In view of some conceptual and practical problems involved in the design of reinforced concrete multi-storey frame structure, such as: frame calculation diagram, determination of seismic grade, short column design, period reduction, etc., according to the current code, the corresponding solutions are put forward by using design theory and combining with practical experience. Next, let's learn about the design of reinforced concrete multi-story frame structure for buildings.

1 Problems in design and structure

(1) The stirrup configuration in the core area of frame joints should meet the requirements. Most designers can pay enough attention to the requirement of the minimum volume stirrup ratio in the stirrup encryption area of frame columns specified in the code, but it is not appropriate to specify the stirrup characteristic values of the core area of frame joints of Grade I, II and III in the Code for Seismic Design of Buildings (GB51111-2111). It is often neglected in design, especially when the axial compression ratio of columns is not large, which often fails to meet the requirements. This regulation is an important structural measure to ensure the ductility of the joint core area and should be strictly observed.

(2) The range of stirrup encryption zone of the bottom frame column should meet the requirements. The Code for Seismic Design of Buildings (GB51111-2111) stipulates that the range of stirrup encryption zone at the bottom column and column root should not be less than 1/3 of the clear height of the column. This is a new requirement, and it should be emphasized in the design that

2 The seismic grade of the structure

In the engineering design, most buildings are based on it. For telecommunications, transportation, energy, fire protection and medical treatment buildings, as well as public buildings such as large stadiums and large retail malls, first of all, we should determine which buildings belong to Class B buildings (and possibly Class A buildings, which are not involved in this article) according to the Sub-standard for Seismic Fortification of Buildings (GB51223-95).

3 Mode combination number of seismic force

For high-rise buildings, when torsional coupling calculation is not taken, at least 3 should be taken; When the number of vibration modes is more than 3, the multiple of 3 should be taken, but it should not be more than the number of layers; When the number of floors of the building is less than or equal to 2, the number of vibration modes can be selected. For irregular structures, when torsional coupling is considered, for high-rise buildings, the number of vibration modes should be ≥ 9; If there are many stories in the structure or the structural stiffness changes suddenly, the number of vibration modes should be more. For example, if the structure has a transfer floor, a small tower at the top, a multi-tower structure, etc., the number of vibration modes should be ≥ 1.2 or more, but not more than 3 times the number of stories in the building.

4 structural period reduction factor

Because of the existence of infilled walls, the actual stiffness of the structure is greater than the calculated stiffness and the calculation period is longer than the actual period. Therefore, the calculated seismic shear force is too small, which makes the structure unsafe. Therefore, it is necessary to reduce the calculation period of the structure, but it is not appropriate to reduce the calculation period of the frame structure or obtain too large a reduction factor. For frame structure, when masonry infilled wall is used, the period reduction coefficient can be 1.6 ~ 1.7; When there are few masonry infilled walls or light blocks are used, 1.7 ~ 1.8 is desirable; When light wall plates are completely used, 1.9 is desirable. Only a pure frame without walls can the calculation period not be reduced.

5 stirrup spacing between frame beams and columns

Articles 6.3.3 and 6.3.8 of the Seismic Code clearly stipulate the minimum stirrup diameter and maximum stirrup spacing in the stirrup encryption area of frame beams and columns with different seismic grades. According to these regulations, it is customary to take the maximum spacing of stirrups in beam and column encryption area as 111mm, and the maximum spacing of stirrups in non-encryption area as 211 mm.. In the general information of computer program, the spacing of stirrup encryption area of beam and column is usually 111mm, and the stirrup area of encryption area is calculated based on this, and the designer should determine the stirrup diameter and limb number according to the specification. In most cases, when there is a secondary beam in the mid-span of the framed beam or there are other large concentrated loads but only two stirrups are provided, the stirrup spacing in the non-encrypted area of the beam is 211mm, which will make the stirrup in the non-encrypted area of the beam insufficient. Therefore, it is suggested that the stirrup of the fixed beam in the program be changed to 211mm in the non-encrypted area of the beam.

6 column part

(1) If the ground is a column, the underground part should be changed to a square column to facilitate construction. The number of cylindrical longitudinal bars shall be at least 8, and the stirrups shall be spiral hoops, and it shall be indicated that the ends shall have one and a half circles of horizontal sections. Square column stirrup should use tic-tac-toe hoop, and encryption according to the specification. Angle column, stair column should increase the longitudinal reinforcement and the whole column height encryption stirrup. Square columns should not be used in kindergartens.

(2) In principle, the longitudinal reinforcement of the column should have a large diameter and a large spacing, but the spacing should not be greater than 211.

(3) Burying pipes in the column, because the longitudinal bars of the beam are anchored in the column, it is generally only possible to bury thicker pipes at the four corners of the column. When the pipe cross-sectional area accounts for less than 4% of the column cross-section, it is unnecessary to check. Heating pipes shall not be worn in the column.

7 Conclusion:

With the increasingly diversified requirements of architectural modeling and architectural functions, all kinds of difficulties encountered in post-holiday design are also increasing, whether for industrial buildings or civil buildings. As a structural designer, it is necessary to boldly and flexibly solve some difficulties and key points in structural schemes under the premise of following various specifications and constantly summarize and improve them in the work.

I believe that after the above introduction, everyone has a certain understanding of the design of reinforced concrete multi-story frame structure for buildings. Welcome to visit Zhongda Consulting for more information.

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