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Section 16: Segmental Spans

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Materials

Use TxDOT Class H concrete with a minimum = 5.0 ksi, and Grade 60 reinforcing steel.

Use 0.6-in., low-relaxation prestressing strand with a specified tensile strength, of 270 ksi.

Use duct as specified in Item 426, "Post-Tensioning" of the Texas Standard Specifications.

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Geometric Constraints

Use minimum cross-section dimensions as specified in Article 5.14.2.3.

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Structural Analysis

Analyze segmental spans in accordance with Article 4.6.2. Analysis must consider time-dependent construction methods; short- and long-term post-tensioning, creep, and shrinkage effects; and secondary force effects due to post-tensioning.

Base live load reactions per lane on the combined effect of the truck loading added to the lane loading.

Round prestress losses to 1 ksi.

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Design Criteria

Check limit states using the Strength I, Strength III, Strength IV, Service I, and Service III load combinations.

In addition to the limit states and loads specified in Chapter 2 and above, apply the construction loads and combinations in Article 5.14.2. Check the cantilever for overturning during erection as specified in Article 5.14.2.4.4.

Satisfy all stress limits for prestressing tendons and concrete as required in Articles 5.9.3 and 5.9.4. Use limits for severe corrosive conditions in areas of the state where de-icing agents are frequently used.

For all considerations other than preliminary design, determine prestress losses as specified in Article 5.9.5, including consideration of the time-dependent construction method and schedule shown in the contract documents.

Check principal stress in webs as required in Article 5.8.5.

Check shear and torsion as required in Article 5.8.6.

Determine tie back reinforcement behind anchorages as required in Article 5.10.9.3.4b. Minimize the amount of tie back reinforcing steel penetrating through segment bulkheads by adjusting the location of anchorages or using hooks for development.

Overhang strength for extreme events, described in Article 9.5.5, is satisfied through TxDOT’s rail crash testing.

Calculate prestress losses for creep, shrinkage, elastic shortening and relaxation as prescribed in Section 4, Pretensioned Concrete I Girders, or by analysis software that has concrete time-dependent capabilities to capture the effect of creep and shrinkage.

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Detailing

Provide 2 in. clear cover to reinforcing steel for entire cross section. Add 1 in. grinding allowance to top slab. Also, increase top slab clear cover to 2.5 in. in areas of state where de-icing agents are frequently used.

Include shear keys in webs of precast and cast-in-place segmental spans. Space shear keys 1 ft. 6 in. along centerline of web.

Provide a minimum 5 ft. tangent length 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.

Include extra longitudinal reinforcement above a horizontal construction joint to mitigate cracking from restraint shrinkage.

Include extra longitudinal reinforcement above a doorway opening or other significant cross-section change, to mitigate cracking from differential shrinkage.

Include provisional post-tensioning ducts as required in Article 5.14.2.3.8. Provisional anchorage hardware is not required, but allowance for future placement of hardware must be considered.


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