HL-93 Highway Bridge Loading Models

Learning Objectives

  • Explain the components of the HL-93 vehicular live-load model and when each is applied.
  • Distinguish the design truck, design tandem, and design lane load.
  • Position concentrated axle loads to maximize a selected structural response.
  • Apply dynamic load allowance only to the load components for which it is specified.
  • Explain multiple loaded lanes and multiple-presence effects.
  • Distinguish strength/service vehicular loading from fatigue-specific loading.
  • Produce a clear loading diagram that can be passed directly into structural analysis.

Highway bridge live loading is not represented by one truck parked arbitrarily on the span. A design loading model combines concentrated axle systems and a distributed lane load, then places them to maximize the response being checked. This topic converts the code loading model into an analysis-ready set of load cases.

Edition and project basis

Reference baseline

The nominal HL-93 values used in this lesson align with the AASHTO LRFD Bridge Design Specifications, 10th Edition (2024). The publication incorporates the September 2025 errata; agency adoption, project supplements, contract documents, and later corrections still govern the actual design.

Numerical load values, factors, exceptions, lane definitions, fatigue provisions, and special-vehicle requirements must be taken from the governing project criteria and adopted bridge specification edition. The workflow below is intentionally edition-aware: identify the governing basis before assigning values.

The HL-93 Loading Concept

Design Truck

A three-axle vehicle model used to represent heavy highway vehicles. The rear axle spacing is positioned within the permitted range to maximize the response under study. Axle loads are treated as concentrated loads at their longitudinal positions.

Design Tandem

A pair of closely spaced concentrated axles used as an alternative concentrated-load model. For the governing vehicular load effect, the design tandem is evaluated with the lane load and compared against the design-truck-plus-lane-load case as required by the adopted specification.

Design Lane Load

A uniformly distributed longitudinal load represents the effect of a stream of vehicles. It acts over the design lane and is combined with the applicable concentrated vehicle model. The lane load is positioned over the response-positive or response-negative region required by the analysis rules.

See the Load Model Before Calculating It

Physical-to-analytical translation

  1. Read the vehicle as an axle system, not as one lumped truck weight.
  2. Track each axle longitudinally as it enters, crosses, and leaves the bridge.
  3. Treat an axle outside the span as visible context but zero influence on that span.
  4. Add the lane load as a separate distributed component over the permitted influence region.
  5. Convert the current placement into response with the applicable influence line or structural model.
  6. Sweep the vehicle and permitted axle spacing until the response envelope stops increasing.

The 3D truck body is illustrative

HL-93 defines an analytical axle system, not a required real-world truck make or body style. The visualization uses a recognizable vehicle body to make the load path easier to see; the calculations are driven by axle loads, axle spacing, lane load, and the governing placement rules.

Analysis-Ready Loading Cases

Build a Vehicular Live-Load Case

  1. Define the bridge span geometry and design-lane arrangement.
  2. Select the structural response to maximize: reaction, shear, positive moment, negative moment, or another effect.
  3. Generate the applicable concentrated-load alternatives: design truck and design tandem.
  4. Add the design lane load according to the governing loading rule.
  5. Move each concentrated-load alternative through all relevant positions rather than choosing a single visual guess.
  6. Apply dynamic load allowance only to the specified concentrated vehicular components.
  7. Apply lane/multiple-presence provisions for the number of loaded lanes being investigated.
  8. Compare alternatives and retain the governing response for each limit-state check.

Dynamic Load Allowance

Dynamic Load Allowance (IM)

Dynamic load allowance increases specified concentrated vehicular effects to account for vehicle-bridge interaction and road-surface irregularity. It is not a blanket multiplier on every live-load component.

Dynamic amplification of an eligible concentrated effect

Applies the governing dynamic allowance to the eligible static concentrated vehicular effect.

QIM=Qstatic(1+IM)Q_{IM}=Q_{static}(1+IM)

Variables

SymbolDescriptionUnit
QIMQ_{IM}Concentrated vehicular effect including dynamic allowance-
QstaticQ_{static}Static effect of the eligible truck or tandem component-
IMIMDynamic load allowance from the governing project basis-

Common modeling error

Do not apply dynamic load allowance indiscriminately to the design lane load, dead load, pedestrian load, or other actions unless the adopted specification explicitly requires it.

Critical Positioning with Influence Lines

Influence lines provide the most transparent way to position moving loads. Concentrated axles are placed where large ordinates of the desired sign occur. Distributed lane load is placed over the influence-line region that contributes to the governing effect according to the applicable loading rule.

Maximum Positive Moment

For a simply supported span, the heaviest axle group generally approaches the region of largest positive moment influence-line ordinates. Exact governing placement should be found by moving the axle system rather than assuming that one axle must sit exactly at midspan.

Maximum Support Shear

High shear generally occurs when heavy axles are close to the support on the response-positive side of the shear influence line. The governing arrangement can differ from the arrangement that maximizes moment.

Continuous-Span Negative Moment

Negative support moment requires loading the spans and influence-line regions that produce negative response at the support. A single-span intuition is insufficient for continuous bridges.

Special Continuous-Span Truck-Train Case

For the AASHTO LRFD continuous-span negative-moment and specified interior-pier-reaction checks, an additional case uses two design trucks with the prescribed minimum headway and fixed rear-axle spacing, together with the applicable percentage of design lane load. FHWA LRFD examples describe the traditional form as 90%90\% of the two-truck effect plus 90%90\% of the lane-load effect. Treat this as a distinct load generator—not as the ordinary one-truck/tandem alternative—and confirm the exact rule in the adopted specification edition and errata.

Longitudinal Placement vs. Transverse Distribution

Longitudinal question

Determine where the truck or tandem sits along the bridge to maximize the selected reaction, shear, moment, or other response. Influence lines and moving-load analysis answer this question.

Transverse question

Determine how lane and wheel effects are shared among girders, boxes, floor beams, or other primary members. Distribution factors or refined three-dimensional analysis answer this different question.

Do not distribute twice

A girder response obtained from a refined model that already represents transverse load sharing should not be multiplied again by a simplified distribution factor unless the adopted method explicitly requires that operation.

Multiple Loaded Lanes

Wide bridges can have several design lanes loaded simultaneously. The engineer must generate the applicable lane combinations, apply the governing multiple-presence treatment, and identify which arrangement maximizes each girder or global bridge response.

Fatigue Loading

Fatigue checks are based on repeated stress range rather than the same maximum-load philosophy used for strength design. Use the fatigue vehicle, dynamic allowance, lane treatment, and cycle assumptions specified by the governing project basis. Keep fatigue load cases separate in the analysis model so that stress-range results cannot be confused with strength envelopes.

From Loading Model to Girder Demand

Live-load demand workflow

  1. Generate the global live-load response envelope for the bridge model.
  2. Determine the fraction of global/lane response carried by the girder using the approved distribution method or refined analysis.
  3. Add applicable dynamic and multiple-presence effects at the correct stage.
  4. Combine the resulting live-load effect with permanent and environmental actions using the governing limit-state load combination.
  5. Pass the factored demand to the member-design topic.

Nominal HL-93 Reference Model

For the standard AASHTO HL-93 highway live-load model, the commonly used nominal SI equivalents are a design truck with axle loads of approximately 35.635.6, 142.3142.3, and 142.3 kN142.3\text{ kN}; 4.27 m4.27\text{ m} from the front axle to the second axle; and a variable second-to-third axle spacing of about 4.274.27 to 9.14 m9.14\text{ m}. The design tandem uses two approximately 111.2 kN111.2\text{ kN} axles spaced about 1.22 m1.22\text{ m} apart, and the design lane load is approximately 9.34 kN/m9.34\text{ kN/m} longitudinally.

Use the adopted edition, errata, and project criteria

These nominal values are provided so the loading geometry can be understood and checked. The governing specification controls exact values, exceptions, dynamic allowance, multiple-presence treatment, negative-moment rules, fatigue loading, permit/special vehicles, and project-specific supplements.

Moving-Load Numerical Controls

Checklist

Interactive visualization

Play the truck or tandem across the bridge, switch between 3D, side, and top views, compare load configurations, change rear-axle spacing, and watch the selected influence-line response update. Use “Show governing searched position” to jump from visual intuition to the numerically searched maximum.

HL-93 Moving-Load Explorer

Concept and model scope

Physical 3D vehicle and influence-line response on a 30 m simply supported span. Axles outside the bridge remain visible but contribute zero to bridge response.

This is an educational simply supported span using nominal SI HL-93 reference values. It is not a project-specific code check. The dynamic allowance toggle applies the illustrative 33% factor to axle effects only; confirm the adopted specification and edition before design use.

Reference vehicle values: truck axles 35.6/142.3/142.3 kN, tandem axles 111.2/111.2 kN, nominal lane load 9.34 kN/m, and optional 33% dynamic allowance on axle effects.

Load case
Response
Front axle10.50 m
Rear axle spacing4.27 m
Current Midspan moment
2074.3 kN·m
Searched maximum
3738.9 kN·m
rear spacing 4.27 m
On span: 3/3 axles
Lane: 1 × 9.34 kN/m
35.6 / 142.3 / 142.3 kN · 4.27 m + 4.27 m
1 loaded lane · 3 of 3 axles on span
35.6142.3142.3influence line · peak ordinate 7.50
Red = axle loads.
Blue = lane load.
Gray axle = off span, zero effect.
Educational simply supported model. The governing specification controls exact HL-93 provisions, multiple-presence treatment, fatigue loading, continuous-span negative-moment cases, distribution to individual girders, and project-specific vehicles.
Key Takeaways
  • HL-93 is a loading system, not a single truck.
  • Truck and tandem alternatives must be compared with the applicable lane load.
  • Critical vehicle position depends on the response being maximized.
  • Dynamic load allowance applies selectively, not universally.
  • Longitudinal vehicle placement and transverse load distribution are different questions and should be solved deliberately.
  • Multiple-lane and fatigue cases must be modeled explicitly and kept traceable to the governing code edition.