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OCR for page 99
99 Where the principal tensile stress, pt, is greater than or equal The SDC C and D design examples for the grouted duct and to 0.11 fc , the larger of the two transverse joint reinforce- cap pocket connections for which additional joint shear reinforcement is required (principal tensile stress, pt, greater ment equations, Eq. 8.13.3-1 and Eq. 8.13.3-2, should be specified for use because the transverse reinforcement than or equal to 0.11 fc ) demonstrate that design speci- requirement of Eq. 8.13.3-2 can become less than that of fications produce full ductility bent cap connections that Eq. 8.13.3-1 in some cases. are constructible and of similar congestion to a CIP For SDCs B, C, and D, where the principal tensile stress, pt, design. The grouted duct connection design process is straightforward. The cap pocket connection design exam- is less than 0.11 fc , minimum joint reinforcement con- ple is similarly straightforward and illustrates a nonitera- sisting of both transverse hoops (per Article 8.13.3 of the tive design approach for satisfying the general equation 2009 LRFD SGS) as well as an area of vertical stirrups inside for pipe thickness as well as the greater pipe thickness the joint, Ajvi s , equal to 0.10Ast, should be used (1). Minimum required when simplified equations from the commen- joint reinforcement should also be considered for SDC A. tary are used. However, requirements for transverse reinforcement in The SDC B design examples for the grouted duct and cap the joint per AASHTO LRFD Bridge Design Specifications pocket connections for which additional joint shear re- (Article 5.10.11.3 and related Articles 5.10.11.4.3 and inforcement is not required (principal tensile stress, pt, less 5.10.11.4.1d) are inadequate and should not be used for than 0.11 fc ) demonstrate two points. First, design spec- SDC B or Seismic Zone 2. Specifications for knee joints should be developed. ifications produce limited ductility bent cap connections that are constructible but more conservative than CIP designs, due to the requirement for minimum joint shear 4.3 Design Flow Charts (transverse) reinforcement and minimum stirrups within and Design Examples the joint. The impact of these more conservative provisions on cost and constructability is expected to be negligible, Based on application of the proposed specifications to var- while the potential impact on seismic performance is sub- ious precast bent cap to column joint configurations, the fol- stantial. Second, the grouted duct and cap pocket connec- lowing conclusions for design flow charts and design examples tion design processes are straightforward. of nonintegral emulative grouted duct and cap pocket connec- SDC A design examples for grouted duct and cap pocket tions can be drawn: connections demonstrate that design specifications produce precast bent cap designs that are more conservative than CIP Design flow charts and design examples developed for all designs, due to the requirement for minimum joint shear SDC levels demonstrate the practical application of the (transverse) reinforcement and minimum stirrups within proposed specifications. These deliverables provide acces- the joint. However, the impact of these more conservative sibility and are expected to encourage implementation of provisions on cost and constructability is considered negli- Accelerated Bridge Construction practices by removing gible, while the potential impact on seismic performance is potential hindrances in design. It is expected that design- considered important. The design example for hybrid bent cap systems provides ers familiar with current methods of AASHTO LRFD seis- mic design and joint detailing will be equally comfortable designers with a simple method to perform the lateral design with the proposed specifications. of these systems, which exhibit different performance char- Designers should consult the design flow charts before acteristics as compared to CIP systems. This design example designing a precast bent cap to column connection. Flow presents methods for predicting the lateral response as well charts provide a clear outline of the design path, thereby as associated detailing requirements for hybrid precast bent reducing the possibility of designer confusion or inadvertent cap systems. The integral design example presented designers with a omission of applicable specifications. Consolidation of SDCs B, C, and D into an integrated flow complete example of the design and detailing require- ments for the implementation of the studied integral sys- chart clearly shows the designer the main provisions that can tem in moderate and high seismic regions. Designers control full and limited ductility design. For example, the familiar with the design of integral bridge systems will be flow chart illustrates that a similar design path is followed for able to adopt the design of this system with relative ease SDCs B, C, and D where the principal tensile stress, pt, is once a widespread understanding of vertical seismic 0.11 fc or larger. This allows the designer to distinguish design requirements is obtained. There is an overall need the various articles of the design specifications and under- in bridge design in high seismic regions to better under- stand the governing seismic provisions. stand the demands associated with vertical loading, as no