National Academies Press: OpenBook

Development of a Precast Bent Cap System for Seismic Regions (2011)

Chapter: Chapter 4 - Conclusions

« Previous: Chapter 3 - Interpretation, Appraisal, and Applications
Page 94
Suggested Citation:"Chapter 4 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2011. Development of a Precast Bent Cap System for Seismic Regions. Washington, DC: The National Academies Press. doi: 10.17226/14484.
×
Page 94
Page 95
Suggested Citation:"Chapter 4 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2011. Development of a Precast Bent Cap System for Seismic Regions. Washington, DC: The National Academies Press. doi: 10.17226/14484.
×
Page 95
Page 96
Suggested Citation:"Chapter 4 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2011. Development of a Precast Bent Cap System for Seismic Regions. Washington, DC: The National Academies Press. doi: 10.17226/14484.
×
Page 96
Page 97
Suggested Citation:"Chapter 4 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2011. Development of a Precast Bent Cap System for Seismic Regions. Washington, DC: The National Academies Press. doi: 10.17226/14484.
×
Page 97
Page 98
Suggested Citation:"Chapter 4 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2011. Development of a Precast Bent Cap System for Seismic Regions. Washington, DC: The National Academies Press. doi: 10.17226/14484.
×
Page 98
Page 99
Suggested Citation:"Chapter 4 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2011. Development of a Precast Bent Cap System for Seismic Regions. Washington, DC: The National Academies Press. doi: 10.17226/14484.
×
Page 99
Page 100
Suggested Citation:"Chapter 4 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2011. Development of a Precast Bent Cap System for Seismic Regions. Washington, DC: The National Academies Press. doi: 10.17226/14484.
×
Page 100
Page 101
Suggested Citation:"Chapter 4 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2011. Development of a Precast Bent Cap System for Seismic Regions. Washington, DC: The National Academies Press. doi: 10.17226/14484.
×
Page 101

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

94 This chapter summarizes all major observations and con- clusions from the precast bent cap connection research con- ducted under NCHRP Project 12-74, including results from the seven bent cap to column connection tests, one girder to bent cap connection test, design specifications including design methodologies, design flow charts and design exam- ples, construction specifications, example connection details, and implementation plan. 4.1 Test Specimens Based on the observed specimen response and data analy- sis, the following conclusions can be drawn. 4.1.1 Cast-in-Place (CIP) Control Specimen • Despite the less conservative design basis used from the 2006 Recommended LRFD Guidelines for the Seismic Design of Highway Bridges (2006 LRFD RSGS) compared to the 2009 AASHTO Guide Specifications for LRFD Seismic Bridge Design (2009 LRFD SGS), including a smaller area of verti- cal stirrups within the joint and smaller area of bent cap longitudinal reinforcement, the CIP specimen satisfied the performance goal of the design—achieving an extensive drift without appreciable strength degradation and exhibit- ing extensive plastic hinging of the column, limited joint dis- tress, and essentially elastic behavior of the bent cap (2, 1). • The CIP specimen provided an appropriate benchmark (control) for comparison with the precast grouted duct and cap pocket specimens. In addition, test results can be reliably used as a supporting basis for developing design and con- struction specifications for seismic precast bent cap systems. 4.1.2 Grouted Duct (GD) Specimen • Despite the less conservative design used from the 2006 LRFD RSGS compared to the 2009 LRFD SGS—including a smaller area of vertical stirrups within the joint and smaller area of bent cap longitudinal reinforcement—the Grouted Duct (GD) specimen satisfied the performance goal of the design, achieving an extensive drift without appreciable strength degradation and exhibiting extensive plastic hing- ing of the column, limited joint distress, and essentially elastic behavior of the bent cap (2, 1). • Emulative performance is concluded for the GD specimen based on the close match between its overall behavior and that of the CIP control specimen, including lateral force- displacement response; plastic hinging; joint shear stiffness; level of joint distress; pattern of joint cracking; strain pat- terns of bent cap and joint reinforcement; integral behavior between the bedding layer, column, ducts, and bent cap; and minor bar slip. • GD response indicates that design specifications for a full ductility grouted duct connection should address vertical joint stirrups inside and outside the joint, horizontal cross ties inside the joint, transverse joint shear reinforcement, and additional longitudinal bent cap reinforcement. • Construction specifications should address fabrication and assembly processes as well as grout used for the connection. 4.1.3 Cap Pocket Full Ductility (CPFD) Specimen • Despite the less conservative design basis used from the 2006 LRFD RSGS compared to the 2009 LRFD SGS—including a smaller area of vertical stirrups within the joint and smaller area of bent cap longitudinal reinforcement—the Cap Pocket Full Ductility (CPFD) specimen satisfied the performance goal of the design, achieving an extensive drift without appreciable strength degradation and exhibiting extensive plastic hinging of the column, limited joint distress, and essentially elastic behavior of the bent cap (2, 1). • Emulative performance is concluded for the CPFD speci- men based on the close match between its overall behavior C H A P T E R 4 Conclusions

95 and that of the CIP control specimen, including lateral force-displacement response; plastic hinging; joint shear stiffness; strain patterns of bent cap longitudinal reinforce- ment; integral behavior between the bedding layer, col- umn, pipe, and bent cap; and minor bar slip. • CPFD response indicates that design specifications for a full ductility cap pocket connection should address vertical joint stirrups inside and outside the joint, pipe thickness based on providing the same circumferential hoop force in the joint as that required by transverse reinforcement provisions of Article 8.13.3 of the 2009 LRFD SGS, supplementary hoop at ends of the pipe, and additional longitudinal bent cap reinforcement (1). • Construction specifications should address fabrication and assembly processes as well as concrete within the cap pocket. 4.1.4 Cap Pocket Limited Ductility (CPLD) Specimen • Despite elimination of the joint reinforcement used in the full ductility specimens (vertical stirrups within the joint, joint-related stirrups and horizontal cross ties external to the joint, and hoops at the ends of the pipe) and reduction of bent cap flexural reinforcement and bent cap transverse reinforcement, the CPLD specimen satisfied the main per- formance goal of the Seismic Design Category (SDC) B design. The CPLD specimen exhibited ductile plastic hing- ing and reached an extensive drift of 5.1% (µ8 nominal), well beyond a displacement ductility of 2.0 (µ2), with only minor (12%) load degradation at maximum drift. This is attributed to the effectiveness of the corrugated steel pipe within the joint. • Extensive joint shear cracking softened the CPLD joint, contributed significantly to column drift, and delayed (but did not prevent) flexural plastic hinging. This response is attributed to the absence of vertical joint stirrups, which permitted unrestrained development, growth, and widen- ing of joint shear cracks. This response was in contrast to the full ductility CIP and CPFD specimens. However, it can be reasonably deduced that similar, or more severe, joint behavior would likely develop for a similarly detailed CIP limited ductility connection because an SDC B CIP joint would incorporate less extensive and less effective trans- verse reinforcement (based on the limited provisions of current AASHTO LRFD Bridge Design Specifications) than that provided by the steel pipe. • Based on the foregoing conclusions, emulative behavior (relative to a limited ductility CIP connection) can be con- cluded for the CPLD specimen. Similarities in performance between the limited ductility and full ductility speci- mens including plastic hinging; lateral force-displacement response; equivalent viscous damping; and integral behav- ior between the bedding layer, column, pipe, and bent cap support this conclusion. • Despite the extensive plastic hinging, the development of significant joint shear damage (but not failure) observed for the CPLD specimen does not match the expressed intent of Article 4.7.1 of the 2009 LRFD SGS for limited ductility structures, including the requirement that “Inelastic action is intended to be restricted to flexural plastic hinges in the column (1).” • CPLD response indicates that design specifications for a limited ductility cap pocket connection should incorporate minimum reinforcement requirements to help produce emulative behavior characterized by flexural plastic hinging with limited effects of joint shear cracking: (1) minimum area of vertical joint stirrups and (2) pipe thickness based on providing the same circumferential hoop force in the joint as that required by minimum transverse reinforcement pro- visions of Article 8.13.3 of the 2009 LRFD SGS (1). In addi- tion, where the principal tensile stress, pt, is greater than or equal to 3.5 psi (or 0.11 ksi), additional joint re- inforcement should be required. • CPLD response also has important implications for CIP design. The following provisions are recommended for inclusion in the LRFD SGS for CIP structures in SDC B (lim- ited ductility) to help produce emulative behavior charac- terized by flexural plastic hinging with limited effects of joint shear cracking: (1) minimum area of vertical joint stirrups and (2) minimum joint transverse reinforcement based on Article 8.13.3 of 2009 LRFD SGS (1). This reinforcement can be determined prescriptively, avoiding extensive seismic analysis, and can result in constructible details. Similar pro- visions can also be adopted for SDC A. 4.1.5 All Emulative Precast Specimens (GD, CPFD, CPLD) Additional analysis is required to develop a new model that fully characterizes grouted duct and cap pocket joint behavior including joint forces, pipe effects, crack patterns, pipe effects, and differences in strain distributions between the specimens and the CIP control specimen. 4.1.6 Conventional Hybrid Specimen • The design methodology used for the conventional hybrid specimen resulted in a system that satisfied performance objectives up to the design level drift. The ultimate lateral deformation capacity was in excess of a 6% drift ratio, with significant reductions in damage and residual offset as com- pared to CIP and emulative systems. • Lateral force-displacement predictions based on the pro- cedures presented by Tobolski (5) and in the attachments ′fc′fc

96 match well with the recorded system response up to the pre- dicted failure point. The predicted ultimate displacement capacity was conservative in comparison to the actual observed lateral capacity. Predictions indicated that the fail- ure of the system would be attributable to the crushing of the confined concrete core whereas the observed failure mode was fracture of column reinforcement. • Use of current joint force transfer models as presented in the 2009 LRFD SGS are reasonable and conservative for the design of joints in hybrid bridge column systems with the consideration of column post-tensioning forces (1). • Larger-than-expected column post-tensioning forces were obtained due to a smaller-than-anticipated anchor set loss in the tendons. Based on observed damage and residual off- sets, it is recommended that the ratio of the neutral axis depth to column diameter be limited to 0.25 to minimize the level of compressive straining in the column and enhance the self-centering capacity of the system. 4.1.7 Concrete Filled Pipe Hybrid Specimen • The design methodology used for the concrete filled pipe hybrid specimen resulted in a system that satisfied perfor- mance objectives up to approximately the design level drift. The ultimate deformation capacity was approximately equal to a 6% drift ratio, with appreciable reduction in damage and residual displacements as compared to the CIP specimen. • Lateral force-displacement predictions based on the pro- cedures presented by Tobolski (5) and in the attachments match well with the recorded system response up to approx- imately a 2% drift ratio. After cycles at a 2% drift ratio, damage was observed in the grout bedding layer, ultimately leading to a continual reduction in the lateral capacity until ultimate fracture of a column reinforcing bar. The reduction in capacity is attributable to the progressive damage to the bedding layer, which resulted in a reduction in the effective column diameter at the base. • Use of current joint force transfer models as presented in the 2009 LRFD SGS are reasonable and conservative for the design of joints in hybrid bridge column systems with the consideration of column post-tensioning forces (1). • Results indicated that the use of fiber-reinforced grout in the bedding layer may enhance overall system performance by maintaining the integrity of the bedding layer and thereby maintaining the column compression toe. 4.1.8 Dual Steel Shell Hybrid Specimen • The design methodology used for the dual steel shell hybrid specimen resulted in a system that satisfied performance objectives up to approximately the design level drift. The ultimate deformation capacity was approximately equal to a 6% drift ratio, with appreciable reduction in damage and residual displacements as compared to the CIP specimen. • Lateral force-displacement predictions based on the pro- cedures presented by Tobolski (5) and in the attachments match well with the recorded system response up to approx- imately a 2% drift ratio. After cycles at a 2% drift ratio, dam- age was observed in the grout bedding layer, ultimately leading to a continual reduction in the lateral capacity until ultimate fracture of a column reinforcing bar. The reduction in capacity is attributable to the progressive damage to the bedding layer, which resulted in a reduction in the effective column diameter at the base. • Use of current joint force transfer models as presented in the 2009 LRFD SGS are reasonable and conservative for the design of joints in hybrid bridge column systems with the consideration of column post-tensioning forces (1). • Results indicated that the use of fiber-reinforced grout in the bedding layer may enhance overall system performance by maintaining the integrity of the bedding layer and thereby maintaining the column compression toe. 4.1.9 All Hybrid Specimens The use of fiber-reinforced grout in the bedding layer of hybrid specimens is expected to enhance overall perfor- mance by maintaining the integrity of the compression toe dur- ing cyclic loading. This is expected to minimize the observed reductions in lateral capacity during larger deformation cycles and enhance the self-centering performance of the systems. 4.1.10 Integral Specimen • During cycling at essentially elastic service demands, the superstructure responded without any observed reduction in stiffness or slip between the girder and reaction block. Under essentially elastic seismic demands, there was simi- larly no observed reduction in stiffness or slip indicating that the system is capable of satisfying operational and service level demands in accordance with LRFD code provisions. • The superstructure connection studied is capable of under- going rotation demands in excess of 0.01 radians in a safe and reliable manner. Under both positive and negative flex- ural loading, the flexural capacity was maintained with sig- nificant energy dissipation under negative loading due to yielding of deck flexural reinforcement. Under positive flex- ural loading, deformations were concentrated at the joint with pronounced joint opening. • At cycles to approximately 0.006 radians, a horizontal crack was observed between the deck and the top of the girder. This crack is attributable to the inadequate anchorage of the girder shear reinforcement provided by traditional 90-deg hooks. It is recommended that headed reinforcement or similarly well-anchored reinforcement is used to minimize

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

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

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

100 capacity design procedures can be used in this loading direction. 4.4 Construction Specifications Based on specifications from Matsumoto et al. (4) and test specimen fabrication and assembly, the following conclusions for construction specifications of precast bent cap connections can be drawn: • Construction specifications are expected to help ensure that precast bent cap connections using precast connections are constructible and also to provide the expected seismic per- formance, durability, and economy. • Fabrication of precast bent caps using grouted ducts and cap pockets is feasible and relatively straightforward, facilitated by the use of readily available, stay-in-place corrugated ducts (grouted duct connection) or corrugated steel pipe (cap pocket connection). • Grouting or concreting of a precast bent cap connection involves procedures, operations, and equipment that may not be familiar to the Contractor, and thus specifications include detailed provisions and commentary to ensure connections are made properly in the field. • Semi-rigid corrugated metal (steel) ducts specified per ASTM A653 should be adopted, based on excellent anchor- age between the column bar, grout, and surrounding con- crete in grouted duct connections. Plastic ducts should only be used based on applicable research and when approved by the Engineer. • Lock seam, helical corrugated steel pipe per ASTM A760, using a pipe thickness that provides equivalent CIP joint hoop reinforcement should be adopted for cap pocket con- nections. This pipe can effectively be used as a stay-in-place form and seismic joint reinforcement, and can also provide excellent confinement and mechanical interlock, allowing column bars to be anchored within lengths comparable to CIP connections. Plastic pipe should not be used. • Special forming is required above and below the steel cor- rugated pipe to form the cap pocket void through the full depth of the precast bent cap. • Fabrication and placement tolerances should be established on a job-specific basis and be considered in the establish- ment of duct and pipe diameters. • Accurate positioning of ducts and column bars may be achieved using templates and/or supplementary reinforce- ment. Guide pipes may be used to facilitate cap setting. • Friction collars and shims may be used to support the cap during placement. Compressible shims should be preferred over steel shims, where possible. Compressible shims such as engineered multipolymer high-strength plastic should have a modulus of elasticity slightly less than the hardened grout at the time of load transfer. Shim stacks should be sta- bilized and prevented from moving during cap setting. • A minimum 500-psi strength margin between the expected compressive strength of the precast bent cap concrete and the specified compressive strength of the connection material (grout for grouted duct and concrete fill for cap pocket) should be adopted to help ensure that the connection does not become a weak link in the system. • For seismic applications, the maximum thickness of a grouted bedding layer should be limited to 3 in to maintain the overall integrity. For hybrid bent caps, polypropylene fibers should be included in the grout matrix to maintain the overall integrity of the joint at a 3 lb/cu yd fraction. • Grout used in vertical joints of integral bridge systems should contain a minimum 3 lb/cu yd fraction of polypropylene fibers to ensure that the essential joint integrity is maintained during loading. • Although lightweight concrete can provide significant advantages for a precast bent cap system, its use should be based on relevant research, including its effect on the seis- mic performance of the connection. • To ensure appropriate mechanical properties, compatibil- ity, constructability, and durability, grout for the grouted duct connection should be specified as shown in proposed Table 8.13.8-1. • Concrete should be sufficiently flowable to fill the pocket and bedding layer and to flow out of air vents at the top of the bedding layer. • To accommodate fabrication and placement tolerances as well as grouting or concreting operations, a bedding layer with transverse reinforcement should be used between the column top and bent cap soffit. Clear spacing between the transverse reinforcement and the formed surfaces should be at least three times the top size of the aggregate, to ensure adequate flow of grout to fill all voids. In addition, bedding layers greater than 3 in should be reinforced based on pro- visions established by the owner. • Uniform spacing between hoops at the top of the column and the bedding layer is critical to ensure that the system ductility is not compromised. Shop drawings should show the intended placement of the first hoop at the top of the col- umn as well as the bedding layer reinforcement. • Contractors should submit a Precast Bent Cap Placement Plan, including (1) a description of bent cap placement, (2) a description of the hardware and method used to hold the bent cap in position, (3) product information for candi- date grouts or concrete mixes, (4) a description of hardware and equipment for grouting or concreting, and (5) the mitigation plan to repair any voids. • Contractors should submit detailed shop drawings, includ- ing (1) the proposed construction sequence; (2) the size and type of ducts or pipes, supports, tremie tubes, air vents, and

drains; (3) bedding layer reinforcement and its location relative to the first hoop at the top of the column; (4) the elevations and geometry for positioning the bedding layer collar for bent cap placement; and (5) the details of grout- ing or concreting equipment and mix design and the method of mixing, placing, and curing. • The trial batch is a key step in achieving the required instal- lation and performance of connection material—grout for grouted duct and concrete fill for cap pocket—and should be specified to (1) determine the required amount of water to be added to achieve acceptable flowability and pot life under expected field conditions, (2) determine the associ- ated compressive strength, (3) examine the material for undesirable properties, (4) establish the adequacy of pro- posed equipment, (5) provide jobsite personnel experi- ence in mixing and handling the connection material prior to actual operations (grouting or concreting), and (6) help the contractor to make a judicious decision regarding selection of connection material (grout brand or concrete mix). • Placement of the grout and concrete fill is a critical step in construction of precast bent cap connections and should be conducted as detailed in the proposed specifications. 4.5 Example Connection Details Based on the development of example connection details for precast bent cap connections in SDCs A through D, the follow- ing conclusions can be drawn: • Example precast bent cap connection details provide clear illustrations and sufficient detail and notes for a thorough understanding of key features of grouted duct and cap pocket connections. • Precast bent cap connection details are expected to be constructible. • Precast bent cap connections incorporate certain details not found in CIP connections that require attention: – Use of a reinforced bedding layer. – Accurate placement and spacing of the hoop at the top of the column and within the bedding layer. – Minimum joint shear (transverse) reinforcement for all SDC levels. – Vertical stirrups inside the joint for all SDC levels. – Cap pocket connections: stay-in-place, partial-depth steel pipe serving as joint shear (transverse) reinforce- ment; concrete fill in cap pocket void and bedding layer; 2-leg vertical stirrups without overlapping within the joint; supplementary hoop for full ductility; column bar anchorage nearly full depth; and optional U-bars for full ductility. – Grouted duct connections: stay-in-place, full-depth steel corrugated ducts; grout in grouted ducts and bedding layer; and column bar anchorage nearly full depth. 4.6 Implementation Plan Based on the development of an Implementation Plan, the following conclusions can be drawn: • The Implementation Plan provides an effective roadmap for implementing NCHRP Project 12-74 research products. • All steps of the Implementation Plan should be closely fol- lowed through the appropriate channels to ensure that the NCHRP Project 12-74 research products are implemented in a timely and comprehensive manner. • The extent of Research Team participation depends on future funding. 101

Next: References »
Development of a Precast Bent Cap System for Seismic Regions Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Report 681: Development of a Precast Bent Cap System for Seismic Regions explores the development and validation of precast concrete bent cap systems for use throughout the nation’s seismic regions.

The report also includes a series of recommended updates to the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications, Guide Specification for LRFD Seismic Bridge Design, and AASHTO LRFD Bridge Construction Specifications that will provide safe and reliable seismic resistance in a cost-effective, durable, and constructible manner.

A number of deliverables are provided as attachments to NCHRP Report 681, including design flow charts, design examples, example connection details, specimen drawings, specimen test reports, and an implementation plan from the research agency’s final report. These attachments, which are only available online, are titled as follows:

Attachment DS—Design Specifications

Attachment DE—Design Examples

Attachment CS—Construction Specifications

Attachment ECD—Example Connection Details

Attachment SD —Specimen Drawings

Attachment TR—Test Reports

Attachment CPT—Corrugated Pipe Thickness

Attachment IP—NCHRP 12-74 Implementation Plan

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!