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Load and Resistance Factor Design

Load and Resistance Factor Design (LFRD) is a 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.

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 3.4.1-2> to 1.25.

Bridge Widenings

Slab Spans

Slab spans can be widened with slab spans. Skewed slabs with main reinforcing perpendicular to the bents will be weak if the edge beam is removed under traffic. The edge should be shored under this condition. Alternatively, dowels can be grouted into the existing slab edge and the widening placed with reinforcing parallel to the centerline of the roadway. Curbs may be removed after the new slab has cured.

Concrete Girders

Concrete girder spans can be widened with concrete girders, prestressed beams, prestressed box beams, or double-tee beams. Prestressed beams are recommended. Box beams may be used if depth is an issue.

Pan Form Girders

Pan form girders can be widened with pan forms, prestressed box beams, or double tee beams. Pan forms are recommended. Alternates using double-tee beams may be appropriate for certain situations.

Steel I-Beam

Steel I-beam spans can be widened with prestressed beams or steel I beams. Prestressed beams are recommended. Steel I beams may be used if depth, framing, or aesthetics is an issue.

Continuous Steel I-Beam

Continuous steel I-beam units can be widened with prestressed beams or steel I beams. Simple-span prestressed beams with the slab continuous are recommended. The slab should have standard reinforcing and be tied to the existing slab.

Cantilever/Drop-In Steel I-Beam

Cantilever/drop-in steel I-beam units can be widened with prestressed beams or continuous steel I beams. Simple-span prestressed beams are recommended with expansion joints over the bents connected by longitudinal open joints to the existing expansion joint at the notches.

Continuous Steel Plate Girders

Continuous steel plate girder units can be widened with continuous steel plate girders or with prestressed beams if the span is 140 ft. or less.

Prestressed Concrete Beams

Prestressed concrete beam spans and units should be widened in kind.

Steel-Reinforced Elastomeric Bearings for Prestressed Concrete Beams

Geometric Constraints

Rectangular pads are preferred over round pads, which make it harder to satisfy rotation.

Structural Analysis

Expanding length of prestressed concrete beam units can be taken as 1/2 total unit length. For highly skewed bridges and very wide bridges, take expanding length on a diagonal between slab corners to obtain the most unfavorable expansion length.

Design Criteria

For Design Method A in <Article 14.7.6>, shape factor S is preferred to be between 10 and 12.

Design Resources

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

Design Examples and Spreadsheets

Elastomeric Bearings

Design Example for Elastomeric Bearings for Prestressed Concrete Beams (Not working templates. PDF files presented in MathCAD format)

Two Shaft Footing
Design Example for Two Drilled Shaft Footing Using Strut-and-Tie Method

FAQ
Why is Texas disregarding the requirement specified for 25 percent of the combined axle loads?

The braking force in the AASHTO LRFD Bridge Design Specifications is significantly larger than is specified in the AASHTO Standard Specifications for highway bridges. This increased load is not justified for Texas.

Why is the minimum spacing for reinforcement in conventional rectangular caps set at 4 inches instead of 3 inches?

The minimum is set at 4 inches instead of 3 inches to allow for adequate consolidation of concrete.

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