National Academies Press: OpenBook

Guidelines for Shielding Bridge Piers (2018)

Chapter: Chapter 1 - Introduction

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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Shielding Bridge Piers. Washington, DC: The National Academies Press. doi: 10.17226/25313.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Shielding Bridge Piers. Washington, DC: The National Academies Press. doi: 10.17226/25313.
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1 Introduction While large trucks do not often strike the pier systems of highway bridges, the potential for catastrophic bridge col- lapse in such collisions makes the design of pier systems and their protection important considerations for highway and bridge designers. Avoiding bridge collapse is of paramount importance because, in addition to the immediate safety consequences, a bridge collapse causes major disruptions to the operation of transportation networks, and repairs are both costly and time consuming. Although the potential for a heavy truck impact with one or more bridge columns or piers to produce catastrophic collapse of a bridge has always been a concern for bridge engineers, only within the last 25 years have the AASHTO bridge design specifications begun requir- ing that piers either be designed to withstand heavy truck impacts or that high-capacity barriers be installed to prevent heavy trucks from striking the columns. The AASHTO Road- side Design Guide (RDG) has generally considered bridge piers the same way as any other roadside fixed objects. This report documents the development of risk-based guidelines for shielding bridge piers and presents the proposed guidance for consideration in the AASHTO LRFD Bridge Design Specifica- tions and the RDG. These approaches proposed for the LRFD Bridge Design Specifications and RDG have been coordinated in this research. These guidelines satisfy the objectives of this project, which include: 1. Developing risk-based guidelines that quantify when bridge piers should be investigated for vehicular collision forces per the AASHTO LRFD Bridge Design Specifica- tions or be shielded with a longitudinal barrier, consider- ing, at a minimum, site conditions, traffic, bridge design configurations, geometry of the roadway section passing beneath a bridge, operations characteristics, and benefit/ cost, and 2. Developing guidelines for barrier selection, length-of-need, and placement for shielding bridge piers and protecting the traveling public. A highway bridge is composed of two major structures: the superstructure, which supports the live load of moving vehicles and pedestrians, and the substructure, which supports the vertical loads of the superstructure. The sub- structure components that transfer the live and dead load from the superstructure to the foundation along the span of the bridge constitute the pier system. There are four general types of bridge piers typically used in highway bridges that cross other highways. There are a variety of other more com- plex pier designs that are more commonly used in waterway crossings, but since the focus of this project is pier protection from heavy vehicle and passenger vehicle traffic, this research is concerned only with bridges that cross over roadways. The term “bridge pier,” as used in this report, is taken to mean the vertical support system of a bridge between the abutments. The Ohio Department of Transportation (ODOT) uses a fairly typical method for categorizing bridge piers, as shown in the following and in Figure 1 [ODOT 2014]. This terminology has been adopted herein: • Pier walls are “a full height, rectangular pier extending from the ground line or streambed up to the bearing elevation. The pier extends the full width of the bridge, supporting all beam members” [ODOT 2014]. • Tee-type or hammerhead piers are “rectangular stem capped with a cantilever-type cap” [ODOT 2014]. • Cap (or bent) and column are a type of pier system that has a series of two or more rectangular or circular columns capped with a bent. • Multiple-column piers use rectangular or circular columns that directly support bridge girders without a cap or bent. Within these broad categories are numerous possible configurations. Configurations like the pier system shown in Figure 2 are particularly vulnerable to vehicle impacts since the pier is unshielded and contains only a single pier column such that the pier system is not redundant. C H A P T E R 1

2 (a) Pier wall (b) Cap and column (c) Tee or hammerhead (d) Multicolumn with no cap Figure 1. ODOT bridge pier types [after ODOT 2014]. Figure 2. Single-column bridge pier system [ODOT 2014].

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TRB’s National Cooperative Highway Research Program (NCHRP) Research Report 892: Guidelines for Shielding Bridge Piers provides proposed load and resistance factor design (LRFD) bridge design pier protection specifications and proposed occupant protection guidelines. Bridge piers are generally close to the roadway to minimize bridge lengths. As a consequence, barriers are normally placed around piers to reduce the potential of vehicle crashes damaging the piers. However, the design and placement of the barriers may not have taken into consideration the possibility that vehicles, particularly large trucks, might still impact the pier. The report also includes four examples that illustrate the use of the proposed specifications and guidelines for shielding bridge piers.

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