Network Wide Assessment of Bridge Barrier Safety

Transcription

1 Network Wide Assessment of Bridge Barrier Safety Andrew Sonnenberg National Bridge Engineering Manager, pitt&sherry Abstract: The need for higher performance bridge barriers has been reflected in changes in barrier design over the last 100 years. These changes have lead to improvements in safety. There exists however many aging bridges with barriers that do not meet current standards. In order to manage the risk of these substandard barriers Asset Owners require tools to rate the risk and to prioritise the upgrade of barriers. pitt&sherry has developed a method for rating the risk of barriers across a network in a manner that allows asset owners to allocate their limited funding where it is most needed. This method has been successfully applied by pitt&sherry to the prioritisation of bridge barrier upgrade for a number of Councils as well as a State Road Authority. 1. Background Bridge barriers in the early 1900 s were designed for pedestrians and not containment of vehicles. It was typical practice to use timber posts with two rails as shown in Figure 1. These barriers provided little protection for road traffic and pedestrians such as children due to their low strength and the large gaps between the rails. Figure 1. Timber barrier pitt&sherry ref: Network wide assessment of bridge barrier safety Andrew Sonnenberg.docx/AS/bw 1

2 Modern barriers are designed for both pedestrians and vehicles. As well as being many times stronger than old timber barriers, modern barriers have added safety features such as climb prevention, appropriately shaped rails and offsets for vehicle safety. A typical modern barrier is shown in Figure 2. Figure 2. Modern Barrier design AS5100 was released in 2004 providing guidelines for the design of bridge barriers. The Code outlines both a site specific risk based assessment and chart based assessment for bridge barrier requirements. The procedures provided by the code are based on AASHTO guidelines with some modification to reflect local conditions. The site specific risk based assessment process provided by the Clause 10 of the code provides minimum barrier performance levels for bridges largely independent of traffic volumes. The clause is intended to apply at medium to high risk sites. It includes factors not covered by the chart based risk assessment method of Appendix B of the Code. These factors include the risk related to railways below, the effect of flood waters, width between kerbs and water depth. The risk based method also considers the type of vehicle that may collide with a bridge barrier with a specific mention of buses. The chart based method accounts for factors such as, traffic volume, percentage of heavy vehicles, rail offset, road type, speed, road grade, curvature, deck height and under structure conditions. The effect of these parameters on risk is reflected by design charts providing a resulting adjusted AADT (annual average daily traffic volume for all lanes) as shown in Figure 3. The ASHTO guidelines reflect the incremental benefit cost ratio between barriers of various performance levels from Low, Regular to Medium containment level. pitt&sherry ref: Network wide assessment of bridge barrier safety Andrew Sonnenberg.docx/AS/bw 2

3 Figure 3. Chart Based barrier assessment 2. Barrier assessment Methodology The first step of assessing an existing network of barriers is to collect information on the barriers. For existing structures a site inspection is undertaken. Information recorded includes: Road grade Curvature Deck height Under structure conditions Rail offset Threshold limits Barrier geometry (height and member sizes) Information from site is then compared with available design drawings. Often differences will be observed between design drawings and actual site conditions, such as rail offsets or land use underneath a structure. The reason for this is land use is changing over time along with lane widths being squeezed onto bridges to allow greater throughput of traffic. Using the geometric properties and design drawings where available an assessment of bridge barrier performance is made and the barrier is classified as: Less than low performance level Low performance level Regular performance level Medium performance level Special/High performance level Traffic survey results are used to understand the current percent heavy vehicle count and AADT. pitt&sherry ref: Network wide assessment of bridge barrier safety Andrew Sonnenberg.docx/AS/bw 3

4 The performance level required to comply with current code requirements is determined in accordance with AS5100. The required performance level is based on the methodology outlined in Appendix B and Clause 10.5 of AS Specific benefit/cost analyses are not undertaken as part of the process but could be considered. For each site, the adjusted AADT is determined using Appendix B of AS The process utilises the chart based level selection including barrier offset, operational speed, road type, gradient and curvature for each site. The bridge with the largest adjusted AADT over that of its current performance level (accounting for % heavy vehicles and speed) is deemed to be of the highest risk. Where two sites have the same adjusted AADT in excess of current performance level then the site which has the lowest current performance level barrier has the higher priority for upgrading. Sites deemed as not requiring barriers are excluded from the assessment. 3. Network wide Modelling To automate the process of barrier assessment pitt&sherry has implemented the methodology into an asset management system AssetAsyst. AssetAsyst has both custom asset modules and tailored modules for asset management. The network wide assessment of barriers is undertaken by the Bridge Module of the software, in conjunction with information sourced from the Roads module. Each road bridge or culvert in AssetAsyst is linked to a road and its segments. The linkage is undertaken by confirming the chainage of the structure along the road. The software then determines the applicable segment and extracts the traffic volume, speed and percent heavy vehicle information stored against the road asset. Against each site the performance level of the barriers can be stored and the offset of the lane from the barriers, road curvature and land use under information. The requirement for upgrade is determined by running a bridge barrier report under the reporting risk menu. A sample report for a selection of bridges previously analysed is provided in Table 1. The example indicates that the highest priority for barrier upgrade is barrier no. 1 on structure BR2345 as it has the highest adjusted AADT in excess of current performance level requirements. Barrier 1 at BR1234 is the next most important followed by Barrier no. 2 at BR2345. The results highlight the complexity of prioritising barrier upgrades due to the large number of factors to be considered and demonstrates the usefulness of the methodology. The methodology allows for a consistent outcome which is useful when considering large networks that need to be managed over many years. Table 1. Sample bridge barrier assessment data. Asset No. Barrier No. Current barrier AADT Truck % R T G D C U Water depth Land occupancy Deck height US Adjust. AADT Speed (km/hr) Rail offset Desired barrier Adjusted AADT in excess of current performance level BR BR low High Medium low Medium Regular BR low High Medium BR low Low- Medium Medium pitt&sherry ref: Network wide assessment of bridge barrier safety Andrew Sonnenberg.docx/AS/bw 4

5 4. Conclusions Barrier performance level requirements prescribed by design codes have changed overtime. It is important that Asset Owners have systems in place to appropriately manage the risk of substandard barriers and plan for their upgrade considering the cost/benefit of each site. Logically the sites with the highest cost/benefit should be addressed first. A methodology has been developed to allow the network wide assessment of bridge barrier performance levels. By implementing the methodology into an Asset Management system a task which normally takes considerable effort can now be repeated with ease allowing assessment to be made regularly when factors such as standards and site conditions change. 5. Acknowledgement I would like to acknowledge the local government organizations and one of the state road authorities that have provided the opportunity to apply the methodology to the management of their bridge barrier assets. 6. References 1. Standards Australia, AS5100 Bridge design Part 1: Scope and general principles, Committee BD 090, April pitt&sherry ref: Network wide assessment of bridge barrier safety Andrew Sonnenberg.docx/AS/bw 5