جستجوی مقالات مرتبط با کلیدواژه "شبکه ی دولایه" در نشریات گروه "عمران"

In order to better understanding the structural response of double-layer flat grids, the effects of using of a reinforced concrete (RC) topping in composite action with double-layer space-grid structures on the buckling behavior of such grids are studied. Both non-composite and composite flat grids (with and without any integral RC topping) with different span lengths are considered. The base models with span lengths of 15, 30 and 60 meters are initially selected and designed employing the well-known weight–density procedure. The finite element models are then developed in the ABAQUS environment with special consideration in the modeling procedure for beam type elements of pin connections at two ends in order to employ the capabilities of the software for buckling and post-buckling analyses. Elastic buckling analyses as-well-as non-linear analyses incorporating both the geometric and material non-linearities emphasizing the structural stability and post-buckling behavior are performed. It was found that the presence of a RC cover in composite action with double-layer flat grids can lead a decrease in the weight of steel around 29 % as-well-as maximum vertical deflection around 61 %. The presence of a RC layer can also improve the elastic buckling capacity up to 94 %. The results showed that the ultimate strength of such structures in composite state increases significantly. Finally, the RC layer can improve the post-buckling behavior which can lead an increase in the structural reliability from progressive failure or sudden loss of the structural strength.

Although natural frequencies and mode shapes can be accurately measured by dynamic tests, error bounds in values of damping estimation can be large. Since damping strongly influences the design and control of structures, efforts are made to find the damping that has the least error. The random and bias errors are the two important types of error in damping estimation of structures via frequency domain methods. These errors can be reduced by choosing appropriate frequency spacing. In this study, the frequency spacing leading to the least bias and random errors for estimation of modal damping is determined. The complexity of the damping phenomenon on the one hand and complexity of the structural behavior of a double layer grid, on the other hand, led to this study. For this purpose, a double layer grid constructed from ball joint system was tested. the modal damping ratios related to the first 6 modes of a double layer grid with the ball jointed system were identified for different frequency spacing via two output only modal identification techniques; namely enhanced frequency domain decomposition (EFDD), curve fit frequency domain decomposition (CFDD). The modal damping ratio estimations identified through the two methods were then compared with the results of the input output identification method of Ibrahim time domain (ITD) as the reference value. The results showed that there is an almost linear relationship between the modal damping ratio estimations and the frequency spacing in each mode. At the frequency spacing of 0.0625 Hz, the modal damping ratios obtained from the output only methods showed the least difference (between 0 to 21.43%) with the reference values. At this frequency spacing, the root mean square deviation of modal damping estimations between enhanced frequency domain decomposition (EFDD) and curve-fit frequency domain decomposition (CFDD) with the corresponding reference values was 0.02.

The dynamic response of the double-layer girds (a kind of space trusses) can be determined by means of the software based on the finite element method. However, it is always needed the approximate techniques by which one can easily and quickly obtain the suitable estimation of the dynamic behavior of the structures. This need may be caused by: 1- providing an appropriate estimation in the first stage of designing the members, and 2- verification of the results of the computer programs.
               In this study, an approximate method is suggested for seismic analyses of the double-layer grids with the spatial square on square layout, the square or rectangular plan and simple supports around the structure. The method is on the basis of the simulation of double-layer grid to the equivalent beam and plate. Here, the smallest circular frequency of the vertical vibration is computed by using Rayleigh’s method. In addition, the seismic forces caused by the vertical acceleration of the earthquake are evaluated by employing the equations of the dynamics of the structures and estimating the shape function of the first mode of the vertical vibration. There are the deflections of the structural nodes in the achieved equation for estimating the circular frequency; therefore, an initial estimation for the sizes of the members of the double-layer gird is required. In fact, since the weight of the structure should be considered in evaluating the applied loads, an approximate design must be performed. For this purpose, a step by step technique is suggested based on the AISC Specification to initially design the structure. In addition, the assessment of the deflection of the double-layer girds is done based on the uniformly distributed loads and non-uniformly distributed loads.
            Finally, to assess the suggested approximate method, 21 models of the double-layer grids with the spatial square on square layout are considered. These models have the horizontal and diagonal (web) members with the equal lengths. The supports of the double-layer girds are simple. The bays have 30 to 60 m with the step of 5 m and the number of 10, 12 and 14 horizontal members along the span. In order to analyze and design the models, the members are considered to be connected to each other as bending free connections. The spectrum analysis is done to take into account the seismic effects. Also, SAP2000 is used to analyze and design the models in question, and the results achieved by this software are considered as exact outcomes. On the other hand, the models are analyze by the suggested approximate method and the obtained results including frequencies, seismic vertical forces of the members of the models are compared with the exact outcomes in diagrams. The findings show that the proposed method can be evaluated the dynamic properties of the double-layer girds with a suitable precision which prove the robustness of the derived scheme. In fact, the maximum error in estimating the frequency is about 15%, and 12% if it is considered the variation of the weight of the double-layer girds and if it is not considered, respectively. Furthermore, the comparison of the achieved forces of the members between the exact and approximate results is revealed that the maximum error is about 11%, which is a good estimation for an approximate method.

Large-scale spatial skeletal structures belong to a special kind of 3D structures widely used in exhibition centers, supermarkets, sport stadiums, airports, etc., to cover large surfaces without intermediate columns. Space structures are often categorized as grids, domes and barrel vaults. Double layer grid structures are classical instances of prefabricated space structures and also the most popular forms which are frequently used nowadays.Topology optimization of large-scale skeletal structures has been recognized as one of the most challenging tasks in structural design. In topology optimization of these structures with discrete cross-sectional areas, the performance of meta-heuristic optimization algorithms can be increased if they are combined with continuous-based topology optimization methods. In this article, a hybrid methodology combining evolutionary structural optimization (ESO) and harmony search algorithm (HSA) methods is proposed for topologyoptimization of double layer grid structures subject to vertical load. In the present methodology, which is called ESO-HSA method, the size optimization of double layer grid structures is first performed by the ESO. Then, the outcomes of the ESO are used to improve the HSA. In fact, a sensitivity analysis is carried out using an optimization method (ESO) to determine more important members based on the cross-sectional areas of members. Then, the obtained optimum cross-sectional areas of members are used to enhance the HSA through two modifications. Structural weight is minimized against constraints on the displacements of nodes, internal stresses and element slenderness ratio. In topology optimization of double layer grid structures, the geometry of the structure, support locations and coordinates of nodes are fixed and this structure is assumed as a ground structure. Presence/absence of bottom nodes, and element cross-sectional areas are selected as design variables. In topology optimization of the ground structure, tabulating of nodes is carried out based on structural symmetry: this leads to reduce complexity of design space and nodes are removed in groups of 8, 4 or 1. The presence or absence of each node group is determined by a variable (topology variable) which takes the value of 1 and 0 for the two cases, respectively. The ground structure is assumed to be supported at the perimeter nodes of the bottom grid. Therefore, these supported nodes will not be removed from the ground structure. In order to achieve a practical structure, the existence of nodes in the top grid will not be considered as a variable. This causes the load bearing areas of top layer nodes to remain constant. Also, discrete variables are used to optimize the cross-sectional area of structural members. These variables are selected from pipe sections with specified thickness and outer diameter. Therefore, in topology optimization problem, the number of design variables is the summation of the number of compressive and tensile element types and the number of topology variables. The proposed approach is successfully tested in topology optimization problem of double layer grid structure. In particular, ESO-HSA is very competitive with other metaheuristic methods recently published in literature and can always find the best design overall. Also, it is determined that HSA method can find better answer in the topology optimization of large-scale skeletal structures, in comparison to optimum structures attained by the GSA and ICA.

Reticular space structures are widely selected as cover for large spaces, and are also used as refuge spaces for earthquake hit areas. Space structures are light and highly indeterminate and, for some time, these two properties caused researchers to take them as a-seismic structures. However, the 1995 Kobe earthquake in Japan showed that although space structures are safer than conventional buildings, they are not completely a-seismic. Double layer grids (DLG) are one of the most frequently used types of space structure. The aim of this research is to introduce some relations for calculating the earthquake induced forces on these structures. The relations comprise of a) assessing the static equivalent vertical base shear of earthquake action on DLGs and b) assessing the distribution pattern of the vertical base shear on the nodes of these structures. These relations make assessment of the earthquake action on the double layer grids very easy and versatile.
In this research, for assessment of the vertical equivalent static earthquake loading on double layer grids, a set of double layer grids are established. These grids have the same conffguration but dierent dimensions. Also, ffve strong ground motions with high vertical components are selected. The acceleration records of the earthquakes are scaled for a very high seismic area. By numerous linear and nonlinear analyses, a formula for assessing the earthquake vertical base shear on DLGs is produced. Also, a set of equations for distribution of the base shear on the nodes of these structures is developed. In the ffnal stage, the versatility of the equations is examined on some double layer grids, and a comparison is made with the results of the equations of the code of practice of China. It is shown that the introduced equations in this research make the DLGs safer against strong ground motions.