Girum S. Urgessa

Dynamic Response of Retrofitted Masonry Walls for Blast Loading

A full-scale blast test was conducted on eight masonry walls reinforced with two and four layers of carbon fibers and two types of polymer matrices. The walls were then subjected to a 0.45-kg pentolite booster suspended from the ceiling of a test structure. The pressure-time history caused by the blast and the resulting displacement response were measured during the test. This paper presents a summary of the test program and the corresponding results from a nonlinear single degree of freedom analysis. The results provide a basis for determining effective means of retrofitting existing masonry walls and designing new structures to withstand blast loads. The paper also outlines a fiber-reinforced polymer retrofit design procedure for walls subjected to blast loading.

Vibration properties of beams using frequency-domain system identification methods

The application of system identification methods to a number of civil engineering structures is increasing in order to improve the understanding of actual structural behaviors and augment traditional analytical assessments. The identification methods are typically used to determine structural dynamics properties (frequencies, damping ratios and mode shapes) and detection of local structural damages and deterioration. This paper presents two frequency domain system identification methods for determining structural vibration properties: eigensystem realization algorithm and the McKelvey frequency domain subspace algorithm. The methods were used in order to formulate a mathematical model that closely matches the frequency response function obtained from a vibration experiment conducted on an uncontrolled cantilever plate. Natural frequencies and damping ratios were determined using the system identification methods and the results are compared with a finite element analysis. A pulse response for a step input is simulated based on the state-space model system matrices obtained from the identification.

Optimum design of 3D reinforced concrete frames using DMPSO algorithm

Abstract Evolutionary optimization algorithms are widely used for solving optimization problems. However, in the realm of real-world structures finding the optimum design would be difficult and time-consuming due to large number of design variables, enormous size of the search space, and availability of numerous constraints. As a result, holistic optimization approaches that consider architectural and other practical requirements in addition to required code provisions are rather limited in real-world structures. This paper presents an innovative algorithm combining multi-criterion decision-making (DM) and Particle Swarm Optimization (PSO), which is called DMPSO, for accelerating convergence toward optimum solution in 3D reinforced concrete (RC) frames. All practical requirements are considered for obtaining directly constructible designs without any further modifications. The effectiveness of the proposed algorithm is illustrated in optimization of 3D RC frames subjected to lateral seismic forces according to ASCE 7 requirements. The results confirm the ability of the proposed DMPSO algorithm to efficiently find optimal solutions for 3D RC optimization problems.

Rebar cage construction and safety (best practices)

This volume describes best practices for construction engineering and construction practice regarding rebar cages. It will be useful to construction engineers working with large columns for the first time as well as construction contractors looking to establish policies and procedures related to rebar column safety.

Significance of Stress-Block Parameters on the Moment Capacity of Sections Under-Reinforced with FRP

Synopsis: For under-reinforced concrete sections reinforced with FRP, failure of a member is initiated by rupture of the FRP bar and the typical ACI compression stressblock might not be applicable. This is because of the fact that the corresponding strain at the extreme fiber of the concrete will not reach the ultimate strain in concrete. Therefore, accurate computation of flexural capacity requires developing equivalent stress-block parameters that represent the stress distribution in the concrete at a particular strain level. While the ACI 440 permits the use of a simplified approach to calculate moment capacities that do not require equivalent stress-block calculations, the significance of this simplification needs to be examined. This paper suggests a family of curves based on the extreme fiber strain in concrete using three existing stress-strain models. The paper highlights the significance of these curves for different values of compressive strengths of concrete.

Review of Quasi-Analytical and Cavity Expansion Methods for Projectile Penetration of Concrete Targets

This paper presents state-of-the art review on the development of quasi-analytical and cavity expansion methods that are used to determine projectile penetration of targets, dating back to the middle of the 20th century. The paper briefly describes the three general approaches used in determining the impact mechanics of targets subjected to projectiles; quasi-analytical, analytical and numerical approaches. Then, the paper provides detailed review of quasi-analytical and analytical methods because they are quite important in characterizing small-scale or large-scale experimental data; developing a basic understanding of the penetration mechanics; and making faster predictions of global parameters such as penetration depth within the limits of applicability of the methods. Quasi-analytical methods are employed when the physical phenomenon being described is highly complex and dependent upon variables that are difficult to isolate and control. These methods often have, at their core, fundamental basis in physics such as Newtons second law of motion. A list of well-known quasi-analytical equations used for determining depth of penetration into concrete targets is provided. The Sandia National Laboratory equations are provided as an example because they are developed from comprehensive database of tests. Analytical methods attempt to provide a correlation between many of the variables in the phenomenon being modeled. They are typically based on solving differential equations of continuum mechanics. However, these methods typically rely on assuming material properties which are necessary to arrive at closed-form solutions. A detailed review of analytical equations that are based on the cavity expansion method, supplemented with experimental data obtained from penetration experiments, is provided. Although the majority of the review includes analytical equations used for concrete targets, these equations trace their origin in the earlier works of penetration of metal and soil targets, which are included in cavity expansion method chronology provided in the paper.

Concrete Material Flow during Projectile Penetration

The penetration of ogive-nose projectiles into concrete targets is investigated using finite element analyses. The Elastic-Plastic Impact Computation (EPIC) is used to examine the velocity vector fields of high and low-strength target material during concrete penetration. The velocity fields represent the flow of the target material around the nose of the projectile, and provide information about the concrete’s penetration resistance behavior. The simulations show that material flow patterns transition during a penetration event. The dependence of this transition point as a function of concrete strength and impact velocity is presented. A Spherical Expansion Comparison Methodology (SECM) is developed to provide possible physical interpretations of the observed transition points. Structural engineers and model developers looking to improve the accuracy of current analytical concrete penetration models will benefit from the findings of this paper.

Computational Model for Internal Relative Humidity Distributions in Concrete

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.

Accumulation and head-loss characteristics of selected pressurized water reactor loca-generated debris

Abstract In the event of a loss-of-coolant accident within the containment of a pressurized water reactor (PWR), piping thermal insulation and other materials in the vicinity of the break will be dislodged by break jet impingement. A series of tests was conducted on two different closed-loop test setups that were specifically designed to study the accumulation of debris and the consequent head loss across sump screens in PWRs. This paper addresses issues related to accumulation of transported debris on the sump screen and the consequent head loss. New test data that provide insights on head loss across a debris bed consisting of fragments of calcium silicate were generated.

Review of Polymer Coatings Used for Blast Strengthening of Reinforced Concrete and Masonry Structures

This paper presents a literature review on the use of polymer coatings in strengthening reinforced concrete and masonry structures against the effects of blast. The use of glass and carbon fiber-reinforced polymers (GFRP and CFRP) in blast strengthening applications has been studied very well in the past two decades. However, experimental and numerical studies of polymer coatings used in blast strengthening applications are scarce in literature. This paper compiles the available experimental and numerical studies that utilized polymer coatings as protective layers, including, but not limited to, polyurea and polyurethane spray-on polymers.