Steel Bridge Design Guide -- AASHTO LRFD

Steel bridges represent the backbone of highway infrastructure, carrying millions of vehicles annually. Unlike building design governed by AISC 360, steel bridge design follows AASHTO LRFD Bridge Design Specifications (9th Edition, 2020) -- a fundamentally different document calibrated for highway live loads, fatigue cycles in the tens of millions, and a 75-year design life. This guide covers the essentials: plate girder proportioning, cross-frame design, fatigue assessment, field splices, and composite deck interaction.

Plate Girder Proportioning per AASHTO LRFD Section 6.10

The plate girder remains the most common steel bridge type for spans from 80 to 400 feet. The designer selects web depth, web thickness, and flange dimensions to satisfy flexural strength, shear strength, fatigue, and serviceability (deflection) limits at every section along the span. The key proportioning limits from AASHTO 6.10.2 are mandatory, not advisory.

Web slenderness: The web depth-to-thickness ratio D/tw must not exceed 150 for webs without longitudinal stiffeners, or 300 for webs with longitudinal stiffeners. These limits prevent web buckling during fabrication and handling. For a typical 72-inch deep girder, D/tw = 150 means tw_min = 72/150 = 0.48 inches -- an important practical constraint: 1/2-inch web plate is the realistic minimum for a plate girder this deep, governing over the shear strength requirement.

Flange proportioning: The compression flange width-to-thickness ratio bf/2tf must satisfy bf/2tf <= 12.0 for Fy = 50 ksi (AASHTO 6.10.2.2-1). This is more restrictive than the AISC compact limit of bf/2tf <= 0.38 x sqrt(E/Fy) = 9.15, reflecting the fact that bridge girder flanges must remain stable through millions of load cycles. For a 16-inch wide flange, tf_min = 16/(2 x 12) = 0.67 inches. The flange must also satisfy bf >= D/6 for constructibility (ensuring the compression flange has adequate lateral stiffness before the deck hardens). For D = 72 inches, bf_min = 12 inches. Additionally, bf <= D/4 to ensure vertical bending behavior rather than lateral buckling.

Flange transitions: Plate girders are fabricated with thicker flanges in high-moment regions and thinner flanges in low-moment regions for economy. Flange thickness transitions must occur with a minimum 1:2.5 taper and the butt weld joining different thicknesses must be ground flush to avoid stress concentrations that would trigger fatigue cracking. The transition location is chosen where the factored moment equals the capacity of the thinner flange -- an optimization problem solved for each span.

Cross-Frame and Diaphragm Design

Cross-frames (X-braced or K-braced frames between adjacent girders) serve two structural purposes: providing lateral stability to the compression flange during construction before the deck hardens, and distributing live loads transversely among multiple girders. They are the most fatigue-sensitive components of a steel bridge.

Spacing governances: AASHTO 6.7.4.2 establishes maximum cross-frame spacing of 25 feet along the span, with additional frames required at all bearing locations, at points of dead-load contraflexure in continuous spans (where the flange stress changes sign, creating a lateral stability risk), and within 20 feet of field splice locations. A typical 120-foot simple span will have cross-frames at 0 ft (bearing), 25 ft, 50 ft, 75 ft, 100 ft, and 120 ft (bearing) -- six frames at 20-25 ft spacing.

Design forces: The cross-frame must resist wind load per AASHTO 3.8 applied to the girder web and deck profile, plus a minimum stability force of 0.2% of the compression flange yield force. This stability force derives from the member out-of-straightness and residual stress effects that real girders exhibit. For a girder with a 16 in x 2 in compression flange (Fy = 50 ksi, area = 32 in^2, yield force = 1,600 kips), the minimum lateral force per cross-frame = 0.002 x 1,600 = 3.2 kips. End cross-frames at abutments also carry wind and seismic lateral forces from the entire superstructure into the bearings and shear keys.

Fatigue in cross-frames: Differential vertical deflection between adjacent girders under truck loading cycles the cross-frame members, inducing stress ranges that must be checked per AASHTO 6.6.1. The cross-frame diagonal connection to the girder web stiffener is a Category E detail (CAFT = 4.5 ksi). For bridges with average daily truck traffic (ADTT) exceeding 5,000, this detail often governs and forces designers to use larger connection plates and improved weld profiles (ground and inspected) to elevate it to Category C.

Fatigue and Fracture -- AASHTO Section 6.6.1

Fatigue is the progressive cracking of steel under repeated load cycles, and it is the governing limit state for most steel bridge components in tension. AASHTO's fatigue design operates on the stress-range concept: it is the change in stress (not the maximum stress) that drives crack growth.

Fatigue load: The fatigue design truck is the HL-93 design truck (32 kip axle + two 8 kip axles) placed to produce the maximum stress range at the detail being checked. The rear-axle spacing is fixed at 30 feet, and the truck is positioned in a single lane with a 1.0 multiple presence factor (not the 1.2 used for strength design). The resulting stress range is multiplied by the fatigue load factor of 0.75 for infinite-life design or 1.00-1.50 for finite-life design depending on ADTT.

Detail categories: Category A (plain rolled surface, no attachments, CAFT = 24.0 ksi) has effectively infinite fatigue life at all stress ranges below 24 ksi -- bottom flanges away from attachments often satisfy this. Category B (longitudinal continuous fillet welds, CAFT = 16.0 ksi) covers the web-to-flange fillet welds in rolled beams and the longitudinal stiffener-to-web welds in plate girders. Category C (transverse stiffener welds to the web or flange with smooth transition, CAFT = 10.0 ksi) covers properly detailed stiffeners and cover-plate end welds with transition radii. Category D (cover-plate end welds without transition radius, CAFT = 7.0 ksi). Category E (cover-plate end welds shorter than 2 inches, CAFT = 4.5 ksi). Category E-prime (full-penetration groove welds with backing bar left in place, CAFT = 2.6 ksi) is the worst category and is essentially prohibited in tension flanges.

Worked example -- fatigue check of a stiffener-to-web weld: A plate girder web (48 in x 3/8 in) has a transverse stiffener fillet-welded at mid-span. The live-load stress range at this location from the fatigue truck is Delta_f = 8.5 ksi. The stiffener-to-web fillet weld is Category C. The CAFT for Category C is 10.0 ksi. Since 8.5 ksi <= 10.0 ksi, this detail has infinite fatigue life and no cumulative damage calculation is needed. If the stress range were 12.0 ksi, finite-life design would apply: N = (A / (Delta_f x gamma))^3 = (44 x 10^8 / 12^3) = 25.5 million cycles. If the bridge ADTT requires only 15 million cycles over 75 years, the detail is acceptable.

Field Splice Design

Field splices connect girder shipping segments at predetermined locations. Per AASHTO 6.13.6, the splice must develop the smaller section's factored resistance but not less than 75% of the flange yield force. The splice is typically located near the dead-load inflection point (approximately L/4 from the support for a simple span) to minimize the design moment.

Flange splices: Bolted cover plates on both sides of the flange, using ASTM F3125 Grade A325 (formerly A325) bolts in slip-critical connections. The number of bolts is determined by the flange force at the splice: P_f = F_yf x A_fn at the strength limit state, or P_f = f_cf x A_fn at the service limit state for slip resistance. The splice plates must develop the full flange force with net section fracture and block shear checks.

Web splices: Double shear plates on each side of the web, designed to carry the shear force plus the portion of the moment attributed to the web. The web moment = M_splice x (I_web / I_girder). Bolt spacing minimum = 3d, edge distance per AASHTO Table 6.13.2.6.6-2.

Composite Deck Interaction

Steel bridge girders act compositely with the reinforced concrete deck through shear connectors (headed studs) welded to the top flange. This composite action approximately doubles the girder's flexural stiffness compared to the non-composite steel section alone. AASHTO 6.10.10 governs shear connector design.

The number of shear studs required between the point of zero moment and the point of maximum positive moment is N = V_h / Q_n, where V_h is the total horizontal shear force (the lesser of 0.85 x f_c-prime x b_eff x t_s or F_y x A_steel), and Q_n is the nominal shear resistance of a single stud = 0.5 x A_sc x sqrt(f_c-prime x E_c) <= A_sc x F_u per AASHTO 6.10.10.4.3. For a 7/8-inch diameter stud in 4,500 psi concrete deck: Q_n = 0.5 x 0.60 x sqrt(4.5 x 3,834) = 0.5 x 0.60 x sqrt(17,253) = 0.30 x 131.4 = 39.4 kips. Fatigue of shear studs must also be checked per AASHTO 6.10.10.2 for bridges with ADTT exceeding 960 trucks/day -- the fatigue shear range per stud p_f = V_f x Q / I (calculated with the short-term modular ratio n), and must be <= 5.5 x d^2 for 7/8-inch studs after 75 years of loading.

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Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. Bridge design must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) familiar with AASHTO LRFD specifications for the specific project location and owner requirements. The site operator disclaims liability for any loss arising from the use of this information.