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Investigation of Steel-stringer Bridges: Superstructures and Substructures, Volume I (Civil Project)

ABSTRACT:

There are hundreds of structurally deficient or functionally obsolete bridges in the state of Iowa. With the majority of these bridges located on rural county roads where there is limited funding available to replace the bridges, diagnostic load testing can be utilized to determine the actual load carrying capacity of the bridge.

One particular family or fleet of bridges that has been determined to be desirable for load testing consists of  single-span bridges with non-composite, cast-in-place concrete decks, steel stringers, and timber substructures. Six bridges with poor performing super structure and substructure from the a fore mentioned family of bridges were selected to be load tested.

The six bridges were located on rural roads in five different counties in Iowa: Boone, Carroll, Humboldt, Mahaska, and Marshall. Volume I of this report presents diagnostic load tests on the super structure of the six bridges. The results of the diagnostic load tests were used to calibrate analytical models of the bridges for rating purposes.

All of the bridges were independently rated by three rating agencies using a codified approach. Those ratings were then compared to ratings calculated using a bridge model calibrated to the actual response of the bridge due to the load test.

The calibrated bridge model was then used to rate the bridges and determine whether an increase in the initial codified bridge ratings was feasible.Volume I of this report focuses on evaluating the super structure for this family of bridges. This volume discusses the behavior characteristics that influence the load carrying capacity of this fleet of bridges.

In particular, the live load distribution, partial composite action, and bearing restraint were investigated as potential factors that could influence the bridge ratings. Implementing fleet management practices, the bridges were analyzed to determine if the load test results could be predicted to better analyze previously untested bridges. For this family of bridges it was found that the ratings increased as a result of the load testing demonstrating a greater capacity than determined analytically.

Volume II of this report focuses on evaluating he timber substructure for this family of bridges. In this volume, procedures for etecting pile internal decay using nondestructive ultrasonic stress wave techniques, correlating nondestructive ultrasonic stress wave techniques to axial compression tests to estimate deteriorated pile residual strength, and evaluating load distribution through poor performing timber substructure elements by instrumenting and load testing the abutments of the six selected bridges are discussed. Also, in this volume pile repair methods for restoring axial and bending capacities of pile are developed and evaluated.

LITERATURE REVIEW

Bridge Rating and Posting:

Before any load testing is undertaken, a visual inspection of the bridge must first be completed.  Any noticeable deterioration or damage should be documented.  Critical bridge components and locations of critical areas on these components should also be determined during the bridge inspection. A bridge obviously should not be tested if catastrophic failure due to loading, such as the yielding of steel girders or failure of a critical member is of any concern.

Load Testing:

Load Testing in the United States and Abroad

In 1999, Schiff and Philbrick conducted a review of current experimental technologies and practices and found that there were several states that conducted load testing on bridges for load capacity calculations including: Texas, Connecticut, New York, Michigan, North Carolina, and Alabama (Schiff, 1999).

Benefits of Load Testing:

The rating factor can be quite conservative when the capacity of the bridge is determined theoretically.  The capacity of the bridge determined using distribution factors, assumed material properties, simply supported conditions, non-composite action, and zero additional stiffness from curbs and railings can often be conservative as these factors could increase the capacity of the bridge.  The capacity of a bridge is often, but not always, determined to be much larger through load testing than can be determined theoretically.

Load Rating Using Load Test Results:

Extrapolation

The test vehicle is rarely the same as the rating vehicles. In most cases, the test vehicle does not weigh as much as the rating vehicles so determining the bridge rating
from the test vehicle cannot be performed directly.

Load Testing Examples:

A number of states have utilized load testing to determine the capacity and ratings of various bridge types.  The summaries from four different reports on bridge testing from the late 1990’s are provided as examples of what has been done in the past in regards to bridge load testing.

BOONE COUNTY BRIDGE (BCB)

Bridge Description:

One of the bridges from the aforementioned family of bridges load tested is located in Boone County, IA. The bridge ( FHWA ID: 77110), henceforth referred to as the BCB, is located on G Avenue approximately 8.5 miles south of Ogden, IA and one mile west of USH 169. Shown in Figure 3.1 is an alignment view of the BCB which was built in 1900 as a 36-foot simple-span, non-composite bridge with six steel girders, a concrete deck, and no skew crossing Little Beaver Creek. The substructure consists of seven timber piles with a double C-channel cap and a timber back wall.

Figure 3.1. BCB Alignment View Looking South.

Figure 3.1. BCB Alignment View Looking South.

Test Setup:

Test Truck

There were three incremental loads, referred to as: an empty truck, a half full truck, and a full truck, used to test the bridge. The incremental loads refer to the amount of material, in this case gravel, the truck was carrying with the full truck increment being close to the maximum amount the truck could legally carry. The truck used for the load test was provided by the county and was a standard maintenance tandem dump truck.

BDI Optimization:

The bridge was modeled using software (WinGEN) provided by Bridge Diagnostics Inc. that utilizes the actual test data to create a model that is close to the actual bridge based on the response of the structure to the truck loadings.

This bridge model modeled each girder separately allowing the moment of inertia for each girder to be optimized separately. This was important due to the partial composite action differences in each of the girders.

Bridge Rating:

  • Conventional Rating
  • Rating Using Optimized Parameters From BDI Software

MARSHALL COUNTY BRIDGE (MCB)

Bridge Description:

The second bridge (FHWA ID: 243470) that was tested is located in Marshall County, IA on Summit Road approximately 3 miles northwest of Marshalltown, IA. The bridge, henceforth referred to as the MCB, is a 40-foot simple-span, non-composite bridge with six steel girders, a concrete deck with a five-inch thick asphalt overlay, and no skew crossing a creek.

Test Setup:

  • Test Truck
  • Testing Plan and Instrumentation

Bridge Analysis:

  • Neutral Axis and Partial Composite Action
  • Load Distribution
  • Moment of Inertia

BDI Optimization:

The bridge was once again modeled using software (WinGEN) provided by Bridge Diagnostics Inc. that utilizes the actual test data to create a model that is close to the actual bridge based on the response of the structure to the truck loadings.  This bridge model consisted of modeling each girder separately so that the moment of inertia for each girder could be optimized.

Bridge Rating:

  • Conventional Rating
  • Rating Using Optimized Parameters From BDI Software

MAHASKA (350) COUNTY BRIDGE (KCB1)

Bridge Description:

The third bridge that was tested is located in Mahaska County, IA on Rutledge Avenue approximately 5 miles northeast of Oskaloosa, IA. The bridge (FHWA ID: 237350),henceforth referred to as the KCB1, is a 33.3-foot simple-span, non-composite bridge with five steel girders, a concrete deck, and no skew crossing a creek. The substructure consists of five timber piles with a double C-channel cap and a timber back wall.

Test Setup:

Test Truck

There were three incremental loads, referred to as: an empty truck, a half full truck, and a full truck, used to test the bridge. The truck used for the load test was provided by the county and was a standard maintenance tandem dump truck.

KCB1 Test Truck.

KCB1 Test Truck.

Testing Plan and Instrumentation

There were three lanes selected for the truck to follow as it crossed the bridge.  Each lane was loaded twice for each load level to ensure repeatability of the test results.  Measurements (strains and deflections) were taken when the centroid of the tandem was at the centerline of each end bearing and at each one-quarter point.

Bridge Analysis:

  • Neutral Axis and Partial Composite Action
  • Load Distribution
  • Moment of Inertia

BDI Optimization:

The bridge was once again modeled using software (WinGEN) provided by Bridge Diagnostics Inc. that utilizes the actual test data to create a model that is close to the actual bridge based on the response of the structure to the truck loadings.  This bridge model consisted of modeling each girder separately so that the moments of inertia for each girder could be optimized.

Bridge Rating:

  • Conventional Rating
  • Rating Using Optimized Parameters From BDI Software

CARROLL COUNTY BRIDGE (CCB)

Bridge Description:

The fourth bridge that was load tested is located in Carroll County, IA on 245 th Street just south of the city limits of Halbur, IA. The bridge (FHWA ID: 94680), henceforth referred to as the CCB, is a 33.3-foot simple-span, non-composite bridge with four steel girders, a concrete deck, and no skew crossing a creek. The substructure consists of six timber piles with a double C-channel cap and a timber back wall.

Test Setup:

  • Test Truck
  • Testing Plan and Instrumentation

Bridge Analysis:

  • Neutral Axis and Partial Composite Action
  • Load Distribution
  • Moment of Inertia

BDI Optimization:

The bridge was once again modeled using software (WinGEN) provided by Bridge Diagnostics Inc. and the actual test data were used to create a model that is close to the actual bridge based on the response of the structure to the truck loadings.  This bridge model consisted of modeling each girder separately so that the moment of inertia for each girder could be optimized.

Bridge Rating:

  • Conventional Rating
  • Rating Using Optimized Parameters From BDI Software

MAHASKA (380) COUNTY BRIDGE (KCB2)

Bridge Description:

The fifth bridge that was load tested, is located in Mahaska County, IA on Osborn Avenue approximately 3 miles northeast of Oskaloosa, IA. The bridge ( FHWA ID: 237380), henceforth referred to as KCB2, is a 37.67-foot, simple-span, non-composite bridge with five steel girders, a concrete deck, and no skew crossing a creek. The substructure consists of five timber piles, a double C-channel cap, and a timber back wall.

Test Setup:

Test Truck

Two incremental loads, referred to as a half full truck and a full truck, were selected for the bridge test. The truck used for the load test was provided by the county and was a standard maintenance tandem dump truck.

Testing Plan and Instrumentation

There were five lanes, selected for the truck to follow as it crossed the bridge. Each lane was loaded twice for each load increment to check repeatability of the test results. Measurements (strains and deflections) were taken when the centroid of the andem was at the centerline of each end bearing and at each quarter point.

Bridge Analysis:

  • Neutral Axis and Partial Composite Action
  • Load Distribution
  • Moment of Inertia

BDI Optimization:

The bridge was once again modeled using software (WinGEN) provided by Bridge Diagnostics Inc. that utilizes the actual test data to create a model that is close to the actual bridge based on the response of the structure due to the truck loadings.  This bridge model consisted of modeling each girder separately so that the moments of inertia for each girder could be optimized.

Bridge Rating:

Conventional Rating

The bridge was rated using the Load Factor Rating (LFR) approach. This analytical rating, in which both the interior and exterior girders were rated, was performed assuming a non composite design with simply supported conditions.

Rating Using Optimized Parameters From BDI Software

Utilizing the strains measured during the load test, the BDI software (WinGEN) was once again utilized to determine the bridge rating using the optimized parameters. Using the modified bridge model, the bridge was rated using the same rating vehicles as the analytical ratings. The rating vehicles were input into the WinGEN software and traversed across the bridge in pre-selected lanes to produce maximum strains in the girders.

HUMBOLDT COUNTY BRIDGE (HCB)

Bridge Description:

The last bridge that was load tested, is located in Humboldt County, IA on 200th Street approximately two miles north of the Humboldt, IA.  The bridge (FHWA ID: 029070), henceforth referred to as HCB, is a 34.4-foot simple-span, non-composite bridge with four steel girders, a concrete deck, and no skew crossing a drainage channel.

Test Setup:

Test Truck

Two incremental loads, referred to as a half full truck and a full truck, were selected for the bridge test. The truck used for the load test was provided by the county and was a standard maintenance tandem dump truck.

Testing Plan and Instrumentation

There were five lanes, selected for the truck to follow as it crossed the bridge. Each lane was loaded twice for each load increment to check the repeatability of the test results.  Measurements (strains and deflections) were taken when the centroid of the tandem was at the centerline of each end bearing and at each quarter point.

Bridge Analysis:

  • Neutral Axis and Partial Composite Action
  • Load Distribution
  • Moment of Inertia

BDI Optimization:

The bridge was once again modeled using software (WinGEN) provided by Bridge Diagnostics Inc. and the actual test data was used to create a model that is close to the actual bridge based on the response of the structure due to the truck loadings.

This bridge model consisted of modeling each girder separately so that the moment of inertia for each girder could be optimized. This was important due to the partial composite action differences in each of the girders.

Bridge Rating:

  • Conventional Rating
  • Rating Using Optimized Parameters From BDI Software

Superstructure Response to Destructive Substructure Testing:

With the cooperation of Humboldt County and the contractor hired to replace the bridge, ISU was provided the opportunity to perform some destructive testing on some substructure elements to determine the load distribution in the pile elements due to loading.

SUMMARY OF LOAD TESTING RESULTS

Factors Influencing Bridge Response and Ratings:

There are many factors that influence a bridge rating; in an attempt to quantify an increased rating for the particular family of bridges investigated in this study, three main factors were investigated: live load distribution, partial composite action, and bearing restraint. The live load distribution was found to be very closely approximated using the analytical equations provided by AASHTO.

Bridge Rating Summary:

Three different rating agencies, each using the Load Factor Rating method, calculated ratings for the six bridges using a codified approach.  The three agencies produced slightly different ratings but provided a good correlation for the superstructure ratings. Using bridge optimization models that utilize the field test strain results for model calibration, Load Factor Ratings were calculated for the six bridges.

CONCLUSIONS

The following conclusions can be deduced from the load testing and analysis of six single span, non-composite concrete-steel bridges:

  • All six bridges exhibited partial composite action without the presence of a mechanical shear connection between the steel girders and the concrete deck.  The degree of partial composite action varied from bridge to bridge and even from girder to girder in each bridge.  The degree of partial composite action from girder to girder for a given bridge ranged from 28% to 114%.
  • There was significant end restraint observed in all of the bridges tested.  With the ends of the girders cast into a concrete diaphragm, the degree of bearing restraint was a significant factor in reducing the induced moment at the midspan of the bridge for some of the bridges but was not consistent in all of the bridges.
  • The live load distribution factors calculated using data from the field tests showed that the AASHTO equations for a single lane loading were slightly conservative but for the interior girders of the two lane loading case the actual live load distribution was less conservative than that predicted using the AASHTO equations.
  • The experimental location of the neutral axis in exterior girders in all of the bridges
    were very close to and often higher than the composite neutral axis location.  The
    curbs and railings were not included in the calculations for the theoretical composite
    neutral axis locations, which is probably the reason for the higher neutral axis locations in the exterior girders.
  • Strains obtained from the optimized bridge models correlated very well with the strain data obtained from the bridge tests.  A scale error between the strains from the optimized model and those obtained from the load test was less than 10% and thus considered to be a good correlation
  • For the most part, there was transverse symmetry observed in the bottom flange
    strains in all the bridges when the test truck was centered on the bridge, but the
    optimized bridge parameters did not produce transverse symmetry across the girders. There was a high degree of variability in the girder moments of inertia.
  • Based on the field data, all of the bridges were determined to have load ratings greater than those calculated using a codified approach. The BCB, KCB1, KCB2, and CCB had ratings that were limited by the exterior girders whereas,the MCB and HCB had ratings that were limited by the interior girders after optimization.
  • The substructure condition did not appear to affect the load rating of the
    superstructure. Removal of pile elements in the HCB demonstrated that the girder
    strains were not affected at either the midspan or the abutment locations.
  • Diagnostic load testing can be utilized to increase the load ratings for this family of
    bridges. All of the bridges had increased ratings due to the results of the load test with the smallest increase being an increase of 29%.
  • Due to the variability of the optimized properties, particularly the girder moments of
    inertia, a reliable factor that could be applied to analytical ratings could not be
    determined. In addition, the sample size of six bridges was determined to be not large enough to produce a statistically reliable factor that could be applied to the theoretical ratings of additional bridges in this family without the aid of a diagnostic load test.

Source: Iowa State University
Authors: Terry J. Wipf | F. Wayne Klaiber | David White | Jeremy Craig Koskie

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