Understanding Highway Vehicle Loadings on Bridges

Designing robust and safe highway bridges is a complex undertaking that requires a thorough understanding of the myriad forces they must endure throughout their lifespan. Beyond the obvious weight of vehicles, bridges are subjected to a complex interplay of static and dynamic forces, environmental impacts, and even unforeseen events. Engineering specifications, such as those laid out by the American Association of State Highway and Transportation Officials (AASHTO), provide comprehensive guidelines for designers to ensure these structures remain stable and functional. This article delves into the various types of highway vehicle loadings and other critical forces that engineers must meticulously consider when crafting these essential links in our road networks.

AASHTO's Framework for Bridge Loadings

The AASHTO Standard Specifications for Highway Bridges mandate that structures are designed to safely carry dead loads, live loads, and the dynamic effect of live loads, known as impact. Furthermore, bridges must be capable of sustaining other forces to which they may be subjected, including longitudinal, centrifugal, thermal, seismic, and erection forces. These various loads are categorised and combined in designated groups to cover all potential scenarios.

The LRFD (Load and Resistance Factor Design) Specification simplifies loads into two primary categories: Permanent and Transient. Understanding these distinctions is fundamental to modern bridge design.

Permanent Loads

Permanent loads are those that are constant or vary only over a very long period. They primarily relate to the inherent weight and long-term effects on the structure itself. Key permanent load designations include:

• DD (Downdrag): Forces resulting from the downward movement of soil around deep foundations.
• DC (Dead Load of Structural Components and Nonstructural Attachments): This refers to the weight of the bridge structure itself, including all its primary components like beams, decks, and piers, as well as any fixed non-structural elements like railings, lighting, and signage. Designers must use the actual dead weights of specified materials, and it's crucial that these loads are meticulously documented on contract plans for future analysis during potential rehabilitations.
• DW (Dead Load of Wearing Surfaces and Utilities): The weight of the road surface (e.g., asphalt, concrete overlay) and any utilities (pipes, cables) that are permanently attached to or supported by the bridge.
• EH (Horizontal Earth Pressure Load): Forces exerted by soil pressure on retaining structures or abutments.
• EL (Accumulated Locked-in Force Effects Resulting from Construction): Residual stresses or forces locked into the structure during its construction process.
• ES (Earth Surcharge Load): Additional pressure on retaining structures from loads placed on the ground surface behind them.
• EV (Vertical Pressure from Dead Load of Earth Fill): The vertical weight of any earth fill supported by the bridge structure.

Transient Loads

Transient loads are temporary or fluctuating forces that act on the bridge. These are often dynamic and can vary significantly in magnitude and application over time. They represent the variable conditions a bridge will encounter.

• BR (Vehicular Braking Force): Longitudinal forces exerted on the bridge deck when vehicles brake, which are then transmitted to supporting members. AASHTO Standard Specifications typically specify this as 5% of the live load in all lanes carrying traffic in the same direction, applied 6 ft above the deck. For LRFD, it's 25% of the design truck or tandem axle weights.
• CE (Vehicular Centrifugal Force): For curved bridges, this is the outward force exerted by vehicles as they navigate the curve. It's applied 6 ft above the roadway surface at the centerline.
• CR (Creep): Long-term deformation of concrete under sustained stress.
• CT (Vehicular Collision Force): Forces resulting from vehicles impacting the bridge structure, particularly piers or abutments.
• CV (Vessel Collision Force): Forces from ships or boats colliding with bridge piers or superstructures, particularly relevant for bridges over navigable waterways. Guidance for this is often based on probabilistic theories considering vessel size and frequency.
• EQ (Earthquake): Seismic forces that can induce significant dynamic stresses. Bridge design in earthquake-prone areas follows specific seismic design specifications aiming to minimise damage and prevent collapse during ground shaking. Steel superstructures, being lighter, generally transmit lower seismic forces to substructure elements.
• FR (Friction): Forces arising from frictional resistance, such as at bearings.
• IC (Ice Load): Forces exerted by ice, either static pressure from accumulated ice or dynamic forces from moving ice floes.
• IM (Vehicular Dynamic Load Allowance): Often referred to as impact, this factor accounts for the dynamic amplification of live loads due to irregularities in the road surface or other disturbances. In earlier AASHTO standards, it was a function of span. For LRFD, it's a Dynamic Load Allowance based on the type of bridge component and is applied only to the truck portion of the live load.
• LL (Vehicular Live Load): This is arguably the most significant transient load, representing the weight of traffic.
• LS (Live Load Surcharge): Additional live load applied to earth fills or retaining structures.
• PL (Pedestrian Live Load): The weight of pedestrians on sidewalks or refuge walks.
• SE (Settlement): Forces induced by differential settlement of foundations.
• SH (Shrinkage): Contraction of concrete due to drying.
• TG (Temperature Gradient): Stresses induced by non-uniform temperature distribution across a structural cross-section.
• TU (Uniform Temperature): Stresses and movements resulting from overall changes in ambient temperature. For steel structures, anticipated extremes can range from -30°F to 120°F in cold climates.
• WA (Water Load and Stream Pressure): Forces exerted by flowing water on piers, calculated based on water velocity and pier shape.
• WL (Wind on Live Load): Wind forces acting on the vehicles present on the bridge.
• WS (Wind Load on Structure): Wind forces acting directly on the bridge's exposed surfaces.

Detailed Examination of Key Transient Loads

Vehicular Live Loads (LL)

The vehicular live load is the primary moving load on a bridge and has evolved significantly over time to reflect changing traffic patterns and vehicle designs. Historically, early bridge designs relied on simple criteria like the weight of horse teams or cattle.

The development of standardised specifications began in the late 19th century with figures like Theodore Cooper, whose publications laid the groundwork for future codes. By the 1920s, various engineering bodies collaborated, leading to the 1928 Specifications for Steel Highway Bridges, which introduced "H" trucks representing lines of vehicles.

Significant changes came in the 1941 AASHO (now AASHTO) Standard Specifications, which introduced the "HS" truck (representing a truck with a semi-trailer) and replaced "truck trains" with either a single design truck per lane or a lane loading. The "HS" trucks also introduced variable axle spacing to better represent actual vehicle configurations.

There are four standard classes of highway vehicle loadings in the Standard Specifications: H15, H20, HS15, and HS20. The "HS" designation signifies a truck with a semi-trailer, while "H" trucks have only two axles. The number (e.g., 15 or 20) relates to the gross weight. For instance, an HS15 loading is 75% of an HS20 loading. Most modern bridge designs, especially for new constructions, require a minimum of HS20 or greater, recognising the increasing weight of commercial vehicles.

For longer-span bridges, lane loadings are used to simulate multiple vehicles in a given lane. These are typically a uniformly distributed load combined with a concentrated load for moment or shear. For example, an HS20 lane load might be 0.64 kips per foot plus an 18-kip concentrated load for moment.

When more than two lanes are loaded simultaneously, AASHTO specifications allow for a reduction in live load (e.g., 10% for three lanes, 25% for four or more) in ASD (Allowable Stress Design) and LFD (Load Factor Design) to account for the decreased probability of all lanes being fully loaded simultaneously. LRFD's approach to live-load distribution also includes specific reduction or increase factors.

The current standard under LRFD is the HL-93 design live load. This is a combination of a design truck (or design tandem for short spans) and a design lane load, specifically chosen to produce the most extreme force effects. The HL-93 design truck has a total weight of 72 kips with specific axle weights and variable spacing for the rear axles. The design tandem consists of two 25-kip axles spaced 4 ft apart. The design lane load is a uniform load of 0.64 kips per foot distributed over a 10 ft width. The HL-93 loading also incorporates "multiple presence factors" which adjust the load based on the number of design lanes, acknowledging the statistical likelihood of multiple heavy vehicles being present in adjacent lanes.

Permit Vehicular Live Loads

Beyond the standard AASHTO loadings, many bridge owners, particularly state highway departments, specify additional "permit vehicular live loads." These account for the movement of exceptionally heavy or oversized vehicles that require special permits to travel on public roads. These loads are typically combined with other loads in specific strength limit states and may assume limited or no other traffic in adjacent lanes due to escort vehicle restrictions. They represent a critical consideration, especially in regions with industries that frequently transport heavy equipment, as they often exceed the standard HL-93 design loads.

Dynamic Load Allowance (IM)

This load component accounts for the dynamic amplification of static live loads caused by vehicle bouncing on uneven road surfaces, bridge deck irregularities, or the dynamic response of the bridge itself. Previously, this was a simple impact factor based on span length. Under LRFD, it's termed the Dynamic Load Allowance and is based on the type of bridge component (e.g., deck, beams) rather than just the span, and it's applied only to the truck portion of the live load.

Live Loads on Bridge Railings

Bridge railings are not merely aesthetic features; they are critical safety components designed to contain and redirect errant vehicles, preventing them from leaving the bridge or colliding with other structures. Design requirements for railings have significantly increased over the decades to reflect real-world impact scenarios.

AASHTO's Guide Specifications for Bridge Railings now require railings to be subjected to full-scale impact tests to achieve specific Performance Levels (PL). These levels are determined by factors such as highway type, design speed, percentage of trucks in traffic, and bridge-rail offset.

• PL-1: Designed to resist impacts from an 1800-lb automobile at 50 mph or a 5400-lb pickup truck at 45 mph, impacting at a 20° angle. Typically for low-volume rural roads.
• PL-2: Resists the same automobile/pickup impacts as PL-1 but at 60 mph, plus an 18,000-lb truck at 50 mph impacting at a 15° angle.
• PL-3: Designed for the highest impact forces, including those from a 50,000-lb van-type tractor-trailer travelling at 50 mph and impacting at a 15° angle, in addition to the PL-2 vehicles. Required for high-volume interstate routes.

Crucially, the performance criteria not only demand resistance to the impact forces but also dictate acceptable vehicle performance post-impact. The vehicle must not penetrate or hurdle the railing, must remain upright, and be smoothly redirected. Due to the difficulty in predicting impact forces accurately, full-scale testing (as per NCHRP Report 350) is strongly preferred for rail systems.

Earthquake Loads (EQ)

For bridges in seismically active regions, earthquake loads are a critical design consideration. The AASHTO Standard Specifications for Seismic Design of Highway Bridges guide engineers in designing structures that can withstand seismic events. Each structure is assigned a Seismic Performance Category (SPC) based on its location and the importance of the highway route. The goal is to allow for some damage during severe earthquakes but with a low probability of collapse. Smaller earthquakes should be resisted within the elastic range of components, avoiding significant damage.

Vessel Impact Loads (CV)

Bridges spanning navigable waters must account for the potential impact of large ships. The AASHTO Guide Specification and Commentary for Vessel Collision Design of Highway Bridges provides guidance based on probabilistic theories, considering the size and frequency of vessels using the waterway. This is an extreme-event limit state under LRFD, crucial for ensuring structural integrity against such rare but catastrophic occurrences.

Thermal Loads (TU, TG)

Temperature variations cause materials to expand and contract, inducing stresses and movements in bridge structures. For steel structures, anticipated temperature extremes can be significant, ranging from sub-zero temperatures to high summer heat. The coefficient of thermal expansion dictates the change in length for bridge members. Designers must incorporate provisions for these movements, often through expansion joints, to prevent excessive stresses. In complex structures like trusses, temperature changes in individual members can induce secondary stresses that require careful analysis.

Sidewalk and Curb Loadings (PL)

For safety, highway structures in urban areas typically include sidewalks and curbs, which must be designed for specific live loads. AASHTO Standard Specifications recommend designing sidewalks and their supporting members for a live load of 85 pounds per square foot (psf). For LRFD, a load of 75 psf is applied to sidewalks wider than 2 ft. Curbs are designed to resist lateral forces, typically applied at the top of the curb.

Wind Loading (WS, WL)

Wind forces are critical, particularly for long-span or tall bridges. AASHTO Standard Specifications consider wind forces as a uniformly distributed, moving live load, acting on the exposed vertical surfaces of all members. These forces are typically based on a 100 mph wind velocity and can be resolved into transverse (perpendicular to the bridge) and longitudinal (parallel to the bridge) components.

• Superstructure: For trusses and arches, wind loads can be 75 psf; for girders and beams, 50 psf.
• Wind on Live Load: An additional force of 0.10 kip per linear foot is applied to the live load, acting 6 ft above the roadway deck.
• Substructure: Wind forces are also applied directly to the substructure, generally at 40 psf for a 100 mph wind.

When wind forces act in combination with live loads, reductions (e.g., 70%) may be applied to the wind load, reflecting the reduced probability of simultaneous maximum wind and live load conditions. Overturning forces due to wind, applied at the windward quarter point of the deck, must also be considered to ensure stability.

Forces of Stream Current, Ice, and Drift (WA, IC)

Piers and other structural elements submerged in water must be designed to resist the forces exerted by flowing water, floating ice, and accumulated drift. The pressure of flowing water is calculated based on water velocity and the shape of the pier. Specific coefficients are applied for different pier types (e.g., circular, square-ended, angle-ended). Ice and drift loads require separate considerations based on AASHTO specifications, accounting for static pressure from ice sheets or dynamic impact from ice floes and debris.

Buoyancy

For substructures, including piling, and where necessary, superstructures, the upward buoyant force exerted by water must be taken into account. This uplift force can reduce the effective weight of the structure, impacting stability and load distribution.

Uplift on Highway Bridges

Beyond buoyancy, other loading combinations can cause an upward force on the bridge, requiring provisions to resist uplift by adequately attaching the superstructure to the substructure. AASHTO recommends engaging a mass of masonry equal to 100% of calculated uplift for certain loading combinations or 150% of calculated uplift at working-load level. Anchor bolts are designed at an increased stress level under these conditions. LRFD specifically mandates hold-down devices in seismic zones to counteract uplift forces.

Frequently Asked Questions (FAQs)

What is the primary difference between H and HS truck loadings?

The main difference lies in the number of axles and the configuration. An 'H' truck typically represents a two-axle vehicle, whereas an 'HS' truck represents a truck with a semi-trailer, meaning it has three axles. The number following the 'H' or 'HS' (e.g., H20, HS20) indicates the gross weight of the design vehicle in thousands of pounds.

Why is the HL-93 load considered the standard for modern bridge design?

The HL-93 load was developed in the 1980s and 1990s through extensive statistical analysis of actual highway traffic patterns and their effects on bridges. It combines the design truck (or tandem) with a design lane load to produce the most critical force effects across various span lengths and bridge types. It also incorporates multiple presence factors to account for the probability of multiple heavy vehicles in adjacent lanes, making it a more realistic and comprehensive design load for today's traffic.

How does AASHTO address extreme events like earthquakes and vessel collisions?

AASHTO addresses extreme events through specific guide specifications and limit states within the LRFD code. For earthquakes, the focus is on seismic design specifications that assign performance categories to minimise damage and prevent collapse during ground shaking. For vessel collisions, guidance is based on probabilistic theories that consider the size and frequency of ships, ensuring that bridges over navigable waters can withstand potential impacts, even if rare.

The comprehensive consideration of these diverse loading types is paramount to the structural integrity and safety of highway bridges. From the constant weight of the structure itself to the dynamic forces of traffic, environmental impacts, and potential extreme events, every load contributes to the complex engineering challenge of bridge design. Adherence to rigorous standards, like those set by AASHTO, ensures that our essential infrastructure can safely serve communities for decades to come, adapting to evolving traffic patterns and environmental conditions.

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