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Publication Number: N/A
Date: June 2013

 

Federal Highway Administration (FHWA) - Long-Term Bridge Performance (LTBP) Program

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Announcement - Selection of Bridges for Mid-Atlantic Prestressed Concrete and Steel Multi-Girder Bridge Clusters


Selection Procedure for Reference and Cluster Bridges

 

 

For questions or comments contact:
Robert Zobel (202) 493-3024 or email at
robert.zobel@dot.gov

 


Bridge Selection Approach - General Steps

  1. Filter all bridges in the selected region using high level criteria

  2. Obtain State prioritization for remaining population

  3. Sort all remaining bridges into “Design of Experiments” subpopulations based on span length, age and Average Daily truck Traffic (ADTT)

  4. Compute the normalized distance measure for each bridge, which defines its experimental “power”

  5. Rank the bridges within each subpopulation based on the distance measure

  6. Select bridges from each subpopulation based on the distance measure and a set of supplemental criteria

  7. Examine the distribution of secondary variables

  8. Iterate until a balance between distance measure, supplemental criteria, and subpopulation variability is achieved

 


Bridge Selection Process

Step 1 Apply High-Level Filters

All candidate bridges adhere to the following criteria:

These criteria were set to ensure that sampled bridges are similar to those likely to be constructed in the future, and to avoid excessive costs or access constraints.

 


Bridge Selection Process

Step 2 Obtain State Prioritization for Filtered Bridge Population

Bridges, which passed the high-level filtering, were submitted to the States within the region for informed prioritization according to the following categories:

 

Low - Data collection for this bridge is not feasible

Medium - Data collection for this bridge is feasible

High - Data collection for this bridge is a high priority*

 

*High priority indicates that the State expressed an interest in including the particular bridge in LTBP Cluster

 


Bridge Selection Process

Step 3 Sort Filtered bridges into “Design of Experiment” Subpopulations”

 

Subpopulation

Span Length

ADTT

Bridge Age

A

+ + +

B

+ +

C

+

D

+ +

E

+ +

F

+

G

+

H

 


Bridge Selection Process

Step 4 Compute the normalized distance measure for all remaining bridges

For each bridge, a distance measure was calculated according to the formula:

The distance measure for a single bridge in a population of bridges is calculated as the square root of the sum of the squares of (1) the difference between bridge age and the median bridge age for the population divided by the median bridge age, (2) the difference between bridge average daily truck traffic and the median average daily truck traffic for the population divided by the median average daily truck traffic, and (3) the difference between bridge mean span length and the median mean span length for the population divided by the mean span length.

 


Bridge Selection Process

Step 5 Rank the bridges within each subpopulation based on the distance measure

 


Bridge Selection Process

Step 6a Select cluster bridges based on distance measure and supplemental criteria

Identification of a set of cluster bridges based on iteration of the following procedure:

  1. Maximize the average distance measure of the set

  2. Maximize the distribution of subpopulations (groups of span length, age, ADTT)

  3. Maximize the distribution of State participation

  4. Maximize the inclusion of “priority” bridges based on feedback from the States

 


Bridge Selection Process

Step 6b Select reference bridges based on distance measure and supplemental criteria

Identification of a pair of reference bridges based on the following procedure:

  1. Minimize the average distance measure of the pair

  2. Select candidates from different subpopulations

  3. Select candidates from different States

  4. Minimize access constraints (i.e., low ADT, sizeable shoulders, etc.)

 


Bridge Selection Process

Step 7 Examine the distribution of secondary variables

This pie charge shows an example distribution of one secondary variable, reinforcement type, for a chosen bridge cluster.  The intent is to make sure that the distribution of secondary variables within a cluster is adequate for experimental purposes.

 


Bridge Selection Process

Step 8 Iterate through process until a balanced set of bridges is achieved and independently verify the selection

All reference and cluster bridges were remotely verified for testing suitability using the LTBP Bridge Portal, including:

All bridges will be verified via site visits prior to final approval

 


Bridge Selection Process

Resulting Mid Atlantic Steel and Prestressed Concrete Clusters – Bare Deck Subpopulation

Click on map for 508 compliance

Red Pin – Steel Reference Blue Pin – Prestressed Concrete Reference
Red Marker – Steel Cluster Blue Marker – Prestressed Concrete Cluster

 

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