Research on the Design and Application of Anti-wind devices for Seismically Isolated Buildings

Specialized anti-wind devices were designed and applied to seismically isolated buildings to solve the problem that seismic isolation buildings in areas with high wind loads could not simultaneously meet the requirements of the seismic damping objectives and the wind bearing capacity calculation. Taking an actual project as an example, anti-wind devices are added to the seismic isolation interface, a finite element analysis model of the structure is established and a time-distance analysis calculation is performed, and a synergistic design is carried out for the seismic damping effect and wind-resistant calculation. The results of the study show that the addition of anti-wind devices to reduce the number of lead rubber bearings can simultaneously satisfy the expected seismic damping effect and wind bearing capacity requirements; Numerical analysis of static loading verifies the effectiveness of anti-wind devices; This anti-wind device can be popularized for use in seismically isolated buildings in areas with high wind loads.


Introduction
Seismically isolated buildings in areas with high wind loads often need to increase the number of leadrubber bearings in order to meet the calculated wind bearing capacity of the seismic isolation interface, resulting in a reduction of the seismic isolation effect.. The standard for seismic isolation design of building(GB/T51408-2021)issued [1] in 2021 stipulates that the seismic isolation interface can be equipped with anti-wind devices that can individually resist the effect of wind loads on the superstructure.These anti-wind devices are required to increase the initial stiffness of the seismic isolation interface under normal wind loads and frequent earthquakes to satisfy the wind load carrying capacity requirements, and to be destroyed and quits work under fortification earthquakes.Some scholars at home and abroad have carried out research on the design and application of anti-wind devices.
Reference [2] proposed a anti-wind device composed of an upper and lower connecting device and a conical anti-wind rod.Experimental research suggests the use of HT250 gray cast iron production of anti-wind rods, the cross-section form using a conical cross-section of the best results, and the conical cross-section angle should be selected 60 °or 90 °.Reference [3] proposed a new series type anti-wind cable composed of gray cast iron and steel wire rope.Analysis results show that under the wind load,tensile force of anti-wind cable is far less than the design value of ultimate tensile force;The cable quits work under frequent earthquake, and post-earthquake repair only needs replace cast iron parts.
Reference [4] proposes a variable energy viscous damper, which closes the piston small hole control valve under normal wind load and utilizes the internal pressure in the cylinder to form stiffness, providing a certain degree of stiffness for the isolation layer and meeting wind resistance requirements.During an earthquake, the piston small hole control valve opens, and the medium in the cylinder is transformed into a viscous damper through the piston, thereby dissipating energy and reducing the stiffness of the isolation layer.In this article, a specialized anti-wind device was designed and applied to seismically isolated buildings.Taking an actual project as an example, anti-wind devices are added to the seismic isolation interface, a finite element analysis model of the structure is established and a time-distance analysis calculation is performed, and a synergistic design is carried out for the seismic damping effect and wind-resistant calculation.Finally, numerical analysis of static loading verifies the effectiveness of anti-wind devices.

Anti-wind device design
The anti-wind device is made of Q235 steel, and each anti-wind device consists of 2~4 pieces of antiwind steel plates and upper and lower connecting plates welded together.The anti-wind steel plate is formed into an X-shape, and the front and rear sides of the center entry area are provided with variable-section arc surface notches, thus forming a weak yield surface.The height of the anti-wind device should not be too high and should be designed as a shear piece.After a number of finite element optimization design, the anti-wind device of the example in this paper adopts 3 pieces of wind-resistant steel plate, and the bearing capacity of each anti-wind device is 250KN.The anti-wind device is shown in figure 1.The size of the anti-wind device is shown in figure 2.

Engineering examples
A school building is a 5-story, RC frame structure with a base isolation.The building plan is shown in figure 3, and the building section is shown in figure 4. The seismic precautionary intensity is 7 degrees (0.15g), the design seismic grouping is Group III, Class II site, the basic wind pressure is 0.80kN/m 2 , and the ground roughness category is Class B. Set the horizontal damping coefficient β < 0.4 .The substructure is based on a short-column scheme with an examine and repair layer of 1600 mm in height.

Arrangement of seismic isolation interface
The seismic isolation interface adopts one column and one seat program, and the diameter of seismic isolator is 500mm and 600mm.After several calculations, the final arrangement of the seismic isolation interface is shown in figure 5. LNR is a natural rubber bearing and LRB is a lead rubber bearing.Xm and Ym are the center of mass of the superstructure and Xs and Ys are the stiff centers of the seismic isolation interface.Finally, the eccentricity between the center of mass of the superstructure and the stiff centers of the seismic isolation interface is controlled to be less than 3%.

Basic period of structures.
The basic periods of isolated structure and seismic structure are shown in table 1.The calculation results show that the isolated structure can significantly prolong the structural basic period, which is about 2.5-3.5 times that of the seismic structure, and therefore can effectively reduce the seismic response of the superstructure.The floor relative displacements of the isolated structure under the rare earthquake are shown in figures 7 and 8.After calculation, the maximum interstory displacements in the Y-and X-directions of the superstructure of the seismic structure are 1/103 and 1/120, respectively, indicating that the seismic structure has undergone large elastic-plastic deformation at this time.On the other hand, the maximum inter-story displacement angles of Y-and X-directions of the isolated structure are 1/592 and 1/713 respectively, indicating that the elasticplastic deformation of the superstructure is small and the damping effect is good.In addition, it can be seen from figures 7 and 8 that the Y-direction displacement of the seismic isolation interface is 80.1 mm and the X-direction displacement is 79.4 mm under the rare earthquake, which are less than the permissible value of 257 mm, and satisfy the limit value of the seismic isolation interface displacement.

Floor acceleration under rare earthquakes.
The floor accelerations in both directions of the structure under the rare earthquake are shown in figures 9 and 10.From figures 9 and 10, it can be seen that the floor acceleration in both directions of the isolated structure is much smaller than that of the seismic structure.After calculation, the seismic reduction rate of each floor is 62.0%~77.2% in Y direction and 59.4%~78.7% in X direction, which is a significant effect of seismic reduction

Wind resistance calculation of seismic isolation interface
According to the requirements of the reference [6], the wind-resistant calculation of the seismic isolation interface of the isolated structure is required to meet the requirement： V is the horizontal force design value of the anti-wind device; wk V is the standard value of horizontal shear force of seismic isolation interface under wind load; is the partial coefficient of wind load, taken as 1.4;After calculation, the horizontal bearing capacity of the lead rubber bearing is 1845kN, and the design value of the horizontal shear force of the seismic isolation interface under wind load is 2746kN, so four wind-resistant devices are added.At the same time, considering that the horizontal shear force on the center of mass of the anti-wind device can be self-balanced as much as possible to reduce the torsional effect of the structure, the anti-wind device should be arranged along the periphery of the structure.The arrangement of anti-wind devices is shown in figure 11.

Numerical simulation of static loading of the anti-wind devices
The anti-wind device adopts Q235 steel, with an elastic modulus of 206GPa, Poisson's ratio of 0.25, yield strength of 260MPa, ultimate strength of 385MPa, and shear stress design value of 125MPa..The finite element analysis software ABAQUS is used to simulate the anti-wind device, in which the upper and lower connecting plates and the wind-resistant steel plate are simulated by C3D8 units, the bottom of the anti-wind device is constrained by solid ends, the top is constrained by the reference point-rigid body, and the horizontal force of 250kN is applied in the X-direction to simulate the actual force situation.The finite element model is shown in figure.12.The force-displacement relationship curve of the anti-wind device is analyzed and shown in figure 13.From figure 13, it can be seen that the yield load of the wind-resistant device is 330 kN, and the ultimate load is 575 kN; the yield displacement is 1.2 mm, and the ultimate displacement is 13.1 mm.The stress cloud of the anti-wind device is shown in figure 14.From figure 14, it can be seen that the maximum shear stress is 119MPa when the horizontal force of anti-wind device reaches 250kN, which is less than the allowable shear stress of steel 125MPa [7].The calculated Y-direction displacement of the seismic isolation interface is 8.30 mm, which is less than the ultimate displacement of the anti-wind device by 13.1 mm under frequent earthquake, and the Y-direction displacement of the seismic isolation interface is 31.9mm, which is greater than the ultimate displacement of the anti-wind device under fortification earthquake.Therefore, it shows that the anti-wind device is involved in the work of the seismic isolation interface under the frequent earthquake, and shear damage occurs under the fortification earthquake.Thus the effectiveness of antiwind device is verified [8].

Conclusions
The following conclusions can be drawn from the above analysis: (1)The addition of anti-wind devices to reduce the number of lead rubber bearings can simultaneously satisfy the expected seismic damping effect and wind bearing capacity requirements .
(2)The anti-wind devices perform their functions in cooperation with lead-rubber bearings under normal application and frequent earthquake conditions,but suspend the working status during the fortification earthquake.
(3) The anti-wind devices in this paper can be generalized and applied in the same type of seismically isolated buildings.

Figure 6 .
Figure 6.Finite element model of isolated structure.

Figure 7 .
Figure 7. Relative displacement of floor in Y direction.

Figure 8 .
Figure 8. Relative displacement of floor in X direction.

Figure 9 .
Figure 9. Acceleration of floor in Y direction.

Figure 10 .
Figure 10.Acceleration of floor in X direction.

Figure 12 .
Figure 12.Finite element model of the anti-wind device.

Table 1 .
Basic periods of structure.
3.3.3.Horizontal damping coefficient.Horizontal damping coefficient β is an important index in the design of isolated structure, which reflects the damping effect of the structure.The horizontal damping coefficients β of Y-and X-directions of the structure under the fortification earthquake are shown in table 2 and table 3.After calculation, the horizontal damping coefficients of Y-and X-directions are both 0.39 to meet the target of β＜0.4.