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1、 NCHRP 12-68 FY 2004 Rotational Limits for Elastomeric Bearings Final Report APPENDIX G John F. Stanton Charles W. Roeder Peter Mackenzie-Helnwein Department of Civil and Environmental Engineering University of Washington Seattle, WA 98195-2700 TABLE OF CONTENTS APPENDIX G PROPOSED DESIGN SPECIFICAT
2、IONS G-1 G.1 Basis G-1 G.2 Proposed Specification Provisions G-5 G.2.1 AASHTO 14.4 Movements and Loads G-5 G.2.1.1 AASHTO 14.4.1 General G-5 G.2.1.2 AASHTO 14.4.2 Design Requirements. G-5 G.2.2 AASHTO 14.7.5 Steel-Reinforced Elastomeric Bearings Method B G-6 G.2.2.1 AASHTO 14.7.5.1 General G-6 G.2.2
3、.2 AASHTO 14.7.5.2 Material Properties G-7 G.2.2.3 AASHTO 14.7.5.3 Design Requirements G-7 G.2.2.3.1 AASHTO 14.7.5.3.1 Scope G-7 G.2.2.3.2 AASHTO 14.7.5.3.2 G-7 G.2.2.3.3 AASHTO 14.7.5.3.3 G-7 G.2.2.3.4 AASHTO 14.7.5.3.4. G-13 G.2.2.3.5 AASHTO 14.7.5.3.5 G-13 G.2.2.3.6 AASHTO 14.7.5.3.6 G-13 G.2.2.3
4、.7 AASHTO 14.7.5.3.7 G-13 G.2.2.3.8 AASHTO 14.7.5.3.8 G-13 G.2.2.3.9 AASHTO 14.7.6.4 Anchorage G-13 G.3 AASHTO 14.7.6 Steel Reinforced Elastomeric Bearings - METHOD A G-14 G.3.1 AASHTO 14.7.6.1 General G-14 G.3.2 AASHTO 14.7.6.2 Material Properties and 14.7.6.3.1 Scope G-14 G.3.3 AASHTO 14.7.6.3.2 C
5、ompressive Stress G-15 G.3.4 AASHTO 14.7.6.3.3 Compressive Deflection G-15 G.3.5 AASHTO 14.7.6.3.4 Shear G-15 G.3.6 AASHTO 14.7.6.3.5 Rotation G-15 G.3.7 AASHTO 14.7.6.3.6 Reinforcement G-15 G.3.8 AASHTO 14.7.6.3.7 Seismic provisions G-15 G.3.9 AASHTO 14.7.6.4 Anchorage G-15 ii G.4 AASHTO M-251 Mate
6、rials Specification. G-15 G.5 Notes. G-17 LIST OF TABLES Table G-1 Summary of Proposed Section Changes in AASHTO Method B Specifications.G-3 iii G-2 APPENDIX G Proposed Design Specifications G.1 Basis The overall objective of this approach is to create a comprehensive specification that is consisten
7、t with the results of the research conducted here, with the performance of existing bearings in the US, and with specifications world-wide. As in previous editions of the AASHTO LRFD Design Specifications, two design methods are provided. Method B includes axial force, rotation and shear, whereas Me
8、thod A represents a simplification of the Method B approach that allows engineers to design bearings without having to consider rotations in detail. Method A was created by estimating the largest rotations likely to occur in practice, and determining the corresponding axial stress that would be allo
9、wed under Method B. The two methods are thus consistent with each other to the greatest extent possible. Some restrictions on the use of Method A are imposed to prevent its use outside the domain of validity of the simplifications on which it is based. The Method B specification is written using the
10、 shear strains caused by axial force, rotation and shear displacement. This approach obviates the need for different equations to address different combinations, such as compression and rotation with or without shear, and is thus conceptually simpler than the existing version. Limiting the total she
11、ar strain is also the principle that underlies the existing specifications. The intent of the proposed design provisions is thus more transparent than that of the 2004 AASHTO LRFD Design Specifications without changing their conceptual basis. Furthermore, future changes can be made relatively easily
12、, should they be needed. The total allowable strain is slightly higher than the one implicit in the 2004 LRFD Specifications, but that fact is partially offset by the presence of a constant amplification factor that is applied to cyclic strains arising from traffic loading. Section G.2 provides desi
13、gn rules for bearings that: are readily satisfied by bearings in common use today, thus meeting a minimum but necessary criterion of reasonability, are consistent with the debonding trends observed in the tests, penalize cyclic loads, in accordance with the findings of the testing program, which sho
14、wed that cyclic loading led to much more debonding than did monotonic loading of the same magnitude, remove the previous restrictions on lift-off, for bearings that have no external plates, and from which the girder can readily separate over part of the bearing surface, introduce a new check for hyd
15、rostatic tension stress, to guard against internal rupture of the elastomer in bearings that have external plates and are subjected to light axial load and large rotations, remove the absolute compressive stress limits (of 1.60 and1.75 ksi) and replace them with an implicit limit related to GS, to e
16、ncourage the use of bearings with G-1 higher shape factors for high load applications. Such bearings performed extremely well in the testing program. An amplification factor of 2.0 is proposed for cyclic loading. This is higher, and therefore more conservative, than the European value of 1.0 or 1.5
17、(value to be chosen by the bridges owner). The European Specification (EN 1337) uses the same total strain capacity of 5.0 that is proposed here, so the existence of a higher cyclic amplification factor makes these proposals inherently more conservative than those of EN 1337. Despite that, they are
18、still simpler, more versatile and more liberal than those in the 2004 AASHTO LRFD Specifications. A change in testing requirements is also advocated. In previous editions of the AASHTO Design Specifications, design by Method B was linked to the requirement for additional, more rigorous testing, and
19、in particular, a long-term test. This testing is relatively expensive and time consuming, and designers were therefore reluctant to use Method B. However, that long-term test has been recently eliminated by the AASHTO T-2 Committee during part of a major re-consolidation of testing requirements from
20、 the AASHTO Construction Specifications into the M-251 Material Specifications. The present status is that the materials in the bearing are to satisfy the physical property tests defined in Section 4 of M-251, and the finished bearings are to be sampled on a lot basis and the sample is to be subject
21、ed to the tests defined in Section 8. At the owners discretion, bearings designed by Method A may instead be subjected to the less rigorous tests of Appendix X1. The researchers appreciate the desire to consolidate all testing requirements in a single document. However, linking the testing requireme
22、nts to the design method has several drawbacks, and a change is therefore suggested. The primary reasons are: The shear strains in a Method A bearing are not necessarily smaller than those in a Method B bearing. Appendix F shows how Method A was derived as a special case of Method B, with the motiva
23、tion of simplification, rather than ensuring lower stresses. In fact, an increase of 25% for the allowable stresses under Method A is recommended. In many cases, the shear strain due to rotation in a Method A bearing will indeed be smaller than the design value implicit in method A, so the total she
24、ar strain will be less than the maximum allowed. But that is true only of some, and not all, Method A bearings. Bearing manufacturers apply the same procedures and standard of care to the fabrication of every bearing. To maintain several different ones would invite errors. Furthermore, the manufactu
25、rers are usually unaware of the design method that has been used. The consequence is that they do not deliberately manufacture Method B bearings to a higher (or lower) standard than Method A bearings. Fabrication problems are most likely to occur in large bearings, for several reasons. First the cur
26、ing becomes more difficult, because the center of the bearing takes longer to heat up and the outer regions risk over-cure before the middle has fully cured. Second, a bearing with many shims has a greater probability of a shim being left out during the lay-up, or shim movement in the mold during cu
27、ring under high temperature and pressure, than is the case with a small number of shims. G-2 The uncertainties in fabrication operations for large bearings are thus greater those associated with the design method. It is therefore proposed that the additional testing should be applied not to bearings
28、 designed by Method B, but rather to large bearings, which are the ones more likely to experience difficulties during fabrication. This will encourage design by Method B and, by implication, the use of higher shape factors. The format of the proposed Method B provisions is simpler than the existing
29、one and therefore reduces the number of sub-sections required in Article 14.7.5.3. The inevitable change in numbering of the sub-sections offers the opportunity to rationalize their sequence as well. Table G-1 shows the existing and proposed sequences. The primary objective is to present the design
30、information in the order in which it will be used. Shear deformations usually control the thickness, for which a trial value is selected first. Then strength requirements (combined axial, rotation and shear) are used to determine plan dimensions and individual layer thicknesses. Reinforcement select
31、ion, compressive deflection calculations and seismic requirements can typically be conducted without affecting the bearing properties selected in previous steps, and so are placed last. Table G-1 Summary of Proposed Section Changes in AASHTO Method B Specifications. SectionOld TitleNew Title 14.7.5.
32、3.1 ScopeScope 14.7.5.3.2 Compressive StressShear Deformations 14.7.5.3.3 Compressive DeflectionCombined Compression, Rotation and Shear 14.7.5.3.4 Shear DeformationsStability 14.7.5.3.5 Combined Compression and RotationReinforcement 14.7.5.3.6 StabilityCompressive Deflection 14.7.5.3.7 Reinforcemen
33、tSeismic Requirements 14.7.5.3.8 Seismic Requirements The proposed Method A Specifications maintain the format of the existing ones. Because that article also addresses cotton duck pads (CDP), plain elastomeric pads (PEP) and fiberglass reinforced pads (FGP), a decision was needed over the use of a
34、cyclic amplification factor. Such a factor is not used for the present Method A, but it does form a part of the proposed Method B. If the proposed new Method A for steel-reinforced bearings were to include cyclic amplification, it would be inconsistent with the procedures for the other pad types in
35、Method A. If it were to be based on non-amplified stresses, then it would be inconsistent with the proposed Method B. Thus it is not possible to be consistent with both procedures. The latter course (no amplification, thereby maintaining consistency with other Method A bearings) was eventually chose
36、n, but a change to the opposite approach would be relatively simple if Committee T-2 sees fit to do so. It should be noted that the researchers see several problems with the testing regime presently defined in M-251. They have also heard complaints from manufacturers along similar lines. They are wi
37、lling to meet with the T-2 Committee members and a representative group of manufacturers to discuss those testing requirements with the goal of improving them. G-3 G-4 G.2 Proposed Specification Provisions This section contains wording of the proposed Design Provisions. Wording in italics indicates
38、comments or operational suggestions, such as new locations for existing text. Strikeouts indicate existing wording to be deleted. Underlines indicate new wording to be added. G.2.1 AASHTO 14.4 Movements and Loads G.2.1.1 AASHTO 14.4.1 General The commentary (paragraph 1) states: “If the bridge deck
39、is cast-in-place concrete, the bearings at a single support should permit transverse expansion and contraction”. This statement should be changed to include precast concrete decks as well. The commentary (fourth paragraph) contains the statement: “The location of bearings off the neutral axes of the
40、 girders can create horizontal forces due to elastic shortening of the girders when subjected to vertical loads”. The meaning of the statement is not clear. The girders do not shorten under vertical loads. If the statement is intended to refer to the fact that rotation at the girder end causes horiz
41、ontal movement at the bottom flange, and that those movements induces shear force in the bearing, then that fact is addressed by the previous sentence, and there is no need to repeat the information. G.2.1.2 AASHTO 14.4.2 Design Requirements. The Commentary (paragraph 1) states: “Live load rotations
42、 are typically less than 0.005 radians, but the total rotation due to fabrication and setting tolerances may be significantly larger than this”. This statement is not consistent with the fact that the rotation allowance for fabrication and placement is 0.005 radians. Either the statement or the allo
43、wance should be changed. Note that 0.005 radians is a very small angle, and corresponds to a movement on a carpenters level of only about one tenth of a bubble length. Bearings are therefore likely to be installed to an accuracy better than this only if an instrument more sophisticated than a carpen
44、ters level is used. That appears unlikely with current construction methods. The Commentary (paragraph 2) states: “As a result, such bearings are permitted temporary overstress during construction. If this was not so, temporary local uplift, caused by light load and large rotation might unreasonably
45、 govern the design”. G-5 Delete these two sentences. The construction condition is now addressed properly by Method B. G.2.2 AASHTO 14.7.5 Steel-Reinforced Elastomeric Bearings Method B G.2.2.1 AASHTO 14.7.5.1 General After the third paragraph (“tapered elastomer layers shall not be used”) add a new
46、 sentence: Plan dimensions used for computing the properties of the bearing shall be taken as the average of the gross bearing dimensions and the shim dimensions. The shape factor of a layer of an elastomeric bearing Change the definitions of L and W to: L = plan dimension of the bearing perpendicul
47、ar to the axis of rotation under consideration W = plan dimension of the bearing parallel to the axis of rotation under consideration These definitions are used in the proposed 14.7.5.3.3 (Combined stress) and 14.7.5.3.4 (Stability). They should also be changed in the list of notation at the start o
48、f Chapter 14. COMMENTARY Make changes as shown. Strike-throughs signify deletions. Underlines signify new wording. The stress limits associated with Method A usually result in a bearing with lower capacity than a bearing designed using method B. This increased capacity resulting from the use of Meth
49、od B requires additional testing and quality control. Steel-reinforced elastomeric bearings are treated separately from other elastomeric bearings because of their greater strength and superior performance in practice (Roeder et al. 1987, Roeder and Stanton 1991). The critical parameter in their design is the shear strain in the elastomer at its interface with the steel plates. Axial load, rotation and shear deformations all cause such shear strains. The design method (Method B) described in this section accounts directly for those shear strains, and