𝐴𝐴𝑐𝑐𝑐𝑐 where: αc = 0.25 for hw/ℓw ≤ 1.5 αc = 0.17 for hw/ℓw ≥ 2.0 For walls subject to a net axial tension, αc Where: Nu is negative for tension. Note that: Alternatively, for walls with hw/ℓw < 2, it shall be permitted to design for in-plane shear in accordance with the strut- and-tie method. 11/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* Minimum reinforcement for walls: 𝐼𝐼𝐼𝐼 𝑉𝑉𝑛𝑛 ≤ 0.04∅ 𝛼𝛼𝑐𝑐 𝜆𝜆 𝑓𝑓𝑐𝑐′ 𝐴𝐴𝑐𝑐𝑐𝑐 Type of nonprestressed Minimum longitudinal , Minimum transverse, Wall type reinforcement Bar/wire size fy, MPa ρℓ ρt ≥420 0.0012 0.0020 Cast-in-place Deformed bars ≤ No. 16 No. 16 Any 0.0015 0.0025 Welded-wire ≤ MW200 or Any 0.0012 0.0020 reinforcement MD200 Precast Deformed bars or welded- Any Any 0.0010 0.0010 wire reinforcement Prestressed walls with an average effective compressive stress of at least 1.6 MPa need not meet the requirement for minimum longitudinal reinforcement ρℓ. In one-way precast, prestressed walls not wider than 3.6 m and not mechanically connected to cause restraint in the transverse direction, the minimum reinforcement requirement in the direction normal to the flexural reinforcement need not be satisfied. 𝐼𝐼𝐼𝐼 𝑉𝑉𝑛𝑛 > 0.04∅ 𝛼𝛼𝑐𝑐 𝜆𝜆 𝑓𝑓𝑐𝑐′ 𝐴𝐴𝑐𝑐𝑐𝑐 ρt ≥ 0.0025 ρℓ ≥ 0.0025 + 0.5(2.5 – hw/ℓw) (ρt – 0.0025) ρl ≥ 0.0025 F- Design of lineal shear wall: - Preliminary sizing and layout 12/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* - Flexible vs. rigid diaphragm 13/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* - Distribution of moment and shear to walls - Additional shear from torsion 14/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* - Minimum wall thickness h Wall type Minimum thickness h 100 mm (a) Bearing Greater of: 1/25 the lesser of unsupported length (b) and unsupported height 100 mm (c) Nonbearing Greater of: 1/30 the lesser of unsupported length (d) and unsupported height Exterior basement and foundation 190 mm (e) Only applies to walls designed in accordance with the simplified design method of Horizontal 15/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim  Libyan Academy for Postgraduate Studies Department of Civil and Architectural Engineering/ Structural Engineering Course: Advanced Reinforced Concrete Design- A(CES605) Lecture (5): Shear wall analysis and Design Based on ACI Code ********************************************************************* C H = hw B = lw 1˂ H/B ˂ 3 H/B ˃ 3 H/B ˂ 1 Shear and Flexure Flexure Shear Edges both 16/15 Best luck folk Fall (2024-2025) Dr. Mohamed karim