کریم عابدی

In the present study, the effect of residual stresses on the performance of braced corrugated steel shear panels (Akbari Hamed and Mofid, 2015a; 2015b; 2015c; 2016; 2017; Amiri et al., 2023) was investigated using numerical modeling, and its results were presented. For this purpose, after validating the finite element modeling method by ABAQUS software, the effect of considering the residual stress on the performance of 12 steel shear panel models with different sizes of sub-panel width, the corrugation angle and the panel thickness was investigated by evaluating some important parameters such as the initial stiffness, the dissipated energy and the strength.

CFRP composites are considered as the one of the most effective methods in strengthening reinforced concrete (RC) members. Although this method has many privileges, the results show that CFRPs use only 30% ~ 40% of their capacity during the loading of a retrofitted concrete beam or slab and suffer from debonding before reaching their maximum capacity. To overcome this issue, prestressing the CFRP composite is proposed and utilized. The aim of this research is to investigate the effect of this strengthening method on the flexural behavior and ductility of RC T-beams. In this regard, eight scaled RC T-beams with dimensions (250×300×3000 mm) in four series, control samples, beams strengthened with non-prestressed CFRP, beams strengthened with CFRP prestressed by 20% and 35%, were produced. For CFRP prestressing, an innovative mechanical arrangement was utilized. Furthermore, the beams were subjected to a four-point bending test and their behavior was investigated. The results indicate that prestressing the composite has significantly increased the bearing capacity of the strengthened beams and improved their failure mechanism compared those of the strengthened by non-prestressed CFRP. Also, the results show that the decrease in beam ductility, when the composite is prestressed by 35%, is less than 10% compared to that of strengthened beam with non-prestressed CFRP. In addition, a numerical method for simulating the behavior of reinforced concrete beams with prestressed CFRP using subroutine coding in ABAQUS is presented.

Tensegrity systems have been used as a class of light space structures in the various forms including flat modular systems, barrel vaults and cable domes. Conventional cable domes achieve their stability through a concrete compression ring, which is inconsistent with the self-support characteristic of tensegrity systems. The previous evidences also showed that the presence of a rigid support system such as a concrete ring around the space structure especially subjected to seismic loads may cause more structural damages in boundary region of structure. In this research, the combination of a Levy-type cable dome with a tensegrity ring created by the semi-regular modules is considered, which leads to a new hybrid cable-strut system. Form-finding of the studied structures is carried out using the force density method. After designing the hybrid structure, its initial collapse behavior is evaluated by the numerical finite elements method. In this regard, static and dynamic analyses are performed on both the hybrid structure and the cable dome attached to rigid supports to evaluate the effect of the tensegrity ring on the cable dome behavior and also the adequacy of static analyses to estimate capacity of the structures. After estimating the initial behavior and the collapse mechanism type of the studied structures, their behavior, are improved by equipping a number of critical struts with Force-limiting devices.

In the present study, the effect of ice-shedding simultaneously at the transmission line's span and the wind was investigated using numerical modeling, and its results are presented. The transmission line has five transmission towers and six power transmission spans related to the 400 kV transmission line of the Khoy-Urmia power plant. The transmission tower's weaknesses have been identified using this study's results, which can be considered in strengthening existing towers and the design of new transmission lines.

Determination of the behaviour factor and displacement amplification factor is one of the most important issues in the seismic design of structures. Seismic codes use the behaviour factor to consider the inelastic behavior of structures. The inelastic behavior of structures is defined by ductility factor. This factor refers to the ratio of the maximum structural displacement to the displacement corresponding to the yield strength. So, the determination of the behaviour factor is directly dependent on the ductility factor so that a greater ductility factor leads to the greater behaviour factor. The behaviour factor is related to other factors such as the overstrength, degrees of redundancy and damping. The calculation of the real lateral displacement of structures including the elastic and inelastic parts is essential for design purposes. Seismic codes use displacement amplification factor for the determination of the maximum real lateral displacement of structures  The main objective of this study is to evaluate the effects of the height of the structure and the length of the link beam on the seismic response modification factors including the behaviour factor and displacement amplification factor in eccentrically braced frames (EBFs). Since the seismic behavior of eccentrically braced frames depends on the link beam length, several eccentrically braced building structures with different lengths of the link beam and various numbers of stories including 2, 5, 10 and 15 story buildings were considered. The ASCE 7-10 code was used for seismic loading and the AISC 360-10 and AISC 341-10 codes were used for the design of structural models. The nonlinear static and nonlinear time history analyses were used to obtain the behaviour factor and displacement amplification factor. The values obtained for the behaviour factor and displacement amplification factor were compared with those prescribed by the ASCE 7-10 code. The seismic codes recommend constant values for the behaviour factor and displacement amplification factor for eccentrically braced systems. The results of this study show that the behaviour factor and displacement amplification factor for eccentrically braced structures significantly depend on the height of the structure and the length of the link beam. So, considering constant values for the behaviour factor and displacement amplification factor for eccentrically braced frames in seismic codes is not reasonable. Then, the effects of the height of the structure and the length of the link beam should be considered in these factors. Finally, with regard to the values obtained in this study, some equations were proposed to calculate the mentioned factors based on the length of the link beam and the fundamental period of structures. The results indicate that the behaviour factor and the displacement amplification factor decrease with the increase in the height of the structure and also by the increase in the length of the link beam.

Free form surfaces are referred to as a combination of double-curved surfaces, which are not necessarily a combination of common geometric shapes such as cubic, spherical and elliptic shapes or a combination of them (Tian, 2004). Two types of instability occur in space structures: limit point instability and bifurcation point instability. The structures that exhibit bifurcation point instability, have more sensitivity to geometric imperfection (in particular the type of node deviation) than limit point instability. Therefore, it is necessary to investigate the sensitivity to imperfection in these structures, particularly single layer dome structures. So far, no research has been conducted on triple domes free form space structures. Therefore, in the present paper, the numerical finite element method is used to analyze and study the instability behavior of triple domes free form single-layer space structure.  In this research, the approximate-perturbed method and the method of generalized conformable imperfection mode were used to implement imperfection.

Space structures are the best solution for covering large areas with few or no intermediate supports. These structures have advantages such as light weight, delicate appearance and ease of erection. Despite these advantages, space structures suffer from progressive collapse due to buckling of compression members. In order to prevent the progressive collapse and improve the double layer space structure’s response, several mechanisms have been developed including force limiting devices (FLDs). The available FLDs have never been practically applied to space structures due to serious drawbacks. To improve the previous FLD’s performance, an innovative type of force limiting device is introduced in this study. This force limiting device which is entitled as Accordion Force Limiting Device (AFLD) is designed based on the idea of buckling restrained braces (BRB). To evaluate the efficiency of this new device, a whole steel accordion force limiting device is designed and constructed to be tested under uniaxial compressive loading. Numerical models are also developed to investigate the effect of applying AFLD on stability of double layer barrel vaults. A parametric study is carried out to study the effect of AFLD, considering Length to span ratio and rise to span ratio in double layer barrel vaults. Results indicated that applying AFLD to critical compression members improves the stability behavior noticeably.

Nonlinear time history analysis (NL-THA) is the most accurate method to estimate the seismic demand of structures and predict their failure. To that end, extensive efforts have been made to develop fast and convenient methods to carry out nonlinear static analyses. In recent years, pushover methods have been widely used as a suitable tool to evalute the seismic performance of structures. Also, various advanced pushover procedures have been proposed to take into account the effect of higher modes and the change in the dynamic properties of structures in the nonlinear phase. Therefore, different pushover procedures have been further developed for this purpose. The nonlinear static analysis  has been widely employed to evaluate the nonlinear behavior of structures. The pushover analysis was first expanded in a number of studies to investigate buildings. Not many studies have been conducted on the seismic demand of latticed space structures. In the present work, therefore, an optimization procedure has been employed to refine the performance of the pushover analysis in estimating the seismic response of double-layer barrel vault roofs with vertical double-layer walls. In the  method proposed herein, the coefficients of the modal load combinations of the studied structures have been optimized using the simplex algorithm to find the optimum load pattern. Fifteen models with various rise-to-span and height-to-span ratios were considered to assess the accuracy of the proposed method in predicting the seismic demand of these structures. The models were analyzed using the OpenSees software. In order to model the buckling behavior of the members, each member was divided into two nonlinear beam-column elements with an initial imperfection of 0.1% at its mid-node.  The models were designed with the dead, snow, temperature, and earthquake loads having been considered. All of the mentioned loads, with the exception of snow load, were applied to the structures as concentrated nodal loads. The snow load, by contrast, was applied to the structures in two symmetric and asymmetric patterns in accordance with the sixth volume of the Iranian national code of buildings. For earthquake loads, the 4th edition of the Iranian code of practice for seismic resistant design of buildings was used. It should be noted that the seismic mass of the roof of each model was calculated by considering the entirety of the dead load in addition to 40% of the snow load. In the design process of each model, the dead, snow, temperature, and earthquake load combinations were formulated based on the AISC-ASD89 standard. The sections of the members of the structures were chosen from hollow tubular sections, with  their slenderness ratios limited to 100. Afterwards, pushover analyses were performed using the optimized load pattern. The obtained results were compared to those of the incremental dynamic analyses (IDA) and two other well-known pushover methods, namely the MPA and the conventional first-mode pushover analysis. The results revealed that the proposed pushover method can provide a good estimation of the base shear and intial stiffness of the structures when compared to dynamic analyses. In addition, an increase in the rise-to-span ratio of the roof causes an improvement in the accuracy of the proposed pushover method. Also, in comparison with the  MPA and conventional pushover procedures, the responses produced by the proposed method are closer to those generated by dynamic analyses. In addition, a comparison of the obtained drift patterns reveals that the results of both the pushover and incremental dynamic analyses along the longitudinal direction of the wall are quite close to each other. Another advantage of the proposed pushover method is that it also produces acceptable results on the nodes on the roof of the space structure. Also, along the transverse direction, the proposed method yields better results.

In current study, a novel retrofitting scheme, hybrid confinement using steel reinforcement profiles and FRP wraps was presented for strengthening of columns and plain concrete piers and verified using the results of an experimental work results. Twenty cylindrical concrete specimens were produced in laboratory conditions and energy dissipation, axial deformation, initial stiffness and load-deflection curve was calculated for each specimen. Two of specimen was used as benchmark, however the other were retrofitted using CFRP wraps, GFRP wraps, near surface mounted steel rods, FRP confinement and proposed retrofitting scheme. Based on the results, axial compressive strength, energy dissipated and ductility was enhanced. The axial force-axial displacement behavior of hybrid confined specimens had two parts. In the first part a parabola curve was observed whereas in the second part the behavior was linear. The failure of specimens was brittle and had explosive nature towards CFRP or GFRP rupture. However the results proved that we will be able to use the proposed novel confinement scheme for shear and flexural strengthening of columns and plain concrete piers of road and railroad bridges.

As an innovative type of composite construction, concrete-filled double skin steel tube (CDFST) has the potential to be used as columns in high-rise buildings. CFDST columns consist of two concentrically-arranged inner and outer steel skins with the annulus between the skins filled with concrete. Of prime features of CFDST columns is the interaction between steel tubes and concrete core. The confinement of the steel tubes on the concrete core benefits the interaction along the interfaces. However, when the interface interaction of steel tubes and concrete core vanish, the deformation starts to loom large. The deformation rate of the columns is increased under fire exposure. The current study aims to investigate the effect of interaction of concrete core and inner steel tube on the fire resistance of CFDST columns considering steel links (longitudinal stiffeners) embedded inside the concrete core welded to exterior surface of the inner tube. Finite element models for CFDST columns were developed using ABAQUS package in this study. A sequentially-coupled thermal-stress analysis procedure is conducted to evaluate the effects of inner steel tube- concrete interaction on the fire performance of these columns. The uses of stiffeners in the CFDST columns increase the fire resistance of the columns effectively. The specimens with stiffeners embedded in the exterior surface of the inner tubes exhibited excellent fire resistance. It can be attributed to the fact that interaction of stiffeners, inner tube and concrete core provides adequate bond and friction at the interface surfaces, contributing to the composite action improving strength during the process of fire exposure. This implies that the interaction of concrete and steel components increases the fire performance of these columns significantly, a striking conclusion, which requires strategies to retain such performance. The conclusions, drawn from this study, can in turn, lead to the suggestion of some guidelines for the design of CFDST columns.

Summary : Tabriz metro twin tunnels in some stations are under ground water level. In addition to the loads from the weight of overburden, the load caused by pore water pressure is applied to them. To enter the current load into model certain boundary conditions must be considered that in this study, the probability of liquefaction and the impact of the earthquake on pore pressure loading structure have been investigated.
Geo-technical design of structures is encountered with a variety of different natural conditions (earthquake, liquefaction, etc.). This will be due to the complexity of behaviors and conditions. These structures during his lifetime, because dealing with different situations such as earthquake will not have fixed characteristics and behavior. Complex Mechanical behavior of the soil under statistical loads can be different and more complicated under dynamic loads. In this respect, the modeling of alternative behavior of saturated soil, especially in the interaction zone of soil – structure, has gained increasing research interest over past decades. Because the estimation of structure response in the soil liquefaction depends on pore water pressure, softening strain and decreasing of soil resistance. For correct investigating, linear dynamic analyze in effective stress is necessary. For this type of analyze couple or semi couple methods could be applied. In this regard Finn & Martin and Byrne could be regarded as semi couple models.
Methodology and Approaches : From aforementioned semi couple models, Byrne model was chosen in this research. After calculation of related indexes, the model implemented in Flac2D software. The possibility of liquefaction take place in structure foundation was investigated by local tests results in effective stress method. For numerical modeling, the Bam earthquake acceleration was selected in DBE level among the existing acceleration and impact of earthquake dynamic load of on project structure pore pressure was investigated.
Results and Using available data at the project structure the probability of liquefaction in the project region was investigated by cyclic stress method. It was estimated that with safety factor of 1.28 for Bam earthquake magnitude earthquake 6.5M, liquefaction will not occur. Nevertheless, due to exist of pore pressure at structure needed parameters was calculated in Byrne model to study simultaneous impact of dynamic loads and pore pressure to be used in numerical modeling. The results showed an increase in pore pressure due to dynamic loads caused by earthquakes so higher load values to the desired location (subway tunnel) is inserted and finally more effective maintenance system for the tunnel is estimated.

In recent years, due to the progress in the field of concrete technology, some types of high-performance concrete and ultra-high-performance concrete have been developed. Reactive powder concrete (RPC), as a new type of ultra-high-performance concrete, was first patented by a French construction company in 1994. The superior properties of RPC such as high compressive strength (more than 150 MPa), high packing density, very low permeability and higher durability are achieved by eliminating the coarse aggregates and substituting them with fine silica sand aggregates, using high amount of silica fume and low water to cement ratio. Steel fibers are another materials used in RPC. The results of researches show that fibers have the main effect on the increasing of flexural strength, ductility and energy absorption, as in some cases, RPC’s flexural strength has been reached up to 40 MPa [1].

Free forms are usually used to refer double curve surfaces which are independent from groups of geometrically or mechanically constrained forms. Geometrically constrained forms are only conditioned by a geometrical definition, like it could be done on basis of simple surfaces. When there is a close relationship between forms and forces, the forms are mechanically constrained [1]. In the present study, behavior of double domes free form single layer space structures as a group of free form structures are investigated. Gaussian curvature of double domes can be positive and negative, unlike regular domes. In single layer reticulated space structures, local instability with nodal snap-through phenomenon could result in propagation in whole structure [2], so stability behavior of double domes free form single layer space structures should be investigated. The parametric study is performed in order to evaluate the effects of different variables on the stability behavior of double domes free form space structures.

Tensegrity systems are self-equilibrium systems that contain discontinuous compressive components and continuous tensile components. Main factor for self-equilibrium of tensegrity systems is the initial self-stresses of the components. Therefore, consideration of the effects of the levels of these self-stresses on the instability behavior and collapse mechanisms has significant importance. Moreover, because of the application of these systems in large spans and crowded places, sudden collapse leads to a lot of kills and damages. Thus this paper is presented in two parts: first part: presentating a method to find “feasible” self-stress states based on a mathematical topic called “simplex method” and using artificial variables; and second part: using a self-stress state obtained from previous part, and scale it in three levels. The effects of the self-stress level on the instability behavior and collapse mechanisms of the barrel vault tensegrity system is considered, and based on the analysis results some suggestionsare presented to determine maximum and minimum self-stress level so the structure doesn’t collapse suddenly and have enough rigidity. The initial state of the system is very specific since it is a self-equilibrated state; moreover the rigidity of the tensioned components is unilateral (no rigidity in compression) and the relational structure is very specific: compressed components are inside a continuum of tensioned components [1]. The equation of static equilibrium of an unconstrained reference node i connected to nodes j and k are given by Where any member (A, B), that connects nodes A and B, has an internal force fa,b and a length la,b; and fext is the external force. A simplified linearised notation qa,b= fa,b / la,b known as tension coefficient or force density is used. Let x = [x1…xn]T, y = [y1…yn] T, and z = [z1…zn]T, be the vectors of coordinates for n nodes along the x, y, and z directions, respectively. Let q be a vector of tension coefficients, with one entry for each of the b members. We can write the matrix form of Eq. (1) by factorizing the projected lengths in the equilibrium matrix A and a vector q of tension coefficients [2] Results of the analysis is shown in Figs. 2-4. In these figures, the label StoCx_y shows the structure with the rigidity ratio of the struts to the cables equal x and self-stress level y%. Figures show that the increasing the selfstresslevel may lead to overall collapse mechanism. It can be said that the self-stress level doesn’t have aneffect on the initial stiffness and final load capacity of the structure. Self-stress is the one of the interesting and important properties of the tensegrity systems. In this study, to obtain a feasible self-stress for the tensegrity structures, an extended method using simplex method and artificial variableswas presented. Then the effects of the self-stress level on the barrel vault tensegrity structures was studied. Results showed that the self-stress level doesn’t affect the initial stiffness of the structure; but it can change the collapse mechanism of the structure from a local collapse to overall collapse.

Reactive powder concrete (RPC), as a new type of ultra-high-performance concrete, has attracted a lot of attention in recent years. This type of concrete with a compressive strength of more than 150 MPa, has high packing density, very low permeability, superior durability and very high abrasion and corrosion resistance. Small steel fibers, used in RPC, cause the increase of flexural strength and ductility. In the present study, it has been tried to produce reactive powder concrete using the most suitable available materials in Iran. Also, the effects of different curing methods on the properties of RPC have been investigated. The results show that using the available materials together with steel fibers and conducting heat treatment, reactive powder concrete with compressive strength up to 160 MPa and flexural strength of 25 MPa was produced.

The concrete-filled steel tube (CFT) column is a composite system that is made with concrete as inner core and thin walled steel as outer tube. The enhancement of load capacity, increase in lateral stiffness, inherent ductility, use in high rise and long span buildings and fire resistance, are some benefits of CFT columns [1].Columns are the most parts of buildings in fire that carry loads of structure. When fire happens in a structure with CFT columns, the steel tube is the first part of these columns, subjected to fire. Therefore, the steel tube expands and concrete core carries remaining loads. It's a main disadvantage of CFT columns [2]. In the present study, finite element modeling of stiffened circular CFT columns is presented. The main characteristic of the stiffened CFT column is internal longitudinal symmetric stiffeners its internal stiffeners that are longitudinal and symmetric. For predicting the load capacity of the proposed sections of CFT columns, the time – temperature curve is applied to the model.2. Methodology2.1. Fire exposure: The temperature of the column rises up (Eq. (1)) and cools down (Eq. (2)) according to the ISO-834 fire curve (Fig. 1) [3-5]. The temperature is applied to nodes of the columns.where T is the fire temperature in ◦C; t is fire exposure time in minute; ‘‘B–C–C’’ is the heating phase; ‘‘C–D’’ is the cooling phase; ‘‘C’’ is the starting point of cooling; To is the ambient temperature in ◦C; th is the fire duration time in min; Th is the maximum fire temperature in ◦C and tp is the total fire exposure time in min.2.2. Heat transfer analysis: The heat transfer process in concrete filled steel tubes includes radiation, convection and heat conduction, all of them are varying with time. There are four heat transfer processes, from fire to outer steel tube surface, from outer to inner steel tube surface, from inner steel tube surface to outer concrete surface and heat transfer into core of concrete from concrete surface [6,7].2.3. Suggested sections: One of the main disadvantages of CFT columns is the expansion of the steel tube when subjected into the fire. Therefore, the confinement of core and finally, the load capacity of CFT column will be decreased. In order to improve the performance of CFT columns, subjected to the fire, a stiffened steel section is suggested in CFT columns as shown in Fig. 2. The stiffeners enhance the confinement of columns by increasing the contact area between steel and concrete. Hence, the confinement of CFT columns is preserved in heating and cooling stage of the fire exposure. Therefore, it is expected that the load carrying capacity of CFT column to be increased.2.4. Finite element modeling: In the present study, finite element modeling of stiffened circular CFT columns is presented. These models have been analyzed using ANSYS (Ver. 11) [8]. In order to investigate into the behavior of the stiffened CFT columns under axial and thermal loadings,FE analyses should be undertaken involving the geometric and material nonlinearities. Three-dimensional solid element (Solid 65) has been used for modeling of the concrete core, shell element (Shell 43) has been used for modeling of the steel tube. Three-dimensional node-to-node contact element (Contact 178) has been used to model the contact between steel wall and concrete core. Modeling of separation, sliding and contact between two nodes during the loading process are the capabilities of this element. Fig. 3 shows the CFT column, modeled in ANSYS.2.5. Verification of finite element modeling: The material properties of concrete and steel are changing when subjected to the fire. Therefore, some researchers define material properties in ambient temperature, heating and cooling phases. In order to verify the accuracy and validity of the finite element modeling, the numerical results, obtained from nonlinear static analysis, have been compared with the experimental results of the CFT columns. Regarding the axial loading, an experimental CFT column with circular section has been used according to the specifications undertaken by Schneider [9] and in order to verify the accuracy and validity of the equations used for applying fire to the CFT column, model specifications are according to Han [10]. Consequently, it has been found that the finite element model is reliable enough to be used to undertake nonlinear analyses for comparative investigation into the behavior of the sections of CFT columns with and without stiffeners.2.6. Material heat coefficients: The thermal coefficients in CFT columns have been investigated by many researchers. In the present paper, the coefficients for concrete and steel tube, provided by Lie and Stringer [11], are adopted. The thermal properties such as conductivity coefficient (k), specific heat(c), density (ρ) and coefficient of expansion (α) have been defined in these equations. Also, they considered the water evaporation of core concrete after sbeing exposed to the fire. In the evaluation of temperature, k and α decrease, and c increases after a few minutes of fire exposure. They also considered these specifications of concrete and steel in their relations.3. A total of 54 CFT stub columns, in three types, including simple, 2 and 4 stiffened circular CFT columns, have been analyzed after subjected to fire. The specimens are divided into two groups (with the same wall thickness and with the same diameter). All of the specimens, have the same wall thickness and the D/t ratio increased in three steps. All of the specimens are subjected to real fire in ANSYS in three time durations, namely 10 minutes (680oC), 20 minutes (780oC) and 30 minutes (840oC) according to standard fire curve, shown in Fig. 2. In suggested sections the area of stiffeners, is 30% of total area of CFT column and L=3810mm. For steel, we have: Fy= 293MPa and Es= 2.01e5MPa. For concrete, we have: fc= 39.6MPa and Ec= 27800MPa.4. In the present study, finite element modeling of stiffened circular CFT columns is presented. The main characteristic of the stiffened CFT column is its internal stiffeners that are longitudinal and symmetric. These models have been analyzed using ANSYS. Having verified the finite element modeling, several different analyses have been undertaken. Based on the results of analyses, the main conclusions are as follows:-Increasing the tube diameter in simple and stiffened CFT columns has increased the load carrying capacity of columns. The increase rate of the load capacity in CFT columns during fire exposure is more considerable than ambient temperature.-Decrease in the wall thickness of CFT columns, in simple and stiffened columns, decreases the load carrying capacity of columns. The decrease rate of the load capacity in CFT columns during fire exposure is more considerable than ambient temperature.-Increasing the number of stiffeners has enhanced the load carrying capacity of CFT columns. However, increasing the number of stiffeners from 2 to 4 dose not have considerable effect on the load carrying capacity of CFT sections (only 2%).Therefore, the use of 2 stiffeners is more economic, and strongly recommended for CFT columns subjected to fire.-The residual strength index (RSI) is defined to quantify the strength of the CFT columns to standard fire. RSI is the ratio of ultimate strength corresponding to the fire duration time into the ultimate strength at ambient temperature [10]. According to the analysis results, decreasing the number of stiffeners and increasing the tube diameter, has enhanced the RSI of CFT columns.

It is undeniable that instability plays a significant role in the design of pipelines. Local buckles induced by excessive force on specific part of pipeline can locally reduce the collapse pressure of a pipeline and cause local failure. Periodic placement of buckle arrestors along the pipeline can ensure that collapse only effect the length of pipe between two arrestors on either side of the initiation site. Arrestor spacing is usually derived by specific practical considerations on the particular line (e.g. installation and repair procedure) but typically spacing of 90-240m has been used. In the present study, the propagation and arresting of dynamic buckle are modeled using the finite element method. For the post buckling analysis, the material and geometrical nonlinearity are considered. The effects of arrestor spacing on cross over pressure have been evaluated for different diameter to thickness ratio of pipeline (D/t), arrestor’s thickness to pipe’s thickness ratio (h/t), and arrestor’s length to pipe’s diameter ratio (La/D).

Space structures are economical and aesthetic in appearance. They provide a unique solution to cover large column free areas. Sports stadia, industrial buildings, exhibition halls, cinema theatres, swimming pools, airport hangars, conference halls, etc, can be economically covered with the use of space structures. Space grids are characterized as two-way or three-way, depending on whether the members intersecting at a node run in two or three directions. Three-way grids are very extremely strong and lead to a very uniform stress distribution. Barrel vault is one of the oldest architectural forms, used since antiquity. The popularity of barrel vaults derived partly from the economy of these structures, as all arches could be constructed as identical members. Braced barrel vaults are developable surfaces of zero Gauss curvature generated by the movement of a curve, known as the. directrix., over a generator straight line. The directrix may be a circular arc, an ellipse, a catenary, a parabola or a cycloid. Single-layer barrel vaults show a range of different instability modes, namely, member instability, node instability, line instability and overall instability, normally each occurring under different loading and boundary conditions. There are two important types of instability, namely: limit point instability and bifurcation point instability. In limit point instability, a structure with nonlinear un-stiffening characteristics loses its stability by a sudden buckling into a mode of deformation which is in accordance with the initial mode of deformation. However, in the bifurcation point instability, a structure with nonlinear softening characteristics may lose its stability by a sudden buckling into a mode of deformation which is quite distinct from the initial unstiffening mode. There are three types of bifurcation point instability, namely: asymmetric bifurcation, stablesymmetric bifurcation and unstable-symmetric bifurcation. However, in most cases, space structures show limit point instability and unstable-symmetric bifurcation point instability. In single-layer braced barrel vaults, the effect of geometric nonlinearity is very important. In fact, due to small angles between the members, deflections and member forces are large and thus significant changes in structural geometry occur. Also, in these structures the possibility of plasticity occurring in the members causes a reduction in their load carrying capacity. Plasticity can occur before the first limit point load in single-layer braced barrel vaults, so despite the fact that the main source of nonlinearity in these structures is geometric, the inclusion of plasticity in the analysis is necessary to safely predict their true collapse load and post-collapse behavior. Therefore, geometric and material nonlinearities should be considered in the analysis of these structures. In the present study, the stability of singlelayer barrel vaults with three-way triangular configuration is investigated by carrying out nonlinear collapse analysis using finite element method. The effects of different parameters, such as initial geometric imperfections, load on a single node, loads on some nodes, unsymmetrical load distribution with different intensities, length to span ratio, height to span ratio, boundary conditions and material yield stress are considered on the stability behavior of single-layer barrel vaults. According to static collapse analyses of studied models, three collapse mechanisms were determined as overall collapse, local collapse with snap-through and localcollapse without snap-through. In the case of local collapse with snap-through, it has been indicated that to evaluate realistic response of the structure, the static collapse analysis is not adequate by itself, and a dynamic snap-through analysis must be performed, as well.

Concrete filled double skin steel tubular (CFDST) members can be recognized as a new kind of CFT construction. They consist of two concentric steel tubes with concrete sandwiched between them, the steel tubes can be circular hollow sections (CHS) or square hollow sections (SHS) where both inner and outer tubes are circular hollow sections. CFDSTs were firstly introduced for vessels to resist external pressure. Strong potential of using CFDST in offshore construction, highways and high-rise bridge piers. All of analysis have been undertaken using ANSYS which is a general purpose finite element programe designed especially for advanced structural analysis. In order to verify the finite element model, a comparison between the experimental results and finite element results was carried out. Generally, good agreement between the experimental and numerical curves is achieved. ¬The effective parameters are the bond between the steel and internal concrete, local buckling, strength of steel tube and concrete, loading conditions of columns. The result of this study clearly exhibit the high strength and ductility of the CFDST columns under axial loading, and they have lighter weight, than their CFT counterparts.

A pipeline is considered as shell structure so its design is based on stability concepts; dueto the exerted high pressure, local instability is probable to occur and so prevention of itsoccurrence and propagation are important subjects in its design. In this paper, dynamic buckle propagation has been modeled by 3D finite element method, results are verified with experimental tests and velocity of dynamic buckle propagation is calculated for pipes with different diameter to thickness ratios. Due to the effect of velocity in designing of marine pipelines, separate relations based on the initiation pressure are derived for the velocity of propagation and the influence of diameter to thickness ratio on the propagation velocity is studied.

Tensegrity systems are spatial reticulated systems rigidified by a state of selfstress. In the present study, the stability of geometrically flexible tensegrity grids, composed of triangular tensegrity prisms, is investigated by carrying out nonlinear collapse analyses using finite element method.
Furthermore, the effects of configuration (continuous or discontinuous strut configurations), type of boundary conditions and stress-strain relation of tensioned members are investigated on the stability of tensegrity grids. The results of analyses indicate that the type of configuration, boundary conditions and stress-strain relation of tensioned members have considerable effect on the type of collapse mechanism, strength and structural stiffness of studied systems.

In this paper, hydrodynamic pressure due to dam-reservoir interaction for an incompressible, irrotational and inviscid fluid with unbounded domain, is analyzed by using finite element approach. In the derivation of boundary condition, it is assumed that dam is vibrating in the normal direction of dam-reservoir interface and this interface is vertical. Moreover, bottom of reservoir is assumed to be rigid and horizontal. The governing equation with related boundary conditions is implemented in the finite element code. The weighted residual standard Galerkin method with 8-node elements is used for developing finite element model. Both sommerfeld boundary condition and perfect damping boundary condition are developed for truncating surface of unboundedfluid domain and the results of two cases are compared with analytical results.