Eurocode traffic loads may look simple on paper. In real bridge models, the challenge is finding the governing load positions, combining them correctly, and keeping the workflow traceable.
For many bridge engineers, the finite element model is only one part of the challenge. The geometry, supports, stiffness distribution, bearings, prestress, construction stages, and boundary conditions all need careful attention. But once the structural model is in place, another question quickly becomes just as important:
How should the bridge be loaded?
Basic load cases, such as self weight, earth pressure, shrinkage, wind load etc are fairly easily defined to an FE-model. Defining the live/traffic loads is a different story. At first glance, the traffic load models in Eurocode can appear relatively straightforward. Road bridges are described using defined load models. Railway bridges use defined train models, axle loads, distributed loads, dynamic considerations, and serviceability checks. The code gives the engineer a structured framework for representing traffic on bridges.
However, once these loads are to be applied to a real bridge model, the practical complexity becomes clear. Traffic loads are not just fixed load cases. They move. They may act in several lanes or tracks. They may need to be placed to maximize bending moment in one section, shear force in another, torsion in a third, or support reaction somewhere else entirely.
And after the most unfavourable traffic effects have been found, they must be combined correctly with permanent actions, temperature, wind, braking and acceleration forces, centrifugal forces, settlements, construction stages, prestress effects, and other relevant actions.
This is where specialized bridge analysis tools can make a major difference.
Eurocode does not state that the bridge structure needs to be checked for every real vehicle that may cross a bridge. Instead, it defines synthetic traffic load models intended to represent the effects of many/most possible traffic situations.
For road bridges, the designer need to consider concentrated axle loads, uniformly distributed loads, tandem systems, lane definitions, load models for global and local verification, special vehicles, braking and acceleration effects, and centrifugal forces depending on the bridge geometry.
For railway bridges, the situation include standard load models, multiple tracks, dynamic amplification, high-speed train models, fatigue trains, braking and traction and checks related to acceleration, deformation, and comfort.
These load models are powerful because they give the industry a common design basis. Still, the load models must be positioned, varied, and combined in ways that capture the governing effects for the specific structure being designed or assessed.
In practice, this means that the engineer is not simply asking: What is the traffic load on this bridge?
The real question is closer to: Where can traffic be placed, which load model is relevant, how should it move, and which position produces the most unfavourable response for each result component and design check?
That is a much more demanding question.
For a simple bridge, the governing traffic position may be intuitive. On a narrow, straight, simply supported single-span bridge, a bridge engineer can make reasonable assumptions about where the traffic should be placed to maximize midspan bending moment or support shear.
But not all bridge designs are limited to ideal textbook cases.
As soon as the bridge has more than one span, is curved in plan, has skew supports, varying stiffness, several traffic lanes, multiple railway tracks, wide decks, edge beams, complex supports, etc, the governing traffic/vehicle position becomes much harder to predict.
A load position that maximizes longitudinal bending moment may not maximize transverse bending. The governing position for torsion may be different from the governing position for vertical reaction. For a curved or skewed bridge, the relationship between load position and internal force response may be far from obvious...and so on.
This is why manual traffic loading can become so time-consuming. The engineer must define a sufficient number of load arrangements, run them, inspect the results, decide whether additional positions are needed, and repeat the process until there is confidence that the relevant governing cases have been captured.
The challenge is not only the amount of work. It is also the risk of missing a critical case.
Finding the worst traffic position is only part of the task. In bridge design, traffic actions must be evaluated together with other actions.
A typical bridge model also include permanent actions such as self-weight, surfacing, barriers, earth pressure, and prestress. It also include variable or imposed actions in addition to traffic, such as temperature gradients, wind, braking and acceleration, centrifugal forces, settlements, etc, depending on the specifics of the project.
For each design situation, these actions must be combined according to the relevant design code. The engineer must account for ultimate limit states, serviceability limit states, characteristic, frequent and quasi-permanent combinations, and project-specific or national annex requirements.
There is an additional consideration: whether a load is favourable or unfavourable may depend on the response component being checked.
For example, the same permanent load may increase compression in one section but reduce tension in another. A settlement case may be unfavourable for one support reaction and favourable for another. Traffic loading may govern one component in one position and another component somewhere else in the bridge structure.
This makes bridge load combinations different from a simple list of manually written equations. The combination process needs to evaluate many possible effects and produce safe, relevant design envelopes.
There are many ways to approach traffic loading and load combinations. At one end of the spectrum is a largely manual workflow.
In such a workflow, the engineer defines traffic arrangements based on experience and engineering judgment. The load cases are analysed, results are reviewed, and combinations are created manually or semi-manually. For simple bridges, this can be reasonable and efficient. It also gives the engineer a very direct understanding of the assumptions used.
At the other end of the spectrum is a specialized bridge analysis workflow. Here, the engineer defines the traffic lanes, tracks or loadable areas, selects the relevant code-defined vehicles or train models, specifies how they may move, and lets the software search for the most unfavourable response. The same environment can then combine these traffic effects with other load groups according to design-code logic.
The automation should not remove engineering judgment. Instead, it moves the engineer’s effort to the right level.
Rather than spending time manually guessing hundreds or thousands of load positions, the engineer can focus on questions such as:
Is the structural model representative?
Are the traffic lanes or tracks defined correctly?
Are the relevant Eurocode load models and national choices included?
Are braking, centrifugal, temperature, wind, prestress and settlement effects represented appropriately?
Are the governing results reasonable and explainable?
Do the envelopes make engineering sense?
This is where automation becomes valuable: not by replacing the engineer, but by reducing repetitive work and making it easier to review the governing cases.
For moving loads, influence-based methods are especially useful.
An influence line describes how a response quantity changes as a unit load moves along a path. For more complex bridge models, influence surfaces extend this idea to two-dimensional loadable areas or traffic lane definitions. Instead of manually placing a vehicle at many possible locations, the software can use influence information to identify where the vehicle should be positioned to maximize or minimize a selected response.
This is a natural fit for bridge engineering because many traffic load problems are fundamentally moving-load problems. The vehicle is not fixed. It travels along the bridge, and the design question is often to find the position that produces the most severe effect.
A traffic load module is only half of the story. The results must also be combined in a way that reflects the design code.
A bridge-specific load combination engine can group actions, apply combination rules, distinguish between permanent and variable actions, account for favorable and unfavorable effects, and generate envelopes for the limit states that matter in design.
This is particularly important when moving traffic loads are combined with other actions. It is not enough to find one maximum traffic effect and place it into one manually written combination. The governing traffic position may vary depending on which response is being checked. The combination should therefore be able to work with envelopes and adverse/non-adverse effects in a controlled way.
For the engineer, a good load combination workflow should provide three things:
Efficiency – fewer repetitive manual load arrangements and fewer hand-built combinations.
Traceability – the ability to understand which load groups and combinations generated the governing result.
Confidence – reduced risk that an important traffic position or combination has been missed.
BRIGADE is developed specifically for bridge and civil structure analysis, and it is reflected in how it handles moving loads and load combinations.
In BRIGADE/Plus and BRIGADE/Standard:
traffic lanes can be defined independently of the mesh.
vehicles can include axle loads with fixed or variable spacing
traffic load effects can be evaluated using influence lines and influence surfaces.
multiple simultaneous vehicles in different lanes can be handled automatically.
BRIGADE includes standardized vehicle libraries, including Eurocode-based models for road and railway bridges. This allows engineers to work from code-defined load models while still retaining the ability to define project-specific or custom vehicles when required.
The load combination functionality is designed for bridge analysis. Users can work with manually defined combinations or built-in design-code libraries, including Eurocode and national annex logic for road and railway bridges. The combination engine account for whether individual loads have adverse or beneficial effects and supports both ultimate and serviceability limit state combinations.
In practice, this means that the engineer can model the bridge, define where traffic is allowed to act, select the relevant load models, include the other actions on the structure, and then evaluate governing traffic effects and code-based combinations in a consistent workflow.
The benefit is not only speed. It is also a clearer path from model assumptions to design results. In addition, BRIGADE/Plus and BRIGADE/Standard differs in their workflow setup. Read more about it here: Choosing the Right Tool for Bridge Analysis
The final objective is not to generate more analysis output. The objective is to create reliable information for design.
Bridge engineers need design results such as and section forces that can be taken further into design checks, reinforcement design, assessment calculations, reporting, or independent verification.
Result interpretation is just as important as load application. A moving load analysis may generate governing positions and envelopes, but the engineer still needs to understand where the critical effects occur and why. A load combination may produce maximum and minimum results, but those results must be presented in a way that supports practical design work.
This is why bridge-specific workflows often include not only traffic loading and load combinations, but also tools for result visualization, reporting, extraction of sectional forces, and export to downstream design processes.
It can be tempting to describe automated traffic loading and load combinations as a way to “save time”. That is true, but it is not the whole story.
The deeper value is that automation helps engineers apply complex design-code requirements consistently across many result components, many load positions, and many design situations.
For simple bridges, experienced engineers can often identify the governing traffic positions manually. For more complex bridges, the number of possible load arrangements and combinations grows quickly. In those cases, relying only on manual assumptions can be inefficient and may introduce unnecessary uncertainty.
A specialized bridge analysis tool does not remove the need for engineering understanding. On the contrary, it makes that understanding more important. The engineer still defines the model, interprets the code, selects relevant load models, reviews the governing results, and decides whether the outcome is structurally reasonable.
But the repetitive search for critical traffic positions and the systematic generation of code-based combinations can be handled more robustly by software designed for that purpose.
Eurocode traffic loads are intentionally simplified representations of real traffic, but their practical application in bridge analysis is not simple. The engineer must consider where traffic can act, how vehicles or trains move, which load models are relevant, which positions govern different response components, and how those effects combine with other actions according to the design code.
For straightforward bridges, much of this can be handled with engineering judgment and a limited number of load cases. For curved, skewed, wide, multi-span or otherwise complex bridges, the task quickly becomes too large for a purely manual approach to be efficient or reliable.
This is where tools such as BRIGADE add value: by combining bridge-specific moving load analysis, influence-based evaluation, predefined traffic models, and design-code-based load combinations in one workflow.
The result is not just faster analysis. It is a more systematic route from Eurocode load definitions to governing design results and ultimately to better-informed bridge engineering decisions.
Here you can read more about how other companies leverage on BRIGADE in their projects.