Modelling Steel Bridges Using Field Testing

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USE OF FIELD TESTING

IN STRUCTURAL MODELLINGOF STEEL BRIDGES

Prof Dr Metwally Abu-Hamd

Cairo University, Egypt



The Problem:

Results of structural models do notrepresent real bridge behavior



Reasons: Unknown bridge conditions related to:

– Load distribution

– End restraint

– Composite action

– Effectiveness of specific bridge details

– Other details contributing to bridge

capacity



Solution

* Use Field Testing to understand thespecific characteristics of each bridge

* Use field collected data to calibrate a

computer constructed structural model of

the bridge

* Use the accurate, calibrated computer model

to determine bridge response to design loads



TYPES OF FIELD TESTS:

1- Material testing

2- Geometric monitoring3- Long term monitoring

4- Static load testing

5- Dynamic testing



1- Material Testing: Using extracted samples or non-

destructive tests to identify the actual

material properties such as density, elasticmodulus, ultimate strength and yieldstrength.

Using ultrasonic or radiographic

examination to identify defect or sectionDeterioration.



2- Geometric Monitoring: Using surveying techniques, GPS, and

remote sensing.

3- Long Term Monitoring:of the structural behavior of the bridge to

identify strain or displacement values under

actual loadings.



4- Static load testing:

To identify strain responses to bridge loads

5- Dynamic testing: To identify acceleration responses to

moving loads or forced excitations.



CALIBRATION PROCEDURE

1- Preliminary inspection

2- Structural Modeling3- Load Test

4- Model Calibration



Step 1: Preliminary inspection:

1) review of bridge plans.2) visual inspection to record any

defects

3) review of previous maintenance andinspection reports



Based on information collected during

this inspection:1) The data needed to construct a

preliminary structural model is collected

2) An instrumentation plan is developed forthe load test so that it provides a better

understanding of the bridge behavior

(e.g.: end restraint, edge stiffening,composite action, load distribution, etc)



Step 2: Structural Modeling:

Types:

1) 1-D: Linear Model; Simple/Continuous Beams

2) 2-D: Grid Model; Plane Structure

3) 3-D: Space Model; Closer to Reality.



Preliminary model:* Represents the best knowledge about the bridge

members, connections, supports conditions,… etc.

* Any of the available analysis software such as STAAD,

SAP, etc. can be used to construct the model.

* The results of the preliminary inspection related to section

and material properties, section deterioration, support

conditions…etc are used in the model input data.

* The live loading of the model should resemble the field

test load. Appropriate critical load paths to inducemaximum live load effects are defined.



Step 3: Load Test: Test Design :

1- Establish purpose /objections of the test application.2- Determine form and duration of the testing.3- Identify the types and quantities of variables to be

measured and their measurement locations.4- Select sensors and data acquisitions components.5-Validate /calibrate the measurement systems.6- Design load cases for controlled tests.

7- Develop procedures for evaluating data quality, dataprocessing and analysis and archival.

8- Create presentation and decision criteria.



The data obtained from field testing must be processedfor use in the parameter estimation module such that

the quality of the data is maintained. Issues that need to be considered are:

1- Data must be validated to ensure that it originated

from a reliable sensor2- Channels should be separated so that parameters

estimation and verification can be performed in clear

checks and balances system; and

3- To ensure consistent conclusions, the results of

different tests must be comparable.



Step 4: Model Calibration :

4-1) Calibration Parameters: The critical parameters of the model are defined

depending on the bridge type and the results of

preliminary inspection; e.g., for a slab-on-girderbridge: Modulus of elasticity of concrete andsteel, area and moment of inertia of spring

supports, deck slab thickness, and moment of inertia of shear link connecting slab to girder.



4-2) Calibration Strategy:1- Evaluate error between field results and model

results.2- Define an Objective Function of the error as:

OF = Error = f (P1,P2,P3,.....,Pn)

where E = total error function, Pi = i’th parameter,

n = number of parameters.3- Find the minimum value of OF using an

optimization method; e.g., gradient method:



Gradient Method:

1- Select initial value of the parameter.2- Find gradient of OF at that point.

3- Proceed along the gradient direction to change the

parameter.

4- Repeat until convergence to the minimum.

5-The parameters from the last iteration represent the

calibrated model. A "good" model will generally

have a correlation coefficient greater than 0.9 and a percent error less than 10%.



Optimization Problem:



Parameter Calibration:



Application to Slab-on-Girder Bridges



STRUCTURAL MODEL:



OPTIMIZATION PROCESS:



CALIBRATION RESULTS:



Example of Parameter Calibration:



Application to Truss Bridges



Bridge Description: Structure Type: Open deck thru-truss with pinned

connections. Span Length(s): 155 feet, Skew Right Member Types: Bottom Chord: Eyebars and built-up sections (riveted). Top Chord: Built-up sections (riveted) End Posts: Built-up sections (riveted) Diagonals: Eyebars Floor Beams: Built-up sections (riveted) Stringers: Built-up sections (riveted) Structural Steel Fy = 36 ksi, E=29,000 ksi (from

material test results)



STRUCTURAL MODEL:



TEST MEASUREMENTS:



TEST RESULTS:



CALIBRATION

RESULTS:



CALIBRATION

RESULTS (Contd.):



References: 1- A. E. Aktan, The Health Monitoring Paradigm For

Infrastructure Management, Seminar at Izmir, May 2005.

2- Bridge Diagnostics, Inc. “Load Test and Rating Report:

Puget Sound & Pacific Railroad Bridge,” 2001. 3- Lei Liu, “An Automatic Calibration Strategy for 3D FE

Bridge Models,” M. Sc. Thesis, University of Cincinnati,

2004.

4- Wang, X., et al, “Overview of a Modal-Based Condition Assessment Procedure”, Journal of Bridge Engineering,

ASCE, Vol. 10, No. 4, July 2005.

Modelling Steel Bridges Using Field Testing

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