نشریه مهندسی سازه و ساخت
سال ششم شماره 3 (پیاپی 27، پاییز 1398)

This paper describes a numerical method for the full-range analysis of prestressed concrete flexural beams strengthened with prestressed glass fiber-reinforced polymper (GFRP) sheets, focusing on ductility and flexural strength behaviour. Prestressing concrete structures cause increased flexural strength of these structures thus increased resistance of structures, increased span length, and reduced displacements of beams are resulted, which is very effective and economically feasible. The study of ductility of these structures that have been used a lot nowadays, has a great importance because it expresses the capability of structure in inelastic deformations before destruction of members. Present paper attempts to investigate the effect of prestressed GFRP sheets on ductility and deformation of beams strengthened by this method. Regarding to this, the modelling of three sample of beams in different conditions was studied using finite element software ANSYS. After comparing the results of numerical analysis of concrete beam with GFRP sheet and the numerical results of two types of prestressed concrete beam with wire and proofing the efficiency of presented model, the displacement and energy ductility index, deformation, resistance and the amount of displacement of concrete beams have been investigated. Three beams with 160mm*280mm*3600mm dimensions have been modelled. In summary, the results express that the application of prestressed GFRP sheet may cause the least displacements of beam and an increase of 10% and 10.4% will be achieved in the flexural stiffness and ductility.

Because Steel Gabled Frame, SGF, is essentially a lightweight, in most cases, in addition to the horizontal earthquake component, their vertical component is also ignored. That's while with an increase of SGF bay, structural mass increases significantly and as a Result, vertical motion can produce large inertial forces in the structural members. Previous research shows destructive effects of this component in both concrete and steel structure systems, but so far the effect of earthquake vertical component on SGF has not been Studied before. Therefore this paper could open a new window of study on this type of Structure against both horizontal and vertical motions. Moreover, reviewing the behavior of SGF under the effect of Soil-Structure Interaction (SSI) can help to achieve the better understanding of the SGF Structural Behavior. For this purpose, in this research 4 types of SGF with 20 and 60 m span, and 6 and 12 m height has been considered. Incremental Dynamic Analysis (IDA) with and without regard to the effects of the vertical component as well as SSI was conducted. Then, results were presented in the form of fragility curves at three different performance levels. Results Indicate that vertical component of an earthquake has an important and decisive role in the behavior of SGFs. For example, with considering both vertical and horizontal motion effects, the SGFs enters nonlinear region immediately in comparison with considering horizontal component only. Moreover, reducing the capacity and increasing the probability of fragility and collapse are the most important effects of vertical and horizontal motion simultaneously. But, Consider Soil-Structural Interaction has a less impact on the seismic behavior of SGFs.

The extensive studies in the field of steel shear walls indicate the initial stiffness, strength, ductility and high energy absorption of this system under cyclic load. These sorts of features obviously show this system is suitable for resisting to seismic loads. Seismic walls are used to design new structures as well as to improve exist structures. The connections of the beams and the columns in these walls could be simple or moment resisting. The basic idea of a thin steel shear wall is the use of a diagonal tension field that is formed after buckling of a steel plate and these shear wall are used in two types of stiffened and unstiffened. The parameters investigated in this research are consist of the distance of the cross sheet to the shear wall and the number of cross sheet stiffeners to the plate of the shear wall. The model was simulated in ABAQUS finite element software to examine the parametric studies of the frame with steel shear walls and cross sheet. Then, the results of the analysis were compared together. According to some of results, by increasing the number of stiffeners between plates and walls by 1.5 times, the stiffness and strength of the frame is reduced by 71% and 51% respectively. Reduction the number of stiffeners by 50% results in decrease of shear strength by 13%. Moreover, decreasing the stiffeners distance by 50% leads to reduction of shear strength by 59%.

The steel shear wall, as a system to resist the lateral loads, buckles when it is vulnerable to the lateral loads applied to the frame before it reaches its ultimate strength. This phenomenon is due to the small thickness of shear wall relative to the adjacent beam and column. Hence, the idea of reinforcing shear wall to control the sheet buckling seems to be quite applicable. In this research, cross sheet have been used to increase loading capacity of reinforced sheets, which are attached to the both sides of the steel shear wall. In this regard, at first the steel shear wall was modeled according to Qomy experimental model. After verification of the developed model conducted by comparing with the results from laboratory test, parametric study was carried out on the thickness and width of the reinforcing sheet. The obtained results of the analysis showed that by increasing the sheeting thickness to 1.5 times, shear strength is enhanced by 7%. Increasing the sheeting width by 1.5 times shear stiffness and shear strength are decreased by 42% and 32% respectively and decreasing of this parameter by 50% leads to decrease of shear strength by 8%. According to the obtained results for the verification model, numerical and experimental results are in good agreement.

Buckling Restrained Braces (BRBs) are installed in buildings to control lateral displacements caused by seismic events. Although, conventional BRBs have various advantages comparing to ordinary bracing systems, their high weight and dimensions because of the restraining units is a dominant drawback. In this paper, a new developed type of BRB named perforated core buckling restrained brace is investigated which resolved conventional BRBs shortcoming. Its core consists of a perforated steel yielding plate which is guided and partially stabilized by the restraining unit. The core is mechanized to obtain two yielding lateral bands which are connected by several equidistant stabilizing bridges. At first part of the paper, the hysteretic behaviour of the tested braces and a large scale brace has been analysed and verified with an FEM model which considers the interaction between the core and the encasing member. The model reproduces the hysteretic response during the first cycles and the influence of friction on the axial strain distribution along the yielding core. In the second part, geometrical parameters such as number of holes and their section in the core of brace were studied and trough hysteretic behaviour, stress distribution, core deformation and its condition under ultimate loads, the optimized core is selected and suggested.

A lot of moment resisting steel frames were damaged, after Northridge earthquake. In these building, the usual rigid weld connection didn’t work properly and fielded. In the rigid weld connections, available Shear force in the flanges and lateral torsional buckling are the main reason for weld fracture.Slotted web connections, by improving many weaknesses present in moment connections were introduced by Seismic Structural Design Associates (SSDA), and has been classified as a special moment connection in the American Institute of Steel Construction (AISC), managed to qualify for the Federal Emergency Management Agency (FEMA) regulations.The Slotted Web connection design rationale that sizes the beam slot length, shear plate, and connection weldments, is based upon successful FEMA/AISC ATC-24 protocol test results and extensive inelastic finite element analyses of the beam and column stress and strain distributions. Incorporated in this rationale, which has been accepted by numerous departments of building and safety, are the UBC and AISC Load and Resistance Factor Design (LRFD) Specifications and the 1997 AISC Seismic Design Provisions for Steel Buildings and its supplements. In this connection, the separation of beam flanges removes the shear force from beam flange and it causes to transfer it on beam web. This connection is using moment resisting steel frame structure with proper behaviour and good seismic performance One of disadvantage of this connection is fast buckling of beam flange, to revise this problem in Slotted web connection we add a couple of vertical stiffness up and down the beam flange. Using of these stiffness, not only prevent fast buckling but also increase ductility and seismic performance and when we use these stiffness we will see, the absorbing of energy in hysteric loop will be more than the usual Slotted web connection.

Nowadays the use of FRP composites for strengthening steel structures has been considered by researchers. In present study, The maximum deformation of steel plates and structural sections before and after strengthening by GFRP plates was evaluated by modelling and static analysis using ABAQUS finite element software. The results indicated the amount of increasing rate in stiffness and load capacity of studied steel plates and I-shaped beams. In this strengthening method, better results would be achieved by installing GFRP plates to the flange of the beam in comparison with installing them to the web of the beam. The results for studied steel hollow sections and I-shaped columns, comparing axial and lateral behaviour of specimens before and after strengthening indicates the increasing rate in axial stiffness and therefore increase in load carrying capacity of columns in comparison with bare specimens. In this research, the influence of geometric imperfection on the reduction of the limit loads of the bare as well as the retrofitted steel plates was also evaluated. The results indicated that the strengthening of steel plates with GFRP plates could be decreased by the sensitivity of them due to presence of initial geometric imperfections, particularly for plates with higher width-to-thickness ratio.

Nowadays, Steel plate shear walls have been considered as the lateral load resisting system in buildings in increasing the strength and stiffness of structures in two sections of seismic reconstruction and improvement. In this paper, the shear strength and stiffness of a stiffened steel plate shear walls under various stiffeners configuration horizontal, vertical and combined structures with finite element method has been studied and finally the proposed equations for determining the unstiffened equivalent thickness of the steel plate The proposed model is used to design a stiffened steel plate shear wall using the proposed equations of the plate frame interaction method. The results indicate a acceptable prediction of the capacity and stiffness of the stiffened steel shear walls using proposed equations and the error rate has been less than 15%.Nowadays, Steel plate shear walls have been considered as the lateral load resisting system in buildings in increasing the strength and stiffness of structures in two sections of seismic reconstruction and improvement. In this paper, the shear strength and stiffness of a stiffened steel plate shear walls under various stiffeners configuration horizontal, vertical and combined structures with finite element method has been studied and finally the proposed equations for determining the unstiffened equivalent thickness of the steel plate The proposed model is used to design a stiffened steel plate shear wall using the proposed equations of the plate frame interaction method. The results indicate a acceptable prediction of the capacity and stiffness of the stiffened steel shear walls using proposed equations and the error rate has been less than 15%.

The moment beam-to-column connections include those connections that can transfer the moment between the beam and the column. Bolted unstiffened and stiffened extended end-plate moment connections are from this category. This type of connection, which was investigated after severe damage to connections in the Northridge earthquake, Due to proper behavior under cyclic loading, was introduced as one of the pre-qualified connections of the AISC code and subsequently by the Iranian Code. Since low cyclic fatigue is another cause of failure in connections under the Northridge earthquake, it is necessary to study the fatigue behavior of this type of connection under the effect of low cycle fatigue. So far little studies have been done on the effect of low cycle fatigue on various types of connections, including end plate connections. In the present study, the S-N fatigue chart (Effective stress to the number of load cycles) that was used to estimate the effect of high cycle fatigue was developed to the low fatigue region for the end plate connections. Then, with using the S-N chart, the effect of low cycle fatigue on a 12-story structure under the influence of various earthquake records was studied to investigate the effect of low-cycle fatigue on structures with end plate moment connections. The analysis of steel structures showed that in end plate moment connections at the plastic hinge position, the effect of the cumulative cyclic fatigue before the plasticization of the connection under severe earthquakes is influential. Since the effects of fatigue are cumulative, it will be significant and destructive for end plate connections of buildings that have experienced several medium and large earthquakes.

Nowadays considering the growth of the population and increasing the need for housing, construction activities have been booming more remarkably than the past. So, the volume of construction waste has increased which has caused many environmental problems. These derbies have potation to reuse in construction activities. Recycling is one of the best ways to manage this debris. Because it reduces the volume of waste. It also confirms to reusing, and it reduces the need to extract and uses natural resources. Considering the importance of this, this waste can be used as replacement of aggregates in concrete. In this study, the effect of different percentages of replacement of construction and demolition waste as coarse aggregate on mechanical properties of concrete has been investigated, and the effect of using these debris on compressive strength, tensile strength and flexural strength concrete was studied. The percentages of replacement are 0%, 10%, 20%, 30%, and 50%, and the water to cement ratio is 0.49. Tests were done for two curing time 7 and 28 days. Also, in order to analysing the results, statistical analysis of one-way ANOVA was carried out. The results show that with increasing the replacement rate, compressive strength, tensile strength and flexural strength are decreased, but these decreases aren’t significant

One of the common methods for estimation of structural failures and earthquake damages is using fragility curves with probabilistic methods. In these methods, a structure such as a bridge can be classified into several structural types in terms of number of spans, type of connections, material used for members, even type of soil, and so on. Also, the seismic load on the bridge can be considered with indicators such as peak ground acceleration (PGA) or spectral acceleration (SA). In this research, the number of span and soil type of concrete bridges with continuous deck have been investigated on the fragility curves. For this purpose, the two, three, and four span bridges were modeled in the finite element software. Then, the fragility curves of each condition were determined by probabilistic method and by performing nonlinear incremental dynamic analysis (IDA) for far field accelerograms and for four soil types of seismic design code of Iran. The results show that the soil type of the bridge site has a significant impact on the fragility curve, and the probability that the bridges will collapse will vary with the type of soil from type I to IV and damage increases as the number of bridge spans increases.

Self-compacting concrete or SCC can be considered as the concrete of the future. The most important feature of self-compacting concrete is flowing under its own weight to fill the mould or framework completely, and also to create a dense and sufficiently homogeneous mix between the bars, without any need for vibration. But the excessive Liquidity of this concrete increases the hydrostatic pressure on the frameworks. So, decrease in the weight of self-compacting concrete could be lead to reducing the pressure on frameworks. It is also clear that decreasing the density of concrete has a significant effect on the reduction of the structure’s weight and will lead to decrease in the size of structural elements. On the other hand, there are general concerns about the segregation of lightweight aggregates during transportation or placing. In this research, the ability of maintaining the homogeneity of the mixture and the mechanical properties of lightweight self-compacting concrete (LWSCC) made with common lightweight aggregates in the country including Scoria, Leca and Pumice, has been investigated on concrete columns, using the Semi-destructive and non-destructive methods. To do this, reinforced concrete columns made by self-compacting concrete were investigated using a semi-destructive core test to determine the compressive strength and impermeability. Non-destructive ultrasonic test at different heights of the columns was also conducted. The results showed that Scoria aggregate had generally better performance in self-compacting concrete compared to other lightweight aggregates.

In this study, seismic behavior of isolated structures supported on different types of friction pendulum bearings namely SFP, DCFP and TCFP subjected to seven records of near fault ground motion have been compared. To investigate the results of different isolators, they are designed in a way to have the same effective period and effective damping as two important parameters. Three-dimensional linear models of three, six and nine story steel moment frames supported on nonlinear FPS, DCFP and TCFP isolators are constructed in the SAP2000 software. By performing nonlinear dynamic analysis and investigating various structural responses such as maximum acceleration of the roof, isolators displacement, base shear and drift of the superstructure, it is showed that the isolation with the TCFP bearing are more efficient than isolation with DCFP and FPS ones. The main reason of this good behavior is five-regime backbone curve of this isolator in which the two last stages of the motion have hardening behavior. In addition, the comparison of the performance of the different isolators suggests the superiority of the TCFP bearing in controlling the displacement of the isolator, acceleration of the roof, and drift of the superstructure. The advantages of TCFP isolator are observed in all the studied superstructures with three, six and nine stories.

Different solutions have been proposed by researchers in order to prevent the breakdown in welded connections between beam flange and column web, using moment connections. The use of post tensioned connections is one of the proposed solutions to reduce the damage in steel moment frame connections. The connection is used for high-resistant steel cables, which are in parallel with the beam flange and pass through the column and are anchored in the outer side of the column flange. The post tensioned force of steel cables causes the beam flange to be compressed against the column flange, which results in a connection resistance moment against the exploited loads. The resistance moment creates a restoring force that maintains the self-centering behavior of the frame. On the other hand, sequential earthquakes can cause damage to the steel structures, which has been investigated by some researchers. Residual relative displacement of the floors caused by the initial vibration of the structures, increases the vulnerability of the structure to the future earthquakes. Therefore, the reduction of this parameter improves the behavior of the structure against seismic sequence. In this study, the performance of five-, eight- and twelve-story frames has been evaluated in two cases of flexural rigid welded-connections and post tensioned connections under the influence of sequential records away from the fault. The results show that the maximum residual relative displacements of floors in the frames with post tensioned connection were reduced by about forty percent compared to frames with simple moment connections. Due to the decrease in the residual relative displacement of the floors in the said structures, the use of a post tensioned connection has led to the improvement of seismic behavior in comparison with moment frames with rigid welded connections against seismic sequence