Wondwosen Ali

Bio: Wondwosen Ali is an academic researcher from George Mason University. The author has contributed to research in topics: Finite element method & Finite difference. The author has an hindex of 3, co-authored 5 publications receiving 21 citations.

Numerical Prediction Model for Temperature Distributions in Concrete at Early Ages

Abstract: A finite element-finite difference numerical model is developed for predicting non-uniform temperature development in hydrating concrete with respect to time and space. The results obtained from this model can be used by structural and construction engineers to predict critical thermal stresses induced due to differential temperatures between the core and the surface of the concrete at early ages and between the zero-stress temperatures and the minimum equilibrating ambient temperatures that the concrete experiences during its service life. The prediction of zero-stress temperatures also enables to quantify the extent of built-in curl developed in concrete structures. The finite element is used to space discretization while the finite difference is used to obtain transient solutions of the model. The numerical formulations are then programmed in Matlab. The numerical results were compared with experimental results found in literature and demonstrated very good agreement.

Structural Capacities of Spherically Voided Biaxial Slab (SVBS)

Abstract: Demand for lighter and safer concrete floor system to improve the span limit resulted from the high weight-to-stiffness/strength ratios of a solid reinforced concrete slab has led to the emergence of spherically voided biaxial slab (SVBS). However, the voids impart a non-prismatic nature in both in-plane directions of this new slab system. Because of this non-prismatic nature, determination of stiffness and structural capacity of a spherically voided biaxial slab is inherently challenging. The inclusion of the voids is intuitively expected to compromise the stiffness and structural capacity of the slab. However, the extent of the compromise is not explicitly known. Therefore, in this paper, the reduction of the stiffness and structural capacities of the spherically voided slab is investigated in comparison to a solid slab with equal dimensions. A 3D non-linear finite element analyses software, namely ANSYS Workbench 14.0, is used to conduct the investigation. The analyses indicated that the flexural capacities of spherically voided biaxial slabs are very close to solid slabs of equal dimension, especially when considering long span slabs. Moreover, the failure mechanisms of SVBS and solid slab system are evaluated and they are found to be almost similar under uniform loads. It is also concluded that the voids around the middle region of the slabs do not significantly compromise the flexural capacity and bending behavior of the slab system. However, the punching shear resistance of SVBS is determined to be considerably less than that of solid slab.

Computational Model for Internal Relative Humidity Distributions in Concrete

Abstract: A computational model is developed for predicting nonuniform internal relative humidity distribution in concrete. Internal relative humidity distribution is known to have a direct effect on the nonuniform drying shrinkage strains. These nonuniform drying shrinkage strains result in the buildup of internal stresses, which may lead to cracking of concrete. This may be particularly true at early ages of concrete since the concrete is relatively weak while the difference in internal relative humidity is probably high. The results obtained from this model can be used by structural and construction engineers to predict critical drying shrinkage stresses induced due to differential internal humidity distribution. The model uses finite elment-finite difference numerical methods. The finite element is used to space discretization while the finite difference is used to obtain transient solutions of the model. The numerical formulations are then programmed in Matlab. The numerical results were compared with experimental results found in the literature and demonstrated very good agreement.

Probabilistic mechanics of explosive fragmentation

Abstract: When explosive munitions (such as artillery shells and bombs) detonate, the casing fragments into larger number of pieces. The fragments produced may have the momentum to perforate and damage structures. The mechanics of the complex loads produced by these fragmentations requires a description of the fragment size, shape, and velocity distributions. The ability to forecast the pattern and size of these fragmentations is a very important aspect of designing structures for penetration effects. This paper presents probabilistic estimation of the properties of high- velocity fragment produced from the metal case of explosively loaded munitions using a computational numerical model in MATLAB. Since protective structures must be designed to withstand the effect of predicted fragments, the fast- running numerical model presented in the paper can be utilized in preliminary design in lieu of advanced analysis such as FEA. The paper includes an example problem where fragment mass distribution and fragment velocity distribution are determined for a given building subjected to explosively fragmenting munitions. Explosively fragmenting munitions produce complex loads on structures. The characterization of these loads requires a description of the fragment size, shape, and velocity distributions. This paper presents probabilistic estimation of impact loads by relatively high-velocity fragments produced from the metal case of an explosively loaded munitions using MATLAB. Curran (1997) reviews predictive theories and models for explosively fragmenting munitions that produces complex loads on nearby targets. He also presented correlations of simpler formulae with fragment aspect ratio data obtained from a scaled explosive experiment. Although the derivation of the formulae was found to be useful, the author highlights the need to validate the fragmentation equations against additional arena test data for wide range of scales and material properties. A technique for predicting performance of explosive fragmentation munitions presented in this work is based on integrating three-dimensional axisymmetric hydrocode analyses with analyses from a newly developed fragmentation computer code MOTT. The validation of the MOTT code fragmentation model was accomplished using the existing munitions arena test data. After having established the crucial parameters of the model, a new explosive fragmentation munitions was designed and optimized. Upon fabrication of the developed munitions, the performance of the new charge was tested in a series of small-scale experiments including the flash radiography, the high-speed photography, and the sawdust fragment recovery. Considering relative simplicity of the model, the accuracy of the MOTT code predictions is rather remarkable. Gold et al. (2006) presented a technique for predicting performance of explosive fragmentation munitions based on integrating three-dimensional hydrocode analyses with a newly developed program called PAFRAG. They validated their program with test data obtained from small-scale experiments including flash radiography, high-speed photography and sawdust fragment recovery. The code implements Mott's fragment distribution for random variations in fragment sizes and it did provide reasonable results when compared to test data. Arnold and Rottenkolber (2008) presented a method for fast data collection of fragmenting shells. They studied fragmentation behavior of metal casings ranging from thin aluminum alloys to mild steel. Their work sketched a method for fast data collection of fragmenting shells and was applied to generic casings of warheads. They have developed an iterative procedure to calculate fragment mass. However, the paper did not provide a development of fragmentation model suitable for axial fracture. Wang et al. (2009) formulated a theoretical method for predicting fragment size caused by blast loads based on fracture mechanics of brittle materials. They developed a finite element model to estimate the material damage, fragment distribution and ejection velocity. A simplified algorithm is presented in their paper in order to predict fragment trajectory and launch distance. They have demonstrated how crack growth velocity is related to strain rate; when the load is applied more rapidly with higher amplitude, a higher level of stress is achieved before flaw

Macromechanical analysis of spherically voided biaxial concrete slabs

Abstract: The use of spherically voided biaxial concrete slab (SVBS) system, which uses hollow plastic balls as infill material, has increased widely because of its reduced weight-to-strength and weight-to-stiffness ratios when compared to solid concrete slabs. However, SVBS is a heterogeneous composite structure in which building a representative continuum model poses a significant challenge. To mitigate this challenge, the feasibility of determining the macromechanical structural behavior of spherically voided biaxial concrete slabs is studied using plate theories, aided by mechanical properties that were determined from a homogenization process of the representative volume element (RVE). This paper presents numerical analysis results of SVBS using both Mindlin-Reissner (thick) and Kirchhoff-Love (thin) plate theories. The results from both theories predicted the slab behavior reasonably well and they were within 10% of each other with the exception of the prediction of the twisting moment. Possible explanation of this deviation is provided in the paper.

Moisture Diffusion in Unsaturated Self-Desiccating Concrete with Humidity-Dependent Permeability and Nonlinear Sorption Isotherm

Abstract: A nonlinear diffusion model for the drying of concrete, previously developed at Northwestern University and embedded in some design codes, was improved and calibrated on the basis of recent...

Seismic Response Reduction of Structures Equipped with a Voided Biaxial Slab-Based Tuned Rolling Mass Damper

Abstract: This paper proposes a novel tuned mass damper (TMD) embedded in hollow slabs of civil structures. The hollow slabs in this context, also referred to as “voided biaxial reinforced concrete slabs,” feature a large interior space of prefabricated voided modules that are necessary in the construction of this special structural system. In this regard, a tuned rolling mass damper system (“TRoMaDaS”) is newly proposed, in combination with hollow slabs, to act as an ensemble passive damping device mitigating structural responses. The main advantage of this TMD configuration lies in its capacity to maintain architectural integrity. To further investigate the potential application of the proposed TRoMaDaS in seismic response mitigation, theoretical and numerical studies, including deterministic and stochastic analyses, were performed. They were achieved by deterministic dynamic modeling using Lagrange’s equation and the statistical linearization method. Finally, the promising control efficacy obtained from the deterministic/stochastic analysis confirmed the potential application of this newly proposed control device.

Structural Capacities of Spherically Voided Biaxial Slab (SVBS)

Abstract: Demand for lighter and safer concrete floor system to improve the span limit resulted from the high weight-to-stiffness/strength ratios of a solid reinforced concrete slab has led to the emergence of spherically voided biaxial slab (SVBS). However, the voids impart a non-prismatic nature in both in-plane directions of this new slab system. Because of this non-prismatic nature, determination of stiffness and structural capacity of a spherically voided biaxial slab is inherently challenging. The inclusion of the voids is intuitively expected to compromise the stiffness and structural capacity of the slab. However, the extent of the compromise is not explicitly known. Therefore, in this paper, the reduction of the stiffness and structural capacities of the spherically voided slab is investigated in comparison to a solid slab with equal dimensions. A 3D non-linear finite element analyses software, namely ANSYS Workbench 14.0, is used to conduct the investigation. The analyses indicated that the flexural capacities of spherically voided biaxial slabs are very close to solid slabs of equal dimension, especially when considering long span slabs. Moreover, the failure mechanisms of SVBS and solid slab system are evaluated and they are found to be almost similar under uniform loads. It is also concluded that the voids around the middle region of the slabs do not significantly compromise the flexural capacity and bending behavior of the slab system. However, the punching shear resistance of SVBS is determined to be considerably less than that of solid slab.

Finite element modeling of behavior of mass concrete placed on soil

Abstract: This dissertation presents the development of a finiteelement model for the prediction of temperatures and cracking potential of massconcrete footings placed on soil. To evaluate the effectiveness of thetemperature predictions from the model, three different bridge pier footings inFlorida weremonitored for temperature developments. The measured temperatures were comparedwith the predicted results obtained from the model. Isothermal calorimetrytesting was done on the cementitious materials of concrete mixtures todetermine the energy released during hydration, which was then converted totemperature rise as inputs for the finite element model. Analysis of influencesof thermal properties of soil on temperature development and cracking in massconcrete footings was conducted. A parametric study on the effects ofdimensions of three types of rectangular footings on the maximum allowabletemperature differential to prevent cracking in concrete was conducted. Auser-friendly computer program called “DIANA Input File Generator” wasdeveloped to provide the needed input files to the TNO DIANA software formodeling of typical mass concrete structures such as rectangular footings andcolumns. The developed finite element model in this study predictedtemperatures reasonably well in mass concrete footings based on the comparisonsof the computed and measured temperatures. The thermal properties of the soilupon which the footing is placed have great influence on the temperature developmentof the concrete and thus the soil needs to be properly modeled in the analysis.From the parametric study conducted, it was found that bottom insulation wouldnot be needed when a mass concrete footing is placed on dry soil, or on soilwith an R-value of 0.41 or greater. Smaller footings do not require a smallermaximum allowable temperature differential to prevent cracking by thermalcontraction. Recommendations on evaluating the thermal properties ofsoil in different in situ conditions, monitoring of footings directly placed onsoil, and development of a data base of rate of heat production of differentcement blends are presented. ( en )