AASHTO LRFD Bridge Design Specifications — 9th Edition (2020)
The governing US standard for bridge design using Load and Resistance Factor Methodology, covering HL-93 live loading, all limit states, steel and concrete superstructures, substructures, foundations, and seismic design.
Scope
The AASHTO LRFD Bridge Design Specifications (9th Edition, 2020) is the governing design standard for all highway bridges in the United States. Published by the American Association of State Highway and Transportation Officials, it covers the LRFD methodology across 14 sections: general design provisions (Section 1), general features (Section 2), loads (Section 3), structural analysis (Section 4), concrete structures (Section 5), steel structures (Section 6), aluminium structures (Section 7), wood structures (Section 8), deck and deck systems (Section 9), foundations (Section 10), abutments and piers (Section 11), bearings (Section 14), and seismic design (Section 4 with Appendix A). The 9th edition includes updates for HL-93 loading refinements, revised seismic design provisions for high seismic zones, updated prestress loss equations, and enhanced provisions for accelerated bridge construction (ABC) and prefabricated bridge elements.
Purpose
The AASHTO LRFD specifications establish a uniform, reliability-based design framework for highway bridges that ensures consistent safety across different bridge types, materials, and span ranges. The LRFD methodology uses load factors and resistance factors calibrated from statistical data on load variability and material strength variability to achieve a target reliability index (beta typically 3.5 for Strength I limit state). The standard is referenced by FHWA and state DOTs for all federally funded bridge projects. The LRFD approach replaced the older Allowable Stress Design (ASD) and Load Factor Design (LFD) methods, offering a more rational treatment of uncertainties and providing consistent reliability across different load types and material resistances.
Engineering Applications
AASHTO LRFD applies to all highway bridge design including steel girder bridges (rolled beams and plate girders, Section 6), concrete slab and box girder bridges (cast-in-place and precast, Section 5), prestressed concrete I-girder and bulb-tee bridges (Section 5), segmental concrete bridges, truss bridges, arch bridges, cable-stayed bridges, and suspension bridges. The standard covers foundation design (spread footings, driven piles, drilled shafts per Section 10), abutment and retaining wall design (Section 11), bearing design (elastomeric, pot, disc, and spherical bearings per Section 14), and seismic design. Bridge design per AASHTO LRFD requires consideration of all applicable limit states and live load configurations (HL-93) for critical force effects in superstructure and substructure elements.
Design Philosophy
AASHTO LRFD is built on the reliability-based design framework. The fundamental equation is: sum(eta_i gamma_i Q_i) ≤ phi R_n where Q_i = nominal load effects, gamma_i = load factors, eta_i = load modifiers (ductility, redundancy, operational importance), phi = resistance factor, and R_n = nominal resistance. The load factors are calibrated to produce uniform reliability across various load scenarios. For example, Strength I (basic vehicular loading) uses gamma_DC = 1.25 for dead load and gamma_LL = 1.75 for live load. The live load model (HL-93) combines the design truck or tandem with the design lane load and includes dynamic load allowance (IM = 33% for all limit states except Fatigue I which uses IM = 15%). The HL-93 loading envelope considers the maximum of: (design truck + lane load), (design tandem + lane load), or (90% of two design trucks + 90% of lane load) for negative moment between points of contraflexure.
Important Requirements
Bridges must be designed for all applicable limit states: Strength I-V, Service I-IV, Extreme I-II, and Fatigue I-II. Live load distribution factors (Table 4.6.2.2.2 for concrete deck on steel/concrete girders) must be calculated using the approximate formulas (lever rule for one lane loaded, refined formulas for multiple lanes). Dynamic load allowance IM = 33% for all limit states except Fatigue I (15%). The deflection limit for steel and concrete bridges is L/800 for vehicular load (including dynamic load allowance) and L/1000 for orthotropic deck bridges per Article 2.5.2.6.2. Prestress losses (Article 5.9.3) include elastic shortening, shrinkage of concrete, creep of concrete, and relaxation of prestressing steel. The refined method uses the time-step method with concrete age correction factors. Fatigue design (Article 6.6.1 for steel) uses the constant-amplitude fatigue threshold delta F_TH for detail categories A through E'. For seismic design (Section 4 with Appendix A), bridges are classified as Critical, Essential, or Other based on importance and must meet performance criteria for the design earthquake (7% probability of exceedance in 75 years for Seismic Zone 2-4).
Key Parameters
The following table summarises the AASHTO LRFD limit states and load combinations (Table 3.4.1-1):
| Limit State | Load Combination | Description | DC/DW Factor | LL+IM Factor |
|---|---|---|---|---|
| Strength I | 1.25DC + 1.50DW + 1.75LL+IM | Basic vehicular loading | 1.25 | 1.75 |
| Strength II | 1.25DC + 1.50DW + 1.35LL+IM | Special permit vehicles | 1.25 | 1.35 |
| Strength III | 1.25DC + 1.50DW + 1.0WS | Wind > 28 m/s, no LL | 1.25 | --- |
| Strength IV | 1.50DC + 1.50DW | High dead-to-live load ratio | 1.50 | --- |
| Strength V | 1.25DC + 1.50DW + 1.35LL+IM + 0.4WS | Wind ≤ 28 m/s with LL | 1.25 | 1.35 |
| Service I | 1.0DC + 1.0DW + 1.0LL+IM | Service-level check | 1.00 | 1.00 |
The following table provides live load distribution factors for interior girders (simply supported, concrete deck on steel or concrete girders):
| Bridge Type | One Lane Loaded (g) | Two or More Lanes Loaded (g) | Parameters |
|---|---|---|---|
| Concrete deck on steel I-girder | 0.06 + (S/4300)^0.4 (S/L)^0.3 (Kg/Lts^3)^0.1 | 0.075 + (S/2900)^0.6 (S/L)^0.2 (Kg/Lts^3)^0.1 | S = spacing (mm), L = span (mm), Kg = longitudinal stiffness |
| Concrete deck on prestressed concrete I-girder | 0.06 + (S/4300)^0.4 (S/L)^0.3 (Kg/Lts^3)^0.1 | 0.075 + (S/2900)^0.6 (S/L)^0.2 (Kg/Lts^3)^0.1 | Same formula, Kg uses girder stiffness |
| Cast-in-place concrete box girder | (1.75 + S/1100) / 2800 | (13/N)^0.3 (S/4300)^0.5 (1/L)^0.25 | N = number of cells |
Mmax = max( Mtruck + Mlane, Mtandem + Mlane, 0.9 x (2 x Mtruck + Mlane) )
Dynamic Load Allowance: IM = 33% (all limit states except Fatigue I)
Fatigue I: IM = 15%
Live Load Distribution Factor (interior, one lane):
g = 0.06 + (S/4300)^0.4 (S/L)^0.3 (Kg/ L ts^3)^0.1
where ts = deck thickness, Kg = n(I + A eg^2)
Note: The HL-93 loading envelope requires checking three distinct live load configurations at each critical section. For simply supported spans, the design truck governs for spans up to approximately 40 m, while the design tandem may govern for shorter spans. The 90% double truck condition specifically targets negative moment at pier supports in continuous bridges. The lane load component is 9.3 kN/m uniformly distributed, and is included in all load configurations. The dynamic load allowance IM = 33% is applied only to the design truck or tandem, not to the lane load.
Practical Engineering Notes
The live load distribution factor formulas in Table 4.6.2.2.2 are approximate methods valid only within specified ranges of girder spacing, span length, and stiffness. The lever rule (Article 4.6.2.2.1) must be used for one-lane-loaded cases when calculating distribution factors manually, while the refined formulas apply for multi-lane loading. The longitudinal stiffness parameter Kg is computed as n(I + A e_g^2) where n = modular ratio (Es/Ec), I = girder moment of inertia, A = girder area, and e_g = distance between girder centroid and deck centroid. For preliminary design, Kg is often assumed and later refined. The multiple presence factor m = 1.20 for one lane loaded, 1.00 for two lanes, 0.85 for three lanes, and 0.65 for four or more lanes. For prestressed concrete girder design, the critical stress checks occur at release (transfer length region) and at service after all losses. The refined prestress loss method (Article 5.9.3) computes time-dependent losses using a step-by-step approach accounting for concrete age at loading, curing method, ambient humidity, and member size. For steel girder design, the constructibility check (Article 6.4) is often critical for non-composite dead loads before the deck cures.
Field Tip: During superstructure design, the most economical steel girder typically achieves 85-95% utilisation in the positive moment region (Strength I) and satisfies deflection limits without stiffeners. For prestressed concrete girders, debonding strands at the ends reduces release stresses at the girder ends and is preferred over harped strands for most standard I-girder shapes (Type III, IV, V, VI). Use the Bending Moment Calculator to verify live load envelopes for HL-93 loading.
Typical Workflow
A typical AASHTO LRFD design workflow: define bridge geometry (span arrangement, bridge width, girder spacing, deck overhang). Determine material properties (concrete f'c, steel Fy, prestressing steel fpu). Compute dead loads DC (components) and DW (wearing surface). Compute live loads per HL-93: position design truck longitudinally for maximum moment/shear at each critical section; add design lane load; apply dynamic load allowance. Apply live load distribution factors (interior and exterior girders separately). Compute load effects for all limit states: Strength I-V, Service I-IV, Extreme I-II, Fatigue I-II. Design deck: use the empirical design method (Article 9.7.2) or the traditional strip method. Design superstructure: for steel girders, check flexure (compact/noncompact), shear, and constructibility; for prestressed concrete, check stress limits at release and service, flexural strength, and shear. Design substructure: pier cap, column, and foundation for maximum reactions from all applicable limit states. Check fatigue for steel details under Fatigue I load combination. Verify seismic requirements per the bridge seismic design category.
Common Mistakes
Warning: Common errors include applying the dynamic load allowance to the lane load (it applies only to truck/tandem), using the wrong distribution factor formula for exterior girders, forgetting the multiple presence factor for multi-lane loading, not checking the Strength IV limit state (dead load only) for long-span bridges where dead load dominates, and neglecting the Fatigue I load combination for steel details. For prestressed concrete, common errors: using incorrect creep and shrinkage coefficients for the local climate, not checking release stress limits (0.60 f'ci for compression, 0.24 sqrt(f'ci) for tension), and forgetting to check the shear design at critical sections per Article 5.8.3. For seismic design, ignoring the minimum support length requirement per Article 4.7.4.4 can lead to unseating during an earthquake.
Best Practices
Use the refined method for prestress loss calculations rather than the approximate lump-sum method, as the refined method better captures the specific concrete mix, curing regime, and environment. For steel bridges, include a minimum of three intermediate cross-frames (diaphragms) per span, spaced at maximum 7.6 m (25 ft). For continuous steel bridges, consider the effects of the construction sequence including the sequence of deck placement. Use refined analysis methods (grillage or finite element) for bridges with significant skew (> 20°), curved alignments, or unusual geometries. For integral abutment bridges, account for thermal movements and cyclic soil pressures. Always document the load rating per AASHTO Manual for Bridge Evaluation (MBE) for the design configuration. Use the Shear Force Diagram Calculator for rapid verification of girder shear envelopes. Use the Prestress Losses Calculator for refined prestress loss estimation.
Limitations
AASHTO LRFD does not cover all bridge types. Long-span bridges (spans exceeding 150 m for concrete and 300 m for steel) may require site-specific load models and advanced analysis beyond the simplified distribution factors. The standard does not cover railway bridges (governed by AREMA), pedestrian bridges (AASHTO Guide Spec for Pedestrian Bridges), moveable bridges, or temporary bridges. The HL-93 live load model may not adequately represent overload permit vehicles on specific routes — route-specific permit vehicle checks (Strength II) are necessary. The seismic provisions are based on the assumption of firm ground and may not capture basin effects, liquefaction, or near-fault pulse effects for sites in high seismic zones. The standard does not provide detailed guidance for construction engineering (falsework, form travellers, launch girders) which require separate engineering analysis.
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References
- AASHTO. AASHTO LRFD Bridge Design Specifications. 9th ed., American Association of State Highway and Transportation Officials, 2020.
- AASHTO. Manual for Bridge Evaluation. 3rd ed., AASHTO, 2019.
- AASHTO Guide Specifications for LRFD Seismic Bridge Design. 2nd ed., 2020.
- Barker, R.M. and Puckett, J.A. Design of Highway Bridges: An LRFD Approach. 4th ed., Wiley, 2021.
- NCHRP Report 796. Calibration of AASHTO LRFD Concrete Bridge Design Specifications. TRB, 2014.