This page provides guidance and recommendations on Load and Resistance Factor Design (LRFD) of specific bridge substructure components.
Terminology and Notation LRFD refers to Load and Resistance Factor Design, a design methodology that makes use of load factors and resistance factors based on the known variability of applied loads and material properties. Bracketed <references> reference relevant sections of the AASHTO LRFD Bridge Design Specifications. Limit states TxDOT recommends the following limit states for design of bridge system components <Article 3.4.1>:
Load Factors TxDOT recommends the following permanent loads <Article 3.5>: The engineer may reduce the maximum load factor for wearing surfaces and utilities <DW in Table 3.4.12> to 1.25. Corrosion Protection MeasuresIn areas of the state where deicing agents are frequently used during winter storms, it is recommended that additional corrosion protection measures be incorporated into the bridge design and details. Districtspecific requirements are available for review. 

Phased Construction Recommendations Do not use abutment, bent, or trestle standard detail sheets for phased structures. Geometric Constraints In most cases, the phase line in an abutment or interior bent will be offset from the phase line for the slab. The phase line should not be under a beam or within a bearing seat. Extend the abutment or interior bent past the slab phase line in order to provide support for the beam or girder. Preferably, the phase line should be a minimum of 4 inches from the bearing seat or edge of beam, whichever is greater. At a minimum, the abutment or interior bent phase line will need to be at the edge of the bearing seat or beam. When phasing an abutment or an interior bent, consider providing enough space between the existing structure and the new construction to accommodate splicing of the reinforcement and formwork. Consider how the next phase of construction will be impacted by the placement of phase lines and reinforcement that extends beyond the phase line. Piles and drilled shaft for the next phase may lie within the length necessary for splicing. Avoid having splices that overlap pile or drilled shaft locations in order to facilitate construction. If unable to provide enough room to splice the reinforcement through traditional overlapping, use welded splices or mechanical couplers. Extend reinforcement that will be spliced by welds or mechanical couplers beyond the end of the cap by at least 1foot. As alternative to splicing or welding the reinforcement, a full depth joint may be used at the phase line. For abutments, if a full depth joint is used, limit the space between abutments to 1inch. Use bituminous fiber to fill the gap between the phases. Use a PVC waterstop across the space along the full height of the cap and backwall. For bent caps, the full depth open joint at the phase line should be at least 1foot wide to allow for forming of the adjacent phases. Individual bent caps would support each phase. If the lower roadway/finished grade allows, use at least two columns per phase. When selecting column or drilled shaft/pile spacing, try to keep the distance from face of column or drilled shaft/pile to the phase line between 0.5 and 4 feet. Overhangs greater than 4 feet can result in high negative moments and permanent deflection of the overhang under loading. The construction of additional phases will not remove this deflection. Phased construction of abutments or bents may require that columns or drilled shafts be spaced at irregular intervals. Offset old bent lines and new bents by at least 5 feet, if possible, to keep from fouling foundations on the existing structure. Structural Analysis When designing bents and abutments to be continuous after phasing, consider all stages of construction (including temporary loads) and the final configuration. Select flexural and shear reinforcement so that loading in all phases can be supported. Design bents and abutments that have full depth joints at the phase line as individual components. Software Use CAP18 with a modified input file adjusted for LRFD (see design examples). Use this spreadsheet for shear design. 

Geometric Constraints Supporting an approach slab on wing walls is strongly discouraged. Compaction of backfill is difficult and loss of backfill material can occur. Without the bearing on the backfill, the approach slab is supported on only three sides (at the two wing walls and the abutment backwall), and the standard approach slab is not reinforced for this situation nor are the wing walls designed to carry the load. The approach slab should be supported by the abutment wall and approach backfill only, and appropriate backfill material is essential. TxDOT supports the placement of a cementstabilized sand (CSS) wedge in the zone behind the abutment. CSS solves the problem of difficult compaction behind the abutment, and it resists the moisture gain and loss of material common under approach slabs. Design Criteria A construction joint is recommended in abutment caps longer than 90 ft. The joint should clear the bearing seat areas. 

Geometric Constraints Cap width should be 3 in. wider than the supporting columns to allow column reinforcing to extend into the cap without bending. Structural Analysis
Software Use CAP18 with modified input file adjusted for LRFD (see design examples). Use this spreadsheet for shear design. Detailing


Geometric Constraints Stem width should be at least 3 in. wider than column width to allow column reinforcing to be extended into the cap without bending. Use a stem height to the nearest whole inch. Ledge depth depends on the punching shear capacity required. Determine ledge width from the development of the ledge tie bars as shown in this figure. Design Criteria
Software Use CAP18 for analysis of primary moment and shear (see design example). Use this spreadsheet for shear design. 

Structural Analysis (for typical bridges only)
Use FRAME11, BMCOL51, PIER, or equivalent applications. 

Structural Analysis (for typical bridges only):
Design Criteria For column design, use Strength I, Strength III, and Strength V limit states. Column design must meet the requirements of <Article 5.7.4>. Software Use BMCOL51 or equivalent applications. 

For additional information on LRFD bridge design as implemented by TxDOT, consult the following resources: 

Inverted Tee Reinforced Concrete Caps
Rectangular Reinforced Concrete Caps
Column for Single Column Bent
Two Shaft Footing
Spreadsheets Foundation Loads 