Abstract: The seismic design of buildings has clear ductility requirements for structural members. Axial compression ratio and shear span ratio are the two most important factors affecting the ductility of members, and they are also a pair of contradictory factors.
Keywords: high-rise seismic short columns
When the story height is constant, reducing the axial compression ratio and improving the ductility will lead to the increase of the column section, and the smaller the axial compression ratio, the larger the section; However, the increase of section leads to the decrease of shear span ratio and ductility of members. Therefore, in the structural design of high-rise buildings, especially super-high-rise buildings, in order to meet the requirements of the code [1] on the limit value of axial compression ratio, the section of columns is often large, and short columns or even ultra-short columns are often formed at the bottom of the structure. In addition, short columns will inevitably appear in the design, such as the stacks of libraries, warehouses with lower floors and underground garages of high-rise buildings. Due to the heavy load, the floor is low. As we all know, the ductility of short columns is very poor, especially ultra-short columns have almost no ductility. When buildings are subjected to earthquakes with local fortification intensity or higher intensity, they are prone to shear failure, leading to structural damage or even collapse, which can not meet the design standard of "repairable under moderate earthquakes, but unable to collapse under strong earthquakes". In order to avoid brittle failure of short columns in high-rise buildings, the author thinks that short columns should be judged correctly first, and then some structural measures or treatment methods should be taken to improve the ductility and seismic performance of short columns.
Correct judgment of 1 short column
Both the specification [1] and the specification [2] stipulate that the ratio of column clear height h to section height h, H/H ≤ 4, is a short column. Many engineers and technicians in the engineering field judge short columns according to this, which is a noteworthy problem. Because the parameter to judge whether it is a short column is the shear span ratio λ of the column, only the column with shear span ratio λ = m/VH ≤ 2 is a short column, and the column with the ratio of column clear height to section height H/H ≤ 4 is not necessarily less than 2, that is, it is not necessarily a short column. The main basis for judging H/H ≤ 4 is: ① λ = m/VH ≤ 2; (2) Considering that the bending points of frame columns are mostly close to the center of the columns, if m = 0.5 VH, λ = m/VH = 0.5 VH/VH = 0.5 h/h ≤ 2, thus H/h ≤ 4. For high-rise buildings, the linear stiffness of beams and columns is relatively small, especially at the bottom, because of the influence of column bottom embedding, the constraint bending moment of beams to columns is relatively small.
When the anti-bending point of a frame column is not in the center of the column, the bending moments of the upper and lower sections of the column are different, that is, Mt≠Mb. Therefore, the shear span ratio of the upper and lower sections of the frame column is also different, that is, λ t = mt/VH ≠ λ b = MB/VH. At this time, which section shear span ratio should be used to judge whether the frame column belongs to short column? The author thinks that the upper and lower sections of frame columns should adopt larger shear-span ratio, that is, λ = max (λ t, λb). The reasons are as follows: the stress of frame column is like a continuous beam under constant axial pressure, and the column height Hn is equivalent to the shear span ratio A of continuous beam. The existing experimental results show that [10]: for continuous beams with constant shear-span ratio a, when the longitudinal reinforcement in the upper and lower sections is the same, shear failure always occurs in the section with large bending moment; For frame columns, critical oblique cracks always occur in sections with large bending moments.
In fact, in the range of column height Hn or shear span ratio A of continuous beam, the maximum shear span ratio appears in the section with large bending moment. The shear bearing capacity of reinforced concrete members decreases with the increase of shear span ratio λ. Therefore, under the same conditions, the shear bearing capacity of the section with larger bending moment is smaller than that of the section with smaller bending moment. Under the load, if shear failure occurs, it can only be in the section with large bending moment. The shear span ratio λ used to judge whether a frame column belongs to a short column should of course be the shear span ratio λ of the section where shear failure may occur.
In general, at the bottom of a high-rise building, the bending point of the frame column is on the upper side, that is, MB > mt At this time, the short column can be judged according to formula (1) or formula (2):
Or HN/h ≤ 2/yn (2)
In the formula, according to the geometric relationship, the height of the bending point of YN-N column can be obtained as follows: YN =1(1+ψ), where ψ = mt/MB, 0 ≤ψ≤1;
Clear height of HN-N column.
Formula (2) is universal. When the inflection point is in the center of the column, ψ = 1, yn = 0.5, and formula (2) becomes HN/h ≤ 4; When the inflection point is in the upper section of the column, ψ = 0, yn = 1, and formula (2) becomes HN/h ≤ 2; If there is no bending point on the frame column, the shear span ratio λ = m/VH ≤ 2 should be adopted to judge the short column. ?
When it is necessary to preliminarily judge whether a frame column belongs to a short column, the height ratio yn of the bending point of the column can be determined by the D value method first, and then the short column can be judged by the formula (2). In the stage of construction drawing design, further judgment can be made according to the computer results.
2 Measures to improve the seismic performance of short columns
When it is judged that the column is not a short column according to the shear span ratio λ, structural measures can be taken according to the seismic requirements of general frame columns; After the short column is determined, the bearing capacity of the short column should be improved as much as possible, the section size of the short column should be reduced, and various effective measures should be taken to improve the ductility and seismic performance of the short column.
2. 1 Composite spiral stirrups are adopted.
The shear bearing capacity of frame columns in high-rise buildings should meet the limit of shear-compression ratio and the requirements of "strong shear and weak bending", and the bending bearing capacity of column ends should also meet the requirements of "strong columns and weak beams". For short columns, as long as the requirements of "strong shear and weak bending" and "strong column and weak beam" are met, the shear failure can be prevented. Therefore, the seismic performance of short columns can be improved by using composite spiral stirrups [4] to improve the shear bearing capacity of columns and the constraint on concrete.
2.2 using a separation column
Because the flexural capacity of short columns is much greater than the shear capacity, they often fail due to shear failure under earthquake action, and their flexural strength cannot be fully exerted. Therefore, the bending strength of short columns can be artificially weakened to make the bending strength correspond to or slightly lower than the shear strength. In this way, under the earthquake, the column will first reach the bending strength, thus showing a ductile failure state.
In order to artificially weaken the bending strength, short columns can be divided into two or four split columns with vertical cracks in them, and each column foot of the split columns is reinforced separately. Some connection keys can be arranged between the legs of the separation column to enhance its initial stiffness and later energy consumption. Generally speaking, the connection keys are straight-through joints, prefabricated partitions, prestressed friction dampers and plain concrete connection keys.
Theoretical analysis and experimental research on the working performance of split columns show that [3 ~ 4]: Although the shear bearing capacity of columns is basically unchanged and the bending bearing capacity is slightly reduced, the deformation capacity and ductility of columns are significantly improved, and the failure mode is changed from shear type to bending type, realizing the idea of short columns becoming "long columns", and effectively improving short columns, especially the shear span ratio λ≤ 1.5. Split column method has been applied in practical engineering [5].
2.3 Steel reinforced concrete columns are adopted.
Steel reinforced concrete columns are composed of steel reinforced concrete and encased concrete. Steel skeleton usually adopts I-shaped, mouth-shaped and cross-shaped sections made by welding or directly binding steel plates.
Compared with the steel structure, the wrapped concrete of steel reinforced concrete column can prevent the local buckling of steel members, improve the overall stiffness of the column, significantly improve the out-of-plane torsional buckling performance of steel members, and give full play to the strength of steel. Using steel reinforced concrete structure can generally save more than 50% steel than steel structure [6]? . In addition, the external concrete increases the durability and fire resistance of the structure. Compared with the reinforced concrete structure, the bearing capacity of the column is greatly improved because of the existence of the steel skeleton, thus effectively reducing the section size of the column; Steel flanges and stirrups have a good restraining effect on concrete, which improves the ductility of concrete. In addition, the good plasticity of the steel skeleton itself makes the column have good ductility and energy dissipation capacity.
Because steel reinforced concrete columns give full play to the characteristics of steel and concrete, and have the characteristics of small cross-section size, light weight, good ductility and superior technical and economic indicators, if steel reinforced concrete columns are used in the lower floors of high-rise or super-high-rise reinforced concrete structures, the cross-section size of columns can be greatly reduced and the seismic performance of structures can be significantly improved.
2.4 using concrete filled steel tubular column
Concrete-filled steel tube is a composite structural material formed by filling concrete in thin-walled circular steel tube, and it is a special form of hoop concrete. Because the concrete in the steel tube is laterally restrained by the steel tube, the concrete is in a three-dimensional compression state, the compressive strength and ultimate compressive strain of concrete are greatly improved, and the ductility of concrete, especially high-strength concrete, is significantly improved. At the same time, the steel pipe is both longitudinal reinforcement and transverse stirrup, and the ratio of pipe diameter to pipe wall thickness is at least below 90, which means that the reinforcement ratio is at least above 4.6%, far exceeding the minimum reinforcement ratio limit required by the seismic code [2] for reinforced concrete columns. Because concrete-filled steel tube has excellent compressive strength and deformation capacity, even under the condition of high axial compression ratio, it can still form a "compression hinge" that develops plastic deformation in the compression zone, and there is no problem that the compression zone is destroyed first, and there is no problem of buckling instability of the compression flange like a steel column. Therefore, it is not necessary to limit the limit value of axial compression ratio in order to ensure the rotation ability of the control section [8]. Regulation [9] stipulates that the bearing capacity of concrete filled steel tubular single limb column can be calculated according to formula (3):
N≤φ 1φeN0(3)
Among them,;
θ = FAAA/FCAC is called the core insertion index, and 0.3 ≤ θ≤ 3;
See regulation [9] for the physical meaning and calculation method of φ 1 and φ e.
It can be seen from Formula (3) that the bearing capacity of columns can be greatly improved by selecting high-strength concrete and appropriate hoop index θ, and the column section is reduced by more than half compared with ordinary reinforced concrete columns, eliminating short columns and having good seismic performance.
3 abstract
1? Whether a short column is short should not be judged by H/H ≤ 4, but by the shear span ratio λ = m/VH ≤ 2. Generally speaking, Equation (2) can be used to distinguish. When it is necessary to preliminarily judge whether it belongs to a short column, the height ratio yn of the reverse bending point can be determined by the D-value method first, and then the judgment can be made by the formula (2) in this paper.
2? When it is judged that the column is not a short column according to the shear span ratio λ, structural measures can be taken according to the seismic requirements of general frame columns; If it is a short column, the bearing capacity of the short column should be improved as much as possible, the section size of the short column should be reduced, and various effective measures should be taken to improve the ductility and seismic performance of the short column. Using composite spiral stirrup and split column technology can effectively improve the seismic performance of short columns; The adoption of new structures such as steel reinforced concrete and concrete filled steel tube can significantly improve the bearing capacity of columns, reduce the cross-sectional size of columns, and avoid short columns, especially ultra-short columns, in the lower part of the structure. Therefore, in the seismic design of high-rise buildings, the above new structures and technologies should be adopted as far as possible according to the specific conditions of the project to avoid brittle failure of short columns.