نشریه سازه و فولاد
سال دوم شماره 3 (پاییز1387)

Geometrical imperfections usually occur during fabrication and erection. Initial out-of-straightness is one of the most common types of these flaws. In this paper, the elastic tangent stiffness matrix of an initially curved beam-column element subjected to nodal forces and transverse member loads is derived. Second-order analysis of planar frames is carried out afterwards. The proposed method introduces a new function for explicit modeling of out-of-straightness imperfection and incorporates it directly into the element's stiffness matrix. In addition, the proposed technique uses only one element for each member and presents a single formulation for both tensile and compressive members. The obtained load-deflection curve is compared with those predicted by other methods and the effect of initial out-of-straightness imperfection is also discussed. Results of numerical examples indicate that initial out-of-straightness imperfection is less important if the stability of frame is controlled by second-order P-Δ effect. On the other hand, it can have a considerable effect on the behavior of structure when second-order P-δ effect is dominant.

One of seismic retrofitting approach of reinforced concrete (RC) frames is using structural steel brace elements. In current paper some existing moment resisting RC frames with intermediate ductility was designed to withstand seismic loads which were predicted by second edition of Standard No. 2800. In other to evaluate rehabilitated RC frames, models were loaded according to third edition of Standard No. 2800. Analyses of frames show stress ratio in most of columns were exceeded allowable value. Selected RC frames were rehabilitated using concentric inverted V-brace elements. Retrofitted frames were evaluated based on Seismic Rehabilitation Code for Existing Building in Iran. Also formation of plastic hinges and provided performance levels were investigated. Displacement control plastic hinges and performance levels of nonlinear modeling criteria were extracted from Seismic Rehabilitation Code. Results present formed plastic hinges with their performance levels and discus about used seismic retrofitting approach efficiency for hazard level of 10% earthquake probability of acceptance in 50 years.

Composite Steel plate shear walls (C.S.S.W.) have been used in structures as a lateral resisting system that it’s building by concrete cover on one or both sides of steel plate. This system that identifed as the 3rd version of the shear walls, by increasing of stiffness, ductility & energy absorption is widely used in tall building, retrofitting of structures and tanks.In this paper, evaluation of concrete layer to steel plate thickness ratio has been investigated on C.S.S.W. behavior. In this regard, some experimental and analytical specimens involved of flexible steel fram, steel shear wall & steel plate shear wall composited with reinforced concret layer are built and tested using a rigid frame and actuator. The variable parameters on composite shear wall were the number of concrete layer, distanse between stuts and ratio of steel plate to of concrete layer thickness. The results show that the stiffness of composite shear wall has direct proportion to concrete layer thickness and has inverse proportion to distance between stuts. Increasing on proportion of thickness of steel plate to thickness of concrete layer until optimum thickness reduced the displacement of out of plate and beyond which there would be no merit. On the other hand, usage of two concrete layers on two side of steel plate was reduced effect of secandary bending.

Analysis and design of space structures are normally time consuming since a large number of members is involved. The problem becomes more involved when optimization is performed, where numerous analysis should be carried out. In this paper, neural network and genetic algorithm are applied for optimization of double steel layer grids. 180 models with three topologies, spans between 10 and 75 m and height between 1 and 2.5 m are analyzed and designed for optimum weight. The results have been used for training and testing of proposed neural networks system to predict the weight of the structures. The genetic algorithm based on neural networks is used for design optimization. Thus, for a site plan with arbitrary length and width, the output of the neuron-genetic system is the selection of a double steel grid with optimized shape based on three available topologies with height, distance between columns and length of horizontal members.

Application of the pushover methods have been increased in last decade. In recent years different advanced pushover methods have been proposed to account the higher modes effects and changes in the modal attribute during inelastic phases. Generally in these advanced methods multiple pushover analyses with lateral load corresponding to the considered elastic mode shapes are conducted separately, and then the total seismic response is estimated by combining the responses due to each modal load (multi-run). Since in the inelastic domain the modes are coupled so using of the modal decomposition concepts are not valid in the nonlinear range. Therefore the multi-run pushover methods encounter with some theoretical problems. These methods are not able to reflect the yielding effect of one mode in the other modes and the interaction between modes in the nonlinear range. For this purpose, some researchers have developed enhanced single-run modal methods in which the structure is pushed with combined modal forces. The modal combination concept is used to define the load pattern rather than to combine the nonlinear responses due to each mode. In this paper a new advanced single-run pushover method has been proposed. The proposed method is applied to SAC-9 building. This provides a good estimation of the peak inelastic drift response.

In the traditional method of bridge design, traffic loads are considered as static loads which are increased by the application of Impact Factor. In current standards, the impact factor is only a function of span length or first flexural natural frequency of the bridge. In this research, the effects of different parameters like vehicle speed, axle distance, number of axles and span length on dynamic responses of steel railway bridges and impact factor are considered. For this purpose, dynamic responses and impact factors are calculated for four simple span steel bridges with 10, 15, 20 and 25 meter length. The vehicle speed varies from 100 to 400 km/h and axle distances from 13 to 24 meter. The results indicate that the impact factors calculated from dynamic analysis in various conditions are greater than those suggested in standards like AASHTO and AREMA so the quantities of impact factors in standards are not sufficient. The impact factor is increased rapidly as the vehicle speed is increased. The results also indicate that the ratio of axle distance to span length affects the quantity of impact factor. The impact factor differs for ratios smaller or greater than one. The number of axles affects the impact factor only in resonance conditions. In this research some formulas are proposed for impact factor considering different parameters such as span length, vehicle speed and axle distance.

Steel shear walls that are used in two (thin and thick) forms, are innovative systems for resistance against lateral wind and earthquake loads. The thin type of steel shear wall has recently attracted the researcher’s interests. Due to good performance of this type of lateral resisting system (for example high stiffness, high ductility index and very much energy dissipation) it can be used for retrofitting of vulnerable structures. In this paper describing some tests on retrofitting a steel frame by a steel shear wall, the advantages and possibility of retrofitting vulnerable structures have been investigated and interaction of the frame and steel shear wall at different stages of loading has been studied. In the tests it was observed that for increase in the load carrying capacity of the shear wall, it is better to use stiffer columns and for increase in energy dissipation, more flexible columns are required.