GB/T 20688.5-2014Rubber bearings - Part 5: Elastic sliding seismic-protection isolator for buildings (English PDF)

Foreword

SAC/TC 35/SC 7 is in charge of this English translation. In case of any doubt about the contents of English translation, the Chinese original shall be considered authoritative.

The GB/T 20688 rubber bearings consists of the following 5 parts under the general title: Part 1: Seismic-protection isolators test methods; Part 2: Elastomeric seismic-protection isolators for bridges; Part 3: Elastomeric seismic-protection isolators for buildings; Part 4: Normal rubber bearings; Part 5: Elastic sliding seismic-protection isolator for buildings.

This part is the fifth part of GB/T 20688. This part is drafted in accordance with the rules given in the GB/T 1.1-2009.

This standard was proposed by China Petroleum and Chemical Industry Federation. This standard was prepared by SAC/TC 35/SC 7 Subcommittee on Rubber Miscellaneous of China National Standardization Technical Committee on Rubber and Rubber Products.

1 Scope

China's national standard for the elastic sliding seismic-protection isolator, the fifth part of the GB/T 20688 family on rubber bearings. Base isolation works by putting a soft, dissipative layer between a building and the ground, so that the shaking arrives filtered rather than whole; but a building supported only on laminated rubber bearings puts too much load on some columns and not enough on others, and lightly loaded points need a bearing that carries vertical load without adding much horizontal stiffness. The elastic sliding bearing is that companion device: a rubber bearing component with a sliding material - PTFE or modified ultra-high molecular weight polyethylene - riding on a stainless steel sliding plate, so the isolator supports its share of the weight and then slips, adding friction damping while the isolation period is set by the elastomeric bearings around it. This part specifies the terms and definitions, symbols, classifications, requirements, test methods, test rules, and marking and labelling of such isolators, and applies to elastic sliding seismic-protection isolators used to protect building structures from earthquake damage. It classifies them by shape, circular or square, and by sliding friction coefficient into low-friction (below 0.03), middle-friction (0.03 to 0.06) and large-friction (above 0.06) types, and fixes the physical properties required of the rubber, the sliding material, the sliding surface and the connecting plates, together with the compressive and shear performance of the assembled bearing. Three normative annexes give the test methods for the linear abrasion coefficient and the compressive elasticity modulus of the sliding material and for the shear performance of the sliding bearing.

This part specifies terms and definitions, symbols, classifications, requirments, test methods, test rules, and marking and labelling of elastic sliding seismic-protection isolators for buildings.

It is applicable to elastic sliding seismic-protection isolators used to provide building structures with protection from earthquake damage.

2 Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

GB/T 528 Rubber, vulcanized or thermoplastic - Determination of tensile stress-strain properties; GB/T 1033.1 Plastics - Methods for determining the density of non-cellular plastics - Part 1: Immersion method, liquid pycnometer method and titration method; GB/T 1040.2 Plastics - Determination of tensile properties - Part 2: Test conditions for moulding and extrusion plastics.

GB/T 3280 Cold rolled stainless steel plate, sheet and strip; GB/T 3398.1 Plastic hardness measurement - Part 1: the ball indentation method; GB/T 6031 Rubber, vulcanized or thermoplastic - Determination of hardness (hardness between 10 IRHD and 100 IRHD).

GB/T 7759 Rubber, vulcanized or thermoplastic - Determination of compression set at ambient elevated or low temperatures; GB/T 7760 Rubber, vulcanized - Determination of adhesion to metal - one-plate method; GB/T 7762 Rubber, vulcanized or thermoplastic - Resistance to ozone cracking - static strain testing; GB/T 15256 Rubber, vulcanized - Determination of low-temperature brittleness (multiple test piece method).

GB/T 20688.1-2007 Rubber bearings - Part 1: Seismic-protection isolators test methods; GB/T 20688.3-2006 Rubber bearings - Part 3: Elastomeric seismic-protection isolators for buildings.

3 Terms and definitions

For the purposes of this document, the following terms and definitions apply.

3.1 elastic sliding bearing, ESB: bearing, for seismic isolation, which consists of sliding material, sliding plate, upper and lower connecting plates.

3.2 rubber bearing component: bearing component, consists of inner vulcanized rubber sheets and reinforcing steel plates.

3.3 sliding material: material which provides sliding functionality, used to compose friction pair with sliding plate. 3.4 sliding plate: plate which provides sliding functionality.

3.5 sliding friction coefficient: ratio of friction force to normal compression force of sliding friction pair.

3.6 failure: either rupture failure caused by compression-shear, distortion of sliding material or sliding plate, or sliding displacement overrun.

3.7 compressive-shear testing machine: machine used to test elastic sliding bearings, which has the capability of shear loading under constant compressive load.

3.8 cover rubber: rubber wrapped around the outside of inner rubber and reinforcing steel plates for the purposes of protecting the inner rubber from deterioration due to oxygen, ozone and other natural elements and protecting the reinforcing plates from corrosion.

3.9 design compressive stress: long-term compressive stress on the elastic sliding bearing and the rubber bearing imposed by the structure.

3.10 effective loaded area: area sustaining vertical load in rubber bearing component, which corresponds to the area of reinforcing steel plates.

3.11 effective width: (square rubber bearing component) the side lengths of inner rubber to which direction shear displacement is not restricted. 3.12 effective diameter: (circular rubber bearing component) the diameter of the inner rubber.

3.13 1st shape factor: ratio of effective loaded area to side area of each inner rubber layer in rubber bearing component.

3.14 2nd shape factor: (circular rubber bearing component) ratio of the diameter of the inner rubber to the total thickness of the inner rubber; (square rubber bearing component) ratio of the effective width of the inner rubber to the total thickness of the inner rubber.

3.15 inner rubber: rubber between multi-layered steel plates inside an elastomeric isolator.

3.16 maximum compressive stress: peak compressive stress in elastic sliding bearings during an earthquake.

3.17 shear properties of elastic sliding bearing: sliding friction coefficient (mu) and initial stiffness (K1) of elastic sliding bearings.

3.18 ultimate properties of elastic sliding bearing: properties at failure of elastic sliding bearing.

4 Symbols

For the purposes of this document, the following symbols applied. A is bearing area of sliding material; As is bearing area of elastic sliding bearing; c is chamfer length of the sliding surface of sliding plate; Dd is outer diameter of circular connecting plate of rubber bearing component; Ds is diameter of circular sliding material or side length of square sliding material; Dr is inner diameter of reinforcing steel plate of circular rubber bearing component; D' is outer diameter of circular rubber bearing component, or outer side length of square rubber bearing component, including cover rubber.

E1 is compression elastic modulus of sliding material; F is compressive failure load of sliding material; Fv is compressive failure load of elastic sliding bearing; fsv is compressive capacity of elastic sliding bearing; fv is compressive capacity of sliding material.

H1 is total height of upper connecting plate and rubber bearing component (excluding sliding material thickness); H2 is thickness of upper connecting plate; H3 is thickness of upper cover plate of rubber bearing component; H4 is thickness of sliding plate; H5 is thickness of lower cover plate of rubber bearing component; H6 is thickness of lower connecting plate.

K1 is initial stiffness of elastic sliding bearing; Kv is vertical compressive stiffness of elastic rubber bearing; L1 is side length of square sliding plate; Lf is side length of square lower connecting plate; Ps is reference diameter of connecting bolt holes of circular rubber bearing component and embedded part; Sd is side length of connecting plate of square rubber bearing component; Sr is effective side length of rubber layer in square rubber bearing component; Ss is center to center distance of bolt holes located near opposite edges of square connecting plate.

The Greek symbols are: horizontal offset of rubber bearing component; difference in rubber bearing component height measured between two points at opposite extremes of 180 degrees angular distance; accumulated compressive strain of sliding material under 0.7 times and under 1.3 times of design compressive load; sliding friction coefficient of elastic sliding bearing; design compressive stress of sliding material and its 0.7 and 1.3 multiples; and flatness of rubber bearing component.

5 Classification

5.1 Classification by shape. Elastic sliding isolators are classified as circular isolator and square isolator (as shown in Figure 1) by shape. Figure 1 shows both shapes in section and plan, keyed as 1 upper junction plate, 2 upper sealing plate, 3 lower sealing plate, 4 lower junction plate, 5 sliding face-plate, 6 sliding material.

5.2 Classification by sliding friction coefficient. Elastic sliding isolators are classified as the following three types by sliding friction coefficient: low-friction elastic sliding isolator, coefficient less than 0.03; middle-friction elastic sliding isolator, coefficient from 0.03 to 0.06; large-friction elastic sliding isolator, coefficient greater than 0.06.

6 Requirements

6.1 Requirements on physical properties of rubber material. The physical properties of rubber material and corresponding test methods shall comply with the specifications in Table 1. The shear modulus of rubber materials should not be less than 0.392 MPa. Table 1 covers tensile properties, ageing properties, hardness, adhesive properties, compressive properties, shear properties, brittleness point, ozone resistance and low temperature crystallization; tensile strength and elongation at break follow the relevant provisions for linear natural rubber bearing in annex B of GB/T 20688.3-2006, the change in tensile strength after ageing is within plus or minus 25 % and the change in elongation at break not more negative than -50 %, the rubber shall not crack in the ozone test, and the remaining items shall stay within the tolerances specified prior to testing. Hardness may be used as a quality control test, but shall not be used for primary design purposes; the ozone resistance is mainly tested for cover rubber.

6.2 Requirements on sliding material. Sliding material of elastic sliding bearing may adopt polytetrafluoroethylene (PTFE) plate or modified ultra-high molecular weight polyethylene plate. The physical properties of sliding material and corresponding test methods shall comply with the specifications in Table 2. Sliding material shall be made from the original powder molding. Turning plates shall not be used. The recycled materials and recovery materials must not be used. The average particle diameter of PTFE material shall not be greater than 50 um.

Table 2 requires, for the PTFE plate and the modified ultra-high molecular weight polyethylene plate respectively: density 2.14 to 2.20 g/cm3 and 0.93 to 0.98 g/cm3; tensile strength at least 30 MPa for both; elongation at break at least 300 % and at least 250 %; ball hardness H132/60 from 23.0 to 33.0 MPa and from 26.4 to 39.6 MPa; line wear coefficient not more than 6 % and not more than 3 %; ultimate compressive strength at least 80 MPa for both; compression elastic modulus at least 360 MPa for both.

Sliding material shall be no dimple. The total thickness shall be not less than 5 mm. The embedded depth shall not be less than half of the total thickness. The protrusion thickness shall not be less than 2 mm. In the test and use process, oil or grease is prohibited to use in the surface of the sliding material. Peeling, cracks, delamination or other damage phenomenon are not allowed. When using PTFE sliding material, the PTFE plate shall be recessed into and bonded in the lower sealing plate after back surface activation treatment. When using modified ultra-high molecular weight polyethylene sliding material, sliding material shall be embedded in the lower sealing plate. If necessary, two kinds of materials can be fixed by mechanical means.

6.3 Requirements on sliding surfaces. Stainless steel shall be used when sliding panel is made of steel. The chromium content shall be greater than or equal to 18 %. Proposal in inland areas: the use of stainless steel in accordance with the requirements of 06Cr19Ni10, 06Cr19Ni13Mo3 specified in GB/T 3280. Proposal in coastal areas and inland corrosive environment: stainless steel in accordance with the requirements of 022Cr19Ni13Mo3 specified in GB/T 3280. When the diagonal length of the sliding panel is less than 1 500 mm, the thickness shall not be less than 2 mm; when the diagonal length of the sliding panel is greater than or equal to 1 500 mm, the thickness shall not be less than 3 mm.

6.4 Material requirement of upper and lower connecting plate. The thickness of upper and lower connecting steel plate using Q235 or Q345 shall not be less than 25 mm. The strength design value of the steel plate is given in GB/T 20688.3-2006.

6.5 Performance requirements of elastic sliding bearing. 6.5.1 Test requirements for mechanical properties of elastic sliding bearing: the mechanical properties of elastic sliding bearing and corresponding test methods shall comply with the specifications in Table 3. Table 3 requires the tolerance of the compressive stiffness to be within plus or minus 30 % and that of the initial stiffness within plus or minus 15 %; for the sliding friction coefficient, the tolerance is within plus or minus 50 % for the low friction elastic sliding seismic-protection isolator and within plus or minus 30 % for the middle and high friction ones. For the dependency of shear properties, the reference compressive stress is equivalent to the design compressive stress, the reference loading rate should be 0.4 m/s, the reference repeated loading cycle is the 3rd cycle with the change in properties after the 50 cycles within 30 %, and the reference temperature is 23 degrees C.