Steel Bridge Design Guide — AASHTO LRFD, EN 1993-2, AS 5100

Steel bridge design encompasses plate girder, truss, and composite deck systems. This guide covers design provisions across AASHTO LRFD Bridge Design Specifications, EN 1993-2, and AS 5100.

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Core calculations run via WebAssembly in your browser with step-by-step derivations across AISC 360, AS 4100, EN 1993, and CSA S16 design codes. Results are preliminary and must be verified by a licensed engineer.

Overview of Steel Bridge Types

Plate Girder Bridges

The most common steel bridge type for spans of 50-200 ft (15-60 m). Plate girders are fabricated by welding steel plates into I-shaped sections (or box sections) with depths typically ranging from 3-15 ft (1-5 m). The girders are composite with a reinforced concrete deck slab through shear connectors. Key design advantages: (1) Customizable depth and flange sizes for optimal material distribution, (2) Economical for the typical bridge span range, (3) Simple to fabricate and erect, and (4) Well-understood behavior with mature design provisions.

Truss Bridges

Truss bridges use triangulated steel members to efficiently carry loads for spans of 100-500 ft (30-150 m). Common configurations include: Pratt truss (diagonals in tension), Warren truss (alternating tension/compression diagonals), and K-truss (for deep trusses). Trusses can be through-type (traffic passes between the trusses) or deck-type (traffic passes on top).

Cable-Supported Bridges

For very long spans: cable-stayed (200-3,000 ft) and suspension (1,000-7,000 ft). These systems use high-strength steel cables or strands as the primary load-carrying elements, with steel girders or orthotropic decks as the stiffening system. Cable-supported bridges require specialized analysis including: cable geometry nonlinearity, aerodynamic stability, and construction sequence analysis.

AASHTO LRFD Bridge Design Specifications

The AASHTO LRFD Bridge Design Specifications (9th Edition, 2020) govern the design of steel highway bridges in the United States. Key sections:

Loads and Load Combinations (AASHTO Section 3)

Live load model: HL-93 consists of an HS20 design truck with a 32-kip axle plus 25% added lane load. The HS20 truck has: 8-kip front axle, 32-kip middle axle, 32-kip rear axle at 14-ft spacing, with 4.3-ft axle width. Additional loads: (1) lane load — 0.64 kip/ft uniformly distributed over the design lane, (2) multiple presence factors — 1.2 for one lane, 1.0 for two, 0.85 for three or more, (3) dynamic load allowance — 33% for the design truck, 75% for deck joints, 0% for lane load, and (4) centrifugal forces for curved bridges.

Fatigue load: Design fatigue truck — one HS20 truck with 30-ft axle spacing and 0.8 kip/ft lane load. The fatigue design life is typically 75-100 years for highway bridges.

Load combinations (Table 3.4.1-1):

Design of Steel I-Girders (AASHTO Section 6)

Flexure (6.10.6): I-girders are designed as either compact or non-compact sections. For compact sections in positive flexure: Dcp ≤ 3.76√(E/Fyc). The nominal flexural resistance Mn = 1.3 × Rh × My for the compression flange, where Rh is the hybrid factor. For the tension flange: Mn = Rh × Fyt × St. Composite sections are checked separately for positive (concrete in compression) and negative (steel in compression, concrete in tension) flexure.

Shear (6.10.9): For unstiffened webs: Vn = C × Vp, where C is the ratio of shear buckling resistance to shear yield strength, and Vp = 0.58 × Fyw × D × tw. For stiffened webs (transverse intermediate stiffeners): Vn = Vp × (C + 0.87(1-C)/√(1+(do/D)²)), where do is the stiffener spacing.

Fatigue and fracture (6.6): Fatigue design per AASHTO Section 6.6 considers: (1) stress range Δf at each detail location, (2) detail category (A through E'), (3) number of cycles (365 × ADTT × years), and (4) finite life vs. infinite life design. For fracture-critical members (FCM), Charpy V-notch (CVN) impact testing at the minimum service temperature is required per AASHTO 6.6.2.

EN 1993-2 (Eurocode 3 Part 2 — Steel Bridges)

EN 1993-2 governs steel bridge design in Europe. Key differences from AASHTO:

Load Model 1 (LM1): Concentrated loads of 300 kN (67.4 kips) per axle (tandem system) plus uniformly distributed load of 9 kN/m² (188 psf). The tandem system represents heavy truck loading.

Load Model 3 (LM3): Special vehicles (e.g., 1,800 kN / 405 kip vehicles) for abnormal loads.

Fatigue (EN 1993-1-9): Uses a similar stress range approach to AASHTO, but with different detail categories and partial factors. The damage equivalent factor λ is used to account for the bridge's traffic composition and span.

AS 5100 (Australian Standard for Bridge Design)

AS 5100 Part 6 (Steel and composite steel-concrete construction) governs steel bridge design in Australia:

Loading: SM1600 (1600 kN design truck) with W80 (80 kN) wheel loads. The M1600 moving traffic load includes a design truck (six axles, 360 kN total) plus lane load (28 kN/m), or a heaviest vehicle load (HVL) of 600 kN (for permit loads).

Steel Bridge Deck Systems

Concrete Deck on Steel Girders

The most common deck system for highway bridges. The cast-in-place reinforced concrete slab (typically 8-10 inches / 200-250 mm thick) is composite with the steel girders through shear studs. The slab spans transversely between girders (typical spacing: 8-14 ft / 2.4-4.3 m). Advantages: economical, proven performance, simple construction.

Orthotropic Steel Deck

An orthotropic deck consists of a steel plate stiffened by longitudinal ribs (closed trough ribs or open bulb ribs) and transverse floor beams. This is the lightest deck system, used for long-span bridges where dead load is critical. Key design considerations:

Steel Bridge Bearings

Bridges require bearings to accommodate rotation and translation from loads and temperature:

Elastomeric bearings: Most common for medium-span bridges (up to 200 ft). Consist of alternating layers of elastomer and steel plates. Accommodate both rotation and translation through elastomer shear deformation. Design per AASHTO Section 14.

Pot bearings: For high vertical loads (up to 5,000 kips). The PTFE (Teflon) sliding surface accommodates translation, while the elastomeric pad in a steel pot accommodates rotation.

Spherical bearings: For multi-directional rotation and translation. Ideal for curved and skewed bridges.

Corrosion Protection for Steel Bridges

Steel bridges require corrosion protection systems based on the environment:

Paint systems: Three-coat system (zinc-rich primer + epoxy intermediate + polyurethane topcoat) is standard for most highway bridges. High-performance coating systems provide 25-40 years of service life.

Weathering steel: For bridges in non-marine environments, weathering steel (ASTM A588) develops a protective patina that eliminates the need for painting. Per AASHTO, weathering steel is not permitted in: (1) marine environments within 1 km of salt water, (2) bridge decks with salt spray, (3) tunnels or enclosed areas, and (4) environments with high chloride or sulfur dioxide concentrations.

Hot-dip galvanizing: For smaller bridges and pedestrian bridges. Provides 50-75 years of maintenance-free service in most environments.

Frequently Asked Questions

What are the main types of steel bridges? Three main types: (1) Plate girder bridges — most common for medium spans (50-200 ft / 15-60 m), fabricated from welded plate I-girders, often composite with concrete deck. (2) Truss bridges — efficient for longer spans (100-500 ft / 30-150 m), using bolted or welded connections. (3) Cable-supported bridges — cable-stayed and suspension for very long spans. Each type has specific design provisions in AASHTO LRFD Section 6, EN 1993-2, and AS 5100.

What are the AASHTO LRFD load combinations for bridges? AASHTO LRFD uses Strength I (basic vehicular), Strength II (permit loads), Strength III/IV (wind), Service I (deflection), Service II (slip-critical connections), Fatigue I/II (fatigue life), and Extreme Event (seismic, collision, ice, flood). The live load model is HL-93 (HS20 design truck + 25% lane load). EN 1993-2 uses Load Model 1 (concentrated and uniformly distributed loads) and Load Model 3 (special vehicles).

How are steel bridge girders designed for fatigue? Per AASHTO LRFD Section 6.6 and EN 1993-1-9: (1) Stress range Δf is computed for each detail category (A through E'), (2) Design life cycles N = (365)(ADTT)(years), (3) Fatigue resistance (ΔF)n = (A/N)^(1/3) for constant-amplitude limit, (4) Finite life check: γ(Δf) ≤ (ΔF)n/φ, (5) Check infinite life: nominal stress range ≤ constant-amplitude fatigue threshold (CAFL). Detail categories differ: welded attachments (Category E), bolted splices (Category B), base metal (Category A).

What is an orthotropic steel deck bridge? An orthotropic steel deck is a lightweight bridge deck system consisting of a steel plate stiffened by longitudinal ribs and supported by transverse floor beams. The steel plate serves as both the structural deck and the wearing surface (with a thin pavement overlay). Orthotropic decks are 30-50% lighter than concrete decks, making them ideal for long-span bridges where dead load reduction is critical. Key design challenges include: (1) fatigue of welded connections between ribs and deck plate, (2) durability of the wearing surface, and (3) buckling of the thin deck plate under wheel loads. AASHTO Section 9 provides orthotropic deck design provisions.

How are steel bridge bearings designed? Steel bridge bearings transfer loads from the superstructure to the substructure while accommodating movements from thermal expansion, shrinkage, creep, and live load deflection. Per AASHTO Section 14: (1) Elastomeric bearings — designed for a maximum compressive stress of 1.0 ksi (6.9 MPa) and a maximum shear strain of 50% from translation; the elastomer thickness (typically 0.5-2 inches) determines the movement capacity. (2) Pot bearings — designed for bearing stress on the PTFE surface of 3-5 ksi (21-35 MPa), with stainless steel mating surface for sliding. (3) Fixed bearings — designed for horizontal force capacity equal to 15% of the vertical load. All bearings must be designed for thermal movement calculated from ΔT = α × L × Δtemp, where α (steel) = 6.5 × 10⁻⁶/°F and Δtemp is per AASHTO Section 3.12.

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This page is provided for general technical information and educational use only. It does not constitute professional engineering advice. All results must be independently verified by a licensed Professional Engineer.