This article explains the origin, meaning, application, and implications of HA and HB loadings. It breaks down the requirements under BD 37/01. It also explores how these loads act on different structural components.
Designing safe and functional highway structures requires a firm understanding of vehicular loading. Highway bridges, culverts, retaining walls, and other elements must resist loads from all vehicles likely to use them. These include routine passenger cars, fully loaded trucks, and even abnormal vehicles. Quantifying these loads using structured models is a necessity for every designer of highway structures. In the United Kingdom and other jurisdictions that reference British standards, the HA and HB loadings classes define how these vehicles impact design.
While modern codes like Eurocode EN 1991-2 now guide highway loading in many parts of Europe, British Standards such as BD 37/01 remain relevant. Many infrastructure assets still rely on the legacy HA and HB loading systems for inspection, maintenance, or upgrade assessments. Also, understanding these systems helps bridge the conceptual gap for engineers transitioning from old to new standards. HA and HB loading are not just historical curiosities. They still influence real-world design, maintenance, and load rating practices.
This article explains the origin, meaning, application, and implications of HA and HB loadings. It breaks down the requirements under BD 37/01. It also explores how these loads act on different structural components. The article helps structural engineers understand how to apply these load models and appreciate their significance in assessing highway structures.
HA Loading
HA loading represents the standard vehicular loading found on typical UK roads. It includes the types of vehicles most commonly encountered. This loading category simulates normal traffic conditions, including both light and moderately heavy vehicles. Designers use HA loading for the majority of road structures, particularly those not expected to experience unusually heavy traffic.
The HA loading model consists of two key components: uniformly distributed loads (UDLs) and knife-edge loads (KELs). These loads simulate the combination of vehicle weight spread over an area and concentrated axle loads. UDLs represent vehicle weight transmitted through tyres and spread through road surfacing. KELs simulate the concentrated effect from single axles, particularly in short-span conditions.
Under BD 37/01, HA loading applies across different lanes with adjusted intensities. The design considers multiple lanes, each carrying traffic in either direction. Designers apply full HA loading in the most critical lane. Adjacent lanes receive reduced load levels to simulate realistic traffic patterns. This staged loading simulates worst-case effects without applying unrealistic loading intensities.
Components of HA Loading: UDL and KEL
HA UDLs acts longitudinally along the bridge deck. Engineers apply these distributed forces across the full width of the traffic lane. The UDL value depends on the loaded length and the number of lanes. For short loaded lengths, the load intensity is higher. As the length increases, the intensity reduces. This variation reflects the distribution of loads along different span lengths.
Knife-edge loads in HA loading act transversely across the deck. Engineers place these point loads to simulate the high concentration from a single axle. KELs create peak effects in bending, shear, and punching shear. They prove particularly important in short-span slabs and transverse elements. Where critical, KELs may even govern the slab or diaphragm design.
The combination of UDL and KEL produces an envelope of effects. Designers check these effects for bending moment, shear force, deflection, and bearing loads. The structure must safely resist all possible permutations of forces. These checks apply to both the main span and ancillary elements such as kerbs, cantilevers, and parapets.
HB Loading
HB loading represents abnormal or exceptionally heavy vehicles. These include military transporters, construction equipment, and abnormal industrial loads. HB loading accounts for extreme but plausible events. Engineers must consider HB loading where such vehicles may use the road or when authorities may grant abnormal load permits.
The HB load consists of a notional wheeled vehicle. The standard HB vehicle has four axles arranged in pairs. Each axle pair has a track width of 3.0 meters and axle spacing of 1.8 meters. The default HB loading is a 30-unit model. Each “unit” corresponds to 10 kN axle load, so a 30-unit HB vehicle exerts 300 kN per axle. The entire vehicle weighs 1200 kN.
Engineers can scale the HB model up or down. For instance, an HB/45 model represents 450 kN per axle or 1800 kN total. The selected unit loading depends on route classification and structure type. Some critical structures may require HB/45 or higher. Less critical ones may suffice with HB/30 or HB/25.
Application of HB Loading in Structural Design
Engineers apply HB loading selectively, often in combination with HA loading. The worst-case scenario governs. In general, if HA causes higher effects, the designer uses HA. If HB governs, the structure must resist HB loading effects. Some cases require the structure to resist both HA and HB with appropriate safety factors.
HB vehicles do not occupy just one lane. Their wide track and long axle base mean they straddle multiple lanes. Designers place the HB vehicle centrally to produce maximum effects. This placement often generates high torsional or transverse effects in the case of a deck. It may also overload bearings or edge beams if improperly accounted for.
HB vehicles travel at reduced speed or under escorted conditions. Designers apply dynamic amplification factors to reflect vibration and impact. These factors vary depending on the type of structure, span length, and component flexibility. In slabs, impact effects may significantly amplify the already concentrated loads.
When to Use HA or HB?
HA loading applies to all public highway structures by default. Every road bridge, culvert, or wall must withstand HA loading unless explicitly excluded. HB loading, on the other hand, applies to structures likely to experience abnormal vehicles. These include routes serving industrial zones, ports, military facilities, or construction corridors.
Design standards like BD 37/01 specify the combination rules. Some structures must resist both full HA loading and one or more HB vehicle cases. Others may use either HA or HB, depending on the load effects produced. This decision depends on span geometry, component design, and critical section locations.
In the case of very short spans, knife-edge effects from HA dominate. On longer spans, the distributed effects from HB become critical. Engineers must assess both and use the more onerous result. The designer should not assume HA loading always governs. Each case must follow code-based comparative evaluation.
Cover Depth and Load Placement
An important consideration is the depth of soil or surfacing between the road and the structure. This depth, called “cover,” determines how load transfers. Shallow covers (less than 600 mm) result in high concentrations. The loading acts almost directly on the structure. In such cases, engineers use unmodified HA and HB loading values.
For cover depths greater than 600 mm, BD 37/01 permits modified load values. The depth spreads the load before it reaches the structure. This results in reduced pressures on slabs or culverts. Tables and charts in BD 37/01 provide reduction factors. Engineers must use these with care. Incorrect application can result in unconservative designs.
Load placement also affects design. Engineers place HA UDLs and KELs to produce maximum effects. HB vehicles must also be positioned for critical outcomes. If the structure includes overhangs or cantilevers, asymmetric placement becomes necessary. This captures worst-case bending, torsion, and shear.
Load Combinations and Safety Factors
Designers must apply HA and HB loadings within structured combinations. BD 37/01 outlines these combinations clearly. For ultimate limit state checks, engineers apply factored loads. These include dynamic effects, partial safety factors, and multiple presence factors. Serviceability checks use unfactored or lightly factored loads.
For instance, a common combination includes:
- 1.1 × HA UDL + 1.1 × HA KEL + dynamic amplification
- Or 1.1 × HB vehicle + impact + temperature + shrinkage
Wind, braking, and thermal effects also combine with traffic loads. These secondary actions can significantly influence slender decks or long-span bridges. Engineers must assess each combination case and document the governing results. Neglecting an interaction term can lead to underdesign or unexpected performance issues.
Comparison with Eurocode Traffic Load Models
Eurocode EN 1991-2 introduces Traffic Load Models (LM1 to LM4). LM1 simulates normal traffic. LM3 and LM4 simulate abnormal or special loads. These models reflect modern traffic patterns and international coordination. However, they retain many concepts from HA and HB loadings.
For example, LM1 includes UDL and tandem systems similar to HA. LM3 represents a heavy multi-axle vehicle, like HB. Load placements, intensities, and influence lengths echo those in BD 37/01. The transition from HA/HB to Eurocode does not require abandoning old logic. Instead, it involves translating familiar systems into modern frameworks.
Engineers working on assessments or strengthening projects still rely on HA and HB. Many existing bridges were designed using BD 37. Understanding HA and HB loadings enables effective rehabilitation, risk assessment, and rating studies. Until every structure upgrades to modern design codes, HA and HB remain relevant.
Also See: Dispersal of Traffic Load on Buried Culverts
Conclusion
HA and HB loading models remain vital for understanding highway structure performance under traffic actions. These models provide structured, calibrated tools for simulating common and exceptional vehicle effects. Engineers use them not just for design but also for load rating, assessment, and maintenance planning. HA loading represents standard vehicles, using distributed and concentrated actions to model real traffic conditions. HB loading simulates abnormal vehicles through a scalable four-axle vehicle model. Engineers must consider both, applying the more severe case and combining them with other actions as specified.
