Section 16: Spliced Precast GirdersAnchor: #i1603360
Use Class H (HPC) concrete for girder elements:
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- Precast Elements: Anchor: #PBKVMCNO
- Minimum fci ́ = 4.0 ksi, Maximum fci ́ = 6.0 ksi Anchor: #OMQVHSJP
- Minimum fc ́ = 5.0 ksi, Maximum fc ́ = 10.0 ksi Anchor: #YGHEQWJP
- Cast in Place Elements: Anchor: #SSXUIJIR
- Maximum fc ́ = 6.0 ksi
Use Class S concrete for cast in place deck (or Class S (HPC) if de-icing chemicals are routinely used at the site):
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- Maximum fc ́ = 4.0 ksi
Use prestressing strand with specified tensile strength, fpu of 270 ksi.
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- Use 0.6 in low-relaxation strands for pretensioning strands. Anchor: #MUJHXNNT
- Use 0.6 in low-relaxation strands for post-tensioning tendons.
Provide post tension system in accordance with Item 426, “Post Tensioning” of the TxDOT Standard Specifications, with the following exceptions:
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- Non-Severe Corrosive Environments: Anchor: #SHNEUHBT
- Galvanized or plastic duct can be used Anchor: #CPFTSXVN
- Meet requirements for Protection Level 1B Anchor: #YHWRLCQR
- Do not use tape-sealed connections Anchor: #OQCHKAEP
- Severe Corrosive Environments: Anchor: #HJWYRLWP
- Use plastic duct only Anchor: #CPYXFJQY
- Meet requirements for Protection Level 2
All stressed tendons in the finished structure must be grouted. All tendons that are stressed at the precast yard must be grouted prior to transport.Anchor: #i1605677
The minimum numbers of girders in any roadway width is as follows:
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- I-Section: 3 girders. If the span is over a lower roadway and the vertical clearance is less than 20 ft., a minimum of 4 girders are required. Anchor: #WVNDTHUH
- U-Section: 2 girders.
Girder designs should meet the following requirements:
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- Use a web or flange thickness that is a whole number and results in the duct not exceeding 0.54 times the least gross concrete thickness at the duct. Anchor: #CQQOGPXT
- Distribute the weight of one railing to no more than three girders. Anchor: #MVKQEINW
- Haunch concrete placed on top of the girder may be considered when determining composite section properties. Anchor: #CIRQSLKJ
- Composite section properties can be calculated assuming either constant modulus of elasticity for the girders and slab, or transforming the sections based upon their respective modulus. Determine respective modulus in accordance with AASHTO LRFD Bridge Design Specifications, Article 184.108.40.206. Anchor: #TXANFQSF
- Live load distribution can be determined from one of the following methods: Anchor: #XACRRPBU
- Must conform to AASHTO LRFD Bridge Design Specifications, Article 220.127.116.11.2 for flexure moment and Article 18.104.22.168.3 for shear when used in conjunction with a line girder analysis. Anchor: #VCQCGQCP
- As determined by use of the lever rule when the span/girder arrangement is out of the applicable range of Articles 22.214.171.124.2 and 126.96.36.199.3 when used in conjunction with a line girder analysis. Anchor: #LBVQECNW
- As distributed by the model when used in conjunction with a grillage, finite element, or other refined model. The model must capture the effects of the complete unit and transfer loads in an acceptable fashion. Anchor: #DFBLXHVH
- When prestressed concrete deck panels or stay-in-place metal forms are allowed, design the girder using the basic slab thickness.
Analysis must consider the effects of the following:
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- Staged construction Anchor: #MNGYBYGV
- Addition and removal of temporary supports Anchor: #GXUCQNON
- Locked in forces Anchor: #ORWGGFLX
- Staged post tensioning Anchor: #CYDWNPRK
- Secondary forces due to post tensioning Anchor: #QMKQDTEO
- Torsion due to horizontally curved alignments Anchor: #SBQETMBG
- Superstructure / Substructure interaction Anchor: #TQQGJKRV
- Temperature variation
Provide a minimum of two tendons per web.
Use diaphragms at all bearing locations.
Provide a full depth diaphragm at all splice and anchorage locations.
Intermediate diaphragm use is not mandatory.
The precast sections must meet the following requirements for transportation:
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- Prestressed Sections:
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- Factor the self-weight load by 1.33 Anchor: #KXCIABKJ
- Strand stress after seating of chucks is not greater than 0.75 fpu for low-relaxation strands Anchor: #OUNHFIGB
- Use the concrete release strength (fci ́)
for the following stress limitations:
Tensile stress < 0.24λ (ksi)
Compressive stress < 0.65 fci ́ (ksi)
- Do not drape pretensioning strands. Debond the strands as needed.
- Non-Prestressed Sections:
- Anchor: #MVMPHTWO
- Factor the self-weight load by 1.33 Anchor: #XNIIEKUA
- Design the section as a reinforced concrete member, subject to the provisions in AASHTO LRFD Bridge Design Specifications, Article 5.6.3. Use the concrete release strength (fci ́) in place of the concrete final strength (fc ́) Anchor: #FYYPDDPJ
- Limit the stress in the reinforcing steel to 36 ksi
The precast sections must meet the following requirements during construction stages:
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- Factor the self-weight load by 1.0 Anchor: #EJWOVSRR
- Include loads to represent weight of form work for splices and strong backs (if applicable) Anchor: #OAKTUWJO
- Use the final concrete strength (fc ́)
for the following stress limitations:
Tensile stress < 0.24λ (ksi)
Compressive stress < 0.6 fc ́ (ksi)
The girder must meet the following requirements in the final (service) condition.
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- Use associated final concrete strengths (fc ́) for the precast sections and cast in place splices Anchor: #JOLGMUSV
- Use effective prestress force after all short and long term losses. Losses can be determined by hand as outlined in Section 4, Pretensioned Concrete I Beams and I Girders, or by analysis software that has concrete time dependent capabilities to capture the effect of creep and shrinkage. Anchor: #CCMYQBIA
- Compressive stress limitations: Anchor: #KMJRROKM
- Tensile stress limitations: Anchor: #UUWDPOUR
- Principal Tensile stress at Neutral Axis
Service III Loading < 0.110λ (ksi)
All post tensioning must be done prior to placement of the deck.
The composite deck is not a prestressed element and is not held to the stress limitations listed above.
The deck must meet the following requirements:
- Anchor: #CVCKVXCM
- Design Load includes effects due to the following: Anchor: #KWIYAUSX
- Longitudinal steel must meet the following
- Anchor: #XUKLTPXG
- Tensile stress in deck concrete is less than (0.9)(0.24)λ (ksi), use No. 4 bars at 9-in spacing Anchor: #XEOEAKYT
- Tensile stress in deck concrete is greater than (0.9)(0.24)λ (ksi), deck reinforcement must equal or exceed 1% of the gross deck cross-sectional area (do not use bars larger than No. 6):
Design shear based upon Strength I Loading for the final condition and in accordance with Article 188.8.131.52 of the AASHTO LRFD Bridge Design Specifications. Use the General Procedure as provided by Article 184.108.40.206.2. Do not use provisions of Appendix B of the AASHTO LRFD Bridge Design Specifications. The effective web width must be reduced by 25% of the outer diameter of the splice coupler.
Design ultimate moment based upon Strength 1 Loading for the final condition.
Refer to Section 4, Pretensioned Concrete I Beams and I Girders for interface shear design of the deck to girder flange interface.
Show predicted slab deflections should be shown on the plans. Compute deflections using the same composite sections (constant modulus for girder and deck, or transformed sections) used in the analysis. Denote on plans the assumed modulus (if constant is used) or the assumed values of fc ́ of the individual elements.
Included in plans the assumed construction sequence that includes the following:
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- Order of construction Anchor: #FAOYQLTG
- Shore tower locations Anchor: #IIIDCXLC
- Shore tower loads Anchor: #AQWTALIV
- Lifting / support points of precast members Anchor: #GKUFLFJV
- Girder elevation points Anchor: #QIABMJGY
- Post tensioning sequence
Require contractor to provide a temporary bracing plan of the girders.
Require contractor to provide shoring and erection plan.Anchor: #HIHDOWSP
Provide 2-in. clear cover to reinforcing steel for entire cross section. Also, increase top slab clear cover to 2.5 in. in areas of state where de-icing agents are frequently used.
Provide a minimum tangent length, dependent on duct size and type, of tendon from the anchorage head before introducing any curvature. Determine minimum radius of curvature for individual duct sizes based on published values from suppliers.
Reference Item 426 “Post Tensioning” in the General Notes for all post tensioning, grouting materials, and construction. Note exceptions if Protection Level 1B is used in the design (galvanized duct allowed)
Provide anchorage zone details per Article 220.127.116.11