Precast prestressed concrete wide-flange (WF) girders are the workhorse of American highway bridge construction. For spans between roughly 40 and 175 feet, they are typically the most economical superstructure choice — competitive first cost, wide contractor availability, and minimal long-term maintenance. Selecting the right section at the preliminary design stage sets the cost and constructability trajectory for the entire project.
This article covers the AASHTO LRFD framework for preliminary girder selection, span-to-depth ratio guidance, transverse layout rules, and the practical factors that often override the pure engineering calculation.
Standard WF girder designations (e.g., WF58TDG) indicate the nominal section depth in inches followed by a fabrication designation. The TDG suffix indicates a double-T groove configuration used for composite deck connections. Most states have adopted the following standard sections, originally developed through state DOT coordination:
| Section | Depth (in) | Bottom Flange Width (in) | Top Flange Width (in) | Approx. Self-Weight (plf) | Typical Span Range (ft) |
|---|---|---|---|---|---|
| WF36TDG | 36 | 26 | 18 | ~590 | 40–65 |
| WF42TDG | 42 | 26 | 18 | ~670 | 55–80 |
| WF50TDG | 50 | 26 | 18 | ~760 | 65–95 |
| WF58TDG | 58 | 26 | 18 | ~855 | 80–115 |
| WF66TDG | 66 | 26 | 18 | ~945 | 90–130 |
| WF74TDG | 74 | 26 | 18 | ~1,035 | 100–145 |
| WF83TDG | 83 | 26 | 18 | ~1,145 | 120–165 |
| WF95TDG | 95 | 26 | 18 | ~1,280 | 140–180 |
Span ranges are approximate and assume HL-93 live loading, normal-weight concrete (f'ci = 6.5 ksi, f'c = 8–10 ksi), and girder spacing of 8–9 ft on-center. Actual span capacity depends on number of draped or debonded strands, concrete strength, and composite deck thickness.
AASHTO LRFD Table 2.5.2.6.3-1 provides recommended minimum depth-to-span ratios for preliminary design. For prestressed concrete I-beams (simple spans), the recommended minimum is:
Where L is the span length in feet and depth is in feet. These are minimums — exceeding them improves deflection performance and typically reduces the number of strands required.
Practical application: for a 100-ft span, the minimum overall depth (girder + deck) for a non-composite section is 0.045 × 100 = 4.5 ft = 54 inches. A WF58TDG (58 in girder) plus an 8-inch deck gives 66 inches total — within range. A WF50TDG (50 in girder + 8-in deck = 58 in total) technically falls below the non-composite minimum but satisfies the composite check. In practice, the WF58 would typically be specified for a 100-ft span.
Once the girder section is selected, the transverse arrangement determines the number of girder lines. The governing parameters under AASHTO LRFD are:
AASHTO LRFD Section 4.6.2.2 provides distribution factor equations for moment and shear. These equations are valid within the following girder spacing limits for prestressed concrete I-beams:
Practical range for most highway bridges: 6.0 ft to 11.5 ft on-center. Below 6 ft, the number of girder lines drives cost up significantly. Above 11.5 ft, the deck slab thickness and transverse reinforcement become the controlling cost element, and the distribution factor equations may fall outside their calibrated range.
A standard 8.0-inch CIP concrete deck works for girder spacing up to approximately 9.5–10.0 ft. For wider spacing (10–12 ft), deck slab thickness typically increases to 9–10 inches, adding dead load and cost. The net effect is that girder spacing beyond about 10 ft on-center rarely saves money on a total-cost basis for typical highway bridges.
AASHTO LRFD requires a minimum of four girder lines for bridges that must remain serviceable after the loss of one girder line (structural redundancy check). Three-girder bridges are permissible but require special load path redundancy analysis. For most state DOT projects, four is the practical minimum regardless of deck width.
Not every state has precasters capable of manufacturing WF83 or WF95 sections. These sections require specialized long-line casting beds (typically 400–500 ft), high-capacity overhead cranes, and specialized stressing jacks. Specifying a section that must be shipped from out of state adds cost and schedule risk. Check with regional precast associations (e.g., Northwest Precast Concrete Association, Southeast Precast Association) before specifying sections above WF74 in most states.
Maximum girder length for standard transportation permits is typically 120–130 ft in most states. Girders up to approximately 160 ft can be moved with special wide-load permits, but costs increase and routing constraints apply. Beyond 160 ft, spliced precast or steel plate girder becomes the practical alternative. If your span requires a WF95TDG at 170 ft, verify transport feasibility before committing to that alternative in a type study.
Long, heavy WF girders require large-capacity cranes. A WF95TDG at 150 ft weighs approximately 90 tons. Erecting it in a confined site, over water, or on an active roadway requires careful equipment planning. Some sites physically cannot accommodate a crane with the required reach and capacity, which can eliminate an otherwise economical precast alternative.
The span ranges in the table above assume concrete compressive strength at release (f'ci) of approximately 6.5 ksi and at 28 days (f'c) of 8–10 ksi. Higher strength concrete (f'c up to 14–15 ksi is achievable with modern mix designs) can extend span capability by 10–15%, but not all precasters can reliably achieve these strengths. Maximum strand count is limited by the bottom flange geometry — WF sections with 26-in bottom flanges can typically accommodate 40–52 strands in the bottom row plus additional rows above.
For a preliminary type study, the precast-to-steel crossover point varies by site and market, but the following conditions generally favor steel plate girder over precast concrete WF:
Bridge Copilot's girder optimization module applies the logic described above — AASHTO LRFD span-to-depth ratios, standard WF section library, practical spacing constraints — to automatically select the controlling girder section for a given span length, deck width, and loading. The output includes:
This automated selection replaces the manual process of looking up span tables, checking spacing limits, and iterating on girder count — tasks that a trained engineer can do correctly but that take time and are prone to transcription errors when repeated across multiple alternatives.
Bridge Copilot selects the optimal WF section and girder spacing for your span automatically, following AASHTO LRFD requirements. Try it free — no download, no setup.
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