Shield, Carol K.

This report presents results from a laboratory study and field implementation of acoustic emission monitoring of fatigue cracks in cover-plated steel bridge girders. The acoustic monitoring successfully detected growing fatigue cracks in the lab when using both source location and a state of stress criteria. Application of this methodology on three field bridges also proved successful by detecting a propagating crack in two of the bridges and an extinguished crack in a third bridge. Researchers tested a double angle retrofit, designed by the Minnesota Department of Transportation, both in the lab and in the field of girder with fatigue cracks in the top flange. This retrofit does not require removal of concrete deck, and only involves bolting the retrofit to the bridge girder web. The double angle retrofit applied to laboratory test girder resulted in a reduction of flange stresses by 42 percent. Field implementation of the retrofit had mixed success. On one bridge, stress ranges in the cracked flange was reduced by 43 percent. However, on a second test bridge, the reduction was only 8 percent, likely due to the inadequate space for proper installation of the retrofit.

This project details the development and evaluation of an acoustic emission (AE) system for monitoring large scale structures, both in the lab and in the field. The system consists of acoustic emission sensors, preamplifiers, filers, an AE monitor, and a digital oscilloscope. The system has been applied successfully to both steel and concrete structures and used to detect brittle fracture and low-cycle fatigue failures in welded steel joints and crack propagation in cover-plated rolled bridge girders, in the field and in the laboratory. The AE system detected initial cracking during the flexural crack testing of two high-strength concrete prestressed bridge girders. The acoustic emission monitoring of bond tests also provided insight into the behavior of the bond between glass fiber reinforced polymer rebar and concrete.

As part of a project at the University of Minnesota to investigate the application of high-strength concrete in prestressed girders, four shear tests were performed on high-strength concrete prestressed girders. Originally constructed in August 1993, the girders, Minnesota Department of Transportation (Mn/DOT) 45M sections were 45 inches deep. Each girder utilized 46 0.6-inch diameter prestressing strands on 2-inch centers. The girders were designed assuming a 28-day compressive strength of 10,500 psi. Later, a 4-foot-wide and 9-inch-thick composite concrete deck was added to each girder using unshored construction techniques. The shear test results were compared with predicted results from ACI 318-95 Simplified Method, ACI 318-95 Detailed Method (AASHTO 1989), Modified ACI 318-95 Procedure, Modified Compression Field Theory (AASHTO LRFD 1994), Modified Truss Theory, Truss Theory, Horizontal Shear Design (AASHTO 1989), and Shear Friction (AASHTO LRFD 1994). The calculated shear capacities were in all cases conservative compared to the actual shear capacity.

This report describes the design, instrumentation, construction, and test set-up of two high-strength concrete prestressed bridge girders. The girder specimens were constructed to evaluate prestress transfer length, prestress losses, flexural fatigue, ultimate flexural strength, and ultimate shear strength. Each test girder was a 132.75-foot long, 46-inch deep, Minnesota Department of Transportation (Mn/DOT) 45M girder section reinforced with 46 0.6-inch diameter 270 ksi prestressing strands. The 28-day nominal compressive strength of the girders was 10,500 psi. Each girder was made composite with a 9-inch thick, 48-inch wide composite concrete deck cast on top with a nominal compressive strength of 4000 psi. Girder I used a concrete mix incorporating crushed limestone aggregate while Girder II utilized round glacial gravel aggregate in the mix with the addition of microsilica. In addition, the two test girders incorporated two different end patterns of prestressing--draping versus a combination of draping and debonding--and two different stirrup configurations--standard Mn/DOT U versus a modified U with leg extensions. More than 200 strain gages were imbedded in each girder during construction. Other reports present flexural and shear testing results.

The I-35W St. Anthony Falls bridge was highly instrumented with over 500 sensors to verify design assumptions; serve as a testbed to examine bridge sensing techniques; and evaluate the effectiveness of different bridge monitoring strategies. The instrumentation deployed on the bridge to investigate the structural behavior included vibrating wire strain gages (VWSGs); thermistors; fiber optic sensors (SOFO); resistance strain gages; linear potentiometers; accelerometers; and corrosion monitoring sensors. This report documented the successes and challenges of the monitoring program over the first ten years of the bridge's life. In particular; the effectiveness of different strain measurement techniques and sensor distributions were addressed. Previous investigations of temperature-dependent and time-dependent behavior were also expanded with the larger data set to better understand the behavior of post-tensioned concrete box girder structures with the potential to impact future designs.

Since the opening of the I-35W Saint Anthony Falls Bridge in 2008; over 500 sensors have been collecting data to better understand the behavior of post-tensioned concrete box girder structures. Recent research in the accelerometers installed on the bridge indicates they can be effectively used in a vibration-based structural health monitoring system; but previous studies have shown that natural frequency alone may not be sufficient to determine the performance of the structure. Vertical displacements were believed to be a simpler performance measure as direct comparisons can be made with design calculations and maintenance guidelines. To avoid the shortcomings of conventional displacement measurement options; this study focuses on using the currently installed accelerometers to estimate the vertical displacements of the southbound bridge. The proposed technique utilizes up-to-date modal parameters within a dual Kalman filter to estimate the vertical displacements of the structure from noisy acceleration measurements. When applied to the I-35W Saint Anthony Falls Bridge; it was found that the dual Kalman filter approach captures only dynamic displacements due to relatively slow loading (e.g.; traffic loading and thermal loading) and the corresponding low-frequency static displacements are likely too small for GPS measurements due to the high stiffness of the structure.

Over time; the southbound exterior girder ends on each side of Pier 4 and Pier 26 of Bridge 27568 suffered significant corrosion damage that exposed transverse reinforcement; prestressing strands in the exterior side of the bottom flange and the sole plate anchorages. The girder ends were repaired in 2013 by encasing supplementary steel reinforcement in shotcrete over a 4 ft. length of the girder. The two repaired girders and two companion girders; removed when the bridge was replaced in 2017; were brought to the University of Minnesota and tested to failure in shear to determine the effectiveness of the repair. The laboratory testing showed that the repair was able to return the girders with significant corrosion damage to the strength of the companion girders; indicating that the repair was effective.

Vibration based structural health monitoring has become more common in recent years as the required data acquisition and analysis systems become more affordable to deploy. It has been proposed that by monitoring changes in the dynamic signature of a structure; primarily the natural frequency; one can detect damage. This approach to damage detection is made difficult by the fact that environmental factors; such as temperature; have been shown to cause variation in the dynamic signature in a structure; effectively masking those changes due to damage. For future vibration based structural health monitoring systems to be effective; the relationship between environmental factors and natural frequency must be understood such that variation in the dynamic signature due to environmental noise can be removed. A monitoring system on the I-35W St. Anthony Falls Bridge; which crosses the Mississippi River in Minneapolis; MN; has been collecting vibration and temperature data since the structures opening in 2008. This provides a uniquely large data set; in a climate that sees extreme variation in temperature; to test the relationship between the dynamic signature of a concrete structure and temperature. A system identification routine utilizing NExT-ERA/DC is proposed to effectively analyze this large data set; and the relationship between structural temperature and natural frequency is investigated.

As shear requirements for prestressed concrete girders have changed, some structures designed using older specifications do not rate well with current methods. However, signs of shear distress have not been observed in these bridge girders and they are often deemed to be in good condition. The primary objective of this research program was to investigate the accuracy of existing shear distribution factors, which are used to estimate bridge system live load effects on individual girders, and provide recommendations on shear distribution to be used in Minnesota with four components: a full-scale laboratory bridge subjected to elastic and inelastic behavior, field testing of bridges, a numerical parametric study, and integration of results to develop a screening tool to determine which structures benefitted from refined analysis. Laboratory bridge inelastic testing indicated shear force redistribution after cracking and before ultimate failure. Use of elastic distribution factors is conservative for shear distribution at ultimate capacity. Elastic laboratory testing was used to validate the finite element modeling technique and study the behavior of a barrier and end diaphragm, which affected shear distribution; ignoring their effects was conservative. Parametric study results indicated that a ratio of longitudinal stiffness to transverse stiffness could be used as a screening tool. If the stiffness ratio was less than 1.5, shear demand from a simple, conservative grillage analysis may be more accurate than shear demand from AASHTO distribution factor methods. Grillage analysis shear demand results due to permit trucks may also be more accurate, regardless of the screening tool ratio.

The time-dependent and temperature-dependent behavior of post-tensioned concrete bridges were investigated through a case study of the St. Anthony Falls Bridge, consisting of laboratory testing of concrete time-dependent behaviors (i.e., creep and shrinkage), examination of data from the in situ instrumented bridge, and time-dependent finite element models. Laboratory results for creep and shrinkage were measured for 3.5 years after casting, and the data were best predicted by the 1978 CEB/FIP Model Code provisions. To compare the in situ readings to constant-temperature finite element models, the time-dependent behavior was extracted from the measurements using linear regression. The creep and shrinkage rates of the in situ bridge were found to depend on temperature. An adjusted age using the Arrhenius equation was used to account for the interactions between temperature and time-dependent behavior in the measured data. Results from the time-dependent finite element models incorporating the full construction sequence revealed that the 1990 CEB/FIP Model Code and ACI-209 models best predicted the in situ behavior. Finite element analysis also revealed that problems associated with excessive deflections or development of tension over the lifetime of the bridge would be unlikely. The interactions between temperature and time-dependent behavior were further investigated using a simplified finite element model, which indicated that vertical deflections and stresses can be affected by the cyclic application of thermal gradients. The findings from this study were used to develop an anomaly detection routine for the linear potentiometer data, which was successfully used to identify short-term and long-term artificial anomalies in the data.