نشریه سازه و فولاد
سال نهم شماره 18 (پاییز و زمستان 1394)

Direct analysis method is a new approach on stability design of steel structures. This method consider second order effects by applying lateral force and reducing stiffnesses, however the effective length method consider these effects by increasing column length. The direct analysis method takes the effective length factor unity for all members. Joints displacement, out of straightness and out of plumbness as initial imperfections can be modeled directly or alternatively by applying notional loads in design considerations. Inelasticity and geometric nonlinearities calculated by reducing member stiffness in design considerations. In heavy oil structures, significant gravity loads and column length lead to more second order effects in compare to typical structures. This study evaluates and compares nonlinear dynamic analysis responses of designed heavy oil structures in both direct analysis method and effective length method based on ANSI/AISC 360-10 provisions.

Earthquakes in recent years showed that design of structures using force method is not a reliable method for design of structures against earthquake. Performance Based plastic Design is developed as a modern way in seismic design of structures in order to refine the seismic performance of stable buildings faced to earthquake. In this way use the balance of internal and external work to calculate the base shear. Also, this method provides a new distribution of internal force that nonlinear effect of higher mods can be considered in the design of structures. Select the target displacement and remove the undesirable mechanisms at the beginning of the design process is another advantage of this method compared to the force method. This paper assesses the force and performance based plastic design method for Special Moment Resisting Frames whit RBS connection. By this point, four buildings with 4, 8, 12, 20 stories of Special Moment Resisting Frames Based on Iran Building Regulations, and for the second time attempted to apply plastic design based on Performance based Design method. Performance based Design Method was applied and Pushover Analysis on The Original Building carried out. Performance and yielding mechanism of the buildings showed that both types of limit states satisfied the Life Safety level, although under severe earthquake record satisfied Immediate Serviceability level. Performance Based Design under stronger excitations reached for target point mechanism, but in the Force Based Design Method was observed that Performance Based Design is as beneficial as Force Based Design Method.

In this paper flexural buckling load for tapered columns has been evaluated (in-plane buckling of frame). The studied issue is a one-bay frame, its two bases are hinged or rigid, and sway is permitted. The analysis is done by finite differences and virtual work methods. First the length column divided into "n" pieces and the deformation differential equation is approximated in form of finite differences. In this process "n-1" linear equations are obtained which are composed of "n" variables. The variables are the lateral displacement of specified points on the length of the column. For the continuation of the problem an equation is required. It can be gained by the virtual work and consideration of the displacement boundary condition in the top of the column. When the corresponding square matrix of the equations collection is available the critical load will be calculated. The equation obtained by equating the determinate of that matrix to zero has "n" roots. The minimum root is the critical load. The results of the analysis are compared with the other methods of other researchers in some examples. The comparison showed that the introduced method can calculate the critical load with a good accuracy. This method can be generalized for other frames made of tapered members.

The application of transformation induced plasticity (TRIP) steels is increasing because of their good strength and formability. The special microstructure of TRIP steel can be obtained by a two stage heat treatment contains intercritical annealing (IA) and intermediate ageing. The temperatures of these two stages have important effect on microstructure and mechanical properties of TRIP steels. In this paper the effect of heat treatment temperature on microstructure and mechanical properties of steels with chemical composition of Fe-0.2C-1.5Si-1.5Mn was investigated. XRd test was applied to determine the austenite volume fraction of microstructure. For measurement of strength and elongation of cases, tensile test was used and then the results were discussed by thermodynamic calculations. By the increasing of intercritical annealing temperature the volume fraction of austenite, its carbon and the strength of steels increased at first and then were decrease. Increasing of intermediate ageing temperature caused to decreasing of austenite volume fraction, its carbon content, strength and elongation of cases.

In this paper, the lateral-torsional and distortional buckling stability of rafters with elastic supports subjected to gravity loads have been studied by means of a numerical method. To fulfill this purpose, the equilibrium equations for elastic rafter members with linear behavior are derived from the stationary condition of the total potential energy. In the applied energy method, effects of initial stresses and load eccentricities from centroid of cold-formed cross-sections are also considered. The Power series expansions are utilized to solve the fourth-order differential equations of considered rafters with variable coefficients. According to aforementioned method, displacements components and geometric properties are approximated by power series form with a known degree. The critical buckling loads are then derived by solving the eigenvalue problem. In order to present the accuracy of the proposed method, several numerical examples including lateral-torsional behavior and distortional buckling analysis of rafters with cold-form sections are considered. The evaluated results are compared to the finite element simulations and other available methods.

Integrity and stability of important structures is threatened by terrorist attacks all-over the world. Hence, analysis and design of such structures against blast and impact loads attracts more attention by researchers nowadays. Beam‑to‑column connections play important role in load‑carrying chain in the structure, so that well‑designed ones can retain stability of the structure after failure of some structural elements and, hence, prevent overall collapse of the structure. Present research focuses on against‑blast behavior of extended end plate beam‑to‑column steel connections. In this way, various configurations of these connections is analysed by nonlinear dynamic analysis in the finite element software, ABAQUS/Explicit taking into account strain‑rate effects. The result of this research indicates that against‑blast performance of steel structures with extended end plates is directly influenced by web thickness of beams and columns in the joint zone, and locating stiffeners has important role in this issue. Strengthening the column in the joint with continuity plates and web doubler plates is an appropriate choice for improving the performance of the joint under blast loads.