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Feb. 10, 2005

Many steel bridges have reached or exceeded their original design life as a result of aging and accumulated stress cycles. The structures have succumbed to increasing traffic volume and weight and deteriorating components. Replacing them or performing major retrofits is extremely costly and often unnecessary. The remaining life of a bridge—whether it is fit for continued service—can be evaluated with reasonable precision, and the results often indicate that the structure is more serviceable than expected.

Many steel bridges have reached or exceeded their original design life as a result of aging and accumulated stress cycles. The structures have succumbed to increasing traffic volume and weight and deteriorating components. Replacing them or performing major retrofits is extremely costly and often unnecessary. The remaining life of a bridge—whether it is fit for continued service—can be evaluated with reasonable precision, and the results often indicate that the structure is more serviceable than expected.

Field measurement is the key to investigating actual behavior and live-load response of existing structures. Typical measurements include strains, displacements and accelerations in critical structural components while the structure is subjected to controlled and regular in-service loading.

The measurements clarify various unknowns in structural analysis, such as load distribution, support conditions, stress fields in connections and stress/loading histograms under actual live load. Combined with finite-element analysis, field measurements can verify and calibrate an analytical model to produce accurate and comprehensive results that help predict loading conditions beyond the test load. Applications of bridge evaluation technology include: remaining fatigue life of steel bridges, evaluation of steel structures with fatigue cracks, bridge load ratings and structural problem diagnosis.

Life left?

AASHTO provisions for fatigue evaluation of steel bridges allow the use of several methods for determining the range of cyclic stresses caused by the live load. The most important factors are the truck weight and the method of analysis for load distribution and stress calculation. While the analytical methods can be enhanced through such devices as weigh-in-motion studies or the use of a three-dimensional computer model, the most accurate method for assessing live-load stresses is a field measurement of strains in identified fatigue-critical members using strain gauges. The measured strains reflect the actual load distribution in the structure as well as the weight, volume and traffic patterns of vehicular loads.

The use of field measurements has been increasingly employed in assessing the remaining life of steel bridges, especially for old structures. The stress ranges determined from the field strain measurement are usually significantly lower than calculated ones because secondary members and the floor system are usually not considered in analytical models. Field measurement also can assess the effect of localized stress increases resulting from the secondary bending at truss joints and section losses caused by corrosion, factors that are difficult to include in analytical models.

In evaluating bridge fatigue, it is essential to accurately determine the live-load-induced stress ranges under normal traffic. The actual stress histories experienced by bridge components can be measured by strain gauges. The measurement results are then processed and expressed in terms of stress-range histograms using cycle-counting algorithms such as rain flow or peak-to-peak. These algorithms detect the peaks and valleys of the strain history and determine the cycle counts for all events of measured stress ranges. The stress-range histograms show the occurrence of stress ranges in terms of the number of cycles for various magnitudes during the test period.

Prior to a field strain measurement, a structural analysis using 2-D or 3-D computer models should be performed to identify the most fatigue-critical members. The selection of members for instrumentation should be based on the results of the analysis, but it must also take into account the physical condition of members and secondary stresses that are not considered in the analysis.

In addition, a calibration test should be conducted with vehicles of known weight to correlate the strain responses with the vehicle load and location. Stress-range histograms under normal traffic must be measured for at least a week to represent the basic unit of load repetition.

Strain service

Constructed in 1959, the Cleveland Central Viaduct, which spans the Cuyahoga River, is an eight-lane, riveted steel structure consisting of 2,722-ft, nine-span, continuous-cantilevered deck trusses flanked by continuous-span girder approaches. A structural analysis following an analytical approach in AASHTO fatigue evaluation provisions indicated insufficient fatigue life for continuing service. As a result, field-strain measurements were made on 19 identified fatigue-critical members. A total of 28 strain gauges were installed to measure the nominal axial strain at member midlength and local stresses near truss joints for bending effects due to joint fixity and laterally framed members, as well as stress increases at corroded sections.

Strain time histories were recorded for a calibration test using a test vehicle of known weight crossing the bridge in each of the eight travel lanes while regular traffic was temporarily blocked. Under normal traffic, stress-range histograms from all gauges were established through real-time rain flow processing over a two-week period. The root-mean-cube effective stress range was calculated based on each stress-range histogram and adjusted for the effects of secondary bending and section loss.

Based on the field-measured stress-range histograms, the evaluation concluded that the remaining fatigue life of the bridge was infinite. It was recommended, however, that measures be taken to rehabilitate corrosion damage at primary truss members.

A cracked case

The presence of fatigue cracks complicates the evaluation of the projected life of bridge spans. It must be determined whether the cracks will continue to grow and cause a fracture and what remedial actions, if any, should be taken. In such cases, the stability of the cracks is assessed by investigating the driving force and fatigue resistance of the details. If appropriate, a repair strategy is devised that will prolong the life of the bridge.

In the case of the Norfolk Southern Railroad Bridge over U.S. Rte. 50 in Salisbury, Md., fatigue cracks in the girder webs were discovered at many intermediate diaphragm locations, and it was estimated that they had been active since 1990. Superstructure repairs to the 70-ft, simple-span structure were made in 1994 by installing structural angles bolted between the connection plate of the diaphragms and the bottom flange of the girders. An inspection in 2001 revealed that some of the existing cracks had grown while six new cracks appeared. A year later, three new cracks were found in the web gap between the intermediate transverse stiffener and the girder bottom flange. However, most existing cracks did not show measurable growth.

A 3-D finite-element model was established to analyze live-load forces in the connections between the girders and diaphragms. The results indicated that the bridge superstructure acted as a two-dimensional composite floor system in both the longitudinal and transverse directions. The railroad loading caused much greater deflections for the girders just underneath than the exterior girders. As a result, the transverse curvature across the bridge width caused high axial and bending forces in the diaphragms. The high connection forces caused by such behavior at the ends of the diaphragms were the driving force for the cracking in the web gaps.

A field test was performed on the bridge to measure strains and displacements in the girders and diaphragms under a passing train to verify the structural behavior predicted by the finite-element analysis. The field measurement indicated high forces in the diaphragms made of 36-in. rolled I-beams. These measurements confirmed the driving force for the cracking and quantified the loading forces for repair design.

A retrofit design was developed to establish a direct load path between the bottom flange of the diaphragm and the bottom flange of the girder. The retrofit connection will transmit all the forces from the end of the diaphragm to the bottom flange of the girder without the distortion of girder web at the web gap area.

Loaded answers

A conventional structural analysis does not produce accurate bridge ratings because of conservative assumptions in load distribution and ignoring the participation of secondary members. Conservative ratings may result in unnecessary posting, detouring, repair or replacement. The problem is more pronounced in old structures that have deteriorated or lack construction plans or material information. Bridge evaluation through field load testing determines the actual live-load distribution and strain and displacement responses of bridge structures under controlled test loads. Bridge load ratings are then derived based on the test results.

Built around 1924 and widened in 1934, the U.S. Rte. 40 bridge over Swan Creek in Harford County, Md., is a 42-ft, simple-span bridge made of multiple reinforced concrete T-beams. A diagnostic field load test was performed since analytical load ratings were insufficient for certain vehicles the bridge needs to carry. Strain gauges were installed at midspan of all eight beams in the original portion of the bridge. Weldable gauges were affixed to the main reinforcing steel, and bondable gauges were mounted on the bottom surface of the concrete deck adjacent to beams.

Two test vehicles were used to cross the bridge in various patterns, and the live-load distribution factor was determined based on the strain responses from all beams. A computer model was established to calculate member forces and check the stiffness of the bridge based on the measured deflection at the midspan. Using the live-load distribution factor and the beam strain responses obtained from the load test, the load ratings were revised and found to be satisfactory for all rating vehicles.

With the aging of our nation’s bridges and the growing vehicular weight they are carrying, it has become increasingly important to assess their fitness for continued service. Experience indicates that structural analysis based on specification loads and distribution factors usually underestimates the remaining fatigue life and structural capacity of existing bridges by overestimating the live-load effects.

URS bridge engineers have performed evaluation and retrofit design of various types of bridges in many states across the country and found that an in-depth evaluation through combined computer modeling and field measurements can more precisely determine a bridge’s serviceability and produce most efficient solutions for repairs or retrofits if determined necessary.

About The Author: Zhou is program manager, bridge evaluation, testing and retrofit, for URS Corp., Hunt Valley, Md.

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