3英文

Evaluation of dynamic loads on a skew box girder

continuous bridge

Part II: Parametric study and dynamic load factor Abstract

Studies on dynamic loads are important for bridge engineering as well as pavement design.

A large number of research studies have indicated that bridge dynamic loads increase road surface damage by a factor of 2–4. Although the field test is the best available approach to understanding actual vehicle-induced dynamic loads on bridges, according to pervious studies there is only a limited amount of field data available on skew box girder continuous bridges. This paper presents an evaluation of vehicle-induced dynamic loads, based on a field test that was carried out on a skew box girder continuous bridge as reported in a companion paper (Part I). The effects of different parameters such as the weight, speed, type, number of axles and position of vehicles on dynamic loads are investigated. Based on the statistical analysis, the use of the dynamic load factor (DLF) is proposed. The dynamic load factor obtained in this study is less than the values provided by most bridge design codes.

Keywords: Skew bridge, Dynamic load factor, Dynamic load allowance, Normal traffic condition

1. Introduction

V ehicles that are expected to cross a bridge are accounted for in the design or evaluation of the bridge through a static design loading and a certain prescribed fraction of it that is referred to as an impact factor, a dynamic load allowance, or a dynamic increment [1].

As opposed to static design loading which is a tangible entity that can be formulated from the static weights of actual or foreseen vehicles, determining the dynamic load is not a straightforward procedure because of the complex nature of the interaction between the bridge and moving vehicles.

Numerous studies on bridge dynamics have been carried out. It has been discovered that the magnitude of the dynamic load depends on several factors, including bridge dynamic behaviors,

road roughness, vehicle dynamic characteristics, vehicle speed, type, weight, number of axles, axle spacing, the position of vehicles on the bridge, and so forth. Although the exact magnitude of this dynamic load can be reasonably estimated based on the static load, it has never been ascertained as it varies from case to case.

Most studies have used the analytical approach to study the bridge–vehicle interaction problem and estimated the dynamic load factor (DLF). Even though the field test is the best available approach to understanding the actual bridge–vehicle interaction to estimate the DLF, the amount of data available from the field on dynamic loads is limited [2]. Billings [3] carried out field measurements of dynamic loads on 27 bridges of various kinds. He found that the dynamic load allowance ranges from 0.05 to 0.10 and from 0.08 to 0.20 for prestressed concrete and steel bridges, respectively. Tests carried out by Cantieni [4] on 226 bridges in which most of the bridges were loaded with the same vehicle, under the same load, and with the same tire pressure showed that a dynamic load allowance as high as 0.7 could be obtained from the field. Chan and O’Connor [5,6] have found maximum value of 1.25 for a dynamic load allowance. Recently, on the basis of the analytical simulations and field tests, Nowak et al. [7] pointed out that the dynamic load allowance is considerably lower than code-specified values. According to their findings, the maximum simulated and measured dynamic load allowances do not exceed 0.17 for a single heavy truck, and 0.10 for two trucks moving side-by-side. It can be understood that, given the limited number of field tests, most of the studies on vehicleinduced dynamic loads on bridges have ended up with different outcomes. This shows the need to conduct more field tests on various bridges using unbiased random samples of vehicles.

In this study, a field test was conducted on a skew box girder continuous bridge to collect vehicle-induced dynamic response data on the bridge and information from various kinds of vehicles under normal traffic conditions. The details about the tested bridge such as the cross section, location of sensors, roadway width are presented in a companion paper [15]. A parametric study was carried out to determine the factors that affect the DLF. To determine the appropriate DLF value for design purposes, a statistical analysis was conducted.

2. Dynamic load factor

2.1. Definition of the dynamic load factor

V arious studies have been carried out to determine and evaluate the magnitude of the