تحلیل اجزا محدود

The negative issues of concrete structures include the large dimensions of concrete members due to the low resistance of concrete compared to steel structures, the high density of reinforcements in some parts, and the brittle behavior of concrete due to weakness in tension. The use of fine aggregate and a lower water-to-cement ratio increases the strength, adding fibers to high-strength UHPC concrete increases the softness and tensile strength of concrete, which is called high-strength concrete with UHPFRC fibers with a compressive strength up to 200 MPa and a tensile strength of up to 14 Mpa. In this research, a technique for the numerical modeling of UHPC concrete is proposed using the LS-DYNA finite element software and modifying the tensile part of a normal concrete material model and the available test data. The validated LS-DYNA model is used to study this concrete on the cyclic behavior of short and thin shear walls. The use of this concrete in the boundary element of the walls has a significant effect in improving their seismic behavior. In this research, the effects of different parameters were investigated. It will be shown that an increment in the percentage of fibers up to 3%, the wall's lateral resistance and initial stiffness increase significantly.

Air temperature variations due to daily, seasonal, and annual changes can affect the bridges. The deformation and stress caused under solar radiation should not neglected in bridge design and their effects can be compared with dead and live loads. Thus, the components of bridges such as bearings, dampers and so on, are seriously affected by the combination of external loads and thermal stress. Different countries have provided temperature gradients in their codes. Almost in all of the codes, the vertical temperature gradient is specified, but unfortunately in none of them, the lateral temperature gradient is presented. Furthermore, in the vertical gradient, there are numerous lacks exist in different codes. For an example, most of the codes do not involve either temperature variations due to annual changes, or not considered the longitude or latitude of the location of the designed bridges. These problems lead the engineers to do the precise study beside the codes provided by each country, for temperature effects on the bridge structures. This paper investigates the effect of vertical and lateral thermal gradient loads for concrete box girder designed based on Iranian Standard Loads for Bridge (ISLB) code, using experimental test and three-dimensional finite element analysis. The ISLB code has two main problems in the field of thermal gradients. Firstly, the vertical temperature gradient provided in ISLB code, cannot used for all bridges in Iran, because each bridge has its unique geographical environment, latitude, longitude, and axis of orientation. Secondly, it does not contain any models for the lateral temperature gradient. To handle these problems, the experimental test is done and thermocouples are installed in different parts of the segment to get the thermal gradients and investigate their effects. In the case of predicting vertical gradient, the recorded data show that, the maximum temperature difference occurs in 1430 hrs with the value of 8.8 °C , while by considering the lateral gradient, the maximum temperature difference occurs at 1130 hrs with the value of 2.9 °C . In this paper, comparison between vertical and lateral gradients leads to consider only vertical gradients in further investigations. Moreover, by applying vertical gradient in the finite element model of the Jenah bridge, maximum thermal stress is occurred in the intersection of the web and top flange with the value of 1.96 MPa and maximum deflection of 4.36 mm in the midspan of the bridge. As a solution for mitigating the negative effects of the thermal gradients, using polyurethane insulation is proposed and modeled in the FE model. Results of simulation reveal that utilizing insolation can reduce the top slab temperatures to 30.5 °C , 29.16 °C , 27.5 °C  and 26.4 °C  from 33.6 °C  in the case of using 2, 3, 5 and 6mm polyurethane insulations, respectively, which results in stress reduction from 1.96 MPa to 1.65, 1.35, 0.63 and 0.38 MPa in the case of using 2, 3, 5 and 6 mm polyurethane insulations, respectively. Furthermore, using insulation can reduce the deflection of the bridge, which in this study, the maximum deflection of the 48 m span is reduced from 4.36mm to 3.57, 2.86, 1.63 and 1.07 mm, by utilizing 2, 3, 5 and 6 mm polyurethane insulations, respectively.

In this research the behavior of thick-walled Circular Steel Tubes (CSTs) with corrosion under axial compression and bending moment have been studied. There are several factors leading to corrosion in steel structures, especially in marine structures that results in strength reduction of whole structure. Steel tubes are widely used in construction of on-shore and off-shore structures such as, oil rigs and platforms, oil and gas transmission lines. Current study focuses on strength reduction of steel tubes with “pitting corrosion”. To this end numerical analyses using finite element software, Abaqus, have been performed. Therefore, utilizing verified models, the simultaneous effects of bending, axial load and corrosion on CSTs have been evaluated. Accordingly, by considering the dimensional characteristics of typical tubular legs of off-shore jackets, a set of parametric study has been undertaken. Pitting corrosion has been characterized in the analyses by relative mass loss ratio index, in companion with morphological indexes of the corrosion, i.e. density and depth of the pits. In accordance with the results obtained and based on the AISC360-22 formula for calculating the load bearing capacity of uncorroded CSTs under combined axial compression and bending moment, two adjustment factors were introduced to scaled down the AISC360-22 formula for accounting the degradation due to the pits. The obtained factors for the relative mass loss ratios vary between 10 to 30 percent showed an average lost bearing capacity of 20 to 50 percent as compared with intact CSTs, with a more profound detrimental effect for deeper pits. The comparison of the proposed method for estimating the lost capacity of the corroded CSTs under combined aforementioned actions in two distinct scenarios, revealed the maximum difference of 11 percent with those obtained from finite element analyses. This showed the applicability of the proposed method.

The steel plate shear wall (SPSW) as a lateral bearing system in high-rise buildings has been focused in recent decades. Today, these lateral bearing members with high ductility, high energy absorption, and high initial stiffness are used in new buildings as well as rehabilitation of existing buildings, especially in earthquake-prone countries. The use of stiffeners in the SPSW reduces the thickness of the infill plate, and increases the strength and stiffness of the SPSW. The aim of this study is to present a new arrangement for stiffeners with high performance in order to achieve higher shear strength and lateral stiffness. For this purpose, after validating the numerical modeling, several SPSWs with common (horizontal-vertical and diagonal arrangement) and proposed arrangement (diagonally along with horizontal-vertical) of stiffeners were modeled in Abaqus finite element software and were subjected under monotonic and cyclic loads. The results showed that the suggested arrangement provided more confinement for the infill plate and enhanced the final shear strength of the SPSW by 47.5%.

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.

In recent years, vibration-based damage detection has been used to assess damage to structures. In this case, damage detection is based on changing the dynamic response before and after damage. In this research, the flexibity matrix method has been used to identify damage in segmental bridge (Imam Khomeini intersection bridge). The advantage of this method is its accuracy and sensitivity to a small number of modes for large structures. The bridge was simulated in Abacus software and after applying weight and prestressing force to the cables, the frequency and modal shapes were extracted for healthy and damaged conditions. Damaged conditions included three scenarios, single (in one place) and multiple (in several places and simultaneously) on the bridge deck as well as damage to the column. Each of the above damages involved a 30% reduction in stiffness. The results showed that this method was able to detect damages in the deck. But more investigation was needed to monitor the health of the column. The accuracy of health monitoring was also assessed using two modes, which had acceptable accuracy compared to the use of 6 modes.

Vertically-mixed structures are buildings constructed with different materials progressing in the vertical direction. These buildings feature a concrete frame on the lower stories but a steel one on top. Reinforced Concrete (RC) column connected to steel column in story that called transition story. The sudden change of stiffness in the transition story in these structures and the mass irregularity prompted research attempts concerning the response modification factor and the analytical period of the structures. There isn’t much research on this issue. Due to below concrete column dimensions upper steel column dimension is limited. To accurately study the transfer of loads, the stiffness of the transition story column, and its failure and seismic behavior, this study investigates full-scale experimental a test specimen under axial and lateral cyclic load. Proposed connection in this study that called trough bolt lap connection had good behavior in seismic load up to 4% drift ratio. No sign of failure and stiffness reduction observed in test results. Analytical finite element study is done for verification and parametric study in transition story column in variable axial load and longitudinal bars ratio to grass section of concrete. Studies have shown no failure in the column connection. Column had force-control behavior in lateral loads.

Nowadays, building structures encounter with challenges such as construction speed and cost, especially in high seismicity zones. To accomplish this, steel structures was developed to accelerate the construction process and other economic issues. According to high strength ductility and energy dissipation, steel structure systems have been used widely in active seismic regions. The idea of application of shear panels has been using from many years ago as systems with high energy dissipation capability in EBFs as link beams and steel shear walls. The purpose of the EBFs design is the yielding of link beam and remaining the adjacent member at elastic region. According to the available criteria in design codes, shear in beams is a force-controlled action that exceeding the specified value as nominal strength is not permissible and the capacity design theory should be considered. Increasing the web thickness is the main effective factor achieving the needed shear strength and leads to the enhancement of plastic flexural capacity. The result of this action is more seismic demands in other structural members to keep in desirable operational level. So the shear plastic hinges is introduced instead of flexural plastic hinges at both ends. At this case because of uniform shear yielding through the web, energy dissipation capability is much better than the flexural yielding which begins from the outer face of the beam located on flanges. The web panels of built-up sections restrained by top and bottom flanges and two-sided transverse stiffeners have the ability to carry further loading beyond the web buckling load. The small lateral web displacements produced by excessive loading are not substantial because of available components to supply more resistance. Using adequate stiff transverse to resist against the out-of-plane deformation resulted from post-buckling; tension field actions are developed in shear panels before reaching the maximum shear strength by forming a truss with tension diagonals and compression verticals fixed by stiffeners. The concept of shear resisting frames with non-prismatic beams were presented with the scope of reduction in link beam rotation, elimination of architectural limitations, restrictions on the ratio of span free length to beam total depth and high energy  dissipation capacity. Shear yielding and out of plane deformations caused by tension action field mainly control the frame behavior and energy dissipation. The proposed system is made up two strong side columns connected to the link element with weaker section in the middle of the frame as shear fuse with non-prismatic beams. Tendency to use haunched beams makes it feasible to achieve any link length ratio especially less than 1.0. This paper presents the introducing, design and performance of 1-story-shear resisting frames with different link length ratios (ranges from 0.5 to 1.6 with 0.1 variations) and shear-controlled behavior. The goal is achieved by implementing pushover and cyclic analyses numerically with ABAQUS software. But at first a verification analysis is done to validate the modeling procedure and reach a good conformity between numerical and experimental results. The outputs are presented in the form of response modification factor, displacement ductility and overstrength factor for pushover analyses and hysteresis behavior, backbone curve, energy dissipation capability and overstrength factor for cyclic analysis. Also at the end, 3, 5 and 7-story-frames were studied through pushover analysis and values of response modification factor and overstrength factor of the total frames presented. The results indicate desirable behavior of 1-story-shear resisting frames from the point of stiffness and strength degradation with high values of response modification factor equal to 9.18.

Many flat slab -column frame structures have been built in many parts of the world، since the beginning of the century. The absence of beams makes the form work simple، increase the clear story height، and decreases total building height. However brittle punching failure is a problem that unnecessarily limits the widespread use of flat plates in active earthquake zones. Many slab -column connections in flat plate structures were damaged and failed in punching shear after the 1985 Mexico City earthquake، the 1989 Loma Prieta earthquake، and the 1994 Northridge earthquake. These shows that slab–column connections are prone to punching shear failure when lateral forces، due to earthquake loading، cause substantial unbalanced moments to be transferred from the slab to the column. Slabs with low or medium reinforcement ratios tend to fail in flexure rather than in punching shear. For slabs with reinforcement ratios of 1% and more، the mode of failure tends to be the punching shear type of failure. Fiber-Reinforced Polymers (FRPs) have gained increasing popularity in retrofit of reinforced concrete members in the last two decades. Using FRP materials to enhance slabs in flexure is very desirable from the application point of view due to the ease of handling and installing. FRP material، unlike steel، are not subject to either corrosion or rust in the long term. There is limited amount of research available on srengthening of slab connections. These studies include investigations where the slabs were strengthened using FRP laminates around a central stub column or bonded over the entire width of the slab. with regard to flexural strengthening، externally bonded FRP strips have been used for the strengthening of one way slabs as well as two way slabs. The determination of the structural behavior of FRP–strengthened concrete slabs requires extensive experimental and/or advanced numerical methods. as far as theoretical methods are concerned، Reitman and Yankelevsky have developed a nonlinear finite element grid analysis based on the yield line theory. Other researchers have employed finite element packages to investigate the structural behavior of strengthened slabs with FRP that a full bond between the concrete and the adjacent strengthening FRP materials was assumed. Recently، Binici and Bayrak reported the test results of a strengthening method using carbon fiber reinforced polymers (CFRPs) as shear reinforcement. Previous studies concentrated on enhancing shear capacity of slab-column connections for new construction. Stirrups، bent up bars and shear studs were used as shear reinforcement in previous studies. This study investigates the application of different methods of strengtheing of flat slabs. At first، slabs were model in ABAQUS in order to study the parameters that influences punching shear capacity. Analytical result indicate that increasing concrete compressive strength improves punching shear capacity. Based on this result، steel reinforcement ratios determines the mode of failure. Then، FEM models of slab were strengthend، and type of strengthening، type of FRP materials، number of strip and layer were investigated. The results show that using FRP strip increases punching shear capacity and reduces energy absorption.