preface

Energy dissipation technology is to install energy dissipation devices in some parts of the structure with relatively large deformation, or design some non load-bearing components as energy dissipation components. Through the energy dissipation devices and energy dissipation components, a large amount of seismic input energy is consumed to achieve the purpose of vibration reduction [1].

Compared with the traditional seismic structure, the energy dissipation structure is safer.The energy dissipation components of traditional seismic structures are the main structural components themselves, such as frame beams in frame structures.Due to the randomness of earthquake and the errors in design, calculation and construction of structures, it is difficult to accurately control the location and degree of damage of structures in earthquakes.Especially in rare earthquakes, it is difficult to ensure the safety of load-bearing structural members.Energy dissipation components are specially set in the energy dissipation structure to consume a lot of seismic energy and reduce the seismic response of the structure, so as to effectively protect the load-bearing components of the main structure.In the energy dissipation structure, the function of different structural components is clear, which is more conducive to improve the seismic performance of the structure.

1Project overview

The project is located in a city of Yunnan Province, with 4 floors of ground floor and 2 floors of basement. The ground floor area is 17029.03m2 and the total height is 29.65m.The basement 2 is the garage, the basement 1 is the supermarket, the 1 ~ 2 floors are commercial, the 3rd floor is KTV, the 4th floor is cinema, the roof is planting roof.The structural design service life of the project is 50 years, the seismic fortification intensity is 8 degrees (0.20g), the design earthquake group is the third group, the site category is class II, the site characteristic period is 0.45s, the seismic fortification category of buildings is the key fortification category, and the steel frame structure system is adopted.

As the project is located in a high intensity area, full attention should be paid to the seismic performance of the building structure, especially the anti collapse performance of the structure under rare earthquake. In order to improve the safety of the structure under earthquake, the energy dissipation design is proposed to strengthen the seismic performance of the structure.

2Energy dissipation and shock absorption scheme

2.1Selection of energy dissipation and shock absorption technology

Steel frame structure and steel frame braced structure are widely used in high-rise buildings. However, the lateral stiffness of pure steel frame structure is limited. Under the action of earthquake and strong wind load, the lateral displacement is large, which limits its application height.Steel frame braced structure can solve the problem of lateral stiffness to a certain extent, but when the brace is pressed under the action of moderate earthquake or strong earthquake, it is easy to produce buckling phenomenon, which is very easy to cause damage or failure of the brace itself and the connection. At the same time, the hysteretic energy dissipation capacity of the brace after buckling becomes poor, which makes it difficult to effectively dissipate energy and reduce the seismic capacity of the structure.In order to solve the buckling problem of braces under compression, foreign scholars have developed a kind of brace which can avoid buckling, called buckling restrained brace (BRB).Buckling restrained brace is composed of core material, outer sleeve and unbonded material in sleeve.Although the BRB core plate is similar to the core plate, the basic form of buckling is limited.The center of the support is the core material. In order to avoid the overall buckling of the core material under compression, that is to say, it can reach the yield under tension and compression, the core material is placed in a steel casing, and then the filling material is poured into the casing. The filling material has certain strength, good compactness and superior durability.Due to overcoming the buckling characteristics of traditional braces under compression, the buckling restrained brace is a diagonal brace member with high strength in the elastic stage, and an energy dissipation component with superior performance after the core member yielding, which can effectively absorb the energy generated by earthquake [2].

The buckling restrained brace (BRB) is used as the energy dissipation component in the energy dissipation design of the project.On the one hand, it can provide enough stiffness for the structure, on the other hand, the damper can dissipate energy under strong earthquake.Although the investment of damper is large, it can enhance the structural safety of the structure, enhance the seismic capacity of the building, and improve the use environment of the building, which is of great significance to the building.

2.2Design contents of buckling restrained brace

Buckling restrained braces (BRBs) absorb seismic energy by using the yield deformation and hysteretic energy dissipation of core materials.For this project, the specific design contents mainly include:

(1) Determine the equivalent support stiffness of the structure in yjk software, determine the parameters and quantity of energy dissipation shock absorber, as well as the installation position and type of energy dissipation shock absorber;

(2) The structural response of the structure with shock absorber under frequent earthquake is calculated

(3) Elastic time history analysis was carried out to check the displacement angle under small earthquakes;

(4) Under rare earthquake, the elastic-plastic displacement is checked, and the components with insufficient bearing capacity are adjusted accordingly. Finally, the design of connecting members and structural members connected with dampers is completed.

2.3Structural damping target and performance target

In this project, the damping targets under frequent and rare earthquakes, as well as the performance objectives and design methods of components and joints connected with energy dissipation dampers

3Analysis of energy dissipation and shock absorption

3.1Layout of buckling restrained braces

Buckling restrained braces (BRBs) can absorb energy through the yield deformation of members to obtain seismic and seismic effects.Therefore, buckling restrained braces should be arranged in the position with larger axial deformation or larger relative displacement under seismic load.Under horizontal earthquake action, the overall torsional failure caused by asymmetric brace system should be avoided as far as possible. Therefore, the layout of buckling restrained brace in frame structure should follow the principles of symmetry, uniformity, periphery and dispersion.Based on the above principles, the buckling restrained braces of the project are mainly arranged at four corners, as shown in Fig. 2. Meanwhile, the buckling restrained braces are arranged continuously from 1 to 4 floors vertically to avoid the weak parts caused by stiffness weakening or sudden change.

A total of 66 buckling restrained braces are used in the project, and the elastic segments of the braces are adjusted to reduce the number of BRBs. The parameters and performances of BRBs are shown in Table 3

3.2 Elastic time history analysis of small earthquakes

In this project, SAP2000 is used to establish the structural model, calculate and analyze.SAP2000 software has the functions of convenient and flexible modeling, simulation and powerful linear and nonlinear dynamic analysis. The buckling restrained brace is simulated by nonlinear element.Through the elastic time history analysis of the damping structure, the stress of the buckling restrained brace under small earthquake is observed, and the inter story displacement angle of the structure under small earthquake is rechecked.In the elastic time history analysis, five actual strong earthquake records and two artificial simulated acceleration time history curves are selected. Seven time history curves (as shown in Figure 4) meet the relevant requirements of seismic design of buildings, such as characteristic period, peak acceleration, duration, etc.The artificial waves are synthesized according to the third group (0.45s) of class II sites, and the natural waves are all taken from the actual records of class II sites.Fig. 5 shows the results of comparison between the average fitting response spectrum of seven time history curves and the standard response spectrum.The calculation results show that the bearing capacity of the support is less than the yield bearing capacity under frequent earthquakes, and the elasticity only provides stiffness. The displacement angle of X direction is 1 / 603, and the displacement angle of Y direction is 1 / 589 (as shown in Fig. 6), which meets the predetermined structural performance target.

3.3 Elastoplastic time history analysis of large earthquakes

The elastic-plastic time history analysis of the original structure and the damping structure with buckling restrained brace under rare earthquake are carried out.In the process of elasto-plastic time history analysis, material nonlinearity is considered; small deformation assumption is adopted; and geometric nonlinearity of structure is not considered.The Hilbert Hughes Taylor step-by-step integration method provided by the program is selected to solve the differential equation of motion.In the process of elastic-plastic time history analysis, the selected seismic wave is amplitude modulated according to the code, and the peak acceleration after amplitude modulation is 400cm / S2.Three seismic waves in elastic time history analysis are selected for large earthquake analysis, and the analysis results are taken as envelope values.Plastic hinge is defined in beam and column of main structure frame.

3.3.1 Interlayer displacement angle

Under rare earthquake, the comparison of story drift angle between the structure with buckling restrained brace and the original structure without brace is shown in Table 4. The results show that the ratio of interlayer displacement angle to original structure story displacement angle under rare earthquake is 70.5% in X direction and 73.5% in Y direction. After adopting energy dissipation design, the collapse resistance of the structure under rare earthquake is greatly improved.

3.3.2 Plastic hinge distribution

In order to ensure “no collapse in a large earthquake”, the structure must have a reasonable energy dissipation mechanism under the earthquake action, allowing some components of the structure to enter the plasticity under the action of large earthquake. The energy consumption of the structure is related to the situation and sequence of the hinge out of the structure.Taking the y-direction seismic condition of San wave as an example, the results of inelastic hinge at the moment of maximum deformation are shown in Fig. 7. The results show that only a small number of frame beams enter into plasticity, and the performance of main components under rare earthquake meets the predetermined seismic performance target.

3.3.3 Output and displacement of shock absorber under strong earthquake

The output of buckling restrained brace under strong earthquake is shown in Table 5.According to the calculation results of elastic-plastic time history, under rare earthquake, the buckling restrained braces of the main structure enter into plastic energy dissipation, which reduces the damage of the main structure.It can be seen from the hysteretic curve of buckling restrained brace (see Fig. 8) that it conforms to the design intent of the structure, and the hysteretic curve is full and plays a role in dissipating seismic energy.

4.Conclusion

Through the research and analysis of engineering examples, the application of buckling restrained brace in the design of high-rise steel structure is introduced

(1) Because there is no compression stability problem, the bearing capacity of buckling restrained brace is 2-10 times higher than that of ordinary brace under small earthquake. On the basis of improving the seismic performance of the structure, the section size of the structure can be greatly reduced, and the purpose of safety and economy can be realized.

(2) Under the action of frequent earthquakes, the buckling restrained brace can provide lateral stiffness for the structure, so that the inter story displacement angle of the structure can meet the requirements of the code and meet the fortification target of small earthquake

(3) Under rare earthquake, as the first line of defense of the structure, the buckling restrained brace is the first to yield and dissipate energy to reduce the damage of the main frame structure. At the same time, the buckling restrained brace has stable bearing capacity and ductility after yielding, realizing the limited increase of structural stiffness and ductile failure mechanism.

(4) Because the buckling restrained braces are arranged outside the building, it can increase the torsional stiffness of the structure, meet the requirements of torsional displacement ratio and cycle ratio, and solve the problem of structural overrun.