Frame structure design

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Design of frame structures: 1. The cross-sections of columns and beams should be appropriate; these are determined by factors such as displacement, axial compression ratio, and reinforcement ratio. Beams with larger spans should have larger cross-sections, while those with smaller spans should have smaller cross-sections. The cross-sectional width of continuous beams should preferably be the same. The cross-section of the columns should be reduced every about 3 floors in order to save costs; each reduction should be no less than 50 mm on each side to facilitate formwork installation, and it should not exceed 200 mm to avoid sudden changes in stiffness. The topmost sections (the few upper floors) can use a size of 300 mm × 300 mm, provided that this meets the computational requirements. By reducing the cross-section of the columns, the usable area can also be increased accordingly. 2. Concrete strength grade: It is advisable to use ≥C25 (to leave some margin); the grade should be the same for columns and beams. The concrete strength grade should not change at points where the column cross-section changes, in order to avoid sudden changes in stiffness. The strength grade of the slab should not exceed C40 (as stipulated in Clause 4.5.2 of the relevant codes); Clause 1.7 of the “Technical Guidelines for Controlling Cracks in Cast-in-Place Reinforced Concrete Floors in Residential Buildings” issued in Shanghai on December 20, 2001, under document number Hu Jian Jian (2001) No. 0907, states that “the concrete strength grade of cast-in-place floors should not be higher than C30”. The “Guide to Controlling Cracks in Reinforced Concrete Structures”, compiled by the Concrete and Prestressed Concrete Committee’s Concrete Quality Subcommittee and the High-Strength and High-Performance Concrete Subcommittee of the Chinese Civil Engineering Society (first edition published by the Chemical Industry Press in April 2004), also recommends that “when ordinary concrete is used for floors and roof slabs, its strength grade should not exceed C30, while for foundation slabs and underground exterior walls, it should not exceed C35”. This is to control the amount of cement used – the higher the concrete strength grade, the more cement is required, which increases the likelihood of cracks forming. 3. Column design: 1) According to Clause 10.3.1, Article 1 of the concrete design code, the reinforcement ratio for longitudinal bars should not exceed 5%. Clause 10.3.2, Article 4 stipulates that when the reinforcement ratio for longitudinal bars is greater than 3%, there are requirements regarding the diameter, spacing, and hooks of the stirrups; these bars can also be welded into a closed loop shape (different from the requirement in Code 89 that they must be welded into a closed loop). Clause 11.1.13 states that in seismic design, this ratio should not exceed 5% ； Article 6.4.4, Paragraph 3 of the Code: It should not be greater than 5%, and certainly not more than 6%; in seismic design, it should not exceed 5%. Article 6.4.9, Paragraph 4 is similar to Article 10.3.2, Paragraph 4 of the Concrete Code, but it does not require that the stirrups be welded into a closed loop shape. 2) The clear spacing between longitudinal bars should be ≥ 50 mm (Article 10.3.1, Clause 3 of the Concrete Design Code); in seismic design, for columns with a cross-sectional dimension greater than 400 mm, the spacing between longitudinal bars should not exceed 200 mm. 3) One cross-section should have a single diameter; symmetric reinforcement is preferred to facilitate construction, and it also simplifies the design process ； The diameter of the rebar should not be larger at the top and smaller at the bottom. There’s a small 2-story building with 16 columns in total, labeled KZ1–16. The reinforcement requirements differ between the 1st and 2nd floors, and there are 32 different cross-section types. Why go to such trouble? 4) Use strong columns and weak beams; the longitudinal reinforcement should not be too small. Except for the frames on the first and second floors where φ16 or φ18 can be used, it is better to use φ20 or larger. 5) Spacing between stirrups: For seismic resistance grade 1, it should not be greater than the larger of 200 mm and 20d (where d is the diameter of the stirrup); for seismic resistance grades 2 and 3, it should not be greater than the larger of 250 mm (300 mm according to Code 89 for grade 3) and 20d; for seismic resistance grade 4, it should not be greater than 300 mm. The code does not define what \"stirrup span\" is; generally, designers consider it to be the distance between two stirrups in the horizontal direction. The spacing between the stirrup limbs should not be too small either; for columns with dimensions of 600×600, 6-limb stirrups are used, for those with 500×500 dimensions, 5-limb stirrups are used, and for columns with 400×400 dimensions, 4-limb stirrups are used. Using too many stirrups is unnecessary and can also affect the pouring of concrete. It is possible to place the main rebar alternately with stirrups in order to save steel. 6) Stirrup ratio: The new code specifies a higher value than the 1989 code; it is related to the axial compression ratio of the column, the concrete strength grade, and the tensile design strength of the stirrups. 7) Use the planar representation; do not use a tabular format (the tabular method in the 03G101-1 manual is also not intuitive and makes review difficult). 4. Beam and slab design: “Discussions on the Construction of Reinforced Concrete Structures” (I)–(IV) in issues No. 5 of 2003 to No. 2 of 2004 of PKPM New World, as well as “Exploration of Measures to Prevent Cracks in Reinforced Concrete Beams” in issue No. 6 of 2001 of PKPM New World, and “Exploration of Control Measures for Cracks in Cast-in-Place Reinforced Concrete Floor Slabs” in issue No. 1 of 2002 of PKPM New World, etc. 5. Stairs in reinforced concrete structures: 1) They cannot be supported by masonry. 2) Supported by a \"small frame,\" the beams and columns should meet the requirements for seismic resistance at level 3 (stirrups ≥ φ6@150). 6. Structural columns (GZ) in reinforced concrete structures: 1) The upper end of the column should be weakly connected to the beam and slab; it is acceptable if there is no connection at all, or a 1φ12 connection can be used. The upper end of the GZ should be separated from the beam and slab by 20–30 mm, otherwise it will affect the stress conditions of the beam and slab at that upper end. 2) The stirrups of GZ do not need to be reinforced; it is not a seismic component (some standard catalogs specify reinforced stirrups). 3) GZ must first have filler walls constructed (with interlocking joints) before the pouring is carried out; it is highly inappropriate for the construction party to carry out the pouring first. 7. Length of the tie bars in masonry infill walls within reinforced concrete structures: The rules applicable to masonry structures cannot be used; instead, clause 2 of section 13.3.3 of the seismic design codes should be followed – for seismic intensity levels 6 and 7, the length of these tie bars should be no less than 1/5 of the wall’s length and no less than 700 mm; for seismic intensity levels 8 and 9, it is advisable for these tie bars to run throughout the entire length of the wall. 8. In reinforced concrete structures, components such as elevator machine rooms and water tanks cannot be supported by masonry; this is a mandatory requirement specified in the relevant codes.