Masood Hajali

Detecting Location of Construction Defects in Drilled Shafts Using Frequency Tomography Analysis of Cross-Hole Sonic Logging

Cross-hole Sonic Logging (CSL) is the standard method used in evaluating the integrity of deep foundations for bridges, such as drilled shafts, and is based on travelling ultrasonic waves between probes in parallel tubes. Several studies have used arrival time and wave speed, such as in cross-hole tomography, to detect construction defects in drilled shaft foundations. A processing method for threecomponent wide-band CSL data is presented—named Frequency Tomography Analysis (FTA). It uses changes in color of the frequency amplitude of the received signal in the location of defects. The method transfers time-domain data to frequencydomain data of the signals propagated between the tubes using Fast Fourier Transform (FFT). The method is performed after a CSL test has determined the high probability of an anomaly in a given area. The procedure improves the location accuracy and further characterizes the features of the defect. The new technique is validated experimentally using two drilled shaft samples constructed with foam pieces inserted throughout the length of the shaft inside the rebar cage before concrete placement, to replicate construction defects. FTA is then utilized after the CSL tests to detect the location of the defects. The technique proves to have a very high resolution and can determine the exact location of any void or defect inside the rebar cage of a drilled shaft. This provides a significant improvement to current techniques used in quality control during construction of bridges.

Determination of Two-Dimensional Plastic Zone Shape and SIF at Crack-Tip Using RKPM

Reproducing kernel particle Method (RKPM) is a meshless technology which has proven very useful for solving problems of elastic-plastic fracture mechanics. The mode I plastic zone shape at the crack-tip in a work-hardening material is obtained using RKPM. Ramberg-Osgood stress-strain relation is assumed and the crack-tip stress intensity factor (SIF) before and after formation of the plastic zone are examined. To impose the essential boundary conditions, penalty method is used. To construct the shape functions in the vicinity of the crack and crack-tip, both the diffraction and visibility criteria are employed. A comparison between two conventional treatments, visibility and diffraction, to crack discontinuity is conducted. The effects of different dilation parameters on SIF under plane-stress and plane-strain conditions are studied. Results including plastic zone shape are compared with finite element method (FEM) to show the accuracy of RKPM. The main objective is to study the effects of different dilation parameters on SIF under plane stress and plane strain conditions and to obtain the mode I plastic zone shape at the crack-tip in a work-hardening material using RKPM.

Behavior of Axially Loaded Drilled Shaft Foundations with Symmetric Voids Outside and Inside the Caging

Abstract Drilled shaft foundations usually carry very high design loads, and often serve as a single load-carrying unit. These conditions have created a need for a high-level of quality assurance during and after construction process. During the construction process, different types of anomalies such as necking, soft-bottom gap at the base, voids and soil intrusions can occur. Anomalies throughout the length can significantly reduce the axial load capacity of the drilled shaft. This paper studies the effect of voids inside and outside the reinforcement cage on the strength and structural capacity of drilled shafts. The objective of this research is to quantify the extent of loss in axial strength and stiffness of drilled shafts due to presence of three different types of symmetric voids throughout their lengths; also, to evaluate the potential for buckling of longitudinal bars within the various types of voids. To complete these objectives, fifteen large-scale drilled shaft samples were built and tested using a hydraulic actuator at the Florida International University’s (FIU) Titan America Structures and Construction Testing (TASCT) laboratory. During the static load test, load-displacement curves were recorded by the data acquisition system (MegaDAC). Results show that the presence of symmetric voids outside the rebar cage (void Type C) that occupy 40% of the cross sectional area of the drilled shafts cause 27% reduction in the axial capacity, while the symmetric voids that penetrate inside the core (void Type B) cause 47% reduction in the axial capacity. The findings indicate that the voids Type B decrease the capacity and stiffness of drilled shafts more than other types due to the resulting inadequate confinement of the concrete and reinforcement.

Comparison between Visibility and Diffraction Criteria on SIF and -Integral Value for Mode Crack Using RKPM

Recently, mesh-free methods are increasingly utilized in solving various types of boundary value problems. Much research has been done on mesh-free methods for solving differential equation problems including crack and also obtained satisfactory results. Among these methods, reproducing kernel particle method (RKPM) has been used increasingly in fracture mechanic problems. The -integral and the stress intensity factor (SIF) are the most important parameters for crack problems. In this study -integral has been used to calculate the SIF in the crack tip. The mode SIF at the crack tip in a work-hardening material is obtained for various dilation parameters using RKPM. A comparison between two conventional treatments, visibility and diffraction on SIF and -integral value, is conducted. Visibility and diffraction methods increase the accuracy of RKPM results and effect on the -integral results at the crack tip. In comparing between the visibility and diffraction methods to modify the shape functions, the diffraction criterion seems to have better results for the -integral and SIF value.

Finite Element Evaluation of Modal Stresses in Cantilever Highway Sign Structures

The dynamic properties of the cantilever sign structures such as its mode shapes and frequency for different modes are being studied under wind load. The analysis considered the dead loads of the structure principally the self-weight of the truss, sign, and walkways, and the wind load. Finite element analysis are conducted to evaluate the modal stresses in the truss and compared to the total stresses. The first three mode shapes in the wind load direction and the gravity load direction are modeled. The modal participating mass ratios of the sign structure for different mode shapes are also calculated. Results show that in chord members with 24 inch length, 50 percent and in chord members with 48 inch length, almost all the stresses (bending + axial) are modal stresses. In interior diagonal members 40 percent, in horizontal diagonal members 75 percent and in column part almost 75 percent of stresses (bending + axial) are modal stresses. The purpose of the study is a modal and static analysis in critical members of a cantilever sign structure under wind loading using finite element method.