Carol K. Shield

Development length of glass fiber-reinforced polymer bars in concrete

Glass fiber-reinforced polymer (GFRP) reinforcing bars have become a viable option for reinforcement in concrete when corrosion is a concern. The objective of this research was to investigate the bond performance of GFRP reinforcing bars in concrete, to evaluate the existing ACI Committee 440 recommendations for development length of FRP bars in concrete, and to develop new design recommendations if needed. An equation for the development length of GFRP reinforcing bars in concrete was formulated by applying a methodology similar to one used to create the development length equation for steel reinforcing bars. Data collected from beam-based bond tests in the literature were used to construct two equations for development length: one based on a splitting mode of failure and the other on a pullout mode of failure. The effects of bar diameter, bar tensile strength, concrete compressive strength, cover, and the presence of transverse reinforcement were investigated. The proposed equations appear to be a conservative yet reasonable means to calculate the development length of GFRP reinforcing bars in concrete given the available test data. The proposed equations are compared to those found in ACI 440.1R-03 and the Japanese Design Guidelines.

Development and implementation of the effective force testing method for seismic simulation of large-scale structures

This paper describes the development and experimental implementation of a real–time earthquake simulation test method for large–scale structures. The method, effective force testing (EFT), is based on a transformation of coordinates, in which case the structure is fixed at the base (similar to the set–up for the pseudo–dynamic (PsD) test method); however, in the case of EFT, the method is based on a force–control algorithm rather than a displacement–control algorithm. Effective forces, equivalent to the mass of each storey level multiplied by the ground acceleration, are applied at each respective storey. As such, the EFT forces are known a priori for any ground acceleration record. As opposed to the PsD test method in which the ground displacements to be imposed are affected by the measured structural response as the stiffness changes. As in the case of the PsD test method, the EFT method is suitable for testing any type of structural system that can be idealized as a series of lumped masses (e.g. building or bridge structures). Research has been conducted on a linear elastic single–degree–of–freedom system at the University of Minnesota to develop and investigate implementation of the EFT method. A direct application of the EFT method was found to be ineffective because of a natural velocity feedback phenomenon between the actuator and the structure to which it is attached. A detailed model of the control, hydraulic and structural systems was developed to study the interaction problem and other nonlinear responses in the system. The implementation of an additional feedback loop using the measured velocity of the test structure was shown to be successful at overcoming the problems associated with actuator–ndash;control–ndash;structure interaction, indicating that EFT is a viable real–time method for seismic simulation studies.

FIELD PERFORMANCE OF INTEGRAL ABUTMENT BRIDGE

The behavior of an integral abutment bridge near Rochester, Minnesota, was investigated from the beginning of construction through several years of service by monitoring more than 180 instruments that were installed in the bridge during construction. The instrumentation was used to measure abutment horizontal movement, abutment rotation, abutment pile strains, earth pressure behind abutments, pier pile strains, prestressed girder strains, concrete deck strains, thermal gradients, steel reinforcement strains, girder displacements, approach panel settlement, frost depth, and weather. In addition to determining the seasonal and daily trends of bridge behavior, live-load tests were conducted. All of the bridge components performed within the design parameters. The effects from the environmental loading of solar radiation and changing ambient temperature were found to be as large as or larger than live-load effects. The abutment was found to accommodate superstructure expansion and contraction through horizontal translation instead of rotation. The abutment piles appeared to be deforming in double curvature, with measured pile strains on the approach panel side of the piles indicating the onset of yielding.

Fabrication of BioInspired Inorganic Nanocilia Sensors

In nature, microscale hair-like projections called cilia are used ubiquitously for both sensing and motility. In this paper, biomimetic nanoscale cilia arrays have been fabricated through templated growth of Co in anodized aluminum oxide. The motion of arrays of Co cilia was then detected using magnetic sensors. These signals were used to prove the feasibility of two types of sensors: flow sensors and vibration sensors. The flow sensors were tested in a microfluidic channel. They showed the ability to detect flows from 0.5 ml/min to 6 ml/min with a signal to noise (SNR) of 44 using only 140 μW of power and no amplification. The vibration sensors were tested using a shake table in the low earthquake-like frequency range of 1-5 Hz. The vibration response was a mW signal at twice the frequency of the shake table.

Comparison of acoustic emission activity in reinforced and prestressed concrete beams under bending

Abstract Three ordinary reinforced and two prestressed concrete beams were cyclically loaded in three point bending until failure. Crack initiation and crack propagation due to bending were monitored for acoustic emission (AE) activity and compared with visual observation. Results of source location data for both types of specimens were compared to visually observed cracks. Some insight into the percentage of AE events that must actually be detected to describe the health of the beams was gained. The existence or absence of a Kaiser effect in concrete was evaluated. Comparisons of the difference in AE behavior between ordinary reinforced and prestressed concrete beams are presented.

Instrumentation and Modeling of I-35W St. Anthony Falls Bridge

The I-35W St. Anthony Falls Bridge was constructed to replace the steel truss bridge that collapsed on August 1, 2007. The design of the replacement bridge featured a smart-bridge system. The smart-bridge concept included instrumentation for long-term monitoring of the structural behavior of the bridge. Truck-load tests were conducted prior to opening the bridge and 26 months later to measure the response of the structure under controlled loading. The measurements were used to validate a FEM model of the bridge constructed to further investigate the behavior of the structure. The correlation between computed and measured results was found to be good. This paper describes the bridge, the instrumentation installed within the bridge, the FEM model, and validation of the model with respect to the truck-load tests. Recommendations are provided for static instrumentation plans of concrete box-girder structures.

LRFD Factors for Pultruded Wide-Flange Columns

Fiber-reinforced polymer (FRP) pultruded profiles are produced by a number of manufacturers worldwide in similar, but nonstandard, wide-flange, I, angle, and tubular profiles. At present there is no American National Standards Institute approved design code in the United States for structural design with pultruded FRP profiles. Manufacturers of pultruded profiles each provide their own design equations, design methods, material properties, and safety factors for their pultruded products. There is a need for standardization of production and design of pultruded profiles to enable mainstream use of these profiles in structural engineering practice. The purpose of this paper is twofold: (1) to provide appropriate resistance factors ( ϕ factors) for wide-flange pultruded columns that are compatible with ASCE 7 load factors and (2) to provide a unified analytical equation for local and global buckling of concentrically loaded axial members, which may be appropriate for a future design code. The resistance fact...

Investigation of Thermal Gradient Effects in the I-35W St. Anthony Falls Bridge

Thermal gradients were measured through the section of the I-35W St. Anthony Falls Bridge, a posttensioned concrete box girder bridge in Minneapolis, Minnesota, over the course of 3 years. The magnitudes and shapes of the measured thermal gradients were compared with various design gradients, and a fifth-order curve was found to best approximate the shape of the gradients. The responses of the structure to the largest measured thermal gradients were compared with stresses and deformations predicted by finite-element modeling given applied design gradients. The measured structural response was found to be best predicted when the finite-element model of the bridge was subjected to a fifth-order design thermal gradient scaled to match maximum top surface temperature values proposed by AASHTO LRFD Bridge Design Specifications for the region. Stresses and deformations predicted by finite-element modeling using the AASHTO LRFD bilinear design gradients were found to be considerably lower than those derived from measured results. Recommendations for design thermal gradients are proposed.

Experimental Performance of Steel Braced Frames Subjected to Bidirectional Loading

Concentrically braced frames (CBFs) are stiff, strong systems frequently used to resist seismic loading. Special CBF (SCBF) behavior is dominated by brace buckling, while buckling restrained braced frames (BRBFs) develop tensile and compressive yielding and avoid brace buckling. Both systems are widely used in seismic design, and both have a number of specific design issues. This paper describes a first of its kind, 2-story, 1-bay by 1-bay frame tested at the University of Minnesota Network for Earthquake Engineering Simulation facility to examine the large-displacement, bidirectional behavior of SCBFs and BRBFs with realistic boundary conditions and to verify the design approach. The SCBF had rectangular hollow steel section (HSS) braces in a single-story X configuration, and the BRBF used a single-diagonal configuration. The design of the gusset plates for the HSS braces followed a previously proposed balanced design procedure with an elliptical clearance to permit out-of-plane rotation caused by brace buckling. The single-story X-brace SCBF concentrated damaged into one-half the brace length, and the first HSS brace fractured at 2% story drift. The BRBF gusset-plate design followed current design standards, and two of the BRB cores fractured at 3.6 and 4.2% story drift prior to any instability in the BRB or system. The SCBF sustained limited damage to the beams and columns; however, the BRBF had much more significant damage to these members because of larger deformations and BRBF behavior. The results indicate that these systems have a stable response to large cyclic deformations and the impact of bidirectional loading on the measured response was minimal.

Structural Health Indices for Steel Truss Bridges

Steel bridges represent a significant portion of the nation’s aging bridge inventory. The United States Army’s inventory has over two hundred steel bridges that are vital to the operation of nearly every Army Installation. The Army regularly inspects their bridges to ensure the continued functionality of the bridges. However, current inspection techniques and schedules may not be able to detect certain defects that could compromise bridge integrity. To supplement current inspection standards, the US Army Engineer Research and Development Center (ERDC), has contracted to install a structural health monitoring (SHM) system on the historic steel truss Government Bridge at the Rock Island Arsenal. ERDC envisions a system that measures strain, acceleration, tilt, and electrical resistance to determine the bridge’s structural health. Research into the usefulness of the Damage Locating Vector (DLV) method, and the damage indices that the method calculates, in detecting corrosion damage has been conducted. Numerical studies on a finite element model of the Government Bridge have indicated that the DLV method has potential as a damage detection algorithm on such a large structure. Laboratory experiments using corroded members in a model truss have additionally shown that the DLV can be used to monitor corrosion induced damage.