Evaluation of dynamic loads for highway bridges

Abstract

The ever increasing weight and number of heavy vehicles in a highway system on one hand and the introduction of the improved design procedures and new materials for bridge design and construction on the other, have made it increasingly important for researchers and engineers to better understand the bridge-vehicle interaction problems, and to accurately estimate the dynamic effects brought about by moving vehicles on the serviceability of existing bridges and in the design of new bridges. This thesis reports the findings of a research project in which an innovative method to identify the dynamic loads of moving vehicles on continuous supported bridges has been developed and consequently the dynamic behavior of highway bridges under moving vehicles axle loads is studied using analytical and experimental methods. In recent years a method called the moving force identification system was introduced to identify the dynamic axle loads of a moving vehicle, and to acquire the corresponding measured dynamic responses of a bridge. The system does not need to instrument any vehicle for investigation. This makes it easy and economical to collect unbiased samples of vehicle data under normal traffic conditions. However the system can only be applied to simply supported bridges, but there exist a huge number of continuous bridges in any highway network. It therefore prompts a need to develop a method which can measure dynamic axle loads using unbiased random samples of vehicles on continuous bridges, so that more samples of vehicle induced dynamic loads could be collected and a better understanding of bridge-vehicle interaction obtained. To address this, a method based on the Time Domain Method (TDM) is formulated to identify vehicle axle loads moving on a continuous bridge. Further, the method is modified to identify moving force using only a selected span of interest (target span) from a continuous bridge, so that it is not necessary to instrument the whole bridge. Numerous studies on bridge dynamics carried out in the past were mostly based on analytical methods or experimental studies in laboratory. Although these studies can provide useful information for better understanding of bridge-vehicle interaction problems, they cannot fully reflect the problem in a real bridge structure. Field testing is one of the methods which can fill this gap, since it includes all the parameters that affect the real behavior of a bridge. In this study a field test was conducted, and based on the collected unbiased sample of field data, vehicles’ induced dynamic loads on the tested bridge were evaluated and parametric studies were carried out to consider the worst combinations (most bridge unfriendly conditions) to produce the largest dynamic loads on bridge. After identifying the critical parameters, a statistical analysis was carried out and appropriate values of dynamic load factor (DLF) were proposed for bridge design. A dynamic load factor of 1.30 and 1.24 were obtained for simply supported and continuous bridges, respectively. The results of dynamic load factors obtained from the bridges investigated under this study are much lower than the dynamic load factor obtained using BD37/01 (2001) and the Structures Design Manual for Highways and Railways (2002),in which an allowance of 1.80 was made in driving the HA loading to consider the dynamic load factor. Based on the information acquired from the measured responses and the identified dynamic axle loads, using the developed moving force identification system, the causes of high dynamic load factor were investigated. It was discovered that the causes of high dynamic loads on a bridge depend upon the frequency contents of the bridge as well as on the dynamic behavior of the vehicle traversing the bridge. This research study provides a better understanding of the problem of bridgevehicle interaction. Based on the method of moving force identification developed, bridge-vehicle interaction problems in continuous bridges may be readily studied.Parameters which affect the dynamic loads on a bridge are identified from unbiased samples of field data. Based on this, relationships between various dynamic loads affecting parameters and dynamic load factor are established. It was also discovered that the Structures Design Manual for Highways and Railways issued by the Highways Department of the Hong Kong SAR is conservative in estimating the dynamic load factor for bridge design. Therefore further study which includes field tests in several representative bridges to establish the dynamic load factor for safe and economical bridge design in Hong Kong is proposed. Methods are established to identify characteristics of bridge friendly vehicles using measured responses of a bridge and identified dynamic axle loads of a vehicle in normal traffic conditions. The results obtained in this study may be used to propose modifications in bridge and vehicle design.