Revisiting Reliability for Rural Bridges

Caroline Bowers Author

07/28/2020 Added

37 Plays

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This project compares bridge management strategies using multi-attribute utility theory to select a cost effective and sustainable method for improving the reliability of rural bridges.

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[00:00:00.696]Hello, my name is Caroline Bowers.
[00:00:02.611]I am a Civil Engineering student working with Dr. Steelman.
[00:00:05.946]My project focuses on Revisiting Reliability for Rural Bridges.
[00:00:10.542]The current state of bridges in America is sub-optimal.
[00:00:14.148]The 2017 Infrastructure Report Card rated bridges at a C+.
[00:00:19.498]According to the report, 9.1% of United States bridges are structurally
[00:00:24.613]deficient meaning they require
[00:00:26.480]significant maintenance, rehabilitation, or replacement.
[00:00:31.046]19.2% of Nebraska bridges have load restrictions or postings.
[00:00:35.846]An example of a posting can be seen in Figure 1.
[00:00:38.778]A problem with bridge postings is that
[00:00:41.145]they can be out of date or an approximation.
[00:00:43.748]This leads to an under utilization of transportation assets
[00:00:47.510]meaning the bridge is posted when it doesn’t need to be.
[00:00:50.827]A load test or long term monitoring could reveal a higher reliability
[00:00:55.009]and replacing or strengthening a bridge could allow heavier vehicles to cross.
[00:00:59.409]Structurally deficient and load restricted bridges lead to
[00:01:03.742]rerouted vehicles to ensure nominally adequate safety.
[00:01:07.175]By removing load postings,
[00:01:08.958]detours can be eliminated and a bridge's reliability can be increased.
[00:01:12.674]This has significant economic, social, and environmental implications.
[00:01:17.691]Rerouted vehicles lead to increased fuel consumption,
[00:01:21.176]travel time and distance, hours logged,
[00:01:23.128]and wear on vehicles and the transportation infrastructure.
[00:01:26.605]Bridge management strategies aim to optimize the life-cycle of bridges.
[00:01:30.872]Primary examples of bridge management strategies include
[00:01:34.221]doing nothing, replacing, and load testing.
[00:01:37.304]Other alternatives are retrofitting and monitoring.
[00:01:40.703]Most bridge management strategies focus solely on the
[00:01:43.920]economic aspect of a project by looking to minimize the cost
[00:01:47.636]using life-cycle cost analysis.
[00:01:50.269]Triple Bottom Line sustainability addresses the
[00:01:53.235]economic, social, and environmental aspects of a project.
[00:01:56.585]Project decision makers following Triple Bottom Line sustainability
[00:02:01.384]seek to choose a cost effective bridge management strategy
[00:02:04.734]which not only results in high performance, but also mitigates
[00:02:08.483]negative environmental and social impacts.
[00:02:11.633]Multi-attribute utility theory is a useful tool in this process.
[00:02:14.956]It evaluates the economic, environmental, and social impacts of a project
[00:02:19.899]while maximizing holistic performance and minimizing total costs
[00:02:24.215]over multiple dimensions of sustainability.
[00:02:27.499]It incorporates risk and utility.
[00:02:29.847]Risk addresses the probability and consequences of bridge failure.
[00:02:33.880]Utility measures how much a particular aspect of a project means to
[00:02:38.346]the decision maker. The risk-attributes associated with sustainability
[00:02:43.493]can be seen in Figure 3.
[00:02:45.212]The objective of this project is to compare bridge management strategies
[00:02:49.412]as they relate to Triple Bottom Line sustainability
[00:02:52.562]using multi-attribute utility theory.
[00:02:56.560]My procedure was based on a previous study.
[00:02:59.242]Using the National Bridge Inventory and Nebraska Department of Transportation,
[00:03:03.177]I gathered information on two rural Nebraska bridges
[00:03:06.076]and the local transportation network including bridge length and width,
[00:03:09.925]detour length and duration, average daily traffic, probability of failure,
[00:03:14.758]carbon dioxide emissions, and energy consumption.
[00:03:18.175]This data was inputted in various equations to calculate risk and utility
[00:03:22.024]to compare bridge management strategies.
[00:03:24.690]What I was looking for was a strategy with both high cost utility
[00:03:28.506]and sustainability utility.
[00:03:30.973]This translates to a bridge with low cost and limited negative
[00:03:34.722]social and environmental impacts.
[00:03:38.507]These are the equations I used to compare bridge management strategies.
[00:03:42.405]They model utility. The utility values range from 0 – 1.
[00:03:47.437]0 represents the least utility, while 1 represents the greatest utility.
[00:03:53.320]This can be seen in Figure 4.
[00:03:56.036]The inputs into the first two equations include the risk attitude of the
[00:04:00.670]decision maker (gamma), so whether they are risk-averse or risk-accepting,
[00:04:05.668]the risk (which is calculated using other equations), and the
[00:04:09.451]minimum and maximum risk values allowed by the decision maker.
[00:04:13.417]The third equation combines the utility of the risk-attributes.
[00:04:17.303]K is the weighting factor. It is the relative importance of the risk to
[00:04:21.133]the decision maker. It is anywhere from 0-1, but
[00:04:24.549]the 3 values must sum to 1.
[00:04:26.866]The graph of the cost utility is shown here.
[00:04:29.382]The utility of a risk averse decision maker is greater than
[00:04:32.965]a risk accepting decision maker.
[00:04:35.614]As mentioned earlier, I studied 2 rural Nebraska bridges.
[00:04:40.117]I primarily looked at a risk averse decision maker as it is the most common.
[00:04:44.830]The results for each bridge can be seen here.
[00:04:47.646]There are several things to point out in these graphs.
[00:04:50.396]The do nothing bridge management strategy has a much lower sustainability utility.
[00:04:55.329]This is due to the assumption of a permanent detour caused by a load posting.
[00:05:00.611]It was assumed that the other strategies would result in the removal
[00:05:04.361]of the load posting and therefore a temporary detour was used.
[00:05:08.127]The monitoring option is more uncertain than other options.
[00:05:12.510]Both the time required to perform the monitoring and the associated
[00:05:16.260]cost are not well-established.
[00:05:18.226]In general, monitoring has a higher probability of removing load postings
[00:05:23.841]than load testing.
[00:05:26.458]For the cost utility, is was assumed that the maximum cost allowed was the cost
[00:05:31.640]to replace the bridge.
[00:05:33.701]Therefore, the replacing strategy has a cost utility of 0 and the do nothing
[00:05:38.173]strategy has a cost utility of 1.
[00:05:40.789]The other strategies are somewhere in between.
[00:05:43.356]Another thing to point out is the variation in the sustainability utility
[00:05:48.189]ranges from Bridge 1 to Bridge 2.
[00:05:50.821]Bridge 2 has a greater sustainability range.
[00:05:53.506]This is most likely due to the greater average daily traffic causing
[00:05:57.071]a much lower sustainability utility.
[00:06:00.005]Based on the assumptions I made regarding the removal of the load postings,
[00:06:04.737]the best strategy is a load test.
[00:06:07.436]It had both a high cost utility and sustainability utility.
[00:06:11.602]I also investigated the effects of different variables.
[00:06:15.035]One by one, I increased detour length, average daily truck traffic,
[00:06:19.135]average daily traffic, and probability of failure.
[00:06:22.551]In all cases, increasing the variable led to a decrease in utility,
[00:06:26.749]with varying levels of sensitivity.
[00:06:29.150]Finally, I looked at the return on investment of a load test.
[00:06:32.566]I calculated how many days it would take for the cost of a detour to exceed the
[00:06:37.765]cost of a load test.
[00:06:39.032]For both bridge 1 and Bridge 2, it would take less than a year for a load test
[00:06:43.065]to be worth the cost.
[00:06:45.291]Further research could be conducted incorporating the fatalities and
[00:06:49.281]costs incurred by car accidents associated with a detour.
[00:06:53.110]Also, additional data on the retrofitting costs and the duration and cost of
[00:06:57.979]monitoring could aid further study.
[00:07:00.029]So to conclude, I investigated different bridge management strategies
[00:07:05.243]aiming to find a strategy which minimized cost and limited
[00:07:09.085]negative social and environmental impacts.
[00:07:11.695]This strategy then could be used to increase the reliability of rural bridges.
[00:07:16.661]Using data collected from the National Bridge Inventory and
[00:07:20.627]Nebraska Department of Transportation, I calculated the risk and utility for
[00:07:24.692]five bridge management strategies.
[00:07:26.659]It was found that a load test can be a cost effective and sustainable way to
[00:07:31.325]increase the reliability of bridges by leading to a removal of a load posting.
[00:07:35.958]Thank you.

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