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OCR for page 20
20 Span 1 Span 2 > Span 1 multi-column bents, the stiffness of the member rep- Integral pier cap resenting the substructure will be taken equal to the sum of the stiffness of all columns in the bent. The transverse moments in the substructure columns may be determined using 2-dimensional frame analy- sis of the columns and the pier cap. The loads trans- mitted from the girders to the substructure should be applied as concentrated loads at girder locations. Loaded areas The live load torsional moment in the pier cap on either side of the column of a single-column integral pier may (a) Loading for Maximum Torsion be taken equal to maximum live load moment acting at the top of the column determined using 2-dimensional Integral pier cap frame analysis as described above. This moment may be assumed constant along the full length of the pier cap. The maximum torsional moment in the pier cap due to seismic loading may be taken equal to one-half the column overstrength moment at the top. The exterior and the first interior girders may be assumed to trans- Loaded area fer 40 and 60 percent of the torsional moment due to (b) Loading for Maximum Column Moment seismic load to the pier cap. In most cases the column of a single-column pier will Figure 16. Live load cases for maximum pier cap torsion be located at mid-width of the structure. This was the and maximum column moment. geometry used in this study. However, site geometri- cal constraints may require offsetting the column from the mid-width of some bridges. The simplifications For a bridge with more than two girders on either side of the provided above are recommended to be used if the column, it is rational to expect that the percentage of the tor- column is offset by no more than 10 percent of the sional moment transferred by the girders is highest for the inte- bridge width. The 10-percent limit is an arbitrary limit rior girder and is lowest for the exterior girder. It can also be based on engineering judgment. rationally expected that the interior girder in a bridge with two For other bridges, 3-dimensional refined analysis should girders on either side of the column transfers more moment be used to determine the forces acting on the compo- than that in a bridge with a larger number of girders. In the nents of both the superstructure and the substructure. absence of analytical studies on bridges with more than two girders on either side of the column, the proposed distribu- tion, which is based on results of a bridge with two girders 3.2.2 Design and Anchoring of on either side of the column, is expected to yield conserva- the Column-to-Pier Cap Connection tive results for bridges with more girders. Based on these observations, and considering the limita- The following were observed during testing of the two test tion of the study, the following recommendations are made: specimens: For I-girder superstructures connected integrally with the The column design forces may be determined using the substructure the following approximations may be used: AASHTO LRFD Specifications (1) supplemented by The girder forces may be determined ignoring the any specific requirements of the owner agency. Accord- effect of the integral connection (i.e., conventional line ing to the AASHTO LRFD Specifications, column design girder computer programs may be used for the analy- moments and associated shears are taken as the largest sis and design of the girders with integral connections). calculated forces from all applicable strength limit states The longitudinal moments acting on the substructure and the extreme event limit states. For the extreme event columns may be determined using 2-dimensional limit state that includes seismic forces, the maximum col- frame analysis. The section properties of the frame umn moment is determined as the lesser of the moment members representing the superstructure and those calculated from elastic analysis and that based on plastic representing intermediate piers or bents should be hinging of the column including consideration of the col- taken equal to the flexural stiffness of the full cross umn overstrength effects. section of the bridge and the flexural stiffness of the Filling the pier cap compartment directly above the col- pier column, respectively. In the unlikely case of using umn (see Figure 17) with concrete and extending the col-

OCR for page 20
21 A A Transfer horizontal shear between column and cap beam Carry shear from beams to column produced by MT Carry shear from beams ML to column produced by ML. Also carry shear from axial load since this is the MT most direct load path to the column from the beams. girder diaphragm Section A-A Figure 17. Shear studs in the integral connection. umn longitudinal reinforcement into the pier cap pro- two extra turns and the ends of the spiral bars were bent vides adequate anchorage to the column. The length of toward the center of the column and were provided with the column longitudinal reinforcement above the bottom a seismic hook. The column longitudinal reinforcement in flange of the pier cap should be sufficient to develop the second specimen did not lose confinement until the these bars. If needed, the bars may be extended through end of the test. Unfortunately, the second specimen failed the top flange of the pier cap into the deck slab. at a lower level of inelastic deformations than the first At high levels of inelastic deformation, the column longi- specimen because of the loss of bond between the longi- tudinal reinforcement in the first test specimen appeared tudinal bars and concrete. It was expected that the perfor- to have not been fully confined at the point these bars mance of the confinement reinforcement in the second passed through the bottom pier cap flange. At this point, specimen would exceed that in the first specimen because the column spiral reinforcement was stopped at either side of the extra anchorage provided. of the pier cap flange plate and was anchored by two extra Providing shear studs inside the pier cap (see Figure 17) turns of the spiral as recommended by Sritharan et al. (15) to transfer the column axial load and moments to the pier which is more than the one and a half turns required by cap provided a satisfactory load path. The moment used the AASHTO LRFD Bridge Design Specifications (1). In to design the connection should be taken as the column the second test specimen, the spiral was anchored using top design moment, including consideration of the over-