Application of fracture mechanics to steel moment connections

The 6.8 magnitude Northridge earthquake that shook California\u27s San Fernando Valley on January 17 in 1994, did not cause the collapses of major steel structures, but steel moment connections, confidently designed and constructed in the past with traditional code simplification and common site welding techniques, were discovered not to meet our expectations. The main purpose of the present study is to show the applicability of fracture mechanics to the analysis of the steel moment connection failures during the Northridge earthquake. Other purposes are to develop the numerical procedure of determining the fracture strength of the steel moment connection using linear elastic fracture mechanics, and the probabilistic analyses of brittle fractures of connections. The steel moment connection failures during the 1994 Northridge earthquake and the design philosophy are reviewed and the post-Northridge earthquake experimental and analytical researches are examined. Possible causes of the steel moment connection failures are categorized into welding-related factors, design-related factors, and material-related factors. For the analyses, the idealizations of the steel moment connections considering each factor are studied. The stress concentration and the sate of stress of the connection are investigated using an idealized five-plate moment connection model. The brittle fracture strength of the welded-flange bolted web connection with backing bars is studied using linear elastic fracture mechanics. Post-Northridge connection tests are simulated using the finite element method. An edge crack is assumed at the column-weld interface to represent crack-like defects and other adverse effects. The energy release rate at the front of the edge crack at the failure load of the test is studied using the J-integral method. From post-Northridge connection test results and a finite element model, the effective fracture toughness of the column-weld interface is investigated. The effective fracture toughness is the maximum energy release rate of the material at the front of the crack at the failure load. A new method of calculating the brittle fracture strength of the connection is suggested using the effective fracture toughness. The validity of the suggested method is investigated through numerical examples. Based on the suggested method, the probability of brittle fracture of the connection is investigated using a sample weld defect distribution and a yield stress distribution. In addition, possible applications of this method are discussed

3D Internal Visualization of Concrete Structure Using Multifaceted Data for Ultrasonic Array Pulse-Echo Tomography

This research proposes a 3D internal visualization using ultrasonic pulse-echo tomography technique to evaluate accurately the state of concrete structures for their efficient maintenance within a limited budget. Synthetic aperture focusing technique (SAFT) is used as a post-processing algorithm to manipulate the data measured by the ultrasonic pulse-echo technique. Multifaceted measurements improve the weakness of the existing ultrasonic pulse-echo tomography technique that cannot identify the area beyond a reflector as well as the area located far away from measuring surfaces. The application of apodization factor, pulse peak delay calibration and elimination of trivial response not only complements the weaknesses of the SAFT algorithm but also improves the accuracy of the SAFT algorithm. The results show that the proposed method reduces the unnecessary surface noise and improves the expressiveness of the reflector’s boundaries on the resulting images. It is expected that the proposed 3D internal visualization technique will provide a useful non-destructive evaluation tool in combination with another structure evaluation method

Pulse Peak Delay-Total Focusing Method for Ultrasonic Tomography on Concrete Structure

An ultrasonic array device like the A1040 MIRA is used to non-destructively visualize the inside of concrete structures. A data set acquired by the ultrasonic array device is so unfocused that an image reconstruction algorithm is required to transform the data set into an understandable image. The image reconstruction algorithm like total focusing method exploits the time-of-flight of an ultrasonic pulse when focusing the image. While a high frequency ultrasonic pulse barely affects the accuracy of results, a low frequency ultrasonic pulse with a long wavelength causes an overall sagging of the resulting image around half wavelength of the pulse, which results in a poor quality of results. In this research, a modified total focusing method called pulse peak delay-total focusing method is proposed to calibrate the sagging in the resulting images due to the long wavelength of the pulse. The simulation of an ultrasonic array signal is implemented to validate the proposed method. The experimental results are compared with the simulation results to validate the proposed method. The simulation using the proposed method shows good agreement with experimental results. Analysis of results using potential damage curve and array performance indicator shows that the proposed method allows the higher accuracy, as well as the increased resolution of resulting images

Engineering Structures Structural behavior of ultra high performance concrete beams subjected to bending

a b s t r a c t Ultra high performance concrete (UHPC) exhibits improved performance compared to conventional concrete. Various experimental tests on structural behavior are of importance in order to establish reasonable design specifications for UHPC. Therefore, this study provides a detailed presentation of experimental test results for the flexural behavior of ultra high performance concrete beams. The experimental parameters included the amount of rebar and the placing method for the UHPC. The flexural behavioral characteristics were examined with respect to test results on UHPC beams with rebar ratios less than 0.02 and steel fibers with a volumetric ratio of 2%. Steel fiber-reinforced UHPC proves to be effective at controlling cracks and exhibits ductile behavior with a ductility index ranging between 1.60 and 3.75. In addition, the method of placing UHPC affects the flexural behavior with regard to the orientation of the steel fibers. The results of this study provide valuable data that can be used in future studies on the development of computational models of the deflection and flexural behavior of UHPC

Inverse Analysis of UHPFRC Beams with a Notch to Evaluate Tensile Behavior

Recently, ultra high performance fiber reinforced concrete (UHPFRC) has been developed to attain considerably increased compressive cracking strength and ductile tensile behavior with high tensile strength through adding straight steel fibers in concrete mixture. Although benefits with UHPFRC were investigated through experimental program, it is difficult to predict structural behavior of UHPFRC members since theoretical approaches are limited. In this paper, inverse analysis procedure has been proposed for a three-point bending test with notched UHPFRC beams so that tensile behavior of UHPFRC could be rationally evaluated. On the inverse analysis procedure, failure mode of the UHPFRC beam was simplified and the simplified diverse embedment model (SDEM) was employed. To verify the proposed inverse analysis procedure, UHPFRC beams with a notch were analyzed with the tensile behavior of UHPFRC evaluated through the inverse analysis procedure. The analytical predictions showed good agreement with the load-crack mouth opening displacement (CMOD) responses measured through the three-point bending test. Consequently, it can be concluded that UHPFRC tensile behavior can be rationally evaluated through the proposed inverse analysis procedure. The proposed inverse analysis procedure can be useful in relevant research areas such as development of advanced design approaches or computational methods for UHPFRC members

Effects of Single and Hybrid Steel Fiber Lengths and Fiber Contents on the Mechanical Properties of High-Strength Fiber-Reinforced Concrete

This paper describes an experimental study on the mechanical properties of high-strength fiber-reinforced concrete (HSFRC). The experimental parameters included the content and length of the steel fiber as well as the use of either a single-type fiber or hybrid steel fibers. The steel fiber contents were 1.0, 1.5, and 2.0% based on the volume of HSFRC, and the steel fiber lengths were 13, 16.5, and 19.5 mm. In addition, hybrid steel fibers incorporating steel fibers of different lengths were used. Compression tests and crack mouth opening displacement tests were performed for each HSFRC mixture with different experimental parameters. The mechanical properties of the HSFRC, such as compressive strength, elastic modulus, and tensile strength, increased with the steel fiber content. The mechanical property results of the HSFRC mixture using a single fiber length of 13 mm were greater than the results of the other mixtures. The compressive strength, elastic modulus, and tensile strength of the HSFRC mixture with hybrid steel fibers were similar to those of the mixtures with a single length of steel fiber. Additionally, based on the test results of the material properties, equations for predicting the elastic modulus and tensile strength of the HSFRC were suggested; the predictions using the proposed formula closely agreed with the experimental results

Estimating the Tensile Strength of Ultrahigh-Performance Fiber-Reinforced Concrete Beams

The tensile behavior of ultrahigh-performance fiber-reinforced concrete (UHPFRC) depends on the dispersion and orientation of steel fibers within the concrete matrix. The uneven dispersion of randomly oriented steel fibers in concrete may cause differences in the tensile behavior between material testing specimens and beams. Therefore, in this study, the tensile behavior was investigated by fitting the analysis result of the moment-curvature curve to the experimental result of a UHPFRC beam. To this end, three UHPFRC mixtures with different compressive strengths were fabricated to test the material properties and flexural behavior of UHPFRC beams. Both a single type of steel fiber and a combination of steel fiber types were used with volume fractions of 1.0% and 1.5%, respectively, in the three mixtures. Based on the design recommendations, the material properties of UHPFRC were modeled. The results ultimately show that by fitting the analysis results to the experimental results of the moment-curvature curves, the tensile strength of UHPFRC beams can be reasonably estimated