As far as lifetime determination is concerned, the fatigue process has traditionally been divided into two stages: nucleation and crack propagation. A different law of behavior is considered in each stage in order to calculate the lifetime span. For non-constant amplitude load, the nucleation life is usually found by means of a cumulative damage rule, whereas the propagation life is determined by using fracture mechanics based methods.
When the load varies in a random fashion there are two ways that might be used to determine the lifetime. The first would be to perform a cycle-by-cycle simulation of the process, and the second would adopt a statistical approach, making use the statistical characteristics of the load records. The major drawback of the direct simulation method is that it is extremely time-consuming. Conversely, the statistical approach is quite straightforward but it fails to describe the sequence effect, although this may be overlooked in many instances.
In their service life, bridges are exposed to traffic loads, sometimes very heavy, especially on the high volumes roads. Fatigue damage affects especially short span bridges where dead load is relatively low, and therefore the live load stress ranges are higher than in the case of long span bridges.
The traffic load models specified in many codes are based on old collected traffic data. This implies that the models do not represent the traffic loads induced by today’s vehicles since vehicle formations and properties have changed a great deal in recent years. Consequently, by using these load models, especially with the intention of repairing or reconstructing existing bridges to meet current design traffic loads could result in a great waste of money. Therefore, it is very important to continuously update the design traffic load models given in codes. A new era has now begun where simulations and extrapolations are used to statistically analyze recorded vehicle data to study different load effects on bridges with the intention of calibrating. The traffic load models given in different codes.