1、Section 7 DESIGN REQUIREMENTS FOR BRIDGES IN SEISMIC PERFORMANCE CATEGORIES C AND D 7.1 GENERAL Bridges classified as either SPC C or SPC D in accor- dance with Table 1 of Article 3.4 shall conform to all the requirements of this Section. 7.2 DESIGN FORCES FOR SEISMIC PERFORMANCE CATEGORIES C AND D
2、Two sets of design forces are specified in Articles 7.2.1 and 7.2.2 for bridges classified as Category C or D. The design forces for the various components are specified in Articles 7.2.3 through 7.2.7. 7.2.1 Modified Design Forces Design forces shall be determined as in Articles 7.2.1(A) and 7.2.1(
3、B). Note that for columns a maximum and minimum axial force shall be calculated for each load case by taking the seismic axial force as positive and negative. 7.2.1(A) ModiJed Design Forces for Structural Members and Connections Seismic design forces specified in this Article shall apply to: (a) The
4、 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 not including the footing, pile cap, or piles. (c) Components connecting the superstructure to the abu
5、tment. Seismic design forces for the above components shall be detehined by dividing the elastic seismic forces ob- tained from Load Case 1 and Load Case 2 of Article 3.9 by the appropriate Response Modification Factor of Arti- cle 3.7. The modified seismic forces resulting from the two load cases s
6、hall 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 negative) and the maximum loading for each component shall be calcu- lated as follows: Group Load = l
7、.O(D + B + SF + E + EQM) (7-1) 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 modified by dividing by the appropriate R-Factor. Each component of the structure shall be designed to withst
8、and the forces resulting from each load combination according to Division I, and the additional requirements of this chapter. Note that Equation (7-1) shall be used in lieu of the Division I, Group VI1 group loading combina- tion and that the y and factors equal 1. For Service Load Design, a 50% inc
9、rease is permitted in the allowable stresses for structural steel and a 33% increase for rein- forced concrete. 7.2.I(B) Seismic design forces for foundations, including 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 divi
10、ded by the Response Modification Factor (R) speci- fied below. These modified seismic forces shall then be combined independently with forces from other loads as specified in the following group loading combination to determine two alternate load combinations for the foundations. ModiJed Design Forc
11、es for Foundations 465 466 HIGHWAY BRIDGES 7.2.1(B) Group Load = l.O(D + B + SF + E + EQF) (7-2) where D, B, E, and SF are as defined in Article 7.2.1 and EQF = the elastic seismic force for either Load Case 1 or Load Case 2 of Article 3.9 divided by an R- Factor equal to 1 .O. Each component of the
12、 foundation shall be designed to resist the forces resulting from each load combination according to the requirements of Division I and to the additional requirements of Article 7.2.6. 7.2.2 Forces Resulting from Plastic Hinging in the Columns, Piers, or Bents The force resulting from plastic hingin
13、g at the top and/or bottom of the column shall be calculated after the preliminary design of the columns is complete. The forces resulting from plastic hinging are recommended for de- termining design forces for most components as specified in Articles 7.2.3 through 7.2.6. Alternate conservative de-
14、 sign forces are specified if forces resulting from plastic hinging are not calculated. The procedures for calculating these forces for single column and pier supports and bents with two or more columns are given in the following subsections. 7.2.2(A) Single Columns and Piers The forces shall be cal
15、culated for the two principal axes of a column and in the weak direction of a pier or bent as follows: Step 1. Determine the column overstrength plastic moment capacities. For reinforced concrete columns, use a strength reduction factor (+) of 1.3 and for structural steel columns use 1.25 times the
16、nominal yield strength. (Note: This corresponds to the normal use of a strength reduction factor for reinforced concrete. In this case it pro- vides an increase in the ultimate strength.) For both mate- rials use the maximum elastic column axial load from Article 3.9 added to the column dead load. S
17、tep 2. Using the column overstrength plastic mo- ments, calculate the corresponding column shear force. For flared columns this calculation shall be performed using the overstrength plastic moments at both the top and bottom of the flare with the appropriate column height. If the foundation of a col
18、umn is significantly below ground level, consideration should be given to the possibility of the plastic hinge forming above the foundation. If this can occur the column length between plastic hinges shall be used to calculate the column shear force. The forces corresponding to a single column hingi
19、ng are: (a) Axial Forces-unreduced maximum and mini- mum seismic axial load of Article 3.9 plus the dead load. (b) Moments-as calculated in Step 1. (c) Shear Force-as calculated in Step 2. 7.2.2(B) Bents with Two or More Columns The forces for bents with two or more columns shall be calculated both
20、in the plane of the bent and perpendicular to the plane of the bent. Perpendicular to the plane of the bent the forces shall be calculated as for single columns in accordance with Article 7.2.2(A). In the plane of the bent the forces shall be calculated as follows: Step 1. Determine the column overs
21、trength plastic moment capacities. For reinforced concrete use a strength reduction factor (4) of 1.3 and for structural steel use 1.25 times the nominal yield strength. (Note: This corresponds to the normal use of a strength reduction factor for rein- forced concrete. In this case it provides an in
22、crease in the ultimate strength.) For both materials use the axial load corresponding to the dead load. Step 2. Using the column overstrength plastic mo- ments calculate the corresponding column shear forces. Sum the column shears of the bent to determine the max- imum shear force for the bent. Note
23、 that, if a partial-height wall exists between the columns, the effective column height is taken from the top of the wall. For flared columns and foundations below ground level, see Article 7.2.2(A) Step 2. For pile bents the length of pile above the mud line shall be used to calculate the shear for
24、ce. Step 3. Apply the bent shear force to the top of the bent (center of mass of the superstructure above the bent) and determine the axial forces in the columns due to over- turning when the column overstrength plastic moments are developed. Step 4. Using these column axial forces combined with the
25、 dead load axial forces, determine revised column overstrength plastic moments. With the revised over- strength plastic moments calculate the column shear forces and the maximum shear force for the bent. If the maximum shear force for the bent is not within 10% of the value previously determined, us
26、e this maximum bent shear force and return to Step 3. The forces in the individual columns in the plane of a bent corresponding to column hinging, are: 7.2.2(B) DIVISION IA-SEISMIC DESIGN 467 (a) Axial Forces-the maximum and minimum axial load is the dead load plus, or minus, the axial load de- term
27、ined from the final iteration of Step 3. (b) Moments-the column overstrength plastic mo- ments corresponding to the maximum compressive axial load specified in (a) above, with a strength re- duction factor of 1.3 for reinforced concrete and 1.25 times the nominal yield strength for structural steel.
28、 (c) Shear Force-the shear force corresponding to the column overstrength moments in (b) above, noting the provisions in Step 2 above. 7.2.3 Column and Pile Bent Design Forces Design forces for columns and pile bents shall be the (a) Axial Forces-the minimum and maximum design force shall either be
29、the elastic design values deter- mined in Article 3.9 added to the dead load, or the val- ues corresponding to plastic hinging of the column and determined in Article 7.2.2. Generally, the values cor- responding to column hinging will be smaller. (b) Moments-the modified design moments deter- mined
30、in Article 7.2.1. (c) Shear Force-either the elastic design value deter- mined from Article 7.2.1 using an R-Factor of 1 for the column or the value corresponding to plastic hinging of the column as determined in Article 7.2.2. Gener- ally, the value corresponding to column hinging will be significa
31、ntly smaller. following: 7.2.4 Pier Design Forces The design forces shall be those determined in Arti- cle 7.2.1 except if the pier is designed as a column in its weak direction. If the pier is designed as a column the design forces in the weak direction shall be as specified in Article 7.2.3 and al
32、l the design requirements for columns of Article 7.6 shall apply. (Note: When the forces due to plastic hinging are used in the weak direc- tion the combination of forces specified in Article 3.9 is not applicable.) 7.2.5 Connection Design Forces The design forces shall be those determined in Articl
33、e 7.2.1 except that for superstructure connections to columns and column connections to cap beams or foot- ings, the alternate forces specified in 7.2.5(C) below are recommended. Additional design forces at connections are as follows: 7.2.5(A) Longitudinal Linkage Forces Positive horizontal linkage
34、shall be provided between adjacent sections of the superstructure at supports and ex- pansion joints within a span. The linkage shall be de- signed for a minimum force of the Acceleration Coeffi- cient times the weight of the lighter of the two adjoining spans or parts of the structure. If the linka
35、ge is at a point where relative displacement of the sections of super- structure is designed to occur during seismic motions, suf- ficient slack must be allowed in the linkage so that the linkage force does not start to act until the design dis- placement is exceeded. Where linkage is to be provided
36、 at columns or piers, the linkage of each span may be at- tached to the column or pier rather than between adjacent spans. Positive linkage shall be provided by ties, cables, dampers, or an equivalent mechanism. Friction shall not be considered a positive linkage. 7.2.5(B) Hold-Down Devices Hold-dow
37、n devices shall be provided at all supports or hinges in continuous structures, where the vertical seismic force due to the longitudinal horizontal seismic load op- poses and exceeds 50% but is less than 100% of the dead load reaction. In this case, the minimum net upward force for the hold-down dev
38、ice shall be 10% of the dead load downward force that would be exerted if the span were simply supported. If the vertical seismic force (Q) due to the longitudinal horizontal seismic load opposes and exceeds 100 percent of the dead load reaction (DR), the net upwards force for the hold-down device s
39、hall be 1.2(Q - DR) but it shall not be less than that specified in the previous paragraph. 7.2.5(C) Column and Pier Connections to Cap Beams and Footings The recommended connection design forces between the superstructure and columns, columns and cap beams, and columns and spread footings or pile c
40、aps are the forces developed at the top and bottom of the columns due to column hinging and determined in Article 7.2.2. The smaller of these or the values specified in Article 7.2.1 may be used. Note that these forces should be calculated after the column design is complete and the overstrength mom
41、ent capacities have been obtained. 7.2.6 Foundation Design Forces The design forces for foundations including footings, pile caps, and piles may be either those forces determined in Article 7.2.1(B) or the forces at the bottom of the columns corresponding to column plastic hinging as 468 HIGKWAY BRI
42、DGES 7.2.6 determined in Article 7.2.2. Generally, the values corre- sponding to column hinging will be significantly smaller. When the columns of a bent have a common footing the final force distribution at the base of the columns from Step 4 of Article 7.2.2(B) may be used for the design of the fo
43、oting in the plane of the bent. This force distribution produces lower shear forces and moments on the footing because one exterior column may be in tension and the other in compression due to the seismic overturning me ment. This effectively increases the ultimate moments and shear forces on one co
44、lumn and reduces them on the other. 7.2.7 Abutment and Retaining Wall Design Forces m. I ne components connecting the superstructure to an abutment (e.g., bearings and shear keys) shall be designed to resist the forces specified in Article 7.2.1. Design requirements for abutments are given in Arti-
45、cle 7.4.3 for SPC C and Article 7.4.5 for SPC D. 7.3 DESIGN DISPLACEMENT FOR SEISMIC PERFORMANCE CATEGORIES C AND D The seismic design displacements shall be the maxi- mum of those determined in accordance with Article 3.8 or those specified in Article 7.3.1. 7.3.1 Minimum Support Length Requirement
46、s for Seismic Performance Categories C and D Bridges classified as SPC C or D shall meet the fol- lowing requirement: Bearing seats supporting the expan- sion ends of girders, as shown in Figure 3.10, shall be.de- signed to provide a minimum support length N (in. or mm), measured normal to the face
47、of an abutment or pier, not less than that specified below. N = (12 + 0.03L + 0.12H) (1 + 0.000125S2) (in.) (7-3A) or, N = (305 + 2.5L + 10H) (I + 0.000125S2) (mm) (7-3B) where, L = length, in feet for Equation (7-3A) or meters for Equation (7-3B), of the bridge deck to the adjacent expansion joint,
48、 or to the end of the bridge deck. For hinges within a span, L shall be the sum of L, and L, the distances to either side of the hinge. For single span bridges Lequals the length of the bridge deck. These lengths are shown in Figure 3.10. S = angle of skew of support in degrees measured from a line
49、normal to the span. and H is given by one of the following: for abutments, H is the average height, in feet for Equation (7-3A) or meters for Equation (7-3B), of columns supporting the bridge deck to the next ex- pansion joint. H = O for single span bridges. for columns and/or piers, H is the column or pier height in feet for Equation (7-3A) or meters for Equation (7-3B). for hinges within a span, H is the average height of the adjacent two columns or piers in feet for Equa- tioii (7-3Aj or rrieiers or Equation (7-3Bj. Positive horizontal linkages shall be provided at all su- perstructure
