Traffic Load Modelling:
Traffic Load Modelling

Many companies have extremely powerful computer models to predict the response of bridge structures to applied loading. However, the loads themselves are far less certain. What the standards specify is only part of the story. Loading standards specify notional models such as the Eurocode Load Model 1 (Normal loading) or the AASHTO LRFD code. These models have been agreed by code development committees and are designed to generate greater bending moments, shear forces, etc. than the true characteristic levels for traffic at that site. Consequently, the notional load models specified in standards are generally highly conservative, appropriately so in most cases. However, in order to account for reserves in the safety of the structure, more realistic loading should be examined using site-specific loading schemes. This allows the current traffic conditions to be assessed but can also be used for predicting the effect of future traffic growth. Within the field of traffic load modelling ROD-IS have extensive expertise in the following fields

 

  • Advanced Bridge Live Loading

  • Micro-simulation of traffic loading

  • Code development

  • Dynamics

 

 

 

 

Bridge live load models for the design of new bridges and the assessment of existing bridges have traditionally been based on standards or codes of practice. However, these standards are generally conservative as the notional traffic load model needs to represent the worst combination of traffic meeting or overtaking on any given bridge and allow for a wide range of traffic conditions, bridge geometries, spans and locations. Typically, there is a great deal of variation between sites, road classes and characteristic load effects in bridges of different spans. This means that the levels of safety that result from a code design might be very different from reality.

The ROD-IS team utilise advanced analytical techniques (i.e. WIM data analysis, Calibration of statistical models, Live load Scenario simulation, Extreme Value statistical extrapolation, Long run simulations etc.) to calculate the actual characteristic load effects for a range of sites in different countries and for different load effects and spans. In effect ROD-IS can develop a site specific live load model for any bridge at any location worldwide. This site-specific approach is particularly useful for the assessment of existing bridges where ‘hidden’ levels of safety for some load effects can mean the difference between the bridge having to be repaired or not.

 

For the design of long-span bridges, it is critical to ensure that an accurate assessment (using probabilistic rather than deterministic methods) of all loads on the bridge structure is achieved. It is well known that, for short-span bridges, the critical loading events involve free flowing traffic with an allowance for dynamics. For longer-span bridges on the other hand, stationary congested traffic tends to govern (with no dynamics). The cut-off point varies considerably and depends on what allowance is being made for dynamics. Using the Eurocode dynamic allowances, it is in the range of 40 to 50 m, i.e. above this span, congested traffic conditions tend to govern the design.

Weigh-in-Motion (WIM) systems are now installed in many countries and provide a wealth of data on the weights and frequencies of heavy vehicles on our roads. Unfortunately, most WIM systems are only accurate for free flowing traffic. There is a dearth of information on the weights and spacing's of vehicles in congested conditions, as governs the design of long span bridges.

The ROD-IS long-span bridge group have developed micro-simulation software to simulate the behaviour of individual vehicles in congested conditions. For example, there is a tendency for cars to pull out from between trucks as traffic becomes congested. This leads to truck convoys (truck only ‘platoons’) and loading situations that are more critical than anything regular (fee-flowing) WIM data would suggest. The micro-simulation software is being used to monitor the sensitivity of bridge loading to driver behaviour (e.g. desired speed, reaction time, headway etc.) and lane changing processes in order to determine more accurately the true traffic loading on long-span bridges. The micro-simulated traffic loading can be combined with ROD-IS ‘advanced live loading’ capabilities to determine extreme congested traffic loading events for the design of long-span bridges.

 

ROD-IS directors have had a significant involvement in the development of the Eurocode for bridge traffic loading. Professor OBrien was chairman for many years of the national committee overseeing all bridge related Eurocodes while Professor O’Connor was seconded to Paris in the 1990’s to recalibrate the Eurocode traffic load model with new WIM data. ROD-IS has been involved in several research projects on bridge traffic loading and is a world leader in the field.

Using statistical principles and a database of WIM data, ROD-IS can determine characteristic maximum-in-lifetime stresses in bridges and derive a simple traffic load model that best represents the traffic in question. Hence, a bridge loading standard can be developed for a continent, a country, a network, a bridge type or even a specific bridge.

ROD-IS has developed state-of-the-art techniques to address some of the challenges of code development and continue to innovate and break new ground in this field, including being the first to identify problems of mixing statistically-dissimilar traffic loading scenarios as a source of inaccuracy in the extrapolations (we showed that 1-truck crossing events are dissimilar to 2-truck meeting events). Equally, in 2009 ROD-IS were the first to demonstrate the benefits of long-run simulations which allows the client to review typical maximum-in-lifetime loading scenarios to ascertain that they are plausible.

 

Dynamics can be a particular problem in bridges where the natural frequency is close to the frequency of a source of excitation, such as traffic loading or wind. In a number of European research projects, ROD-IS directors have investigated extensively dynamic vehicle/bridge interaction and have demonstrated that the allowance for dynamics in most standards/codes of practice is quite conservative. It is now clear that there is a decreasing trend in dynamic amplification as the vehicle (or combination of vehicles) gets heavier. In the extreme loading case at the Serviceability Limit State, the dynamic amplification of stresses in most bridges is quite low. This means that there is an additional level of safety in most bridges designed to modern standards in their allowance for vehicle/bridge interaction.

ROD-IS has the capability to do a comprehensive statistical analysis that combines static and dynamic loading to find the characteristic total stresses, including allowance for dynamics. This is particularly useful for ‘lively’ bridges that are vibrating excessively. Equally the analysis can differ between road and rail bridges. For example, for road bridges the surface profile has a significant effect and small differences in the transverse location of the moving truck can be important. The dynamic properties of the vehicles (i.e. trucks) can also be difficult to determine accurately as they vary considerably with manufacturer, tyre inflation and make etc. Rail bridges, however, tend to have a relatively smooth surface and vehicle properties are generally known as standard carriages are generally the norm. On the other hand, it is important to be able to combine analytical models with field measurements and there tends to be more data (i.e. WIM data) available for road bridges than rail bridges.