1、Section 6 DESIGN REQUIREMENTS FOR BRIDGES IN SEISMIC PERFORMANCE CATEGORY B 6.1 GENERAL Bridges classified as SPC B in accordance with Table 3.4 of Article 3.4 shall conform to all the requirements of this section. 6.2 DESIGN FORCES FOR SEISMIC PERFORMANCE CATEGORY B 6.2.1 Design Forces for Structur
2、al Members and Connections Seismic design forces specified in this subsection shall apply to: (a) The superstructure, its expansion joints and the connections between the superstructure and the sup- porting substructure. (b) The supporting substructure down to the base of the columns and piers but n
3、ot including the footing, pile cap, or piles. (c) Components connecting the superstructure to the abutment. Seismic design forces for the above components shall be determined by dividing the elastic seismic forces ob- tained from Load Case 1 and Load Case 2 of Article 3.9 by the appropriate Response
4、 Modification Factor of Arti- cle 3.7. The modified seismic forces resulting from the two load cases shall then be combined independently with forces from other loads as specified in the following group loading combination for the components. Note that the seismic forces are reversible (positive and
5、 negative) and the maximum loading for each component shall be calcu- lated as follows: Group Load = l.O(D + B + SF + E + EQM) (6-1) where, D = dead load B = buoyancy SF = stream-flow pressure E = earth pressure EQM = elastic seismic force for either Load Case 1 or Load Case 2 of Article 3.9 modifie
6、d by di- viding by the appropriate R-Factor. Each component of the structure shall be designed to withstand the forces resulting from each load combination according to Division I, and the additional requirements of this section. Note that Equation (6-1) shall be used in lieu of the Division I, Grou
7、p VI1 group loading combina- tion and that the y and factors equal 1. For Service Load design, a 50% increase is permitted in the allowable stresses for structural steel and a 33% increase for rein- forced concrete. 6.2.2 Design Forces for Foundations Seismic design forces for foundations, includiog
8、 foot- ings, pile caps, and piles shall be the elastic seismic forces obtained from Load Case 1 and Load Case 2 of Article 3.9 divided by the Response Modification Factor (R) from Ar- ticle 3.7 and modified as specified below. These modified seismic forces shall then be combined independently with f
9、orces from other loads as specified in the following group loading combination to determine two alternate load combinations for the foundations. Group Load = l.O(D + B + SF + E + EQF) (6-2) where D, B, E, and SF are as defined in Article 6.2.1, and EQF = the elastic seismic force for either Load Cas
10、e 1 or Load Case 2 of Article 3.9 divided by one-half of the Response Modification Factor for the substructure (column or pier) to which the foundation is attached. EXCEPTION: For pile bents, the Response Modification Factor shall not be reduced by one-half. 459 460 HIGHWAY BRIDGES 6.2.2 If a Group
11、Load other than Equation (6-1) governs the design of the columns, seismic forces transferred to the foundations may be larger than those calculated using Equation (6-2), due to possible overstrength of columns. Each component of the foundation shall be designed to resist the forces resulting from ea
12、ch load combination ac- cording to the requirements of Division I and to the addi- tional requirements of Article 6.4. 6.2.3 Design Forces for Abutments and Retaining Walls The components connecting the superstructure to an abutment (e&, bearings and shear keys) shall be designed to resist the force
13、s specified in Article 6.2.1. Design requirements for abutments are given in Arti- cle 6.4.3. 6.3 DESIGN DISPLACEMENTS FOR SEISMIC PERFORMANCE CATEGORY B The seismic design displacements shall be the maxi- mum of those determined in accordance with Article 3.8 or those specified in Article 6.3.1. 6.
14、3.1 Minimum Support Length Requirements for Seismic Performance Category B Bridges classified as SPC B shall meet the following requirement: Bearing seats supporting the expansion ends of girders, as shown in Figure 3.10, shall be designed to provide a minimum support length N (in. or mm) mea- sured
15、 normal to the face of an abutment or pier, not less than that specified below. N= (8 + 0.02L + 0.08H) (1 + 0.000125S2) (in.) (6-3A) or, N= (203 + 1.67L + 6.66H) (1 + 0.000125S2) (mm) (6-3B) where, L = length, in feet for Equation (6-3A) or meters for Equation (6-3B), of the bridge deck to the adja-
16、 cent expansion joint, or to the end of the bridge deck. For hinges within a span, L shall be the sum of LI and L2, the distances to either side of the hinge. For single span bridges L equals the length of the bridge deck. These lengths are shown in Figure 3.10. S = angle of skew of support in degre
17、es, measured from a line normal to the span. and H is given by one of the following: for abutments, H is the average height, in feet for Equation (6-3A) or meters for Equation (6-3B), of columns supporting the bridge deck to the next ex- pansion joint. H = O for single span bridges. for columns and/
18、or piers, H is the column or pier height in feet for Equation (6-3A) or meters for Equation (6-3B). for hinges within a span, H is the average height of the adjacent two columns or piers in feet for Equa- tion (6-3A) or meters for Equation (6-3B). 6.4 FOUNDATION AND ABUTMENT DESIGN REQUIREMENTS FOR
19、SEISMIC PERFORMANCE CATEGORY B 6.4.1 General This section includes only those foundation and abut- ment requirements that are specifically related to seismic resistant construction in SPC B. It assumes compliance with all requirements that are necessary to provide sup- port for vertical and lateral
20、loads other than those due to earthquake motions. These include, but are not limited to, provisions for the extent of foundation investigation, fills, slope stability, bearing and lateral soil pressures, drainage, settlement control, and pile requirements and capacities. Foundation and abutment seis
21、mic design requirements for SPC B are given in the following subarticles. 6.4.2 Foundations 6.4.2(A) Investigation In addition to the normal site investigation report, the Engineer may require the submission of a report which describes the results of an investigation to determine po- tential hazards
22、 and seismic design requirements related to (1) slope instability, (2) liquefaction, (3) fill settlement, and (4) increases in lateral earth pressure, all as a result of earthquake motions. Seismically induced slope instability in approach fills or cuts may displace abutments and lead to significant
23、 differential settlement and structural dam- age. Fill settlement and abutment displacements due to lateral pressure increases may lead to bridge access prob- lems and structural damage. Liquefaction of saturated co- hesionless fills or foundation soils may contribute to slope and abutment instabili
24、ty, and could lead to a loss of foun- dation-bearing capacity and lateral pile support. Lique- 6.4.2(A) DIVISION IA-SEISMIC DESIGN 46 1 faction failures of the above type have led to bridge fail- ures during past earthquakes. 6.4.2(B) Foundation Design For the load combinations specified in Article
25、6.2.2, the soil strength capable of being mobilized by the foun- dations shall be established in the site investigation report. Because of the dynamic cyclic nature of seismic loading, the ultimate capacity of the foundation supporting medium should be used in conjunction with these load combination
26、s. Due consideration shall be given to the magnitude of the seismically induced foundation settle- ment that the bridge can withstand. Transient foundation uplift or rocking involving sepa- ration from the subsoil of up to one-half of an end bearing foundation pile group or up to one-half of the con
27、tact area of foundation footings is permitted under seismic loading, provided that foundation soils are not susceptible to loss of strength under the imposed cyclic loading. General comments on soil strength and stiffness mobi- lized during earthquakes, foundation uplift, lateral load- ing of piles,
28、 soil-structure interaction and foundation de- sign in environments susceptible to liquefaction are provided in the Commentary. 6.4.2(C) Special Pile Requirements The following special pile requirements are in addition to the requirements for piles in other applicable specifica- tions. Piles may be
29、used to resist both axial and lateral loads. The minimum depth of embedment, together with the axial and lateral pile capacities, required to resist seismic loads shall be determined by means of the design criteria established in the site investigation report. Note that the ultimate capacity of the
30、piles should be used in designing for seismic loads. All piles shall be adequately anchored to the pile foot- ing or cap. Concrete piles shall be anchored by embed- ment of sufficient length of pile reinforcement (unless special anchorage is provided) to develop uplift forces but in no case shall th
31、is length be less than the development length required for the reinforcement. Each concrete- filled pipe pile shall be anchored by at least four reinforc- ing steel dowels with a minimum steel ratio of 0.01 em- bedded sufficiently as required for concrete piles. Timber and steel piles, including unf
32、illed pipe piles, shall be pro- vided with anchoring devices to develop all uplift forces adequately but in no case shall these forces be less than 10% of the allowable pile load. All concrete piles shall be reinforced to resist the de- sign moments, shears, and axial loads. Minimum rein- forcement
33、shall be not less than the following: 1. Cast-in-Place Concrete Piles. Longitudinal rein- forcing steel shall be provided for cast-in-place concrete piles in the upper one-third (8 feet or 2.4 meters minimum) of the pile length with a minimum steel ratio of 0.005 provided by at least four bars. Spir
34、al reinforce- ment or equivalent ties of % inches (6 millimeters) diameter or larger shall be provided at 9 inches (225 mil- limeters) maximum pitch, except for the top 2 feet (610 millimeters) below the pile cap reinforcement where the pitch shall be 3 inches (75 millimeters) maximum. 2. Precast Pi
35、les. Longitudinal reinforcing steel shall be provided for each precast concrete pile with a min- imum steel ratio of 0.01 provided by at least four bars. Spiral reinforcement or equivalent ties of No. 3 bars or larger shall be provided at 9 inches (225 millimeters) maximum pitch, except for the top
36、2 feet (610 mil- limeters) below the pile cap reinforcement where the pitch shall be 3 inches (75 millimeters) maximum. 3. Precast-Prestressed Piles. Ties in precast-pre- stressed piles shall conform to the requirements of pre- cast piles. 6.4.3 Abutments 6.4.3(A) Free-Standing Abutments For free-st
37、anding abutments or retaining walls which may displace horizontally without significant restraint (e.g., superstructure supported by sliding bearings), the pseudostatic Mononobe-Okabe method of analysis is recommended for computing lateral active soil pressures during seismic loading. A seismic coef
38、ficient equal to one-half the acceleration coefficient (kh = 0.5A) is recommended. The effects of vertical acceleration may be omitted. Abutments should be proportioned to slide rather than tilt, and provisions should be made to accommodate small horizontal seismically induced abutment displace- men
39、ts when minimal damage is desired at abutment sup- ports. Abutment displacements of up to 10A inches (250A millimeters) may be expected. The seismic design of free-standing abutments should take into account forces arising from seismically induced lateral earth pressures, additional forces arising f
40、rom wall inertia effects and the transfer of seismic forces from the bridge deck through bearing supports which do not slide freely (e.g., elastomeric bearings). For free-standing abutments which are restrained from horizontal displacement by anchors or batter piles, the magnitudes of seismically in
41、duced lateral earth pres- sures are higher than those given by the Mononobe- - Okabe method of analysis. As a first approximation, it is recommended that the maximum lateral earth pressure be computed by using a seismic coefficient kh = 1.5A in conjunction with the Mononobe-Okabe analysis method. 46
42、2 HIGHWAY BRIDGES 6.4.3(B) 6.4.3(B) Monolithic Abutments For monolithic abutments where the abutment forms an integral part of the bridge superstructure, maximum earth pressures acting on the abutment may be assumed equal to the maximum longitudinal earthquake force transferred from the superstructu
43、re to the abutment. To minimize abutment damage, the abutment should be de- signed to resist the passive pressure capable of being mo- bilized by the abutment backfill, which should be greater than the maximum estimated longitudinal earthquake force transferred to the abutment. It may be assumed tha
44、t the lateral active earth pressure during seismic loading is less than the superstructure earthquake load. When longitudinal seismic forces are also resisted by piers or columns, it is necessary to estimate abut- ment stiffness in the longitudinal direction in order to compute the proportion of ear
45、thquake load transferred to the abutment. 6.5 STRUCTURAL STEEL DESIGN REQUIREMENTS FOR SEISMIC PERFORMANCE CATEGORY B 6.5.1 General Design and construction of structural steel columns and connections shall conform to the requirements of Di- vision I and to the additional requirements of this section
46、. Either Service Load or Load Factor design may be used. If Service Load design is used the allowable stresses are permitted to increase by 50%. 6.5.2 P-delta Effects Where axial and flexural stresses are determined by considering secondary bending resulting from the design P-delta effects (moments
47、induced by the eccentricity re- sulting from the seismic displacements and the column axial force), all axially loaded members may be propor- tioned in accordance with Division I, Article 10.36 or 10.54. EXCEPTIONS : 1. The effective length factor, K, in the plane of bend- ing may be assumed to be u
48、nity in the calculation of Fa, Fl, F, or Fe. 2. The coefficient C, is computed as for the cases where joint translation is prevented. 6.6 REINFORCED CONCRETE DESIGN REQUIREMENTS FOR SEISMIC PERFORMANCE CATEGORY B 6.6.1 General Design and construction of cast-in-place monolithic reinforced concrete c
49、olumns, pier footings and connec- tions shall conform to the requirements of Division I and to the additional requirements of this section. Either Ser- vice Load or Load Factor design may be used. If Service Load design is used the allowable stresses are permitted to increase by 33%. 6.6.2 Minimum Transverse Reinforcement Requirements for Seismic Performance Category B For bridges classified as SPC B, the minimum trans- verse reinforcement requirements at the top and bottom of a column shall be as required in Article 6.6.2(A). The spacing of the transverse reinforcement shall be as re- q
