Elastomeric seismic protection isolators for bridges

Transcription

1 Nippon Gomu Kyokaishi, No. 4, 2012, pp 131 xx Elastomeric seismic protection isolators for bridges E. Kobayashi and K. Kaneko Industrial Materials Technology Department, Industrial Products Division, Yokohama Rubber Co. Ltd., 2-1 Oiwake, Hiratsuka, Kanagawa Selected from International Polymer Science and Technology, 39, No. 6, 2012, reference NG 12/04/131; transl. serial no Translated by K. Halpin Introduction Bridge bearings are installed between the bridge abutment or pier substructural components and the beams constituting the superstructure. The bearings have three basic functions: (1) a load transmission function whereby the vertical loads from bridge beams, vehicles, etc, and lateral loads from earthquakes and wind are transmitted to the abutments or piers; (2) a horizontal translation function enabling the beam structure to accommodate expansion in response to temperature change, creep in the concrete, etc; and (3) rotational capability, to accommodate the rotational deformation at support points due to deflection of the beams during passage of vehicles. In addition to their basic functions, many bearings have in recent years been expected to fulfil both an isolating function (aimed at lengthening the natural period and reducing inertial force in an earthquake by providing flexible horizontal support to the superstructure) and a damping function, to enhance the earthquake-resistant performance of the bridge as a whole. Bearings are thus structural members of great importance to the bridge. Bearings divide broadly into steel bearings and elastomeric bearings. Steel bearings are mainly castings, or are fabricated from rolled steel, and have hitherto been used as ordinary bridge bearings. Steel bearings are constructed to provide a load transmission function, horizontal translation function or rotational capability by Figure 1. Bridge structure and nomenclature of structural elements 2012 Smithers Rapra Technology T/7

2 mechanical means; they cannot by themselves provide isolation or damping and must therefore be used in combination with other devices. Elastomeric bearings, on the other hand, are made from vulcanised rubber and steel plates and realise load transmission, horizontal translation and rotation functions by virtue of their rubbery elasticity. Moreover, because of the large deformation capacity and buffering properties they owe to their elasticity, elastomeric bearings have outstanding earthquake-resistance and no major damage to such bearings was reported in the 1995 Hanshin-Awaji earthquake [1]. Another special feature of elastomeric bearings is that they can support isolator and damping functions by themselves, and since the Hanshin-Awaji earthquake an increasing number of elastomeric bearings have been deployed, primarily for bridges spanning high specification roads such as expressways and national highways. This review covers the structure and characteristics of elastomeric bearings, and elastomeric bearing design and quality control procedures. Structure of elastomeric bearings An elastomeric bearing is composed of elastomeric pads, inner steel plates, and top and bottom steel plates. Lateral protrusion of elastomer under compressive loading is prevented by the tensile stiffness of the steel plates and the bonding force between elastomer and plates, the structure thus enhancing load bearing capacity in the compressive direction. For lateral loads, on the other hand, the steel plates offer no constraint to shear deformation of the elastomer and the bearing deforms through the elasticity of the elastomer itself. An elastomeric bearing is hence stiff in compression and flexible in lateral shear. The plan geometry of elastomeric bearings is circular for buildings but generally square or rectangular for bridges. As shown in Figure 3, an elastomeric bearing is also provided with members such as anchor bolts for anchoring the bearing to the superstructure and substructure, along with upper and lower shoe plates, etc, forming a structure that reliably transmits the vertical and lateral loads acting on the superstructure to the substructure. The sides of elastomeric bearings are provided with cover rubber to shield the bearing pad elastomer and steel plates from the external environment and prevent degradation and corrosion. The cover rubber is generally integrally moulded and vulcanized with the pad elastomer and steel. Types of elastomeric bearing Figure 2. Functions required of bearings Elastomeric bearings may be broadly grouped into four types according to the method of supporting lateral forces and their damping capability: fixed elastomeric bearings, movable elastomeric bearings, horizontal force dispersal bearings, and seismic isolation bearings. Table 1 lists the bearing types and functions required. We review here only bearings of the horizontal seismic force dispersion and seismic isolation types that provide isolation and damping functions. Horizontal force dispersal bearings Figure 3. Example of elastomeric bearing structures At the same time as supporting the weight of the superstructure, the horizontal force dispersal bearing disperses the inertial force arising in the superstructure in an earthquake to a plurality of substructures utilising the shear stiffness of the bearing. Since bearings of this type have an isolating function, bridges that make use of such bearings have a longer natural period than bridges using fixed or movable bearings, allowing seismic acceleration to be reduced and T/8 International Polymer Science and Technology, Vol. 39, No. 8, 2012

3 Table 1. Classification of elastomeric bearings [1] Class of elastomeric bearing Fixed Moveable Horizontal seismic Seismic isolation force dispersion Load transmission Vertical support O O O O Lateral support O O O O Displacement Horizontal translation - O O O absorption Rotation O O O O Isolation - - O O Damping O Functions required affording an improvement in the earthquake-resistant performance of the bridge. Figure 4 shows the relation between standard acceleration and natural period used in seismic design on type II (moderately weak) ground [2]. The standard acceleration in an inland near-field earthquake (corresponding to Hanshin-Awaji earthquake level) is 1750 gal (17.5 m/s 2 ) for a natural period of 0.8 s, but is reduced to 1206 gal (12.1 m/s 2 ) for a natural period of 1.5 s. Furthermore, the lateral force borne by a substructure can be regulated by setting the shear spring constant of the elastomeric bearing at the desired value. The shear spring constant is expressed by Equation (1) [1]. On the other hand, since the seismic lateral displacement increases, the dimensions of the elastomeric bearing unit are larger than those of a fixed or movable bearing. bearing because of the energy absorbed; the earthquakeresistant performance of the bridge can therefore be improved even further. Over a three year period from 1989, the Public Works Research Institute collaborated with twenty-eight private companies in Japan on a study of isolator design, leading to the construction in 1991 of the Miyakawa Bridge (Shizuoka Prefecture), the first seismic isolation bridge in the country to use lead-rubber Ks = Ge.Ae/Σte (1) where Ks: shear spring constant of elastomeric bearing (N/mm) Ge: shear modulus of rubber (N/mm 2 ) Ae: area of elastomeric bearing unit, excluding the cover rubber (mm 2 ) Σte: overall rubber thickness (mm) Figure 4. Relation between natural period and seismic design acceleration [2] Seismic isolation bearings Seismic isolation bearings have a dual function: an isolating function that exploits the shear stiffness of the elastomeric bearing unit, and a damping function based on the energy absorbing properties of the elastomer. Typical examples are lead-rubber bearings and high damping elastomeric bearings. As with the horizontal force dispersal bearings, they are able to disperse the inertial force of the superstructure in an earthquake to a plurality of substructures utilising the shear stiffness of the bearing. In addition to the reduction in seismic acceleration due to lengthening of period, the isolation bearing achieves a greater decrease in lateral seismic force than a dispersal type Figure 5. Comparison of response in dynamic analysis of horizontal force dispersal bearing and seismic isolation bearing 2012 Smithers Rapra Technology T/9

4 bearings, and the construction in 1992 of the Yamaage Bridge (Tochigi Prefecture), the first to use high damping elastomeric bearings [1]. Since the Hanshin-Awaji earthquake of 1995, there has been a sharp increase in the deployment of seismic isolation bearings to improve earthquake-resistant bridge performance, with especially enthusi astic deployment on high specification roads such as expressways and national highways. Seismic isolation bearings of lead-rubber type have lead plugs inserted in vertical holes through the interior of the elastomeric bearing, as illustrated in Figure 6a. In an earthquake, the lead plugs deform accompanying shear deformation of the elastomer, seismic energy thus being absorbed by elastic-plastic deformation of the plugs. A high damping elastomeric bearing is a seismic isolator that uses elastomeric material in which damping properties have been imparted to the elastomer itself; it thus functions as an isolation bearing that combines horizontal spring characteristics due to the restoring force generated by the elastomer with hysteresis damping due to energy absorption. High damping elastomeric bearings are of two kinds: HDR (high damping rubber) bearings, and HDR-S (high damping rubber super) bearings, which have greater damping performance than HDR bearings. Figure 7 shows examples of the shear stress-shear strain hysteresis loops of HDR-S at 100%, 175% and 250% shear strain [3]. superstructure and substructures, therefore, the beam expansion spacing must be increased. Consequently, the expansion device installed at expansion spacing points increases in size, to the detriment of vehicular traffic. Moreover, the thickness needed for the elastomeric bearing to provide the desired horizontal displacement capacity increases, and the greater compressive deflection experienced adversely affects vehicle travel. Both the lateral force and horizontal displacement in bridges using horizontal force dispersal bearings and seismic isolation bearings tend to have acceptable values when the natural period is in the range 1-2 s, and it is more effective to use G10 or G12 rubber of large shear modulus to set a suitable shear spring constant. The basic elastomeric properties of elongation at break and tensile strength specified in the Manual of Highway Bridge Bearings are listed in Table 2. Table 3 shows the specifications for age resistance and endurance. Since elastomeric bearings are used outdoors for up to years, a high level of endurance and weatherability are required of the rubber materials. Ozone resistance is especially important. Elastomeric bearings are constantly subject to compressive deformation from supporting the superstructure, and shear deformation due to expansion and contraction of the bridge beams with change in temperature. The bearing cover rubber is continually under tensile deformation, increasing the risk of ozone Rubber materials used for elastomeric bearings Specifications for the rubber materials used in elastomeric bearing assemblies are given in the JRA Manual of Highway Bridge Bearings [1]. Elastomeric bearings of the horizontal force dispersal type and lead-rubber type use natural rubber, while high damping elastomeric bearings use high damping rubber. The design values of the static shear modulus are N/mm 2 (nominal grade G6- G14) for natural rubber and N/mm 2 (nominal grade G8-12) for high damping rubber, the materials used thus having a higher shear modulus than in elastomer laminates for buildings. If material of low shear modulus is used, the horizontal displacement increases owing to the reduced shear stiffness; to avoid collision between Figure 7. Lateral hysteresis characteristics of high damping rubber super (HDR-S) bearing [3] Figure 6. Construction of seismic isolation bearings T/10 International Polymer Science and Technology, Vol. 39, No. 8, 2012

5 Table 2. Basic specifications of elastomeric materials [1] Item Material Designation Shear modulus (N/mm 2 ) Basic properties Natural rubber (NR) High damping rubber (HDR HDR-S) Elongation at break (%) Tensile strength (N/mm 2 ) Test method G JIS K 6251 G Tensile test G G G G G G Table 3. Ageing and endurance specifications of elastomeric materials [1] Item Unit Specification Test method Ageing test Change in stress, 25% % -10 to +100 Heat ageing JIS K 6257 elongation (70 C 72 hr) Change in elongation % -50 (70 C 72 hr) Ageing/endurance Compression set NR % 35 (70 C 24 hr) HDR HDR-S 60 (70 C 24 hr) Ozone resistance - Crack-free to naked eye (40 C 96 hr) (50 pphm, 20% elongation) Ozone resistance (cold regions) - Crack-free to naked eye (-30 C 96 hr) (50 pphm, 20% elongation) Water resistance (weight change) % 10 (distilled water 55 C) (72 hr immersion) Cold resistance - Cold embrittlement temperature -30 C (-40 C in cold regions) Compressive set JIS K 6262 Static ozone ageing JIS K 6259 Immersion test JIS K 6258 Part 4 Low temperature impactbrittleness JIS K 6161 cracking in the superficial part of the cover. Ways of combating this are to increase the proportion of age resistor in the recipe and to employ a rubber compounded from material of superior ozone-resistance such as CR or EPDM for the cover rubber. However, care must be taken with bonding to the bearing elastomer if cover rubber of a dissimilar material is used. Design of elastomeric bearings The procedure for verifying the capacity of elastomeric bearings is specified in the Manual of Highway Bridge Bearings; this compares the response (stress resultant, deformation) generated by loading with the intrinsic performance of the bearing element (yield strength, stress, deformation capacity, etc) to verify that intrinsic performance exceeds the response [1]. It is verified that, in normal circumstances, the stress or strain arising in the bearing unit and fixing members is no greater than the allowable stress or strain in the relevant member so that brittle fracture or buckling, decline in performance due to fatigue or deterioration, etc, do not occur. Postulating a major earthquake of near-field type at the Hanshin-Awaji earthquake level and a plate boundary earthquake of Great Kanto level (known as level 2 motion), it is then verified that under the action of horizontal or perpendicular force due to seismic motion, the stress resultant arising in the bearing unit and fixing members is no greater than the yield strength of the relevant member, or the allowable deformation of the bearing. The earthquake-resistant design of bridges using horizontal force dispersal bearings or seismic isolation bearings entails non-linear dynamic analysis, and the shear properties of the bearings must therefore be represented by a suitable model. The horizontal force dispersal bearing has a weak non-linear hysteresis characteristic and weak strain dependence but is modelled as a linear member of zero strain-dependence 2012 Smithers Rapra Technology T/11

6 within the range used for design to compute the shear spring constant. The hysteresis damping effect is given as 3% of the equivalent damping constant. A seismic isolation bearing, on the other hand, has a strong nonlinear hysteresis characteristic and strain-dependence; the first-order stiffness and second-order stiffness as the bilinear model shown in Figure 8 (a non-linear hysteresis model where the load versus deformation relation is represented by two straight lines) are therefore calculated taking into account strain dependence. The design values in dynamic analysis are specified as the maximum lateral momentum and horizontal force acting on the bearing. The design checks carried out on an elastomeric bearing unit are listed in Table 4 and the design allowances are given in Table 5. The plan dimensions of elastomeric bearings are determined by verifying the vertical support function while thickness is determined by verifying the displacement tolerance function as expressed by the horizontal translation and rotation functions. In addition, Figure 8. Comparison of actual hysteresis loop with design model for a seismic isolation bearing (HDR-S G12) [3] for both horizontal force dispersal bearings and seismic isolation bearings, the shear spring constant of the elastomeric bearing needed to regulate the lateral force acting on the respective substructures in an earthquake is determined and used in determining the dimensions of the elastomeric bearing. The design criteria for bridges that affect the determination of bearing dimensions are many and diverse, and design of the elastomeric bearing must ensure the most compact geometry is obtained within those criteria. A prominent characteristic of elastomeric bearings for bridges is that there are no standard dimensions - all bearings have to be made to order. Confirmatory tests for elastomeric bearing capacity Methods for the quality control of elastomeric bearings are prescribed in the Manual of Highway Bridge Bearings [1]. Although quality control companies such as NEXCO have their own quality control schedules, the basic approach is the same. Standard test methods for bridge elastomeric bearings are ISO22762, introduced in 2005 [4, 5], and JIS K 6411, introduced in Table 6 shows the content of the routine confirmatory tests specified in the Manual of Highway Bridge Bearings. Since the tests are in principle conducted on the actual product, elastomeric bearing manufacturers maintain biaxial loading testers capable of simultaneously loading the largest elastomeric bearings in both the vertical and horizontal directions. Elastomeric bearings are used outdoors for 50 years or more during which they must realise the prescribed isolator function or damping function in the event of a major earthquake. Endurance and stability in different service environments must therefore be confirmed along with the ultimate capacity of the bearing. The tests are summarised in Table 7. Table 4. Design verification checks on elastomeric bearing units [1] Function Item checked Level of effect on dimensions Plan area Thickness General design verification Vertical support Maximum compressive stress O Compressive stress amplitude O Buckling stability O Compressive displacement at end supports O Tensile stress on inner steel plates O Horizontal translation Shear strain O Rotation Rotational displacement O O Fatigue endurance Local shear strain O Seismic design verification Vertical support Tensile stress O Buckling stability O Tensile stress on inner steel plates O Horizontal translation Shear strain O T/12 International Polymer Science and Technology, Vol. 39, No. 8, 2012

7 Table 5. Design allowances for elastomeric bearings [1] Property Allowable Tensile stress Maximum compressive S 1 <8 s maxa = 8.0 N/mm 2 stress 8 S 1 <12 s maxa = S 1 N/mm 2 12 S 1 s maxa = 12.0 N/mm 2 Minimum compressive stress s mina = 1.5 N/mm 2 Stress amplitude S 1 8 Ds a = 5.0 N/mm 2 8<S 1 Ds a = (S1-8.0) Subject to max 6.5 N/mm 2 Normal condition g sa = 70% Shear strain Storm g wa = 150% Earthquake Level 1 motion g ea = 150% Level 2 motion g ea = 250% Local shear strain in normal condition g ta = g u /1.5 g u = Shear elongation of rubber Compressive stress Normal condition s ta = 0.0 N/mm 2 Storm G6 s ta = 0.9 N/mm2 G8 s ta = 1.2 N/mm 2 G10,G12,G14 s ta = 1.5 N/mm2 Earthquake G6 s ta = 1.2 N/mm2 G8 s ta = 1.6 N/mm2 G10,G12,G14 s ta = 2.0 N/mm2 Note. S 1 : first shape factor, restrained area of rubber divided by free area Table 6. Performance confirmatory tests on elastomeric bearing units (routine inspection) [1] Test Content Criteria Compressive tests Compressive displacement Vertical load corresponding to design maximum is applied to test machine and the compressive displacement is measured Not less than design displacement Compressive spring constant For bridges designed taking into account the compressive spring constant, a vertical load corresponding to N/mm 2 compressive stress is applied to the machine and the compressive spring constant is measured Within ±30% of design value Shear tests Shear spring constant, equivalent stiffness Equivalent damping coefficient With a vertical load corresponding to the dead load applied to the test machine, a horizontal displacement corresponding to 175% shear strain is imposed and the shear spring constant (equivalent stiffness) and equivalent damping coefficient are measured Within ±10% of design value Not less than design value Table 7 Performance confirmatory tests for endurance, stability and ultimate capacity of elastomeric bearings [1] Test Content Compressive fatigue Confirms change in shear spring constant (equivalent stiffness) and equivalent damping endurance coefficient in respect of cyclic vertical loading with live load. Shear fatigue endurance Confirms change in shear spring constant (equivalent stiffness) and equivalent damping coefficient in respect of repeated shear deformation from beam expansion and contraction due to temperature change in normal states Shear cycle dependence Confirms change in shear spring constant (equivalent stiffness) and equivalent damping coefficient in respect of repeated shear deformation in earthquakes Period dependence Confirms change in shear spring constant (equivalent stiffness) and equivalent damping coefficient in respect of differences in seismic wave period Temperature dependence Confirms change in shear spring constant (equivalent stiffness) and equivalent damping coefficient in respect of temperature change in the service environment Surface pressure dependence Confirms change in shear spring constant (equivalent stiffness) and equivalent damping coefficient in respect of fluctuation in vertical reaction operating on the bearing Endurance Safety Ultimate capacity Ultimate shear capacity Vertical load capacity Confirms the deformation and horizontal force at rupture or buckling in shear deformation under vertical loading Confirms the load capacity in the tensile and compressive directions up to failure 2012 Smithers Rapra Technology T/13

8 Figures 9 and 10 show the results of ultimate shear capacity tests with a full scale model of an HDR-S G12 bearing (plan dimensions mm, elastomer thickness 29 mm 7 pads = 203 mm). Machined down from the largest mm plan dimensions capable of production to the testable plan dimensions of mm, the bearing resisted rupture despite shear deformation to 711 mm, corresponding to 350% shear strain; the tests thus confirmed that the bearing has ample shear deformation capacity in respect of the design shear strain of 250%. Figure 9. Ultimate shear capacity testing of HDR-S G12 bearing Conclusions Demand for elastomeric bearings has increased sharply since the 1995 Hanshin-Awaji earthquake. At the same time, with much experimentation and numerical analysis, manufacturers of elastomeric bearings have been developing the constituent technologies in an effort to characterise bearing performance and advance bearing technology. There have in the meantime been several major earthquakes in Japan, notably the 2004 Niigata-Chuetsu earthquake, in which no examples of damage to elastomeric bearings have come to light, demonstrating a high level of earthquake resistance. Even in this year s Great East Japan earthquake, most elastomeric bearings remained sound, though bearing damage was reported from some bridges [3]. Since an elastomeric bearing has a role analogous to a circuit fuse, fatal damage to bridges in the shape of beam collapse or pier failure did not arise; however, why the elastomeric bearings were damaged will have to be elucidated, the design-quality control-installation guidelines further consolidated and the technology overhauled. References Figure 10. Results of ultimate shear capacity test on HDR-S G12 bearing 1. Japan Road Association: "Manual for Highway Bridges Bearings" Japan Road Association: "Specifications for Highway Bridges", Research Committee on Menshin Design of Bridges: "Manual for Menshin Design and Passive Control Design of Highway Bridges". Public Works Research Center Elastomeric Seismic-protection Isolators, Part 1: Test Methods, IS : 2010 (E) 5. Elastomeric Seismic-protection Isolators, Part 2: Applications for Bridges, Specifications, IS : 2010 (E) T/14 International Polymer Science and Technology, Vol. 39, No. 8, 2012