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CONTENTS
Volume 47, Number 2, July25 2013
 

Abstract
Flexible behaviors in new aerospace structures can lead to a degradation of their control and guidance system and undesired performance. The objectives of the current work are to analyze the vibration resulting from the propulsion force on a Single Stage to Orbit (SSTO) launch vehicle (LV). This is modeled as a follower force on a free-free Euler-Bernoulli beam consisting of two concentrated masses at the two free ends. Once the effects on the oscillation of the actuators are studied, a solution to reduce these oscillations will also be developed. To pursue this goal, the stability of the beam model is studied using Ritz method. It is determined that the transverse and rotary inertia of the concentrated masses cause a change in the critical follower force. A new dynamic model and an adaptive control system for an SSTO LV have been developed that allow the aerospace structure to run on its maximum bearable propulsion force with the optimum effects on the oscillation of its actuators. Simulation results show that such a control model provides an effective way to reduce the undesirable oscillations of the actuators.

Key Words
Single Stage to Orbit Launch Vehicle (SSTO LV); beam stability; follower force; vibration reduction; adaptive algorithm

Address
O. Kavianipour : Department of Mechanical Engineering, Damavand Branch, Islamic Azad University, Damavand, Iran
A.M. Khoshnood and S.H. Sadati : Department of Aerospace and Mechanical Engineering, K.N.T. University of Technology, Tehran, Iran

Abstract
Methods for estimating structural reliability using probability ideas are well established. When the residual ultimate strength of a buried pipeline is exceeded the limit, breakage becomes imminent and the overall reliability of the pipe distribution network is reduced. This paper is concerned with estimating structural failure of underground flexible pipes due to corrosion induced excessive deflection, buckling, wall thrust and bending stress subject to externally applied loading. With changes of pipe wall thickness due to corrosion, the moment of inertia and the cross-sectional area of pipe wall are directly changed with time. Consequently, the chance of survival or the reliability of the pipe material is decreased over time. One numerical example has been presented for a buried steel pipe to predict the probability of failure using Hasofer-Lind and Rackwitz-Fiessler algorithm and Monte Carlo simulation. Then the parametric study and sensitivity analysis have been conducted on the reliability of pipeline with different influencing factors, e.g. pipe thickness, diameter, backfill height etc.

Key Words
probability of failure; flexible pipes; Hasofer-Lind and Rackwitz-Fiessler algorithm; Monte Carlo simulation; deflection; buckling; wall thrust; bending stress

Address
Kong Fah Tee, Lutfor Rahman Khan and Hua-Peng Chen : Department of Civil Engineering, University of Greenwich, UK

Abstract
In flexural strength design of normal-strength concrete (NSC) beams, it is commonly accepted that the distribution of concrete stress within the compression zone can be reasonably represented by an equivalent by a and B to stipulate the width and depth of the stress block. Currently in most of the reinforced concrete (RC) design codes, a and B are usually taken as 0.85 and 0.80 respectively for NSC. Nonetheless, in an experimental study conducted earlier by the authors on NSC columns, it was found that a increases significantly with strain gradient, which means that larger concrete stress can be developed in flexure. Consequently, less tension steel will be required for a given design flexural strength, which improves the ductility performance. In this study, the authors\' previously proposed strain-gradient-dependent concrete stress block will be adopted to produce a series of design charts showing the maximum design limits of flexural strength and ductility of singly-and boubly- NSC beams. Through the design charts, it can be verified that the consideration of strain gradient effect can improve significantly the flexural strength and ductility design limits of NSC beams.

Key Words
beams; concrete stress block; design; ductility; normal-strength concrete; strain gradient; strength

Address
J.C.M. Ho : School of Civil Engineering, The University of Queensland, QLD 4072, Australia
J. Peng : Department of Civil Engineering, The University of Hong Kong, Hong Kong

Abstract
A procedure employing a Teaching-Learning Based Optimization (TLBO) method is developed to design discrete pin jointed structures. TLBO process consists of two parts: the first part represents learning from teacher and the second part illustrates learning by interaction among the learners. The effectiveness of the TLBO method is demonstrated on the four design optimization problems. The results are compared with those obtained using other various evolutionary optimization methods considering the best solution, average solution, and computational effort. Consequently, the TLBO algorithm works effectively and demonstrates remarkable performance for the optimization of engineering design applications.

Key Words
optimization; algorithms; trusses; structural design; teaching learning based optimization

Address
Vedat Toğan : Karadeniz Technical University, Department of Civil Engineering 61080 Trabzon, Turkey

Abstract
The term \"constructability\" in regard to cast-in-place concrete construction refers mainly to the ease of reinforcing steel placement. Bar congestion complicates steel placement, hinders concrete placement and as a result leads to improper consolidation of concrete around bars affecting the integrity of the structure. In this paper, a multi-objective approach, based on the non-dominated sorting genetic algorithm (NSGA-II) is developed for optimal design of reinforced concrete cantilever retaining walls, considering minimization of the economic cost and reinforcing bar congestion as the objective functions. The structural model to be optimized involves 35 design variables, which define the geometry, the type of concrete grades, and the reinforcement used. The seismic response of the retaining walls is investigated using the well-known Mononobe-Okabe analysis method to define the dynamic lateral earth pressure. The results obtained from numerical application of the proposed framework demonstrate its capabilities in solving the present multi-objective optimization problem.

Key Words
reinforced concrete cantilever retaining wall; constructability; reinforcing bar congestion; multi-objective optimization; non-dominated sorting genetic algorithm

Address
A. Kaveh, M. Kalateh-Ahani and M. Fahimi-Farzam : Centre of Excellence for Fundamental Studies in Structural Engineering, School of Civil Engineering, Iran University of Science and Technology, Narmak, Tehran-16, Iran

Abstract
It is evident that torsional resistance of a reinforced concrete (RC) member is attributed to both concrete and steel reinforcement. However, recent structural design codes neglect the contribution of concrete because of cracking. This paper reports on the results of an experimental and numerical investigation into the torsional capacity of concrete beams reinforced only by longitudinal rebars without transverse reinforcement. The experimental investigation involves six specimens tested under pure torsion. Each specimen was made using a cast-in-place concrete with different amounts of longitudinal reinforcements. To create the torsional moment, an eccentric load was applied at the end of the beam whereas the other end was fixed against twist, vertical, and transverse displacement. The experimental results were also compared with the results obtained from the nonlinear finite element analysis performed in ANSYS. The outcomes showed a good agreement between experimental and numerical investigation, indicating the capability of numerical analysis in predicting the torsional capacity of RC beams. Both experimental and numerical results showed a considerable torsional post-cracking resistance in high twist angle in test specimen. This post-cracking resistance is neglected in torsional design of RC members. This strength could be considered in the design of RC members subjected to torsion forces, leading to a more economical and precise design.

Key Words
cracking torque; post-cracking torsional resistance; RC beams; longitudinal rebars; FE analysis

Address
A.H. Khagehhosseini, R. Porhosseini, R. Morshed : School of Civil Engineering, Yazd University, Yazd, Iran
A. Eslami : School of Civil Engineering, The University of Queensland, Brisbane, Australia

Abstract
A study on economic performances of consecutive multi-span suspension bridges is carried out. In this part of the study, material amount and structural cost estimation formulas of the bridges is derived based on the structural ultimate carrying capacity. The bridge cost includes the part of superstructure and the part of substructure. Three types of bridge foundations, bored piles, concrete caissons and floating foundations, are considered in substructure. These formulas are to be used for the parametric study of the bridge cost in order to define its more economical layout under different conditions in the part two of the study.

Key Words
multi-span suspension bridge; economic performance; economical layout; cost estimation formula

Address
Li-wen Zhang, Ru-cheng Xiao, Bin Sun, Yang Jiang, Xue-yi Zhang : Department of Bridge Engineering, Tongji University, 1239 Siping Rd., Shanghai 200092, China
Dong-li Zhuang, Yun-gang Zhou : Architectural Design & Research Institute of Tongji University (Group) Co., Ltd., Shanghai 200092, China
Xue Tu : LIN Tung-yan & LI Guo-hao Consultants LTD., Shanghai 200092, China

Abstract
Economic performances of consecutive multi-span suspension bridges are studied. The material amount and cost estimation formulas of the bridges have been derived in the part 1 of the study. A parametric study is carried out based on the formulas for investigating the different factors\' effect on the bridge cost. The factors include the bridge sag, the bridge span, the bridge foundation and the environment condition, etc. Then, an economical layout of the bridges is proposed for different conditions. Lastly, a selection of suspension bridge types is discussed based on the economy of bridges.

Key Words
multi-span suspension bridge; economic performance; economical layout; cost estimation formula

Address
Li-wen Zhang, Ru-cheng Xiao, Bin Sun, Yang Jiang, Xue-yi Zhang : Department of Bridge Engineering, Tongji University, 1239 Siping Rd., Shanghai 200092, China
Dong-li Zhuang, Yun-gang Zhou : Architectural Design & Research Institute of Tongji University (Group) Co., Ltd., Shanghai 200092, China
Xue Tu : LIN Tung-yan & LI Guo-hao Consultants LTD., Shanghai 200092, China


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