Development of a Precast Bent Cap System for Seismic Regions (2011) / Chapter Skim
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Pages 62-93
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From page 62... ...
. A major proposed change is to revise Article 8.13, "Joint Design for SDCs C and D" of the 2009 LRFD SGS to include precast bent cap connections (grouted duct and cap pocket)
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From page 63... ...
Where the joint principal tensile stress indicates joint cracking is not expected (less than 0.11 ) , precast bent cap connections in SDCs B, C, and D still require minimum transverse reinforcement and vertical stirrups within the joint.
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From page 64... ...
For elements that are flexurally dominated, the analytical plastic hinge length can be reasonably approximated as one-half of the element depth in the direction of loading. Therefore, the following is a recommended addition to the 2009 LRFD SGS as Article 4.11.6.2: where: Lps = analytical plastic hinge length for integral concrete superstructures (in)
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From page 65... ...
To prevent the premature fracture of column reinforcement in hybrid systems, the reinforcement must be intentionally debonded to accommodate the localized joint opening at the ultimate displacement capacity. A provision is recommended for inclusion in the LRFD SGS as Article 8.8.14 to explicitly enforce this requirement: Longitudinal reinforcement in hybrid columns shall be intentionally debonded from the surrounding concrete at hybrid column end connections.
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From page 66... ...
is the following: The bottom flange of integral precast girders shall be reinforced with closed hoops within the region from the face of the bent cap equal to a distance equal to the superstructure depth. These hoops shall be spaced with the girder shear reinforcement, with spacing not to exceed 8 in.
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From page 67... ...
3.2.9 Joint Definition Specimen test results and related research provide a sufficient basis for safe, constructible, durable, and economical design of nonintegral emulative precast bent cap systems using grouted duct or cap pocket connections and hybrid connections in all SDC levels. However, as shown in Chapter 2, testing was limited to interior joints of multicolumn bent caps.
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From page 68... ...
This indicates that SDC B joint design for precast connections should be based on a check of principal tensile stresses and that all SDC B joints should include at least minimum joint shear reinforcement, defined as transverse reinforcement and joint stirrups. The proposed LRFD SGS adopts the more conservative provisions that principal tensile stresses be checked for SDC B and that joint design depend on this check.
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From page 69... ...
Minimum Anchorage Length Column longitudinal bars should be extended into joints a sufficient depth to ensure that the bars can achieve approximately 1.4 times the expected yield strength of the reinforcement, i.e., a level associated with extensive plastic hinging and strain hardening up to the expected tensile strength. For SDCs C and D, Article 8.8.4 of the 2009 LRFD SGS requires that column longitudinal reinforcement be extended into cap beams as close as practically possible to the opposite face of the cap beam and that for seismic loads, the anchorage length into the cap beam satisfy the following (1)
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From page 70... ...
Anchorage of reinforcing bars in cap pocket connections can be based on prior precast bent cap research on grout pocket connections using trapezoidal prism-shaped pockets without a stay-in-place form (7, 8)
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From page 71... ...
In addition, this equation applies to #11 column reinforcing bars or smaller ones. As for CIP connections, the proposed specifications for grouted duct and cap pocket connections require that column longitudinal reinforcement be extended into precast bent caps as close as practically possible to the opposite face of the bent cap.
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From page 72... ...
than the 2009 LRFD SGS equation for the example, although Figure 3.3 shows the change in anchorage length with compressive strength. The proposed grouted duct equation is based on both tension cyclic and monotonic tension tests and includes a factor of safety of at least 2.0.
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From page 73... ...
Minimum joint shear reinforcement refers to transverse reinforcement within the joint region in the form of column reinforcement, spirals, hoops, intersecting spirals or hoops, or column transverse or exterior transverse reinforcement continued into the bent cap. For precast connections, minimum transverse joint reinforcement is required to help ensure that the connection does not become a weak link in a precast bent cap system.
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From page 74... ...
This avoids iteration in design calculations, but may result in thicker gage pipe used in design. Where the principal tensile stress in the joint, pt, specified in Article 8.15.2.1, is less than 0.11 , the thickness of the steel pipe, tpipe, may be determined from the following: where: f ′c = nominal compressive strength of the bent cap concrete (ksi)
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From page 75... ...
lac = anchored length of longitudinal column reinforcing bars into precast bent cap (in) fyp = nominal yield stress of steel pipe (ksi)
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From page 76... ...
The transverse provisions for nonintegral bent caps are described in more detail in subsequent sections of this report. Horizontal Stirrups The 2009 LRFD SGS requires the placement of horizontal stirrups around the vertical stirrups within the bent cap (1)
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From page 77... ...
Comparison of joint reinforcement for various seismic guide specifications -- SDCs C and D Reinforcement Type Term Specimen Quantity 2006 LRFD RSGS 2009 LRFD SGS Proposed Guide Proposed Specification Specification Proposed Specification 2009 LRFD SGSTest Specimen Transverse Hoop -A max of A, B 1.00B Vertical Joint 0.27 0.20C 0.175 0.175 0.65D 1.00 0.089E - 0.135 GD: 0.135 CPFD: 0.12 GD: 1.52 CPFD: 1.35 GD: 1.00 CPFD: 0.89 Additional Bent Cap Longitudinal 0.0 0.0 0.245 0.245 F 1.00 Horizontal J-bar 0.13G 0.10G Every other intersection in joint Every other intersection in joint G GD: 1.00 CPFD: H Bedding Layer Hoop - - - - Reinforcement per specification I I Notes: A GD test specimen used hoops close to minimum per 2006 LRFD RSGS, and CPFD used a steel corrugated pipe thickness based on 2006 LRFD RSGS.
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From page 78... ...
The 2006 LRFD RSGS used for the design of the prototype bridge and full ductility test specimens did not distinguish between integral and nonintegral bent cap systems, nor between the design of vertical stirrups inside the joint region and outside (i.e., adjacent to) the joint region (2)
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From page 79... ...
. As mentioned previously, additional joint shear reinforcement for precast bent caps should be based on principal tensile stress exceeding 0.11 (i.e., likely joint shear cracking)
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From page 80... ...
SDC B As for other additional joint shear reinforcement, the additional longitudinal bent cap reinforcement stipulated in the previous section for SDCs C and D is conservatively required for SDC B where the principal tensile stress, pt, for a precast bent cap connection is 0.11 or larger.
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From page 81... ...
The proposed design specification is as follows: Bedding layers between columns and precast bent caps shall be reinforced with transverse reinforcement, as shown in Figure 8.15.5.2.2-1 and Figure 8.15.5.2.3-1. Bedding layer reinforcement shall match the size and type of transverse reinforcement required for the column plastic hinging region and shall be placed evenly through the depth of the bedding layer.
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From page 82... ...
, are as follows: • Attachment DS1: Revised Article 2.1 Definitions – Revision of current article to include definitions of emulative and hybrid systems • Attachment DS2: Revised Article 4.3.3 Displacement Magnification for Short Period Structures – Revised Article to account for hybrid systems • Attachment DS3: Revised Article 4.7.2 Vertical Ground Motion, Design Requirements for SDC D – Expanded Article to include explicit requirements for consideration of vertical excitation with integral precast bent caps discontinuous at bent • Attachment DS4: Revised Article 4.11.6 Analytical Plastic Hinge Length – Revised Article to account for integral concrete superstructures • Attachment DS5: Proposed Article 5.6.6 Ieff for Hybrid Systems – New Article for hybrid systems • Attachment DS6: Revised Article 8.4.2 Reinforcing Steel Modeling – Revised Article for hybrid systems • Attachment DS7: Proposed Article 8.8.14 Lateral Reinforcement Requirement for Columns Connecting to a Precast Bent Cap – New Article to ensure spacing between the hoop at top of column and the bedding layer hoop does not compromise system ductility • Attachment DS8: Revised Article 8.5 Plastic Moment Capacity for SDC B, C, and D – Revised Article for hybrid systems • Attachment DS9: Revised Article 8.8.1 Maximum Longitudinal Reinforcement – Revised Article for hybrid systems • Attachment DS10: Revised Article 8.8.2 Minimum Longitudinal Reinforcement – Revised Article for hybrid systems • Attachment DS11: Proposed Article 8.8.14 Minimum Debonded Length of Longitudinal Reinforcement for Hybrid Columns – New Article for hybrid systems • Attachment DS12: Revised Article 8.10 Superstructure Capacity Design for Longitudinal Direction for SDC C and D – Revised Article for integral precast systems • Attachment DS13: Proposed Article 8.13 Joint Design for SDC A – New Article for SDC A precast bent cap connection design • Attachment DS14: Proposed Article 8.14 -- Joint Design for SDC B – New Article for SDC B precast bent cap connection design • Attachment DS15: Revised Article 8.15 -- Joint Design for SDCs C and D Figure 3.7. View of column bar buckling at interface between the bent cap and column–CPFD (after removal of concrete post-test)
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From page 83... ...
• Attachment DE4: SDCs B, C, and D Design Flow Chart – Flow chart for design of precast bent cap connections in SDCs B, C, and D • Attachment DE5: SDC B Design Example -- Grouted Duct Connection – Design example for grouted duct connection in SDC B (minimum joint reinforcement) • Attachment DE6: SDC B Design Example -- Cap Pocket Connection – Design example for cap pocket connection in SDC B (minimum joint reinforcement)
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From page 84... ...
The grouted duct connection uses corrugated ducts embedded in the precast bent cap to anchor individual column longitudinal bars. The ducts and bedding layer between the cap and column or pile are grouted with high-strength, non-shrink cementitious grout to complete the precast connection.
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From page 85... ...
For example, Table 3.4 requires the 28-day grout compressive strength to have a minimum 500-psi margin over the 28-day expected bent cap concrete compressive strength. This margin accounts for the likelihood that the actual concrete strength will exceed its specified strength as well as the possibility of a low grout strength.
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From page 86... ...
Concrete shall satisfy Article 8.13.8.5.5a to ensure pocket and bedding layer are completely filled and without voids. The probable concrete strength for the cap pocket fill is required to provide a minimum 500-psi margin over the expected bent cap concrete compressive strength to ensure that the cap fill concrete is not the weak link in the connection.
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From page 87... ...
Precast Bent Cap Placement Plan per Article 8.13.8.4.2, (2) Design Calculations for Construction Procedures per Article 8.13.8.4.3, and (3)
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From page 88... ...
Precast Bent Cap Placement Plan The Precast Bent Cap Placement Plan is specified as follows: The Precast Bent Cap Placement Plan, at a minimum, shall contain the following items: (a) Step-by-step description of bent cap placement for each bent, including placement of the bent cap on the columns or piles and the proposed method for forming the bedding layer, placing grout in ducts or concrete in cap pockets, and ensuring that grout or concrete is properly consolidated in the connection and bedding layer.
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From page 89... ...
The trial batch is a key step in achieving the required installation and performance of a grouted duct connection. The purposes of a trial batch are to do the following: • Determine the required amount of water to be added to a particular grout brand to achieve acceptable flowability per Table 3.4 and pot life under the temperature and humidity conditions expected in the field; • Determine the grout cube strength corresponding to the flow achieved; • Examine grout for undesirable properties such as segregation; • Establish the adequacy of proposed grouting equipment such as the mixer, pump, tremie tubes, and vent tubes; • Provide jobsite personnel experience in mixing and handling grout prior to actual connection grouting; and • Help the contractor to make a judicious decision regarding grout brand and its use.
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From page 90... ...
All additional materials required to ensure proper connection of bent cap to column, such as but not limited to bedding layer hoops, shall be properly placed according to shop drawings. All surfaces to be in contact with the grout shall be cleaned of all loose or foreign material that would in any way prevent bond prior to setting bedding layer forms.
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From page 91... ...
Concrete pumping shall be required for connections that cannot be completed by other methods within the pot life established for the concrete during the trial batch. All additional materials required to ensure proper connection of bent cap to column, such as but not limited to bedding layer hoops, shall be properly placed according to shop drawings.
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From page 92... ...
Major sections of the construction specification address the following: • Materials – portland cement concrete for the precast bent cap and cap pocket fill; – hydraulic cement (non-shrink) grout; – corrugated metal duct; – lock seam, helical corrugated steel pipe; – connection hardware; • Contractor submittal including a Precast Bent Cap Placement Plan; and • Construction methods including grouting of grouted duct connections and concreting of cap pocket connections (trial batch, placement, and material testing)
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From page 93... ...
• Attachment ECD5: SDCs B, C, and D -- Grouted Duct Connection – Example bent cap details for grouted duct connection in SDCs B, C, and D (additional joint reinforcement required) • Attachment ECD6: SDCs B, C, and D -- Cap Pocket Connection – Example bent cap details for cap pocket connection in SDCs B, C, and D (additional joint reinforcement required)
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