Summary
Highlights
The video presents an example of designing a bearing wall, focusing on calculating its axial load capacity using a specific formula. It explains how to determine effective wall length (b) and wall thickness (t), and how these values are used in the formula. The design must ensure that the wall's axial load strength (strength) is greater than the applied load (enemy). It also covers the check for bearing strength at concentrated load points, ensuring the wall can withstand localized pressure. Reinforcement details for bearing walls are discussed, including minimum vertical and horizontal steel ratios and maximum spacing limits to prevent excessive cracking.
The discussion covers minimum reinforcement requirements, including vertical and horizontal steel ratios for different rebar sizes. It also details reinforcement spacing limits, emphasizing that rebar should not be spaced more than three times the wall thickness or 450 mm. For thicker walls (over 250 mm), two layers of reinforcement are required on both faces, parallel to the wall's surfaces. Minimum wall thickness requirements are also specified for bearing walls (1/25 of the smaller clear distance between supports or 100 mm) and basement walls (200 mm).
The segment transitions to shear wall design, highlighting their role in resisting lateral forces like wind and seismic loads. It explains how floor slabs transfer horizontal shear to shear walls and how this load is distributed based on the walls' stiffness. The video briefly mentions the importance of considering torsion in irregularly shaped buildings or those with asymmetrical shear wall layouts. It also notes that shear forces, moments, and axial loads increase in lower stories due to cumulative effects of lateral loads.
The video begins by introducing the importance of walls in Korean apartment buildings, which are characterized by extensive use of walls. It differentiates between 'In-plane' and 'Out-of-plane' walls based on the direction of applied loads. In-plane loading refers to forces acting along the wall's strong axis, maximizing its moment of inertia, while Out-of-plane loading refers to forces acting perpendicular to the wall. Shear walls are primarily designed to resist in-plane loads like seismic forces, while out-of-plane walls resist loads like wind pressure, soil pressure, and water pressure. Bearing walls are primarily designed to carry vertical loads.
A detailed example of shear wall design is provided. The first step involves checking the overall shear strength (Vn) of the wall, ensuring it exceeds the applied shear force (Vu). The effective depth (d) of the shear wall for calculating shear strength is defined as 0.8 times its total length. The concrete's contribution to shear strength (Vn) is calculated separately, and if insufficient, additional reinforcement (Vs) is added. The formula for calculating the required shear reinforcement (As) is derived based on the difference between the applied shear and the concrete's shear capacity.
The video continues with the calculation of required horizontal reinforcement, demonstrating how to use a specific formula to determine the spacing of D16 rebars for shear resistance. It emphasizes that the calculated spacing must not exceed the maximum allowed spacing (400 mm or 3 times the wall thickness). The minimum horizontal and vertical reinforcement ratios for shear walls (both 0.0025) are also checked, confirming that the designed reinforcement meets these minimum requirements.
The final part of shear wall design focuses on flexural strength. The video explains that the lateral force (Vu) acting on the wall creates a significant overturning moment (Mu = Vu * h), which causes compression on one end and tension on the other. This moment can lead to crushing at the compression end if not properly designed. To assess the wall's flexural capacity, the neutral axis (c) is determined by balancing internal compressive forces from concrete and steel with tensile forces from steel. This calculation involves solving an equilibrium equation (sum of forces = 0) and considering the stress-strain behavior of concrete and steel.
After determining the neutral axis (c), the nominal moment capacity (Mn) of the section is calculated. The design moment strength (Mn) is then compared to the applied overturning moment (Mu). In the example, the initial design is found to be insufficient in flexural strength. To overcome this deficit, additional vertical reinforcement is introduced at the ends of the wall. These reinforced regions at the ends of shear walls are called 'Boundary Elements,' providing additional confinement and flexural capacity to resist the high compressive forces. The number of additional reinforcing bars (e.g., 4D22 on each side) is calculated to meet the required moment strength.
The video summarizes the key steps in shear wall design: checking overall shear strength, ensuring concrete and main reinforcement contributions, and verifying flexural strength, with additional reinforcement in boundary elements if needed. The visual representation of a shear wall with horizontal and vertical reinforcement and boundary elements is presented. The next session will delve into PM (axial force-moment) interaction diagrams for walls and the design of basement walls, including the calculation of soil pressure. The instructor emphasizes the importance of understanding soil pressure calculations for architects, as this responsibility often falls to the architectural department in building design.