12.2.1 Seismic isolation design shall select proper seismically isolated layer composed of seismic isolation device and wind resistance device according to the anticipated vertical bearing capacity, horizontal seismic absorbing coefficient and drift control requirements.
Check and calculation about the vertical bearing capacity and horizontal drift under rarely earthquake for seismic isolation support shall be carried out.
Horizontal earthquake action of structure above the seismically isolated layer shall be determined according to the horizontal shock absorbing coefficient; its standard Horizontal earthquake action of structure above the seismically isolated layer shall be determined according to the horizontal shock absorbing coefficient; its standard value of vertical earthquake action shall not be less than 20%, 30% and 40% of the representative values of gross gravity loads above the seismically isolated layer respectively for Intensity 8 (0.20g), Intensity 8 (0.30 g) and Intensity 9
12.2.2 Computational analysis on seismically isolated building design shall meet the following requirements:
1 Mass points composed of seismically isolated support and top beam slab shall be added in the calculation diagram of seismically isolation system; structure with deformation behavior of shear pattern may be adopted with shearing model (Figure 12.2.2); where the mass centre of structure above the seismically isolated layer and the seismically isolated layer rigidity center are not overlapped, twisting effect influence shall be counted in. Beam and slab structure on the top of seismically isolated layer shall be calculated and designed as part of its upper structure.
Figure 12.12.2 Calculation Diagram for Seismically Isolated Structure
2 Typically, time-history analysis method should be adopted to calculate; response spectrum characteristic and quantity of input seismic waves shall be in accordance with those specified in Article 5.1.2 of this code and the calculated results should adopt envelope values; where it is located within 10 km of earthquake fault, near-field influence coefficient shall be considered for input seismic waves and it should adopt 1.5 within 5 km, not less than 1.25 for those beyond 5 km.
3 The masonry structure and the structures similar to the basic cycle may be simply calculated according to Appendix 1 of this code.
12.2.3 Rubber seismically isolated support of seismically isolated layer shall be in accordance with the following requirements:
1 Limit horizontal drift of seismically isolated support under pressure stress detailed in Table 12.2.3 shall be greater than the relatively larger value among 0.55 times of its effective diameter and 3 times of the total rubber thickness inside the support.
2 The rigidity and damping characteristic variations of seismically isolated support shall not be greater than ±20% of the initial value after endurance test on corresponding design reference period; the creep shall not be greater than 5 % of the total rubber thickness inside support.
3 Vertical pressure stress of rubber seismically isolated support under representative values of gravity load shall not be greater than those specified in Table 12.2.3.
Table 12.2.3 Limit of Pressure Stress for Rubber Seismically Isolated Support
Building category Category A building Category B buildings Category C building
Pressure stress limit (MPa) 10 12 15
Note:
1 Design value of pressure stress shall be calculated according to the combination of permanent load and variable load; thereinto, the floor live load shall be multiplied by the reduction coefficient according to those specified in the current national standard "Load Code for the Design of Building Structures" (GB 50009).
2 Where the overturning check on structure is carried out, the effect combination of horizontal earthquake actions shall be included; for the structure requiring vertical earthquake action calculation, effect combination of vertical earthquake actions shall also be included.
3 Where the second shape coefficient (ratio of effective diameter to total rubber layer thickness) of rubber support is less than 5.0, the limit of pressure stress shall be reduced; where it is less than 5 and greater than or equal to 4, it shall be reduced by 20 %; where it is 4, less than 4 or greater than and equal to 3, it shall be reduced by 40%.
4 For the rubber support with external diameter less than 300mm, the pressure stress limit of Category C building shall be 10MPa.
12.2.4 Layout, vertical bearing capacity, lateral rigidity and damping of seismically isolated layer shall be in accordance with the following requirements:
1 Seismically isolated layer shall be installed at the bottom or under the structure; its rubber seismically isolated supports shall be arranged at the position where larger force is exerted and the space should not be oversized; its specification, quantity and distribution shall be determined through calculation according to the requirements of vertical bearing capacity, lateral rigidity and damping. Seismically isolated layer under rarely earthquake shall remain stable and should not have nonrenewable deformation; the tension stress of its rubber support under the action of both horizontal and vertical seismic of rarely earthquake shall not be greater than 1MPa.
2 Horizontal equivalent rigidity and equivalent viscous damping ratio of seismically isolated layer may be calculated according to the following formula:
h
K
=∑
Kj (12.2.4-1) epζ
=∑
Kjζ
j/Kh (12.2.4-2) Where: epζ
——the equivalent viscous damping ratio of seismically isolated layer;h
K
——the horizontal equivalent rigidity of seismically isolated layer;j
ζ
——the equivalent viscous damping ratio of seismically isolated supportj
determined through test; where the damping device is installed, corresponding damping ratio shall be included;
j
K ——the horizontal equivalent rigidity of seismically isolated support
j
(including energy-dissipated devices) determined through test.3 Where design parameters of seismically isolated support are determined through test, pressure stress limits of vertical load detailed in Table 12.2.3 shall be kept; for the calculation of horizontal shock absorbing coefficients, equivalent rigidity for 100% shear deformation and equivalent viscous damping ratio shall be adopted; for rarely earthquake check, equivalent rigidity for 250 % shear deformation and equivalent viscous damping ratio should be adopted; where the diameter of seismically isolated support is larger, equivalent rigidity for 100% shear deformation and equivalent viscous damping ratio may be adopted. Where time-history analysis is adopted, hysteretic curve acquired through test shall be regarded as reference.
12.2.5 Earthquake action calculation of structure above seismically isolated layer shall meet the following requirements:
1 For multilayer structure, the horizontal earthquake actions may be distributed along height according to the representative values of gravity load.
after seismically isolation may be determined according to those specified in Article 5.1.4 and 5.1.5 of this code. Thereinto, the maximum value of horizontal seismic influence coefficient may be calculated according to following formula:
1 max
α
=βα
max/ϕ
(12.2.5) Where: 1 maxα
——the maximum value of horizontal seismic influence coefficient after seismic isolation;max
α
——the maximum value of horizontal seismic influence coefficient of non-seismically isolation, which may be adopted according to those specified in Article 5.1.4 of this code.β
——the horizontal seismic absorbing coefficient; for multistoried building, it is the maximum shear force ratio of seismically isolation to non-seismic isolation in each layer acquired through elasticity calculation. For the tall building structure, the maximum over-turning moment ratio of seismically isolation and non- seismic isolation in each layer shall also be calculated then shall be compared with the maximum ratio of shear force between layers and finally the relatively larger value shall be adopted;ϕ
——the adjustment coefficient; it shall adopt 0.80 for general rubber support; it shall adopt 0.85 for support shearing performance with a deviation being Category S-A; and it shall correspondingly reduce 0.05 where damper is equipped with seismically isolated device.Note:
1 During elasticity calculation, simplified calculation and response spectrum analysis should be carried out according to the performance parameters where the horizontal shear strain of seismically isolated support is 100%; where time-history analysis method is adopted, it shall be calculated according to the design basic acceleration input of ground motion;
2 The shearing performance deviation of support shall be determined according to the current national standard "Rubber Bearing-Part 3: Elastomeric Seismic-protection Isolators for Buildings" (GB 20688.3).
3 The total horizontal earthquake action of structure above seismically isolated layer shall be greater than or equal to that of the non-seismic isolation structure for Intensity 6 precaution and shall be seismically checked; the horizontal earthquake shear force in each storey shall also be in accordance with those specified for the minimum seismic shear force coefficients of precautionary intensity in this area in Article 5.2.5 of this code.
4 Where it is for Intensity 9 and Intensity 8 as well as the horizontal seismic absorbing coefficient is not greater than 0.3, vertical earthquake action of structure above seismically isolated layer shall be calculated. Where the standard value of structure vertical earthquake action above seismically isolated layer is calculated, each storey may be regarded as the mass point and the distribution of standard values for vertical earthquake actions along the height shall be calculated according to Formula (5.3.1-2) of this code.
12.2.6 Horizontal shear of seismically isolated support shall be distributed according to the horizontal equivalent rigidity of each seismically isolated support based on the horizontal shear of seismically isolated layer under rare earthquake; where it is calculated according to torsion coupling, torsional rigidity of seismically isolated layer shall be counted in.
Horizontal drift of seismically isolated support corresponding to the horizontal shear of rare earthquake shall be in accordance with the following requirements:
i
u
≤[u
i] (12.2.6-1) iu
=η
iu
c (12.2.6-2) Where: iu
——the horizontal drift ofi
th seismically-isolated support where taking into account of the torsion under the rare earthquakes;[
u
i]——the horizontal drift limit of thei
th seismically isolated support; for the rubber seismically isolated support, it shall not exceed the smaller value among 0.55 times of the effective diameter of this support and 3.0 times of the total rubber thickness inside the support;c
u
——the horizontal drift of seismically isolated layer at mass centre part under rare earthquake or where the torsion is not taken into account;i
η
——the torsion influence coefficient ofi
th seismically-isolated support, the ratio of the calculated drifts ofi
th support with regard to and without regard to torsion shall be taken into consideration. Where the centroid of the structure above the seismically-isolated layer and the rigidity center of the seismically-isolated layer are not eccentric in both main axial directions, the torsion influence coefficient of the side support shall not be less than 1.15. 12.2.7 Seismically isolated measures of seismic insulation structure shall meet the following requirements:1 The following measure not to hamper the seismically isolated layer from large deformation under rare earthquake shall be taken for the seismic insulation structure:
1) Vertical isolating joint shall be installed at the periphery of superstructure and the joint width should not be less than 1.2 times of the longest horizontal drift of each seismically isolated support under rare earthquake and should not be less than 200mm. The joint width for two adjacent seismic isolation structures shall adopt the sum of the largest horizontal drift values and shall not be less than 400mm.
2) Full-through horizontal isolating joint shall be installed between superstructure and substructure and the joint height may adopt 20mm and shall be filled with flexible materials; where it is difficult to install horizontal isolating joint, reliable horizontal sliding cushion course shall be installed.
3) Porch, stairway, elevator, carriageway and other parts passing the seismically isolated layer shall be protected from possible crashing.
2 The relevant requirements for seismic measure of structure above seismically isolated layer under non- seismic isolation shall not be reduced where the horizontal seismic absorbing coefficient is greater than 0.40 ( 0.38 where damper is installed); Where the horizontal seismic absorbing coefficient is not greater than 0.40 (0.38 where damper is installed), requirements of non-seismically isolated building specified in this code may be properly reduced; however the intensity reduction shall not exceed Intensity 1 and the details
of seismic design relevant to vertical earthquake action resistance shall not be reduced. Here, details of seismic design may be carried out according to Appendix 1 of this code for masonry structure.
Note: Seismic measures related to vertical earthquake action resistance refer to the requirements of axial compression ratio for wall and column; the relevant requirements of minimum size and ring beam for end wall of external wall.
12.2.8 Connection of seismically isolated layer with superstructure shall meet the following requirements:
1 Beam slab floor shall be installed on the top of seismically isolated layer and shall be in accordance with the following requirements:
1) Cast-in-place concrete beam and slab structure shall be adopted on the relevant positions of seismically isolated support and the cast-in-place plate thickness shall not be less than 160mm;
2) Rigidity and bearing capacity of beam and plate on the top of seismically isolated layer should be greater than those of general floor beam slab.
3) Diecutting and partial bearing of beam and column close to seismically isolated layer shall be calculated; stirrup shall be thickened and steel mesh reinforcement shall be configured as circumstances demand.
2 Connection structure of seismically isolated support and damping device shall be in accordance with the following requirements:
1) Seismically isolated support and damping device shall be installed at the positions convenient for maintenance staff;
2) The connecting piece between seismically isolated support and superstructure or substructure shall be able to transfer the maximum horizontal shear and bending moment of lower support under rare earthquake;
3) Reliable rust prevention measures shall be provided for exposed embedded parts. The anchored reinforcement of embedded parts shall be solidly connected with steel plate; anchor length of anchored reinforcement should be greater than 20 times of the anchored reinforcement diameter and shall not be less than 250mm.
12.2.9 Structure and foundation of structure below seismically isolated layer shall be in accordance with the following requirements:
1 Vertical force, horizontal force and force moment of seismically isolated support bottom of seismically isolated structure under rare earthquake shall be adopted to check.
2 Relevant component parts directly supporting structure above seismically isolated layer among structures (including base plate under basement and seismically isolated tower) below seismically isolated layer shall meet the requirements of built in rigidity ratio and precautionary seismic capacity after seismically isolation and the shear resistant bearing capacity shall be checked according to rare earthquake. Storey drift angle limit structure below seismically isolated layer and above ground under rare earthquake shall be in accordance with those specified in Table 12.2.9.
3 Seismic check and base treatment for seismically isolated building base foundation shall be carried out according to the seismic precautionary intensity in this area and the anti-liquefaction measures for Category A and B buildings shall be
determined by boosting a liquification grade till all the liquefaction settlements are cleared up.
Table 12.2.9 Elasto-plastic Storey Drift Limit of Structure below Seismically Isolated Layer and above Ground under Rare Earthquake Action
Substructure type [
θ
p]Reinforced concrete frame structure and steel structure 1/100 Reinforced concrete frame-seismic wall 1/200 Reinforced concrete seismic wall 1/250