Shear wall structure

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Shear wall structure: 1. Regarding shear wall structures, the \"Code for Seismic Design of Buildings\", the \"Code for Design of Concrete Structures\", and the \"Technical Regulations for Concrete Structures in High-Rise Buildings\" all contain relevant provisions. The provisions in the Code for High-Rise Buildings are more extensive, and there are specific rules regarding short-limb shear walls (Article 7.1.2 with a total of 8 sub-paragraphs). For ordinary shear walls, the value hw (the height of the wall section; in the author’s view this should be referred to as the \"length of the wall section\"), divided by bw (the thickness of the wall section), is generally greater than 8. The height of the wall section should not exceed 8 meters, and longer shear walls should have openings made in them (i.e., so-called structural openings) (as specified in Article 7.1.5 of the High-Rise Building Code). For short-leg shear walls, the ratio hw/bw ranges from 5 (the author believes that a value of 4 is more appropriate based on traditional practices) to 8; the seismic resistance rating should be increased by one level. When hw/bw＜5 (the author believes 4 is more reasonable according to old practices), it is considered an irregular column. For L-shaped, cross-shaped shear walls, etc., as long as one of their limbs meets the requirements of a regular shear wall, they should not be considered short-limb shear walls. 2. Clause 7.1.1 of the High-Rise Building Code stipulates that \"the lateral stiffness of shear wall structures should not be too high.\" A recently reviewed residential building with a shear wall structure had 26 floors and a total height of 79.50 meters; it was entirely constructed using shear walls, with no filler walls except for those around doors and windows. Its first period was only 1.02 seconds. The high lateral stiffness led to excessive seismic loads, which was uneconomical and unreasonable. 3. Regarding the thickness of the underlying shear walls: Clause 7.1.2 of the High-Rise Building Code stipulates that \"high-rise building structures should not use shear wall systems composed entirely of short-leg shear walls.\" When there are many short-leg shear walls, Paragraph 2 of this clause states that \"in seismic design, the bottom seismic overturning moment exerted on the core and ordinary shear walls should not be less than 50% of the total bottom seismic overturning moment.\" When performing calculations, the SATWE program evaluates the height-to-thickness ratio of each wall segment separately; those with a hw/bw ratio of 5 to 8 are classified as short-leg shear walls. As a result, the base seismic overturning moment experienced by such short-leg shear walls in the first mode of vibration can easily exceed 50%. In the TAT method, when performing calculations, if even one of the limbs of shear walls shaped like an L or similar structures meets the requirements for ordinary shear walls, such structures are not classified as short-limbed shear walls. In a given structure, the base seismic overturning moment exerted on short-limbed shear walls calculated in this way may be no more than 50% of the corresponding value; therefore, the author recommends using the TAT method to calculate this parameter. 4. The calculated reinforcement for shear walls should correspond to the reinforcement amount at one end of the wall section. In a design project carried out in Shanghai in 2000 by a certain design firm, half of the calculated reinforcement amount was used at one end; after the construction was completed, inspections for design quality in Shanghai revealed this issue, making it very difficult to carry out corrections. 5. In shear wall structures with a large number of short-leg shear walls, it is inappropriate for most designers to consider the shorter wall sections as restrained edge members or structural edge members, and to distribute the longitudinal reinforcement required for calculations evenly throughout those sections. This is because the reinforcement located near the mid-axis of the wall sections does not function effectively; therefore, the longitudinal reinforcement should be concentrated at the ends of the wall sections. It is advisable to review the calculation results regarding the reinforcement in the edge members of the shear walls. Article 6.4.9 of the Seismic Design Code stipulates that: \"When the length of a seismic wall section is not more than 3 times the wall thickness, it shall be designed in accordance with the requirements for columns, and stirrups shall be arranged densely throughout the entire height.\" Programs such as SATWE also follow this provision in their calculations. If the wall thickness is 200 mm and the length of the wall sections is 600–800 mm, although the length of the wall sections is 3–4 times the wall thickness, the author believes that it is still appropriate to use reinforcement similar to that used in columns. 6. Some people enter a reinforcement ratio of 0.30% for the vertical distribution bars in the computerized information system (the code requires a minimum of 0.25% for shear walls of grades 1, 2, and 3, and a minimum of 0.20% for shear walls of grade 4; these are mandatory requirements), but the actual reinforcement level is less than 0.30%. This is incorrect, as the reinforcement ratio of the vertical distribution bars affects the calculations related to the reinforcement of shear walls (see clauses 7.2.8–7.2.12 of the relevant code). The vertical and horizontal reinforcement bars in shear walls do not need to be very large either. For walls with a thickness of 200 or 250 mm, it seems unnecessary to use φ12@200 double-row reinforcement bars for both vertical and horizontal directions (with a reinforcement ratio of 0.565–0.452%); however, the spacing between the reinforcement bars should be ≤200 mm, which helps prevent cracking in the shear walls.