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Section 5: Concrete Culverts

This section applies to cast-in-place and precast concrete box culverts, inlets, junction boxes, and manholes.

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Materials

For cast-in-place concrete culverts, use Class C concrete (fc ́ = 3.6 ksi) with the following exceptions: use Class S concrete (fc ́ = 4.0 ksi) for top slabs of culverts with overlay, with 1-to-2 course surface treatments, or with the top slab as the final riding surface (i.e., "direct traffic"). Refer to district-specific corrosion protection requirements when designing "direct traffic" culverts for regions where bridge decks are exposed to de-icing agents and/or saltwater spray with regularity. If thus required, use Class S (HPC) concrete for the top slab.

For precast concrete culverts, use Class H concrete (fc ́ = 5.0 ksi).

Use Grade 60 reinforcing steel or deformed welded wire reinforcement (WWR) meeting the requirements of ASTM A1064. Refer to district-specific corrosion protection requirements when designing "direct traffic" culverts for regions where bridge decks are exposed to de-icing agents and/or saltwater spray with regularity. If corrosion resistant reinforcement is required, Grade 60 hot-dip galvanized reinforcing steel may be used in accordance with Item 440.

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Geometric Constraints

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  • The maximum skew angle for box culverts is 45°.
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  • Provide 1.5 in. of clear cover for reinforcement in cast-in-place concrete culverts.
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  • Provide 1.0 in. of clear cover for reinforcement in precast concrete culverts.
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  • Provide 2.0 in. of clear cover above the top layer of reinforcement in the top slab of "direct traffic" concrete culverts.
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Structural Analysis

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  • Analyze each box culvert as a simply supported two-dimensional frame. Do not apply the edge beam requirement in Article 12.11.2.1.
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  • For culverts with less than two feet of fill (i.e., "direct traffic"), distribute the weight of curbs and railing over the entire top slab width if the slab is no wider than 32 ft. Otherwise, distribute the curb and railing loads over 16 ft.
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Design Criteria

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  • Do not apply the provisions for design tandem as described in Article 3.6.1.3.1.
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  • Do not apply the variable axle spacing described in Article 3.6.1.2.2. Set the spacing between the two 32.0-kip axles equal to 14.0 ft.
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  • Unless site-specific information is available, assume cohesionless soil with unit weight equal to 120 pcf and friction angle equal to 30°. Use Rankine's active earth pressure coefficient to compute lateral earth pressure. Precast box culverts may be designed for the earth loads defined in ASTM C1577.
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  • Do not apply the water loads indicated in Table 3.4.1-1 for structural analyses of concrete culverts, unless warranted by site conditions.
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  • Limit the equivalent height of soil, heq, from Table 3.11.6.4.1 to a maximum value of 4.0 ft.
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Detailing

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  • Place reinforcement for skewed ends as directed in the SCC-MD, SCP-MD, or MC-MD standard sheets.
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