Section 5 Risk Assessment

Chapter 11

Abstract

resilience

scour.

185

1. Introduction

Foundations

Multi-Hazard Assessment of

Bridges with Unknown

Xuan Guo, Zhaohua Dai and ZhiQiang Chen

Seismic and Scour Effects on Rural

This chapter proposes a probabilistic framework for assessing seismic and scour

effects on existing river-crossing bridge structures. The emphasis is on bridge structures in rural areas, for which it has been recognized that a large number of rural bridges have unknown foundation types and further are subject to both flooding-induced scour and seismic damage. With a review of the US-based rural bridges, this chapter presents a probabilistic framework for bridge performance assessment. Using a representative rural bridge model, the fragility results for the bridge reveal that scour tends to be beneficial in reducing structural damage at slight to moderate seismic intensities and to be detrimental in increasing collapse potential at high-level intensities. The demand hazard curves further quantify probabilistically the occurrence of local damage and global collapse, and systematically reveal the complex effects of scour as a hydraulic hazard on bridge structures.

Keywords: bridge, unknown foundation, flood-induced scour, multi-hazard,

There are over 484,500 highway bridges built over river channels in the U.S., among which over 20,904 are regarded scour-critical [1]. A distinct feature of flood-induced scour is that once it starts forming around a foundation, it may accumulate or vary over the bridge's service life. Hence it is intuitive that a potentially more severe risk is that scour is combined with other extreme hazards, such as earthquakes, which may threaten bridges serving in both earthquake and flooding active regions, such as Alaska, Oregon, and California in the U.S. On the other hand, it is recognized that a large number of bridges that are in service for tens of years have unknown foundation, in the meantime, been suffering from flooding induced

In a recent National Cooperative Highway Research Program (NCHRP) report published in 2006 [2], it was identified that a very large proportion of the bridges built in 1950 and 1980 had unknown foundations, although it was surprising that even 69 bridges built during 2000–2005 were concluded with the label of bridges

## Chapter 11
