Hossein Agheshlui

Tensile Behavior of Groups of Anchored Blind Bolts within Concrete-Filled Steel Square Hollow Sections

AbstractIn this paper, the tensile behavior of groups of Ajax anchored blind bolts used within concrete-filled steel square hollow sections is investigated. Using Ajax anchored blind bolts moment-resisting bolted connections to concrete-filled steel hollow section columns will be possible. Extensive experimental and numerical studies were undertaken. Bolt sizes and section sizes suitable for medium-rise commercial buildings were used. It was concluded that the groups of Ajax anchored blind bolts can reach the ultimate capacity of equivalent groups of standard structural bolts. The location of an Ajax anchored blind bolt relative to the section side walls has a significant influence on its behavior. For bolts located close to the side walls of a section, concrete struts developed and transferred the loads to the corner of the sections. Bolt diameter, concrete grade, and strut angle were found to be the most influential factors in the stiffness of an Ajax anchored blind bolt. A simple theoretical model, bas...

Stress Influence on Fracture Aperture and Permeability of Fragmented Rocks

The influence of far-field stresses on fracture apertures in a fragmented rock layer is investigated using finite element analysis of a three-dimensional mechanical model. The model implements realistic boundary conditions, interactions between the fragmented layer and neighboring plastic rock layers, and frictional interfaces between the rock blocks. Stress-strain analysis is conducted to obtain stress variations within the fragmented rock layer and the block displacements and rotations. The fracture apertures are calculated using the local stress states instead of the far-field stresses simply being projected on the fractures. It is observed that fracture apertures can vary for the fracture segments over the individual blocks. Ensemble permeability is calculated by running a single-phase flow analysis considering the obtained fracture apertures for fracture segments. The influence of the rock block displacements, rotations and deformations, difference between the mechanical properties of the rock layers, and the orientation of the horizontal stresses is investigated on the ensemble permeability. It is demonstrated that the compressibility of the neighboring layers and block rotations and deformations have significant influence on the permeability of the fragmented rock layer. These effects, which may be ignored in simpler aperture calculation models, can result in considerable inaccuracies in the estimation of fracture apertures and ensemble permeability. Hence, such methods may only be used as indicative tools.

The Impact of Layering and Heterogeneity on Stresses around Boreholes

An accurate understanding of orientation and magnitude of the stresses surrounding a borehole is decisive for the identification of a stable well path and the successful design of completions and stimulation measures including fracking stages. However, measurement of the in situ stress is challenging: Current engineering practice favours two approaches: 1) borehole break-out – drilling-induced fracture interpretation complemented by extended leak-off tests; 2) stress estimation from shear-wave slowness measured with advanced wireline tools. Both methods rely on the applicability of Kirsch’s (1898) stress perturbation equations and laboratory measurements of elastic moduli that are correlated with rock properties that can be logged. Method (2) can be applied where breakouts are absent. Yet little is known how lithologic layering and spatial variations in the mechanical properties of the rock affect its results and the detection limit of stress anisotropy. Here we present finite-element simulations of borehole-related stress perturbations in multi-layer composites with realistic scattering and spatial variations in elastic moduli. Using stress magnitude-shear wave correlations from the literature, travel times are calculated for borehole-parallel trajectories. These results are interpreted in terms of the minimum differential stress that needs to exist in order to be able to detect stress directions. The uncertainty of measurements of stress magnitudes obtained with this method is analysed as well. Our results show that the stress field around boreholes is strongly affected by lithological variations. “Ideal” Kirsch-compatible conditions where the well is aligned with one of the eigenvectors of the stress field, layers are perpendicular to the well, and far-field stresses are Andersonian is rare. More common scenarios are going to be illustrated with a series of simulations including deviated wells. These will also elucidate how rock stress responds to fluid pressure changes in the well.

Effect of Bracing Configurations on the Lateral Behavior of Jacket Type Offshore Platforms, Considering Pile-Soil-Structure Interaction

The configuration of braces has a considerable influence in the lateral behavior of pile supported steel jacket type offshore platforms. In this paper nonlinear static push over analyses and hysteresis curves are used to investigate lateral behavior of a jacket type offshore platform considering different configurations for vertical bracings of the jacket. One sample platform, constructed in Persian Gulf, is precisely modeled using the finite element program Opensees. Since the lateral response of offshore platforms is completely dependent on their foundations’ behavior, an accurate model is used for modeling the foundation of this structure. Soil-pile-structure interaction is considered using Beam on Nonlinear Winkler Foundation model (BNWF). Lateral and vertical soil stiffness and end bearing were considered using p-y, t-z and q-z nonlinear models, respectively. Moreover, the buckling behavior of the braces is considered in the simulation of the platform to consider the behavior of the platform after buckling, and redistribution of the shear forces in the structure due to the bracings failures. The structure was modeled using nonlinear beam-column elements which have the ability to consider the spread of plasticity along the elements. The sections used for elements are fiber sections which are suitable for considering composite section of pile-grout-leg. Displacement controlled nonlinear static pushover analysis and cyclic loading analysis are conducted applying lateral load which its pattern is according to the predominant vibration modes. Three different configurations for lateral bracings are assumed and the behavior of the platform using these three bracing forms is investigated. The first configuration considered is the original bracing of the platform which is a combination of X and chevron braces; the second one is a case in that X braces were used in all of the bays of the jacket; and in the third form chevron braces are used for all of the bays. According to the push over and hysteresis curves, it is concluded that in the jacket modeled using X vertical braces, lateral load capacity, ductility, residual strength ratio and the absorbed energy in cyclic loadings are considerably more than the jackets which were modeled using a combination of chevron and X braces or just chevron braces. In comparison with the jacket modeled using only chevron braces, the jacket constructed using a combination of X and chevron bracings presents better lateral behavior and capacity.Copyright