Shock and Vibration

Volume 2015, Article ID 829380, 12 pages

http://dx.doi.org/10.1155/2015/829380

## Study Based on Bridge Health Monitoring System on Multihazard Load Combinations of Earthquake and Truck Loads for Bridge Design in the Southeast Coastal Areas of China

^{1}Institute of Engineering Mechanics, China Earthquake Administration, Harbin 150080, China^{2}College of Civil Engineering and Architecture, Zhejiang University, Hangzhou 310058, China

Received 8 August 2014; Accepted 1 October 2014

Academic Editor: Bo Chen

Copyright © 2015 Dezhang Sun et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Similar to American *LRFD Bridge Design Specifications*, the current Chinese bridge design code is fully calibrated against gravity load and live load. Earthquake load is generally considered alone and has its own methodology, however, which is not covered in the code in a consistent probability-based fashion. Earthquake load and truck load are the main loads considered in the basis of bridge design in more than 70% of seismic areas in China. They are random processes, and their combination is the main subject of this paper. Seismic characteristics of southeast coastal areas of China are discussed and an earthquake probability curve is calculated through seismic risk analysis. Using measured truck load data from a Bridge Health Monitoring System, the multimodal characteristics of truck load are analyzed and a probability model for a time interval *t* is obtained by fitting results and reliability theory. Then, a methodology is presented to combine earthquake load and truck load on a probabilistic basis. To illustrate this method, truck load and earthquake load combinations are used. Results conceptually illustrate that truck load and earthquake load are not dominant in southeast coastal areas of China, but the effect of their combination is. This methodology quantitatively demonstrates that the design is controlled by truck load in most ranges; that is, truck load is more important to bridge design in the region.

#### 1. Introduction

In the current bridge design specifications of China [1] a typical bridge is designed for 100 years’ service life and the design limit states are only fully calibrated for dead and live loads. Consideration of earthquake load has its own unique approach, principally because data and statistics are rare. Truck load data are old and may not suit the current situation, and they therefore need to be updated. This fact makes it difficult to properly consider both truck load and earthquake load in a consistent fashion. A research project is currently being carried out to establish a methodology to systematically combine truck load and earthquake load. Principle emphasis is given to establishing the proper “demand.” In order to pursue the demand side of bridge design specifications, there are numerical challenges that must be overcome in addition to the fact that very limited historical data are available. Truck load and earthquake load are time-variant random processes. Truck load occurs once in a typical time span of minutes or seconds on a common bridge, while earthquake load occurs once in a typical time span of years or decades. Furthermore, there is no evidence that the occurrence of earthquake loads follows normal distributions. Fortunately, many bridge health monitoring systems (BHMS) have been established, which provide a convenient way to get useful data for this research [2].

To overcome the challenges, many efforts [3–9] have been made in the past decades. However, because numbers of assumptions have to be made in each model, no general conclusions can be drawn about satisfactory approach to deal with load combination of earthquake load and truck load. In more recent papers, a methodology is proposed by Liang and Lee [10, 11]; however, its accuracy is yet to be substantiated.

One objective of this paper is to describe a methodology to handle truck load and earthquake load combinations. Earthquake load is modeled using seismic risk analysis. Truck load is modeled using Stationary Poisson processes based on the BHMS and statistical analysis. Two numerical examples of truck load and earthquake load combinations are used to illustrate the methodology.

#### 2. Earthquake Load

A number of variables describe the effects that earthquakes have on bridges, such as the intensity of acceleration, the rate of earthquake occurrences, the natural period of the bridge, the seismic response coefficient, and the response modification factor. In order to explain the methodology of load combinations, only the intensity of acceleration and the rate of occurrence are chosen as the main variables.

Based on the Poisson process assumption, the probability of exceedance () in a given exposure time () is related to the annual probability of exceedance () by [12],Because the number of earthquakes varies widely from site to site, they are converted to Peak ground acceleration (PGA) and the return period curve (, is return period). The cumulative probability of an earthquake in time can be written asThe PGA and frequency of exceedance curve can be obtained from US Geological Survey (USGS) mapping in the United States but cannot be obtained in China. Therefore, seismic risk analysis is used to calculate earthquake probability curve. The procedures are presented just as follows.

For more than one potential seismic source zone, suppose the parameters of the earthquake are random distributions and the probability over 1 year is a stable Poisson process. Based on the total probability theorem, the probability of exceeding a given earthquake intensity in one site can be expressed by (3), by considering the uncertainties of occurrence and the upper limit magnitude: where is the probability of the th upper limit magnitude exceeding a given earthquake intensity in potential seismic source , is the th year occurrence probability of the th upper limit magnitude in potential seismic source , is the weight of the th year occurrence probability in potential seismic source , and is the weight of the th upper limit magnitude in potential seismic source .

The earthquake intensity could be acceleration, velocity, or displacement. For acceleration, ( is earthquake intensity; is acceleration).

Because of the uncertainties of direction impact of potential seismic source zones, for year, the probability of exceedance iswhere is the conditional probability of the th upper limit magnitude.

For disperse potential seismic source areas, the probability of the th upper limit magnitude can be expressed as where is the area of the potential seismic source ; is the area of zone . If the occurrence probability of 1 year is divided by the weights in each upper limit magnitude of potential seismic source zone, the exceedance probability of the th upper limit magnitude of potential seismic source can be given as According to the seismic belt materials and reports, the southeast coastal area of China has two I degree seismic areas, namely, the South China seismic area and the South China Sea seismic area. The seismic belt of southeast coastal areas of China is located south of the middle Yangtze River seismic belt, bordering on the seismic region of the Tibetan Plateau on the west, and includes Kwangtung province, Hainan province, most of Fujian and Guangxi provinces, and part of Yunnan, Guizhou, and Jiangxi provinces. Crustal thickness ranges between 28 and 40 km, gradually increasing from the southeast coastal area of China to the northwest mountains. An internal secondary elliptical gravity anomaly is relatively developed in the earthquake zones. There are no obvious banded anomalies except the gravity gradient zones of southeast coastal areas and Wuling Mountain. In the zones, magnetic anomalies change gently and there are no larger banded anomalies. Because the southeast coastal areas of China are in the same seismic belt and most of the areas in the zone have a PGA seismic fortification level of 0.1 g, Shenzhen city is then used for the basic earthquake probability calculation and comparison in this paper. The South China belt is shown in Figure 1. From Figure 1, it can be seen that there are many higher than Ms 6.0 earthquakes in the southeast coastal areas of China present in the seismic analysis. Earthquake load is still the main load considered for bridge design in these areas.