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Section 6: Columns for Multi-Column Bents

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Materials

Use TxDOT Class C concrete (fc ́ = 3.6 ksi) and Grade 60 reinforcing steel. Higher concrete or steel strengths may be required in special cases.

Higher reinforcing steel grades may be used, provided their use satisfies requirements in AASHTO.

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Geometric Constraints

The minimum size column and drilled shaft for grade separation structures is 36-in diameter unless a larger size is noted elsewhere. Column and drilled shaft sizes smaller than 36-in diameter are permissible for widenings only for similitude.

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Structural Analysis

Analysis and design is not required for round columns supporting multi-column bents when the following conditions are met:

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  • Column spacing does not exceed 18 ft.
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  • Column height, measured from bottom of the cap to top of drilled shaft or footing, does not exceed 12 times the column diameter (measured in feet). Example: 36’ height limit for a 3-ft diameter column. For drilled shaft foundations in stream crossings, the bottom of the column is to be taken at the bottom of the scour envelope.
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  • Columns are reinforced with the minimum amount of reinforcement, both longitudinally and laterally, as prescribed in AASHTO.
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  • Columns meet these size requirements based on superstructure type:
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    • Slab spans, 24 in. for stream crossings, 36 in. for grade crossings.
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    • Pan form spans, 24 in. for stream crossings, 36 in. for grade crossings.
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    • Slab beam and spread slab beam spans, 24 in. for stream crossings, 36 in. for grade crossings.
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    • Box beam and spread box beam spans, 36 in.
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    • Types Tx28 through Tx54 girder spans, 36 in.
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    • Types Tx62 and Tx70 girder spans, 42 in.
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    • U40 and U54 beam spans, 36 in.
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    • For other beam types, compare drilled shaft load to what would be expected using one of the preceding superstructures, and use a column diameter as appropriate.

If these conditions are not met, column design and analysis, including second order effects and stiffness reduction from cracked concrete, is required.

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Design Criteria

For columns subjected to bending under unfactored dead load, satisfy the minimum reinforcement requirements of Article 5.6.7, using an exposure factor consistent with the site and other bridge elements.

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Detailing

For non-contact lap splices between the column and its foundation, meet the requirements of Article 5.10.8.4.2a.

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Column Collision

When the design choice is to redirect the collision load, follow the requirements given in Chapter 2 - Limit States and Loads. When the design choice is to provide structural resistance, design for the 600-kip equivalent static load as described in Article 3.6.5.

Design the column to withstand the collision force in shear only, not flexure, and do not consider the transfer of this force to the other elements such as bent caps, footings, piles, or drilled shafts. Use a 0.9 load factor for all dead loads and no live load for the axial compression used for shear design. The number of shear planes used in design is dependent on the bent configuration as described below:

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  • Bents with three or more columns that provide sufficient redundancy may use two shear planes to distribute the collision force.
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  • Bents with a lack of redundancy, such as two or fewer columns or large column spacing, such as straddle bents, require special consideration. For these, consider only one shear plane for the resistance of the collision force.

Use of a deflection wall between the columns is permitted, if necessary.

No further analysis is required for columns with a gross cross-sectional area no less than 40 sq. ft., a least dimension of no less than 5 ft. and column transverse reinforcement is composed of at least No. 4 ties at 12-in. maximum spacing or a No. 4 spiral at 9-in. maximum pitch.

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