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OCR for page 97
97 shear slip during flexural joint opening. Although shear slip use because the transverse reinforcement requirement of was observed, the overall resistance of the system did not Eq. 8.13.3-2 can become less than that of Eq. 8.13.3-1 in appear to be adversely affected. some cases. Predictions of the moment-rotation response based on tra- Instead of hoops, cap pocket connections should use a thick- ditional moment-curvature analysis and an assumed effec- ness of corrugated steel pipe, tpipe, based on the average joint tive hinge length equal to one-half the superstructure depth confining hoop force provided by the transverse reinforce- yielded reasonable predictions for use in design. ment required per the 2009 LRFD SGS (1). The proposed general equation for tpipe results in a reasonable pipe thick- 4.2 Design Specifications ness for design. The general equation for tpipe may also be conservatively replaced by the simplified equations provided The conclusions that follow for design specifications of non- in the proposed commentary; however, use of the simplified integral emulative grouted duct and cap pocket connections equations can result in a significantly thicker pipe require- are based on test specimen results and analysis of test results, ment in some cases. related research, and existing specifications. In SDCs B, C, and D, where the principal tensile stress, pt, is less than 0.11 fc , minimum transverse joint shear re- 4.2.1 Design Methodology inforcement should be required. The current design methodology for CIP joint shear design For SDC A, minimum transverse joint shear reinforcement in the 2009 LRFD SGS for SDCs C and D can be reasonably and should be conservatively required, without calculation of conservatively modified for design of integral and nonintegral, the principal tensile stress. emulative, and hybrid precast bent cap grouted duct and cap pocket connections (1). 4.2.4 Integral Bent Caps The 2009 LRFD SGS has discrepancies between the required 4.2.2 Principal Tensile Stress Calculation joint shear reinforcement for integral bent cap systems in the For SDCs C and D, precast bent cap connections should transverse direction and the required joint shear reinforcement require calculation of the principal tensile stress, pt, in the joint for nonintegral bent caps (1). For consistency in design prac- to establish the need for additional joint shear reinforcement, as tice, it is recommended that the integral bent cap requirements required for CIP joints by the 2009 LRFD SGS (1). However, to be updated for consistency. Integral bent caps will require re- incorporate a reasonable safety margin, design of precast con- inforcement along the face of the bent cap based on longitudi- nections should adopt the more conservative provisions iden- nal flexural response; however, additional reinforcement tified in the 2009 LRFD SGS (Articles 4.11.1 and C4.11.1) for based on the 2009 LRFD SGS nonintegral specifications should SDC B (1). Therefore, calculation of the principal tensile stress, be required. pt, for SDC B joints is required to establish the need for addi- tional joint shear reinforcement, as required for SDCs C and D. 4.2.5 Additional Joint Reinforcement In addition, where additional joint shear reinforcement is not for Grouted Duct Connections required (principal tensile stress, pt, less than 0.11 fc ksi), minimum transverse joint shear reinforcement (hoops) and Based on the emulative response of the grouted duct spec- joint stirrups are conservatively required to help ensure that imen, design specifications for grouted duct connections joints resist forces in an essentially elastic manner and do not should adopt the 2009 LRFD SGS provisions for additional become a weak link in the earthquake resisting system. joint shear reinforcement (Asjvi, Asjvo, Asjl, and horizontal J-bars) used in joint shear design (1). 4.2.3 Minimum Transverse Joint 4.2.6 Additional Bent Cap Shear Reinforcement Longitudinal Reinforcement For SDCs B, C, and D, precast bent cap connections should The 2009 LRFD SGS requirement of an additional area of require minimum joint shear reinforcing (transverse hoops), bent cap longitudinal reinforcement, Asjl, equal to 0.245Ast , as required for CIP joints in SDCs C and D per the 2009 should be required for cap pocket connections but may be LRFD SGS (1). However, where the principal tensile stress, excessive for grouted duct (and CIP) connections (1). However, pt, is greater than or equal to 0.11 fc ksi, the larger of the this requirement should be conservatively adopted for all pre- two transverse joint reinforcement equations, 2009 LRFD cast connections until a potentially lower value is determined SGS Eq. 8.13.3-1 and Eq. 8.13.3-2, should be specified for through further research.

OCR for page 97
98 4.2.7 Additional Vertical Joint Stirrups for CIP bent caps in SDCs C and D, and the AASHTO LRFD Outside the Joint Bridge Design Specifications equation do not apply to precast bent cap connections. Use of plastic ducts in precast bent cap The 2009 LRFD SGS requirement of an additional area of connections should be based on provisions developed in vertical stirrups outside the joint, Asjvo, equal to 0.175Ast, is con- ASTM A929/A929M-01(2007) (25) and the owner's approval. servative for precast bent cap connections and should be adopted (1). Future development of a new joint force transfer model may assist in determining whether this requirement is 4.2.13 Provisions for Knee Joints too conservative. Following the precedent for CIP joints, the proposed design specifications and detailing for precast bent cap connections 4.2.8 Additional Vertical Joint Stirrups are limited to interior joints of multicolumn bent caps. Spec- Inside the Joint ifications for knee joints in precast bent cap systems should be developed together with CIP knee joint provisions. The 2009 LRFD SGS requirement of an additional area of vertical stirrups inside the joint, Asjvi, equal to 0.135Ast, is appro- priate for grouted duct connections and should be adopted (1). 4.2.14 Alternative Connection Design A smaller Asjvi requirement equal to 0.12Ast is conservative for and Details for SDC A cap pocket connections and should be adopted. For SDC A, when SD1 is less than 0.10, alternative precast bent cap connections and specifications may be used, as detailed in 4.2.9 Vertical Joint Stirrups Inside the Joint Matsumoto et al. (7) and ASTM A929/A929M-01(2007) (25). However, minimum vertical stirrups in the joint are recom- For cases in which the additional joint shear reinforcement mended, as proposed for the seismic precast bent cap connec- is not required for SDCs B, C, and D (principal tensile stress, tions in SDC A, as well as extension of column longitudinal pt, less than 0.11 fc ), an area of vertical stirrups inside the reinforcement as close as practically possible to the opposite joint, Asjvi, equal to 0.10Ast, is conservative for grouted duct and face of the bent cap. cap pocket connections and should be adopted. In SDC A, a reduced area of vertical joint stirrups inside the joint, Asjvi, equal 4.2.15 Reinforcement in Bedding Layer to 0.08Ast, is conservative for grouted duct and cap pocket and at Column Top connections and should be adopted. An adequate area and precise placement of transverse re- inforcement in the bedding layer and at the column top are 4.2.10 Horizontal J-bars Inside the Joint required for precast bent cap connections to ensure that the Horizontal J-bars are not required for cap pocket connec- required system ductility is achieved. Specifications should tions to achieve emulative response. Use of horizontal J-bars is adopt these requirements. also not practical due to the presence of the pipe and therefore should not be adopted. 4.2.16 Recommended Modifications to the 2009 LRFD SGS for CIP Joint Design 4.2.11 Supplementary Hoop for The following changes to the 2009 LRFD SGS (1) are rec- Cap Pocket Connections ommended for joint shear design of CIP joints: Use of a supplementary hoop at each end of the steel corru- gated pipe in a cap pocket connection helps limit dilation and Integral joint design in the transverse direction should be potential unraveling and should be adopted. updated for consistency with the provisions for noninte- gral systems in the transverse direction. The general mech- anism of transverse response in a multicolumn bent cap 4.2.12 Anchorage Length of Column Bars for both integral and nonintegral bent caps is similar and The depth of a precast bent cap should accommodate col- therefore should adopt the same detailing provisions. umn bar anchorage. The proposed equations for anchorage In SDC B, joint shear (transverse) reinforcement and addi- length of column bars in grouted duct and cap pocket connec- tional joint shear reinforcement should be based on calcu- tions are conservative for all SDCs, require anchorage lengths lation of principal tensile stress, pt. This will help produce comparable to the lengths required for CIP connections, and emulative behavior of limited ductility systems character- should be adopted, subject to the stated limitations. However, ized by flexural plastic hinging with limited effects of joint the 2009 LRFD SGS (1) anchorage length equation, developed shear cracking.