1、Section 3 GENERAL REQUIREMENTS 3.1 APPLICABILITY OF SPECIFICATIONS These Specifications are for the design and construction of new bridges to resist the effect of earthquake motions. The provisions apply to bridges of conventional steel and concrete girder and box girder construction with spans not
2、exceeding 500 feet (150 meters). Suspension bridges, cable-stayed bridges, arch type and movable bridges are not covered by these Specifications. Seismic design is usu- ally not required for buried type (culvert) bridges. The provisions contained in these Specifications are minimum requirements. No
3、detailed seismic analysis is required for any single span bridge or for any bridge in Seismic Performance Cat- egory A. For single span bridges (Article 3.11) and bridges classified as SPC A (Section 5) the connections must be designed for specified forces and must also meet minimum support length r
4、equirements. 3.2 ACCELERATION COEFFICIENT The Acceleration Coefficient (A) to be used in the ap- plication of these provisions shall be determined from the contour maps of Figures 3.2A and 3.2B. (Note: An en- FIGURE 3.2A Acceleration Coefficient-Continental United States (An enlarged version of this
5、 map, including counties, is given at the end of Division-I-A.) 447 448 HIGHWAY BRIDGES 3.2 I ALASKA PUERTO RICO HAWAII FIGURE 3.2B Acceleration Coefficient-Alaska, Hawaii, and Puerto Rico 3.2 DIVISION IA-SEISMIC DESIGN 449 larged version of Figure 3.2A is given at the end of Divi- sion I-A.) Values
6、 given in Figures 3.2A and 3.2B are ex- pressed in percent. Numerical values for the coefficient A are obtained by dividing contour values by 100.0. Local maxima (and minima) are given inside the highest (and lowest) contour for a particular region. Linear interpola- tion shall be used for sites loc
7、ated between contour lines and between a contour line and local maximum (or mini- mum). The seismic loads represented by the acceleration coefficients in Figures 3.2A and 3.2B have a 10% proba- bility of exceedance in 50 years (which is approximately equivalent to a 15% probability of exceedance in
8、75 years). This corresponds to a return period of approxi- mately 475 years. Special studies to determine site- and structure-specific acceleration coefficients shall be per- formed by a qualified professional if any one of the fol- lowing conditions exist: (a) The site is located close to an active
9、 fault. (b) Long duration earthquakes are expected in the region. (c) The importance of the bridge is such that a longer exposure period (and therefore return period) should be considered. The effect of soil conditions at the site are considered in Article 3.5. 3.3 IMPORTANCE CLASSIFICAIION An Impor
10、tance Classification (IC) shall be assigned for all bridges with an Acceleration Coefficient greater than 0.29 for the purpose of determining the Seismic Perfor- mance Category (SPC) in Article 3.4 as follows: 1. Essential bridges - IC = I 2. Other bridges - IC = II Bridges shall be classified on th
11、e basis of Social/Sur- viva1 and Security/Defense requirements, guidelines for which are given in the Commentary. 3.4 SEISMIC PERFORMANCE CATEGORIES Each bridge shall be assigned to one of four Seismic Performance Categories (SPC), A through D, based on the Acceleration Coefficient (A) and the Impor
12、tance Classifi- cation (IC), as shown in Table 3.4. Minimum analysis and design requirements are governed by the SPC. 3.5 SITE EFFECTS The effects of site conditions on bridge response shall be determined from a Site Coefficient (S) based on soil profile types defined as follows: TABLE 3.4 Seismic P
13、erformance Category (SPC) Importance Classification (IC) A I II A 5 0.09 A A 0.09 A 5 0.19 B B O. 19 A 5 0.29 C C 0.29 A D C Acceleration Coefficient SOIL PROFILE TYPE I is a profile with either- 1. Rock of any characteristic, either shale-like or crystalline in nature (such material may be characte
14、r- ized by a shear wave velocity greater than 2,500 feetheconds (760 metersheconds), or by other appro- priate means of classification); or 2. Stiff soil conditions where the soil depth is less than 200 feet (60 meters) and the soil types overlying rock are stable deposits of sands, gravels, or stif
15、f clays. SOIL PROFILE TYPE II is a profile with stiff clay or deep cohesionless conditions where the soil depth exceeds 200 feet (60 meters) and the soil types overlying rock are stable deposits of sands, gravels, or stiff clays. SOIL PROFILE TYPE III is a profile with soft to medium-stiff clays and
16、 sands, characterized by 30 feet (9 meters) or more of soft to medium-stiff clays with or without intervening layers of sand or other cohesionless soils. SOIL. PROFILE TYPE IV is a profile with soft clays or silts greater than 40 feet (12 meters) in depth. These mate- rials may be characterized by a
17、 shear wave velocity less than 500 feetheconds (150 metersheconds) and might in- clude loose natural deposits or synthetic, nonengineered fill. In locations where the soil properties are not known in sufficient detail to determine the soil profile type with confidence, the Engineers judgement shall
18、be used to se- lect a site coefficient from Table 3.5. l that conservatively represents the amplification effects of the site. The soil profile coefficients apply to all foundation types including pile supported and spread footings. A site coefficient need not be explicitly identified if a site-spec
19、ific seismic response coefficient is developed by a qualified professional (Article 3.6). 3.5.1 Site Coefficient The Site Coefficient (S) approximates the effects of the site conditions on the elastic response coefficient or spec- trum of Article 3.6 and is given in Table 3.5.1. 450 HIGHWAY BRIDGES
20、3.6 TABLE 3.5.1 Site Coefficient (S) Soil Profile we I II III IV The value of C, need not exceed 2.5A. For Type III or Type IV soils in areas where the coefficient A 2 0.30, C, need not exceed 2.OA. S 1.0 1.2 1.5 2.0 EXCEPTIONS: 3.6 ELASTIC SEISMIC RESPONSE COEFFICIENT A seismic response coefficient
21、 is specified in this Arti- cle which defines the earthquake load to be used in the elastic analysis for seismic effects. These requirements may be superseded by a 5% damped, site-specific, response spectrum developed by a qualified professional. Such a spectrum shall include the effects of both the
22、 local seismology and the site soil conditions. 3.6.1 Elastic Seismic Response Coefficient for Single Mode Analysis The elastic seismic response coefficient C, used to de- termine the design forces is given by the dimensionless formula: 1.2AS c, = - TU3 (3-1) where, A = the Acceleration Coefficient
23、from Article 3.2, S = the dimensionless coefficient for the soil profile characteristics of the site as given in Article 3.5, T = the period of the bridge as determined in Articles 4.3 and 4.4 or by other acceptable methods. The value of C, need not exceed 2.5A. For Soil Profile Type III or Qpe IV s
24、oils in areas where A 2 0.30, C, need not exceed 2.OA. 3.6.2 Elastic Seismic Response Coefficient for Multimodal Analysis The elastic seismic response coefficient for mode “m,” C, shall be determined in accordance with the following formula: 1.2AS c, = - Tg3 (3-2) where T, = the period of the mth mo
25、de of vibration. 1. For Soil Profile Type III or Type IV soils, C, for modes other than the fundamental mode which have periods less than 0.3 seconds may be determined in ac- cordance with the following formula: C, = A(0.8 + 4.OT,) (3-3) 2. For structures in which any T, exceeds 4.0 seconds, the val
26、ue of C, for that mode may be determined in accordance with the following formula: 3AS c, = - T;21 (3-4) 3.7 RESPONSE MODIFICATION FACTORS Seismic design forces for individual members and con- nections of bridges classified as SPC B, C, or D are deter- mined by dividing the elastic forces by the app
27、ropriate Response Modification Factor (R) as specified in Article 6.2 or 7.2. The Response Modification Factors for various bridge components are given in Table 3.7. These factors shall only be used when all of the design requirements of Sections 6 and 7 are satisfied. If these requirements are not
28、satisfied, the maximum value of R for substructures and connections shall be 1 .O and 0.8, respectively. 3.8 DETERMINATION OF ELASTIC FORCES AND DISPLACEMENTS For bridges classified as SPC B, C, or D the elastic forces and displacements shall be determined indepen- dently along two perpendicular axe
29、s by use of the analy- sis procedure specified in Article 4.2. The resulting forces shall then be combined as specified in Article 3.9. Typi- cally, the perpendicular axes are the longitudinal and transverse axes of the bridge but the choice is open to the designer. The longitudinal axis of a curved
30、 bridge may be a chord connecting the two abutments. 3.9 COMBINATION OF ORTHOGONAL SEISMIC FORCES A combination of orthogonal seismic forces is used to account for the directional uncertainty of earthquake 3.9 DIVISION IA-SEISMIC DESIGN 45 1 TABLE 3.7 Response Modifications Factor (R) Substructure R
31、 Connections3 R Wall- type pie? Reinforced concrete pile bents a. Vertical piles only b. One or more batter piles Single columns Steel or composite steel and concrete pile bents a. Vertical piles only b. One or more batter piles Multiule column bent 2 Superstructure to abutment 0.8 3 span of the sup
32、erstructure 0.8 2 3 to cap beam or superstructure4 1.0 1.0 Expansion joints within a Columns, piers or pile bents Columns or piers to foundations4 5 3 5 The R-Factor is to be used for both orthogonal axes of the substructure. A wall-type pier may be designed as a column in the weak direction of the
33、pier provided all the provisions for columns in Article 6.6 or 7.6, as appropriate, are followed. The R-Factor for a single column may then be used. Connections are those mechanical devices which transfer shear and axial forces from one structural component to another. They generally do not include
34、moment connections and thus comprise bearings and shear keys. The R factors in this Table are applied to the elastic forces in the restrained directions For bridges classified as SPC C or D, it is recommended that the connections be designed for the maximum forces capable of being developed by plast
35、ic hinging of the column or column bent as specified in Article 7.2.5. These forces will often be signincantly less than those obtained using an R-Factor of 1. only. motions and the simultaneous occurrences of earthquake forces in two perpendicular horizontal directions. The elastic seismic forces a
36、nd moments resulting from analy- ses in the two perpendicular directions of Article 3.8 shall be combined to form two load cases as follows: LOAD CASE 1: Seismic forces and moments on each of the principal axes of a member shall be obtained by adding 100% of the absolute value of the member elastic
37、seismic forces and moments resulting from the analysis in one of the perpendicular (longitudinal) directions to 30% of the absolute value of the corresponding member elas- tic seismic forces and moments resulting from the analy- sis in the second perpendicular direction (transverse). (NOTE: The abso
38、lute values are used because a seismic force can be positive or negative.) LOAD CASE 2: Seismic forces and moments on each of the principal axes of a member shall be obtained by adding 100% of the absolute value of the member elastic seismic forces and moments resulting from the analysis in the sec-
39、 ond perpendicular direction (transverse) to 30% of the ab- solute value of the corresponding member elastic seismic forces and moments resulting from the analysis in the first perpendicular direction (longitudinal). EXCEPTION: For SPC C and D when foundation andior column con- nection forces are de
40、termined from plastic hinging of the columns (Article 7.2.2) the resulting forces need not be combined as specified in this section. If a pier is designed as a column per Article 7.2.4 this exception only applies for the weak direction of the pier when forces resulting from plastic hinging are used.
41、 The combination specified must be used for the strong di- rection of the pier. 3.10 MINIMUM SEAT-WIDTH REQUIREMENTS All bridges, regardless of Seismic Performance Cate- gory (SPC) and number of spans, shall satisfy minimum support length requirements at the expansion ends of all girders. These supp
42、ort lengths are defined in Figure 3.10 as dimension N. The minimum value for N is given for SPCAin Article 5.3; for SPC B in Article 6.3; and for SPC C and D in Article 7.3. 3.11 DESIGN REQUIREMENTS FOR SINGLE SPAN BRIDGES The detailed analysis and design requirements of Sec- tions 4,5,6, and 7 are
43、not required for single span bridges. In lieu of rigorous analysis, the connections between the bridge span and the abutments shall be designed to resist the tributary weight at the abutment multiplied by the Ac- celeration Coefficient and the Site Coefficient for the site. This force must be consid
44、ered to act in each horizontally restrained direction. The minimum support lengths shall be as specified in Article 3.10. 452 HIGHWAY BRIDGES 3.12 COLUMN OR PIER LNJ ABUTMENT *EXPANSION JOINT OR END OF BRIDGE DECK FIGURE 3.10 Dimensions for Minimum Support Length Requirements 3.12 REQUIREMENTS FOR T
45、EMPORARY BRIDGES AND STAGED CONSTRUCTION The requirement that an earthquake shall not cause col- lapse of all or part of a bridge as stated in Article 1.1, ap- plies to temporary bridges which are expected to carry traffic and/or pass over routes that carry traffic. It also ap- plies to those bridge
46、s that are constructed in stages and ex- pected to carry traffic and/or pass over routes that cany traffic. However, in view of the limited exposure period, the Acceleration Coefficient given in Article 3.2 may be reduced by a factor of not more than 2 in order to calcu- late the component elastic f
47、orces and displacements. Note that Acceleration Coefficients for construction sites that are close to active faults shall be the subject of special study. Further, the Response Modification Factors given in Article 3.7 may be increased by a factor of not more than 1.5 in order to calculate the desig
48、n forces. This factor shall not be applied to connections as defined in Table 3.7. The minimum seat-width provisions of Article 3.10 shall apply to all temporary bridges and staged construction. Any bridge or partially constructed bridge that is ex- pected to be temporary for more than 5 years shall be de- signed using the requirements for permanent structures and shall not use the provisions of this article.
