MPC
Research Projects (2009-10)

Identifying Number

MPC-326

Project Title

Rapid Load Rating of Short Rural Bridges

University

Colorado State University

Project Investigators

John van de Lindt

Description of Project Abstract

Short rural bridges are defined as bridges less that 20 feet long and are not typically kept on state bridge inventories. These types of short bridges often span a culvert and are either one or two lanes because they carry very low volume routes. This is a problem that is region specific since the Mountain Plains has thousands of these bridges combined with very harsh winters and chemical deicers which can contribute to rapid deterioration of bridge beams and decks. Although the chemical deicers may not be applied directly to the rural routes, they are transported by vehicles returning from trunkline and larger roadways. Although these rural bridges carry a low volume of traffic and are in rural regions their importance to the communities that they provide service to is high. Because there are limited resources for counties who typically have oversight for these, it is proposed to develop a rapid, concise procedure for load rating these bridges. The approach will use as input the results of a visual inspection with moderate measurements of deteriorated locations taken. Then, based on a series of design charts and multi-dimensional interpolation rules (which will be checked during the development process proposed here) a reduction factor for moment and shear capacity can be determined almost immediately. If an original load rating is not known, then a simple flow diagram provides the load assessment which can be combined with the reduction factor to determine the rating. J. van de Lindt has extensive experience with bridge load factor design calibration for the state of Michigan and will apply those principles to this project for the Mountain Plains region. Further, a student will work on the project and have an opportunity to coordinate with the Larimer County Engineering Department in Colorado. Therefore it is proposed in the last task (see below) to use Larimer County, Colorado as a test bed for the approach.

Project Objectives

The primary objective of this work is to develop a simplified approach for load rating low-volume short-span rural bridges without the need for extensive engineering calculations.

Project Approach/Methods

The approach taken in this work can be separated into the following four tasks.

Task 1: Categorization and Method
This task will consist of the literature review which will include the study by Gutkowski and colleagues on inspection of short bridges as well as any relevant NCHRP reports, TRB papers, journal and conference papers, and other transportation center reports. Then the task focus will turn to categorizing the inventory of bridge types typical of rural areas in the Mountain Plains states. This will include contacting between 6 and 10 counties and procurement of their databases. It is anticipated that most or all will be in the state of Colorado. However, the approach developed herein will be applicable to any rural area particularly within the Mountain Plains region.

Task 2: Finite Element Models Combined with Existing Program
Perhaps one of the most important contributions of applied research is performing the complex analyses so others don't have to do it. This task focuses on conducting a series of analyses with various levels of damage to short-span bridge members in order to compute reduction factors based on simple measurements taken during visual inspection. These will be combined with an influence line program that was developed for CDOT in 2007 to develop a "database" of bridge load ratings based on standard structural analysis techniques.

Task 3: Development of Design Charts
Design charts will be developed that enable the user to assign a bridge load rating based on visual inspection and several measurements if deterioration is present in the girders or bridge beams. These will be a function of bridge type, beam type, span, girder spacing, etc. Reasonable ranges for each of these will be developed based on the short-span rural bridge inventory of the 6 to 10 counties discussed in task 1 above. The design charts will enable the user to interpolate with a slight level of engineering conservatism built into the values. Further, within this task the interpolation scheme will be developed and verified based on the same numerical approach as outlined in Task 2.

Task 4: Case Study and Verification
The final research task will be to work with Larimer County in Colorado to test the method on several rural bridges that they have completed full load rating analysis on in the last year. We will apply our methodology "blindly" for a true comparison to see how accurately the rapid assessment and rating method predicts what they calculated using their more extensive assessment.

MPC Critical Issues Addressed by Research

This work addresses MPC focus areas:

Focus Area #1: Low-Cost Safety Improvements - Safety of rural bridges, i.e. their condition, is important to maintaining the rural routes they carry. By making inspections and load rating faster (1) they can be inspected more routinely, and (2) those that are not inspected can be with minimal expense.

Focus Area #16: Infrastructure Longevity - bridges, even short rural bridges, must be maintained. The only way to provide routine maintenance is to inspect regularly. However, because county budgets are usually stretched to the breaking point, not all bridges are able to be inspected on a regular basis. Development of a rapid method for inspection and particularly load rating will enable more routine inspection and subsequently the needed maintenance.

Contributions/Potential Applications of Research

The results of this research can be applied at the county level for inspectors and engineers. The project results and program will be made available on a project website. Assistance trying the method out will be provided to counties as needed. In addition, a mailing immediately following the project will be made to the 6 to 10 counties who made their data available for the study to let them know of the report and charts.

Technology Transfer Activities

The results of this work will be made immediately available to the rural counties within the MPC via the project web page which will be created and linked to the MPC web page.

Time Duration

July 1, 2009 through June 30, 2010

Total Project Cost

$68,602.00

MPC Funds Requested

$31,224.00

TRB Keywords

Load rating; condition assessment; rural bridges