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CONTENTS
Volume 16, Number 6, June 2014
 

Abstract
A new mathematical model and its finite element formulation for the non-linear stress-strain analysis of a planar beam strengthened with plates bolted or adhesively bonded to its lateral sides is presented. The connection between the layers is considered to be flexible in both the longitudinal and the transversal direction. The following assumptions are also adopted in the model: for each layer (i.e., the beam and the side plates) the geometrically linear and materially non-linear Bernoulli\'s beam theory is assumed, all of the layers are made of different homogeneous non-linear materials, the debonding of the beam from the side-plates due to, for example, a local buckling of the side plate, is prevented. The suitability of the theory is verified by the comparison of the present numerical results with experimental and numerical results from literature. The mechanical response arising from the theoretical model and its numerical formulation has been found realistic and the numerical model has been proven to be reliable and computationally effective. Finally, the present formulation is employed in the analysis of the effects of two different realizations of strengthening of a characteristic simply supported flexural beam (plates on the sides of the beam versus the tension-face plates). The analysis reveals that side plates efficiently enhance the bearing capacity of the flexural beam and can, in some cases, outperform the tensile-face plates in a lower loss of ductility, especially, if the connection between the beam and the side plates is sufficiently stiff.

Key Words
externally plated beam; side-plated beam; tension-face plated beam; longitudinal and transverse slips; Reissner beam; reinforced concrete beam; numerical model

Address
(1)Tomaž Hozjan, Aleš Kroflič, Miran Saje, Igor Planinc
University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova 2, SI-1115 Ljubljana, Slovenia;
(2) Jerneja Kolšek:
ZAG - Slovenian National Building and Civil Engineering Institute, Dimičeva 12, SI-1000 Ljubljana, Slovenia.

Abstract
An exact dynamic stiffness method is introduced for investigating the free vibration characteristics of the steel-concrete composite beams consisting of a reinforced concrete slab and a steel beam which are connected by using the stud connectors. The elementary beam theory is used to define the dynamic behaviors of the two beams and the relative transverse deformation of the connectors is included in the formulation. The dynamic stiffness matrix is formulated from the exact analytical solutions of the governing differential equations of the composite beams in undamped free vibration. The application of the derived dynamic stiffness matrix is illustrated to predict the natural frequencies and mode shapes of the steel-concrete composite beams with seven boundary conditions. The present results are compared to the available solutions in the literature whenever possible.

Key Words
steel-concrete composite beams; classical beam theory; free vibration; dynamic stiffness method; analytical models

Address
Key Laboratory of High Performance Ship Technology of Ministry of Education, School of Transportation, Wuhan University of Technology, Wuhan, China.

Abstract
The paper is reporting some comparisons between experimental and numerical results in terms of failure mode, failure time and ballistic properties of mild steel sheet. Several projectile shapes have been considered to take into account the stress triaxiality effect on the failure mode during impact, penetration and perforation. The initial and residual velocities as well as the failure time have been measured during the tests to estimate more physical quantities. It has to be noticed that the failure time was defined using a High Speed Camera (HSC). Thanks to it, the impact forces (average and maximum level), were analyzed using numerical simulations together with an analytical description coupled to experimental observations. The key point of the model is the consideration of a shape function to define the pulse loading during perforation.

Key Words
impact; perforation; dynamic failure; experiment; simulation; impact force

Address
(1) Tomasz Jankowiak:
Institute of Structural Engineering, PUT, Poznan, Poland;
(2) Alexis Rusinek, K.M. Kpenyigba:
LaBPS, National Engineering School of Metz, Metz, France;
(3) Raphaël Pesci:
LEM3 UMR CNRS 7239, ENSAM-Arts et Métiers ParisTech CER of Metz, Metz, France.

Abstract
A single plate shear connection, or shear tab, is a very popular shear connection due to its merit in ease of construction and material economy. However, problems in understanding the connection behavior, both in terms of strength and ductility, have been well-documented. Suggestions or design model for single plate connections in AISC Design Manual have been altered several times, with the latest edition settling down to giving designers pre-calculated design strength tables if the connection details agree with given configurations. Results from many full-scale tests and finite element models in the past suggest that shear strength of a bolt group in single plate shear connections might be affected by yield strength of plate material; therefore, this research was aimed to investigate and clarify effects of plate yield strength and thickness on shear strength of the bolt group in the connections, including the validity of using a plate thickness/bolt diameter ratio (tp/db) in design, by using finite element models. More than 20 models have been created by using ABAQUS program with 19.0- and 22.2-mm A325N bolts and A36 and Gr.50 plates with various thicknesses. Results demonstrated that increase of plate thickness or plate yield strength, with the tp/db ratio remained intact, could significantly reduce shear strength of the bolt group in the connection as much as 15 percent. Results also confirmed that the tp/db ratio is a valid indicator to be used for guaranteeing strength sufficiency. Because the actual ratio recommended by AISC Design Manual is tp/db + 1.6 (mm) for connections with a number of bolts less than six and plate yield strength in construction is normally higher than the nominal value used in design, it is proposed that shear strength of a bolt group in single plate connections with a number of bolts equal or greater than seven be reduced by 15 percent and the tp/db ratio be limited to 0.500.

Key Words
ABAQUS; finite element; shear strength of bolt group; single plate shear connection

Address
Department of Civil Engineering, King Mongkut's University of Technology Thonburi, Bangkok 10140, Thailand.

Abstract
The mechanical characteristic of reinforced concrete filled steel tubular (RCFT) structures are differed from that of concrete filled tubular steel (CFT) structures because the reinforcement in RCFT largely affects the performance of core concrete such as ductility, strength and toughness, and hence the performance of RCFT should be evaluated differently from CFT. To examine the effect axial reinforcement on bending performance, an investigation on RCFT beams with varying levels of axial reinforcement is performed by the means of numerical parametric study. According to the numerical simulation results with 13 different ratios of axial reinforcement, it is concluded that the reinforcement has obvious effect on bending capacity, and the neutral axis of RCFT is different from CFT, and an evaluation equation in which the effect of axial reinforcement is considered for ultimate bending strength of RCFT is proposed.

Key Words
RCFT; axial reinforcement; numerical simulation; neutral axis; bending strength

Address
(1) Alifujiang Xiamuxi, Akenjiang Tuohuti:
College of Architecture and Civil Engineering, Xinjiang University, Urumqi 830047, China;
(2) Akira Hasegawa:
Department of Environmental and Civil Engineering, Hachinohe Institute of Technology, Hachinohe, Japan.

Abstract
This paper describes a series of compression tests performed on cold-formed steel channel sections with perforations in the web (thermal studs) fabricated from a galvanized steel plate whose thickness ranged from 1.0 mm to 1.6 mm and nominal yield stress was 295 MPa. The structural behavior and performance of thermal studs undergoing local, distortional, or flexural-torsional buckling were investigated experimentally and analytically. The compression tests indicate that the slits in the web had significant negative effects on the buckling and ultimate strength of thin-walled channel section columns. The compressive strength of perforated thermal studs was estimated using equivalent solid channel sections of reduced thickness instead of the studs. The direct strength method, a newly developed and adopted alternative to the effective width method for designing cold-formed steel sections in the AISI Standard S100 (2004) and AS/NZS 4600 (Standard Australia 2005), was calibrated to the test results for its application to cold-formed channel sections with slits in the web. The results verify that the DSM can predict the ultimate strength of channel section columns with slits in the web by substituting equivalent solid sections of reduced thickness for them.

Key Words
thermal studs; slits; perforations; ultimate strength; equivalent thickness; direct strength method

Address
(1) Young Bong Kwon:
Department of Civil Engineering, Yeungnam University, Gyongsan, 712-749, Korea;
(2) Gap Deuk Kim:
Department of Steel Research, RIST, Incheon, Korea;
(3) In Kyu Kwon:
Department of Fire Protection Engineering, Kangwon National University, Samcheok, Korea.

Abstract
The main aim of this study is to propose an efficient meta-heuristic algorithm for topology optimization of geometrically nonlinear single layer domes by serially integration of computational advantages of firefly algorithm (FA) and particle swarm optimization (PSO). During the optimization process, the optimum number of rings, the optimum height of crown and tubular section of the member groups are determined considering geometric nonlinear behaviour of the domes. In the proposed algorithm, termed as FA-PSO, in the first stage an optimization process is accomplished using FA to explore the design space then, in the second stage, a local search is performed using PSO around the best solution found by FA. The optimum designs obtained by the proposed algorithm are compared with those reported in the literature and it is demonstrated that the FA-PSO converges to better solutions spending less computational cost emphasizing on the efficiency of the proposed algorithm.

Key Words
optimal design; meta-heuristic; firefly algorithm; lattice dome; geometrical nonlinearity

Address
Department of Civil Engineering, Urmia University, Urmia, West Azerbaijan Province, Iran.

Abstract
The initial clamping forces of high strength bolts subjected to different faying surface conditions drop within 500 hours regardless of loading, any other external force or loosening of the nut. This study develops a mathematical model for relaxation confined to creep on a coated faying surface after initial clamping. The quantitative model for estimating relaxation was derived from a regression analysis for the relation between the creep strain of the coated surface and the elapsed time for 744 hours. This study establishes an expected model for estimating the relaxation of bolted joints with diverse coated surfaces. The candidate bolts are dacro-coated tension control bolts, ASTM A490 bolt, and plain tension control bolts. The test parameters were coating thickness, species of coating. As for 96, 128, 168, and 226 μm thick inorganic zinc, when the coating thickness was increased, relaxation after the initial clamping rose to a much higher range from 10% to 18% due to creep of the coating. The amount of relaxation up to 7 days exceeded 85% of the entire relaxation. From this result, the equation for creep strain can be derived from a statistical regression analysis. Based on the acquired creep behavior, it is expected that the clamping force reflecting relaxation after the elapse of constant time can be calculated from the initial clamping force. The manufacturer's recommendation of inorganic zinc on faying surface as 75 μm, appears to be reasonable.

Key Words
slip critical joint; high-strength bolts; relaxation; coating; creep

Address
(1) Hwan-Seon Nah, Hyeon-Ju Lee:
Smart Energy Lab., Korea Electric Power Corporation Research Institute, 65 Munji-Ro Yusung-Gu, Daejon, 305-380, Korea;
(2) Sung-Mo Choi:
The University of Seoul, Department of Architectural Engineering, 163 Siripdaero, Dongdaemun-gu, Seoul, 130-743, Korea.


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