Guidelines for Shielding Bridge Piers (2018) / Chapter Skim
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From page 3... ...
3 Literature Review 2.1 Guidelines and Specifications 2.1.1 AASHTO Bridge Design Specifications The guidance for protecting structures from vehicle impacts has evolved over the past several decades, as reflected in AASHTO bridge design specifications. The first edition of the AASHTO LRFD Specifications for the Design of Highway Bridges was published in 1994.
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4 a risk analysis, however, implies that quantifying the likelihood of a catastrophic impact is a key consideration, and that consideration would certainly depend on the particular site conditions. The 400-kip design load that first appeared in 2007 was based on information from crash tests with an 80,000-lb tractor-trailer truck and load estimates for train impacts developed by Hirsch [Hirsch 1985]
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5 currently employed and the material referenced in the development of these approaches. Chapter 13 of the Wisconsin Department of Transportation Bridge Design Manual states that piers shall resist loads applied directly to them, including vehicle impact loads [WIDOT 2013a]
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6 AASHTO LRFD Bridge Design Specifications has been used as the basis for the state bridge design standards. A few states provide different test-level barriers for different offsets of the piers and travel lanes.
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7 that the AASHTO vehicle collision provisions were not adequate because the peak dynamic force (PDF) is always more than the AASTHO collision provisions, and the ESF exceeds the AASHTO provisions between impact speeds of 75 and 100 km/h.
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8 Figure 5. Force velocity relationship for single-unit trucks (SUTs)
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9 work suggests that even pickup trucks might pose a risk of bridge column collapse if the pickup truck is traveling very fast (i.e., 85 mph or more) and strikes a circular column less than about 40 in.
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10 operating conditions, technology, and so forth. NCHRP Project 22-14(2)
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11 exceptional events and provide anecdotal information that is useful in developing an improved understanding of the factors that influence catastrophic failure and the development of the guidelines. Cases from a variety of sources will be examined in the following sections.
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12 2.5.2 Crashes Investigated by the National Transportation Safety Board The NTSB investigates and determines the probable cause of significant crashes on highways and in other modes of transportation, with the goal of promoting transportation safety and preventing future similar crashes. In total, the NTSB investigates approximately six highway crashes per year, with each investigation lasting approximately 20 months.
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13 analytical procedures substantiated by such test for each design to increase the compatibility of barriers with both light and heavy vehicles. The standard should also contain requirements regarding the placement of the barriers in the field to assure that compatibility of the vehicle/barrier is not compromised by adjacent environment [NTSB 1973]
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14 Year/ Location Events Injuries Image Ref/ Source 2003/I-275 north ramp bridge at I-40 east, Knoxville, Tennessee A tractor trailer overturned on the I-275 north ramp toward I-40 east. The vehicle fell to the roadway below, striking a ramp support.
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15 Year/ Location Events Injuries Image Ref/ Source 2007/ Chatfield Road over I-35, Navarro County, Texas A tractor trailer carrying home construction materials struck the northernmost 30-in.-diameter column of the two-column pier after the driver fell asleep and drifted into the median. The bridge did not collapse; however, the collision caused severe cracking of the column.
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16 Year/ Location Events Injuries Image Ref/ Source 2008/ Milepost 519 bridge over I-20, Canton, Texas An unloaded tractor trailer struck the westernmost column of the two-column pier located in the shoulder of I-20 eastbound. The column failed; however, the bridge did not.
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17 2.5.4 Summary These crashes illustrate that heavy vehicles have more than sufficient energy to cause considerable damage to bridge piers that can result in the structural failure of pier components and, in some cases, the catastrophic collapse of a bridge. Protecting the bridge structures from heavy-vehicle, high-energy crashes is essential for maintaining the structural integrity of a bridge, especially for bridges with nonredundant pier systems or noncontinuous superstructures.
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18 insufficient barrier shielding, where the length of need was too short or a TL-3 barrier was not adequate for a truck impact. Details of barrier selection, length of need, and barrier placement are important considerations when designing a bridge pier protection strategy that will not only protect the bridge but protect the motoring public.
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19 Appendix C: Survey of Practice, respondents indicated that "engineering judgement" was the method most often used for accounting for site and traffic conditions. The respondents indicated that the most important characteristics were (in priority order)
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20 bridge railings to be used to estimate the probability of barrier penetration. NCHRP Project 22-12(03)
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21 both societal benefits and agency costs such that the benefits are maximized while making the best possible use of agency funds. The disadvantage is that, since costs are explicitly included, regional and temporal variations in the cost elements can make the same solution cost beneficial in one region and not cost beneficial in another.
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