Simple Design Model Research Articles

Simple Plastic Model for Shear Critical SFRC Beams

A simple physical model, for prediction of ultimate shear strength of steel fiber-reinforced concrete (SFRC) beams is developed on the basis of a plastic approach originally proposed for reinforced concrete beams without stirrups. It is founded on the hypothesis that cracks can be transformed into yield lines, and thus is know as crack sliding model (CSM). First, the CSM is improved to take into account the shear strength increase for deep beams, due to the arch effect. Then, the effectiveness factors for fibrous concrete under biaxial stresses are evaluated, taking into account the postcracking tensile strength of SFRC and its ability to control slippage along shear cracks. With the aim of providing a handy and fast tool for design of SFRC beams without stirrups, a simplified design model is also derived. Finally, the proposed models are validated by the results of a large set of experimental tests taken from literature.

Nonlinear relationships between parameters of simplified diffusion-based model for service life design of concrete structures exposed to chlorides

Abstract This paper presents an approach toward performance-based durability design of concrete structures based on a simplified diffusion-based model, which overcomes some of the limitations of the current prescriptive rules of existing design standards. Numerical nonlinear relationships and relative sensitivities between the four parameters that govern the time to corrosion initiation of reinforced concrete structures are determined, including chloride diffusion coefficient, chloride threshold value of reinforcement, concrete cover and surface chloride exposure condition. To achieve a specified design life, it is found that in aggressive chloride-laden environments: (i) increasing the minimum concrete cover is more effective than selecting a corrosion-resistant steel; (ii) it is necessary to use both high-performance concrete and corrosion-resistant steel; (iii) a relative decrease in the concrete cover has to be compensated by a much greater increase in the corrosion resistance of steel; and (iv) the variations in chloride diffusion coefficient have a large impact on required concrete cover and chloride threshold value, especially in increasingly aggressive environments. These relationships can be used to identify the most relevant parameters and serve as a guide for service life-based design for durable concrete structures.

Development of a fast empirical design model for PEM stacks

A simple and fast empirical design model for a 5 kW proton exchange membrane (PEM) stack is presented in this paper. The performance analysis of the PEM stack operating on a membrane humidifying method is made through a series of experiments, including current–voltage–power characteristics, uniformity of cell unit voltages, gas pressure impact and air flux impact. Based on the above analysis, an empirical predicted model for the PEM stack has been developed by the combination of mechanistic and empirical modeling approaches to characterize and predict the voltage–current characteristics without examining in depth all physical/chemical phenomena. The good agreement between the predicted and experimental results covering a range of optimal operating conditions shows that the proposed model provides an accurate representation of the behavior for the PEM stack.

On the design of phosphors for high-temperature thermometry

The temperature-dependent luminescence of certain thermographic phosphors has been used in surface thermometry for a number of years. Of the two practical methods by which temperature can be deduced from the emission [Heyes, A.L., Thermographic Phosphor Thermometry— Physical Principles and Measurement Capability, In: C.H. Sieverding, J.-F. Brouckaert (Eds.), VKI Lecture Series on Advanced Measurement Techniques for Aero and Stationary Gas Turbines. 2004, von Karman Institute for Fluid Dynamics, Brussels, (Vol. Lecture Series 2004–04)], the one considered herein relies on thermally activated non-radiative transitions from the excited to ground state making the decay rate in luminescence temperature dependent. The time constant of the exponential decay in luminescence can then be used to derive the temperature. The authors and co-workers have been working for some years now on the application of this method for condition monitoring and non-destruction evaluation in the hot sections of gas turbines. Turbine entry temperature is linked to efficiency and thereby to CO 2 emissions so that there is a clear imperative to increase it. Current entry temperatures exceed the melting points of the metals from which local components such as combustion liners, nozzle guide vanes and turbine blades are made. Thermal management of these components is therefore critical to the operation of the engine and by implication sensors and instrumentation are required to aid the design of insulation and cooling schemes and to monitor their performance in service. Thermographic phosphors typically consisting of a ceramic host (e.g. YAG) with a rare-earth dopant are excellent candidates. These materials are stable at the required temperatures and the temperature dependence of their luminescence can provide a dynamic range that matches the expected environment. In this paper, the physics of the multi-phonon non-radiative decay process are reviewed and used to construct a simple design model for thermographic phosphors. Comparison with experimental data shows general agreement but there are several anomalous cases. These are reviewed and alternative thermal quenching mechanisms considered.

State of Research on Seismic Retrofit of RC Beam-Column Joints with Externally Bonded FRP

A considerable amount of research has been directed recently toward understanding and promoting the use of externally applied fiber-reinforced polymer (FRP) for the seismic retrofit of reinforced concrete (RC) structures. In this paper, a comprehensive review and synthesis of published experimental studies on the seismic rehabilitation of RC frame beam-column joints with FRP is presented, and the issues that need to be addressed for further research are discussed. In addition, the paper presents a simple design model for predicting the contribution of the FRP to the shear strength of retrofitted joints. The key element in the model is the derivation of an expression for the effective FRP strain, based on the calibration of test data reported in the literature. A total of 54 tests carried out worldwide were considered in the review, and a database of the published studies, encompassing all relevant design parameters, was assembled. The reported test results confirm the structural effectiveness of the FRP s...

The experimental validation of a simplified PEMFC simulation model for design and optimization purposes

Geometric design, including the internal structure and external shape, considerably affect the thermal, fluid and electrochemical characteristics of a polymer electrolyte membrane fuel cell (PEMFC), which determines the polarization curves as well as the thermal and net power responses. In order to predict the response of PEM fuel cells according to the variation of manufacturing materials physical properties, operating and design parameters, a reliable simulation model (and computationally fast) is necessary, which accounts for the power losses due to pressure drops in the gas channels. In this paper, a simplified and comprehensive PEMFC mathematical model introduced in previous studies is experimentally validated. Numerical results are obtained with the model for an existing set of ten commercial unit PEM fuel cells. The model accounts for pressure drops in the gas channels, and for temperature gradients with respect to space in the flow direction, and current increase that are investigated by direct infrared imaging, showing that even at low current operation such gradients are present in fuel cell operation, and therefore should be considered by a PEMFC model, since large coolant flow rates are limited due to induced high pressure drops in the cooling channels. The computed polarization and power curves are directly compared to the experimentally measured ones with good qualitative and quantitative agreement. The combination of accuracy and low computational time allow for the future utilization of the model as a reliable tool for PEMFC simulation, control, design and optimization purposes.

Local buckling of aluminium structures exposed to fire. Part 1: Tests

This paper describes an experimental investigation into local buckling of compressed aluminium alloy sections at elevated temperatures. Stress–strain relationships are derived based on uniaxial tensile tests. A special test set-up with a furnace is developed to test slender square hollow sections and angular sections in compression. The results of the compression tests are compared with simple design models. Methods are presented to determine geometrical imperfections and residual stresses. The test programme has resulted in a set of test data that can be used to validate numerical or analytical models.

A Simplified Design Model for Random Process Variability

A method for modeling the effects of random process variation through measured transistor current is introduced. The methodology culminates by modeling random current variation at a given operating point above threshold as a zero-mean Additive White Gaussian Noise (AWGN) current source with a standard deviation dependent on nominal operating current and design variables, the transistor sizes, W and L, and the transistor operating points, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gs</sub> and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ds</sub> . The model has a simple posynomial form and is accurate compared to measured data within an RMS error of 5.4% for narrow-, wide-, short-, and long-channel transistors. The model's simplified form bridges the gap between existing statistical methods and circuit design. The efficacy of the model in the circuit design space will also be presented. In the analog domain, we will show that the contributions of this model can be used as a replacement for Monte Carlo methods for calculating circuit voltage and current variances. In the digital space, we will investigate the calculation of timing delay distributions. Results calculated via an alpha-power (α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ) law model for average current show that scaling the supply voltage by γ results in an approximately 1/γ scaling in the path-delay standard deviation for a 65-nm process.

Model Study on a Submerged Catalysis/Membrane Filtration System for Phenol Hydroxylation Catalyzed by TS-1

This research is focused on the development of a simple design model of the submerged catalysis/membrane filtration (catalysis/MF) system for phenol hydroxylation over TS-1 based on the material balance of the phenol under steady state and the reported kinetic studies. Based on the developed model, the theoretical phenol conversions at steady state could be calculated using the kinetic parameters obtained from the previous batch experiments. The theoretical conversions are in good agreement with the experimental data obtained in the submerged catalysis/MF system within relative error of ±5%. The model can be used to determine the optimal experimental conditions to carry out the phenol hydroxylation over TS-1 in the submerged catalysis/MF system.

Behavior of RC Beams Subjected to High Rates of Concentrated Loading

The work described in the present article is concerned with the numerical investigation of the dynamic response of reinforced concrete (RC) beams subjected to high rates of transverse loading. Of interest herein is the case of localized impact loading, such as that encountered in the case of contact, impact, and ballistic problems, rather than the case of loads distributed over the entire span of RC structural elements arising particularly from far field explosions. The investigation is based on the use of a commercially available general purpose finite-element (FE) package for nonlinear static and dynamic analysis of three-dimensional FE models. A key feature of our study is the hypothesis that the material properties of concrete and steel reinforcement are independent of the loading rate. Based on this assumption, the effects of the applied loading rate on the exhibited structural response are primarily attributed to the inertia forces that develop within the beam and not to the loading rate sensitivity of the mechanical characteristics of the materials involved. This hypothesis constitutes a major departure from currently accepted design and numerical modeling practices, which adopt exactly the opposite view, thus, providing an alternative explanation as to the causes that affect the complex inelastic response of RC structural elements under high loading rates, as well as the cracking patterns, observed during testing. The results obtained correlate closely with the experimental observation that structural performance, in the form of stiffness, load carrying capacity, and deformability, depart significantly from those recorded under quasi-static loading as certain thresholds of applied loading rates are surpassed, with these changes becoming more pronounced as the rate of loading increases. From the analysis of numerical and experimental predictions, the causes that lead to the above change in behavior are established and a relatively simple design model is proposed, which is able to quantify the observed increase in load-carrying capacity exhibited by RC beams with increasing rates of applied loading as well as the concentration of damage.

A simplified dynamic model for front-end loader design

The front-end loader is an indispensable machine for the off-road construction equipment industry. It is a classic example of a working machine with complex interactions between its subsystems (hydraulic, mechanical, and electrical). Dynamic models of the full-scale vehicle coupled with event-based operator models are currently used to help quantify the overall system performance, efficiency, and operability. However, these models are complex and not always necessary to characterize the response of individual subsystems. There is great value added to the design process — especially in prototyping of new vehicle platforms — in development of simpler models that can quickly and accurately define first-order measures of system loads and performance. This paper presents a subscale dynamic model, which isolates the boom and bucket manipulator systems of the front-end loader for the purpose of design load characterization. The model includes state equations governing the hydraulic dynamics across the control valves and in the cylinders, as well as soil—tool interaction loads (passive earth loads) at the bucket cutting edge. The governing equations of motion for the multi-rigid body model of the bucket linkage are developed using Kane's method. The proposed model is intended to accelerate the structural design and analysis of the boom and bucket linkage subsystems and may yield useful information for optimization purposes. The output from the dynamic simulation is compared with the field test data of the machine.

An adequate theory for the shear strength of reinforced concrete structures

Traditional shear design procedures for concrete structures rely on empirical equations derived from laboratory experiments and lack the type of adequate theory that forms the basis of flexural design. As shear resistance depends on so many variables and because laboratory tests do not cover the full practical range of parameters, empirical shear design equations can result in structures with inadequate shear strength. Recent failures of such structures have highlighted the need for shear design provisions solidly based on an adequate theory. This paper introduces such a theory and explains the simple design models derived from the theory, which include a strut-and-tie model for disturbed regions and a sectional model for flexural regions. These models form the basis of the shear provisions on the 2004 Canadian standard for the design of concrete structures. In the current paper, results from nine major experimental series are compared with predictions from the new shear provisions and with predictions from the shear provisions of the Eurocode and the American Concrete Institute code. It is demonstrated that the new shear design provisions are capable of predicting the shear strength of reinforced concrete members and prestressed concrete members with considerably greater reliability.

Recent developments in numerical modelling of subsurface flow constructed wetlands

Numerical modelling of subsurface flow constructed wetlands (CWs) gained increasing interest during the last years. The main objective of the modelling work is, on the one hand, to increase the insight in dynamics and functioning of the complex CW system by using mechanistic or process based models that describe transformation and degradation processes in detail. As these mechanistic models are complex and therefore rather difficult to use there is, on the other hand, a need for simplified models for CW design. The design models should be premium to the currently used design guidelines that are mainly based on rules of thumb or simple first-order decay models. This paper presents an overview of the current developments on modelling of subsurface flow CWs based on the modelling work and model developments presented at the WETPOL 2007 symposium. Three kinds of models have been presented: simple transport and first-order decay models, complex mechanistic models, and a simplified model that has been developed for design of CWs. The models are presented and selected results are shown and discussed in relation to the available literature.

External beam–column joints: design to Eurocode 2

In practice external beam–column joints are seldom designed for monotonic loading. The current authors believe that this is an oversight which should be addressed. This paper presents a simple strut and tie model for the analysis and design of external reinforced concrete beam–column joints. The strut and tie model is developed from first principles using the concrete design strengths given in Eurocode 2. The main difficulty in developing strut and tie models for beam–column joints is in determining the node dimensions. The novel feature of the authors' analysis is that the joint strength is related to the flexural capacity of the beam at the face of the column which is defined in terms of the maximum moment which can be transferred through the joint into the upper and lower columns. The model is shown to give better predictions of joint shear strength than existing simple design models. A case study is presented which shows that it is often sufficient to provide only minimum shear reinforcement in beam column joints.

A simplified model for lateral response of large diameter caisson foundations—Linear elastic formulation

The transient response of large embedded foundation elements of length-to-diameter aspect ratio D/ B=2–6 is characterized by a complex stress distribution at the pier–soil interface that cannot be adequately represented by means of existing models for shallow foundations or flexible piles. On the other hand, while three-dimensional (3D) numerical solutions are feasible, they are infrequently employed in practice due to their associated cost and effort. Prompted by the scarcity of simplified models for design in current practice, we here develop an analytical model that accounts for the multitude of soil resistance mechanisms mobilized at their base and circumference, while retaining the advantages of simplified methodologies for the design of non-critical facilities. The characteristics of soil resistance mechanisms and corresponding complex spring functions are developed on the basis of finite element simulations, by equating the stiffness matrix terms and/or overall numerically computed response to the analytical expressions derived by means of the proposed Winkler model. Sensitivity analyses are performed for the optimization of the truncated numerical domain size, the optimal finite element size and the far-field dynamic boundary conditions to avoid spurious wave reflections. Numerical simulations of the transient system response to vertically propagating shear waves are next successfully compared to the analytically predicted response. Finally, the applicability of the method is assessed for soil profiles with depth-varying properties. The formulation of frequency-dependent complex spring functions including material damping is also described, while extension of the methodology to account for nonlinear soil behavior and soil–foundation interface separation is described in the conclusion and is being currently investigated.

Ring CMOS NOT-based Oscillators: Analysis and Design

This paper presents design hints in CMOS ring oscillators based on NOT gates. The NOT gate is used as vehicle to introduce basics on signal propagation and also to present simple design models. Both simulation and experimental results are presented in order to show the usefulness of the design models. However, for high frequency oscillators several design strategies that are translated at layout level are also described not only to minimize undesirable effects, but also for testing the circuit under study (CUS).

Simplified design methods of reverse flow catalytic combustors for the treatment of lean hydrocarbon–air mixtures

Three simplified models for reverse flow reactor design proposed in the literature have been compared with a detailed and experimentally validated 1D heterogeneous dynamic model. The model proposed by Cittadini et al., has been found to be the most accurate at the considered operation conditions. Maximum reactor temperature, an important parameter for preventing damage to the catalyst or detecting reactor extinction, has also been determined for all the designs. The costs associated to different RFR design alternatives for the combustion of the compounds studied are also evaluated, in order to determine the optimal economical design.

Broadband optical coupling between microstructured fibers and photonic band gap circuits: Two-dimensional paradigms

We demonstrate high efficiency, broadband coupling between photonic crystal waveguides and photonic crystal fibers PCFs using simple two-dimensional design models. We demonstrate an effective largebandwidth small-footprint beam collimator for light exiting a two-dimensional subwavelength scale photonic crystal waveguide consisting of an air core. This collimator relies on destructive interference between diffracted light from the mouth of the waveguide and the light emitted by surface resonators. We demonstrate efficient coupling between air-core photonic crystal waveguides and various two-dimensional models of photonic crystal fibers. A hollow-core photonic crystal fiber, described by an air defect with a Bragg stack cladding on either side, yields a coupling efficiency of better than 94% over a bandwidth of 25% of the center frequency, with peak transmittance exceeding 98%. A small-mode-area PCF, consisting of a subwavelength solid core attached by spokes to the PCF cladding, is modeled by a slab waveguide. In this latter case, we demonstrate coupling efficiency better than 94% over a bandwidth of 17%, with peak transmittance exceeding 99%. Combining collimation at the photonic crystal exit port and a nonadiabatic taper in a small-mode-area PCF, we obtain over 98% coupling efficiency over a bandwidth of 135 nm centered at a wavelength of 1.5 m. These results provide valuable paradigms for efficient transfer of optical information between photonic band gap PBG microcircuits and PCF-enabled telecommunication networks.

Comparison of Modeling Approaches for Full-scale Nonlinear Viscous Dampers

A study is presented comparing several identification approaches, both parametric and nonparametric, for developing reduced-order nonlinear models of full-scale nonlinear viscous dampers commonly used with large flexible bridges. Such models are useful for incorporation into large-scale computational models, as well as for use as part of structural health monitoring studies based on vibration signature analysis. The paper reports the analysis results from a large collection of experimental tests on a 1112 kN (250 kip) orifice viscous damper under a wide range of frequency and amplitude oscillations. A simplified parametric design model is used in the parametric phase, as well as two different nonparametric methods: the Restoring Force Method, and artificial neural networks. The variations of model parameters with the excitation and response characteristics are investigated, and the relative accuracy and fidelity of the modeling approaches are compared and evaluated.

A new statistical approach for joint shear strength determination of RC beam-column connections subjected to lateral earthquake loading

Reinforced concrete (RC) joint shear strength models are constructed using an experimental database in conjunction with a Bayesian parameter estimation method. The experimental database consists of RC beam-column connection test subassemblies that maintained proper confinement within the joint panel. All included test subassemblies were subjected to quasi-static cyclic lateral loading and eventually experienced joint shear failure (either in conjunction with or without yielding of beam reinforcement); subassemblies with out-of-plane members and/or eccentricity between the beam(s) and the column are not included in this study. Three types of joint shear strength models are developed. The first model considers all possible influence parameters on joint shear strength. The second model contains those parameters left after a step-wise process that systematically identifies and removes the least important parameters affecting RC joint shear strength. The third model simplifies the second model for convenient application in practical design. All three models are unbiased and show similar levels of scatter. Finally, the improved performance of the simplified model for design is identified by comparison with the current ACI 352R-02 RC joint shear strength model.